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Energy Storage Systems:
A Comprehensive Guide
First Edition
Abdellatif M. Sadeq
I
@Abdellatif M. Sadeq, 2023
Energy Storage Systems: A
Comprehensive Guide
First Edition: September 2023
@ Copyright with Author
All publishing rights (printed and ebook version) reserved by the author. No part of this
book should be reproduced in any form, Electronic, Mechanical, Photocopy or any
information storage and retrieval system without a prior written permission.
DOI: 10.13980/RG.2.2.33942.55771
ISBN: 979-8-9907836-5-2
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@Abdellatif M. Sadeq, 2023
PREFACE
Greetings to the domain of Energy Storage Systems (ESS), a sphere where inventive ideas
blend with eco-friendliness to influence the trajectory of energy. This book embarks on an
exploration through the vibrant terrain of energy storage, a vital facilitator in
revolutionizing how we create, apply, and oversee our energy reserves.
The necessity for effective energy storage has never been more prominent. With the rise of
renewable energy sources, an escalating need for electrification, and an ever-expanding
appetite for sustainable solutions, ESS have emerged as critical cornerstones in the global
energy shift.
This comprehensive guide is carefully crafted to impart a profound comprehension of
various energy storage technologies and their diverse applications. Whether you are a
student embarking on an academic expedition, a researcher on the verge of groundbreaking
innovation, an engineer crafting sustainable solutions, a policymaker steering energy
directives, or an industry specialist navigating the swiftly changing energy landscape, this
book is tailored to meet your requirements.
The journey commences with the basics—an exploration of historical origins and the
significance of ESS across multifaceted sectors. It progresses into a thorough analysis of
Thermal Energy Storage (TES), Mechanical Energy Storage (MES), Chemical Energy
Storage (CES), Electrochemical Energy Storage (EcES), Electrical Energy Storage (EES),
and Hybrid Energy Storage (HES) systems.
The book presents a comparative viewpoint, allowing you to evaluate the strengths and
weaknesses of each technology, aiding informed decision-making for specific applications.
Real-life instances and case studies are intricately woven, delivering pragmatic insights
into the present state of ESS and providing glimpses into the prospective horizon.
Equipped with this knowledge, we envision a future where energy storage seamlessly
integrates into our daily lives—where clean, sustainable energy is efficiently harnessed,
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and our collective dedication to a greener planet is powered by innovative energy storage
solutions.
This book extends an invitation to explore, learn, and contribute to a future where energy
is not only stored but optimized and utilized for a sustainable and promising future.
Welcome to this enlightening expedition into the captivating realm of ESS.
Abdellatif M. Sadeq
The Book Author
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ABOUT THE AUTHOR
Dr. Abdellatif M. Sadeq has earned his B.Sc. (2015), M.Sc. (2018), and Ph.D. (2022) in
mechanical engineering from Qatar University. On September 2023, he has obtained a
second M.Sc. in hybrid and electric vehicles design and analysis. He has been working
as a graduate teaching and research assistant at Qatar University since 2015. He has over
eight years of experience teaching undergraduate students various general and
mechanical engineering courses. His areas of expertise in research are internal
combustion engines, premixed turbulent combustion, alternative fuels, fuel technology,
hydrogen production, storage, utilization and combustion, electric and hybrid electric
vehicles design and analysis, renewable energy utilization, energy storage techniques,
system modelling and simulation, automotive wiring harness, battery technology, heat
transfer, and HVAC.
Author’s Signature
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@Abdellatif M. Sadeq, 2023
USING THIS BOOK
This well-organized book on ESS offers a valuable tool for individuals and professionals
aiming to grasp, navigate, and excel in the swiftly evolving domain of energy storage. The
book provides an extensive understanding of diverse energy storage technologies and their
applications, making it an indispensable resource for scholars, researchers, engineers,
policymakers, and industry experts.
Grasping the Basics: The initial segment of the book establishes a strong base by
examining the historical backdrop and importance of ESS across various sectors. Readers
acquire insights into the pivotal role ESS plays in tackling the hurdles of integrating
renewable energy and ensuring grid stability.
Surveying Various Energy Storage Technologies: The subsequent chapters
systematically unravel the complexities of Thermal Energy Storage (TES), Mechanical
Energy Storage (MES), Chemical Energy Storage (CES), Electrochemical Energy Storage
(EcES), Electrical Energy Storage (EES), and Hybrid Energy Storage (HES) systems. Each
technology is thoroughly explored, assisting readers in comprehending their distinct
features, applications, and potential benefits.
Analyzing and Contrasting for Informed Decision-making: Chapter 8 stands out by
providing an all-encompassing comparison of various energy storage technologies. This
analysis equips readers with the knowledge required to make informed decisions based on
specific use cases, efficiency, scalability, and other crucial parameters.
Practical Insights and Anticipating the Future: The book also encompasses a chapter
dedicated to the current state of energy storage systems, showcasing real-life examples and
case studies. Furthermore, Chapter 10 offers a peek into future trends and challenges,
enabling readers to anticipate what the dynamic field of energy storage holds.
Leveraging the Knowledge: Armed with the insights from this guide, readers can
effectively contribute to the advancement of energy storage technologies, devise
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sustainable energy strategies, and make informed choices regarding the implementation
and integration of ESS in diverse applications. Additionally, researchers can employ the
comparative analysis and future projections to identify research gaps and propel innovation
in this critical field.
In summary, this book serves as an extensive and essential guide, equipping individuals
with the understanding needed to navigate the intricate world of energy storage. Whether
aiming to enhance academic comprehension, drive innovation, or make strategic decisions
in the energy sector, this guide offers a comprehensive and informative resource.
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TABLE OF CONTENTS
INTRODUCTION............................................................................................................. 1
CHAPTER 1: INTRODUCTION TO ENERGY STORAGE SYSTEMS (ESS) ........ 2
1.1 The Importance of Energy Storage Systems ................................................................. 2
1.2 Historical Overview of Energy Storage Systems.......................................................... 4
1.3 Significance of Energy Storage in Different Sectors .................................................. 10
CHAPTER 2: THERMAL ENERGY STORAGE (TES) SYSYEMS ....................... 12
2.1 Introduction to Thermal Energy Storage Systems ...................................................... 12
2.2 Sensible Heat Storage (SHS) System ......................................................................... 13
2.3 Latent Heat Storage (LHS) System............................................................................. 35
2.4 Thermochemical Energy Storage (TCES) System ..................................................... 44
CHAPTER 3: MECHANICAL ENERGY STORAGE (MES) SYSTEMS ............... 47
3.1 Introduction to Mechanical Energy Storage Systems ................................................. 47
3.2 Pumped Hydro Energy Storage (PHES) System ........................................................ 48
3.3 Gravity Energy Storage (GES) System ....................................................................... 50
3.4 Compressed Air Energy Storage (CAES) System ...................................................... 52
3.5 Flywheel Energy Storage (FES) System..................................................................... 56
CHAPTER 4: CHEMICAL ENERGY STORAGE (CES) SYSTEMS ..................... 60
4.1 Introduction to Chemical Energy Storage Systems .................................................... 60
4.2 Hydrogen Energy Storage System .............................................................................. 60
4.3 Synthetic Natural Gas (SNG)...................................................................................... 63
4.4 Solar Fuels .................................................................................................................. 65
CHAPTER 5: ELECTROCHEMICAL ENERGY STORAGE (EcES) SYSTEMS 69
5.1 Introduction to Electrochemical Energy Storage Systems.......................................... 69
5.2 Battery Energy Storage (BES) System ....................................................................... 70
5.3 Flow Battery Energy Storage (FBES) system ............................................................ 90
5.4 Paper Batteries .......................................................................................................... 101
5.5 Flexible Batteries ...................................................................................................... 105
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CHAPTER 6: ELECTRICAL ENERGY STORAGE (EES) SYSTEMS ................ 107
6.1 Introduction to Electrical Energy Storage Systems .................................................. 107
6.2 Capacitors ................................................................................................................. 108
6.3 Supercapacitors ......................................................................................................... 111
6.4 Superconducting Magnetic Energy Storage (SMES) System ................................... 116
CHAPTER 7: HYBRID ENERGY STORAGE (HES) SYSTEMS .......................... 120
CHAPTER 8: COMPARISON AMONG THE ENERGY STORAGE SYSTEMS 125
CHAPTER 9: CURRENT STATUS OF ENERGY STORAGE SYSTEMS .......... 128
CHAPTER 10: FUTURE TRENDS AND CHALLENGES...................................... 131
INTRODUCTION
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INTRODUCTION
In today's pursuit of progress and environmental responsibility, the importance of effective
ESS cannot be emphasized enough. ESS serve as the vital link between generating and
utilizing energy, playing a critical role in managing the variability of renewable energy
sources and fortifying the stability of power grids.
Our expedition commences by comprehending the fundamental significance of ESS,
delving into their historical context, and highlighting their relevance across diverse sectors.
Subsequent chapters proceed to examine distinct categories of energy storage systems,
shedding light on their potential and varied applications.
Chapter 2 ushers us into the domain of Thermal Energy Storage (TES) systems, elucidating
the complexities of Sensible Heat Storage (SHS), Latent Heat Storage (LHS), and
Thermochemical Energy Storage (TCES). Simultaneously, Chapter 3 navigates
Mechanical Energy Storage (MES) systems, encompassing Pumped Hydro Energy Storage
(PHES), Gravity Energy Storage (GES), Compressed Air Energy Storage (CAES), and
Flywheel Energy Storage (FES).
Continuing our journey, Chapter 4 delves into Chemical Energy Storage (CES),
spotlighting hydrogen, Synthetic Natural Gas (SNG), and Solar Fuels. In Chapter 5, we
delve into Electrochemical Energy Storage (EcES) systems, encompassing Battery Energy
Storage (BES), Flow Battery Energy Storage (FBES), Paper Batteries, and Flexible
Batteries. Chapter 6 introduces Electrical Energy Storage (EES) systems, showcasing
capacitors, supercapacitors, and Superconducting Magnetic Energy Storage (SMES).
In Chapter 7, we investigate Hybrid Energy Storage (HES) systems, amalgamating diverse
technologies to optimize energy storage solutions. Chapter 8 conducts a comparative
examination of different energy storage technologies, assisting in informed decision-
making for specific applications. Chapter 9 offers a glimpse into the present status of ESS
through real-world instances and case studies.
Our odyssey reaches its pinnacle in Chapter 10, where we prognosticate future trends and
confront the challenges in the continually evolving sphere of energy storage. Secure your
seatbelts as we navigate the dynamic and transformative universe of ESS, steering you
towards a future fueled by harnessed energy. Welcome to the prospective landscape of
energy.
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CHAPTER 1: INTRODUCTION TO ENERGY STORAGE
SYSTEMS (ESS)
In an era marked by swift technological progress and escalating sustainability concerns,
the relevance of ESS looms large. Chapter 1 lays the groundwork for a comprehensive
exploration of ESS, encompassing three pivotal facets: the significance of ESS (Section
1.1), a historical survey of these systems (Section 1.2), and their profound importance
within various sectors (Section 1.3). Within this voyage, the pivotal role that ESS assumes
in molding the energy landscape is uncovered, tracing its historical lineage, and
understanding its transformative influence across a spectrum of industries and applications.
1.1 The Importance of Energy Storage Systems
Energy storage systems hold a pivotal position in today's quest for sustainable and efficient
energy utilization. These systems are reshaping the energy sector by addressing critical
challenges in power generation and distribution. As the world advances toward renewable
energy sources and grapples with grid stability concerns, the significance of energy storage
systems becomes increasingly apparent [1-5]. ESS are imperative for ensuring a
sustainable and dependable energy future as they contribute to the followings:
Incorporating Renewable Energy
One of the principal rationales behind the growing importance of ESS lies in their role in
assimilating renewable energy sources, such as solar and wind, into the power grid.
Renewable energy generation is inherently sporadic, contingent on weather conditions and
daylight hours. ESS can amass surplus energy generated during periods of high production
and discharge it when generation is low, ensuring a steady and consistent energy supply.
This not only diminishes reliance on fossil fuels but also bolsters grid resilience.
Grid Stability and Dependability
ESS bolster the stability and reliability of electrical grids. They function as a buffer,
steadying grid frequency and voltage by absorbing excess energy during peak production
and delivering it during peak demand, mitigating the risk of blackouts and voltage
fluctuations. This guarantees an uninterrupted power supply, even when confronted with
abrupt fluctuations in energy generation or unforeseen contingencies.
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Management of Peak Demand
Energy demand fluctuates throughout the day, with peak demand typically occurring
during the evening when people return home from work. ESS can discharge stored energy
during peak demand periods, diminishing the necessity for costly peaker plants used
exclusively during peak demand hours. This not only reduces costs but also curtails
greenhouse gas emissions linked to peak power generation.
Decentralization of Energy Grids
ESS facilitates the decentralization of energy grids. Distributed energy storage systems can
be installed at various grid points, including residences, commercial establishments, and
industrial facilities. This decentralization diminishes the need for lengthy transmission
lines and trims energy losses during transmission. It also elevates grid resilience by
lowering vulnerability to widespread outages.
Shifting Renewable Energy in Time
ESS enable the temporal relocation of renewable energy. Surplus energy generated during
periods of low demand or high renewable energy production can be stored and harnessed
when required. This adaptability permits a better synchronization between energy supply
and demand, curtailing waste and optimizing resource utilization.
Backing Electric Vehicles
As Electric Vehicles (EVs) gain traction, ESS become indispensable in supporting the
burgeoning EV market. They provide a means to efficiently charge EVs, manage the
augmented demand on the grid stemming from widespread EV adoption, and facilitate
intelligent charging solutions that balance energy consumption between vehicles and the
grid.
Grid Resilience during Extreme Events
ESS play an instrumental role in reinforcing grid resilience during extreme weather events
like hurricanes, wildfires, or severe winter storms. In such circumstances, ESS can supply
backup power to vital infrastructure, emergency shelters, and medical facilities,
guaranteeing the continued operation of critical services, even when the primary grid is
compromised.
ESS are indispensable elements for nurturing a sustainable and dependable energy future.
They facilitate the integration of renewable energy sources, bolster grid stability, manage
peak demand, encourage the expansion of EVs, and provide resilience in the face of
extreme events. As the world endeavors to reduce its carbon footprint and transition to a
CHAPTER 1: INTRODUCTION TO ENERGY STORAGE SYSTEMS (ESS)
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cleaner energy landscape, the centrality of ESS in achieving these objectives cannot be
overstated. Their significance is paramount, and sustained research and investment in this
domain are imperative for realizing a greener, more efficient, and robust energy future.
1.2 Historical Overview of Energy Storage Systems
Throughout history, the development of ESS has been a cornerstone of humanity's journey
to efficiently harness and manage energy resources. From rudimentary storage methods to
the contemporary, high-tech solutions of today, the evolution of ESS has significantly
influenced human ability to power industries, illuminate homes, and drive progress. This
historical overview delves into the progression of ESS, highlighting pivotal milestones and
technological advancements that have paved the way for modern energy storage solutions
[6-8].
Early Beginnings
The origins of ESS can be traced back millennia to the moment humans first mastered the
art of fire. The ability to control fire represented an early form of energy storage, allowing
early civilizations to accumulate and release energy as needed for heating and cooking. As
societies progressed, innovations such as water wheels and windmills emerged, harnessing
kinetic energy for mechanical work and grain milling.
The Emergence of Batteries
In the 18th century, a groundbreaking moment occurred with the invention of the inaugural
true battery by the Italian scientist Alessandro Volta in 1800. Volta's creation, known as
the voltaic pile, consisted of a stack of alternating copper and zinc disks separated by
cardboard soaked in brine, generating a continuous flow of electricity. This discovery
marked a significant turning point in energy storage, enabling the controlled storage and
release of electrical energy.
Rise of Lead-Acid Batteries
During the mid-19th century, French engineer Gaston Planté introduced the lead-acid
battery, a technology that remains in active use today. Lead-acid batteries represented a
groundbreaking development in portable energy storage, finding application in early
telegraphy and eventually powering the emerging automobile industry. These batteries
were instrumental in propelling the automotive sector forward, ushering in the era of
internal combustion engine vehicles.
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Advancements in Chemical Storage
The 20th century witnessed remarkable strides in chemical energy storage, including the
advent of Nickel-Cadmium (Ni-Cd) and Nickel-Metal Hydride (NiMH) batteries. These
technologies found application in a wide range of electronic devices, from portable radios
to early laptop computers. However, it was the introduction of Lithium-ion (Li-ion)
batteries in the late 20th century that brought a transformative shift to portable energy
storage. Li-ion batteries offered superior energy density, extended cycle life, and reduced
environmental impact compared to their predecessors.
Modern Energy Storage Solutions
The 21st century has seen the proliferation of diverse energy storage technologies, driven
by the mounting demand for integrating renewable energy, bolstering grid stability, and
promoting electric mobility. Li-ion batteries remain at the forefront, powering ubiquitous
devices such as smartphones, laptops, and EVs. Additionally, emerging technologies like
solid-state batteries, flow batteries, and supercapacitors continue to push the boundaries of
energy storage capabilities.
Grid-Scale Energy Storage
Grid-scale energy storage systems have gained prominence as the world shifts toward
renewable energy sources like solar and wind. These systems employ various technologies,
including Li-ion batteries, pumped hydro storage, and compressed air energy storage, to
capture surplus energy during periods of high generation and release it when demand
surges. Grid-scale energy storage enhances grid stability and facilitates the integration of
intermittent renewable energy sources.
Future Prospects
The historical journey through the development of ESS underscores the persistent drive for
more efficient, sustainable, and adaptable methods of capturing, storing, and harnessing
energy. As technological progress continues, the future holds promising prospects,
including advancements in energy storage materials, increased energy density, and
innovative storage solutions that can cater to the evolving energy needs of an ever-changing
world.
The historical survey of ESS highlights the central role these technologies have played in
shaping human society over time. From the initial mastery of fire to today's state-of-the-art
energy storage solutions, the pursuit of efficient energy utilization and management has
been a driving force behind progress and innovation across the ages. Looking forward,
energy storage systems will continue to be instrumental in addressing the energy challenges
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of tomorrow, supporting the adoption of renewable energy, ensuring grid stability, and
driving the electrification of transportation. The evolution of ESS is detailed in
chronological sequence within Table 1 [9-22].
Table 1: The evolution of energy storage systems.
Year
ESS
Description
Reference
1839
Fuel Cell
In 1839, Sir William Robert Grove introduced
the inaugural simplistic fuel cell. His innovation
encompassed the amalgamation of hydrogen
and oxygen within an electrolyte, resulting in
the generation of electricity and water.
[9]
1859
Lead-Acid
Battery
French physicist Gaston Planté is credited with
conceiving the inaugural practical iteration of a
rechargeable battery founded on lead-acid
chemistry.
[10]
1883
Flywheel Energy
Storage
For military applications, John A. Howell
devised the inaugural Flywheel Energy Storage
in 1883.
[11]
1899
Nickel-Cadmium
Battery
A Swedish scientist, Waldemar Jungner, is the
visionary behind the Nickel-Cadmium battery,
a rechargeable innovation characterized by
nickel and cadmium electrodes immersed in a
potassium hydroxide solution.
[12]
1907
Pumped Hydro
Energy Storage
The utilization of pumped storage commenced
in 1907 at the Engeweiher pumped storage
facility, situated near Schaffhausen,
Switzerland.
[13]
1960
Sodium-Sulfur
Battery
The first Sodium-Sulfur battery emerged in the
1960s, originally developed by the Ford Motor
Company.
[14]
1969
Superconducting
Magnetic Energy
Storage
In 1969, Ferrier introduced the concept of
Superconducting Magnetic Energy Storage to
serve as an energy source capable of
accommodating diurnal fluctuations in power
demands.
[15]
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1977
Borehole Thermal
Energy Storage
A Borehole Thermal Energy Storage facility
comprising 42 boreholes was constructed in
Sigtuna, Sweden, in 1977.
[16]
1978
Compressed Air
Energy Storage
Germany witnessed the installation of the
world's inaugural utility-scale Compressed Air
Energy Storage plant, boasting a capacity of
290 MW, in 1978.
[17]
1982
Supercapacitor
The Pinnacle Research Institute (PRI) played a
pivotal role in the development of the
supercapacitor in 1982, notable for its low
internal resistance, initially intended for
military applications.
[18]
1983
Vanadium Redox
Flow Battery
In 1983, M. Skyllas-Kazacos and colleagues
pioneered the Vanadium Redox Flow Battery at
the University of New South Wales, Australia.
[19]
1983
Polysulfide
Bromide Flow
Battery
The inception of the Polysulfide Bromide Flow
Battery can be attributed to Remick et al. in
1983.
[20]
1991
Lithium-ion
Battery
Sony's release of the inaugural commercial
lithium-ion battery in 1991 marked a significant
milestone in the realm of energy storage.
[21]
2007
Paper Battery
Dr. Robert Linhardt, Dr. Omkaram Nalamasu,
and Dr. Pulickel Ajayan from Rensselaer
Polytechnic Institute, New York, innovated the
concept of paper batteries in 2007.
[22]
It is worth noting that the deployment of ESS started in the 19th century and has undergone
significant evolution to reach their current state. ESS can be categorized based on various
factors, including the type of energy they store, their intended applications, storage
duration, and efficiency, among others. This book specifically concentrates on classifying
ESS based on the type of energy they store, which can include thermal, mechanical,
chemical, electrochemical, electrical, and magnetic forms. Additionally, ESS can
sometimes store energy in hybrid configurations, combining two distinct forms. Table 2
provides a comprehensive list of the ESS discussed in this book.
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Table 2: Classification of energy storage systems according to the type of stored energy.
ESS
Types
Thermal Energy
Storage (TES)
Sensible Heat Storage (SHS)
Liquid
Solid
Latent Heat Storage (LHS) or Phase Change Materials (PCM)
Thermochemical Energy Storage (TCES)
Pumped Thermal Energy Storage (PTES)
Mechanical Energy
Storage (MES)
Pumped Hydro Energy Storage (PHES)
Gravity Energy Storage (GES)
Compressed Air Energy Storage (CAES)
Flywheel Energy Storage (FES)
Chemical Energy
Storage (CES)
Hydrogen Energy Storage
Synthetic Natural Gas (SNG) Storage
Solar Fuel
Electrochemical
Energy Storage
(EcES)
Battery Energy Storage (BES)
Lead-Acid
Lithium-ion
Nickel-Cadmium
Sodium-Sulfur
Sodium-ion
Metal-Air
Solid-State Batteries
Flow Battery Energy Storage (FBES)
Vanadium Redox Battery (VRB)
Polysulfide Bromide Battery (PSB)
Zinc‐Bromine (ZnBr) Battery
Paper Battery
Flexible Battery
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Electrical Energy
Storage (EES)
Electrostatic Energy Storage
Capacitors
Supercapacitors
Magnetic Energy Storage
Superconducting Magnetic Energy Storage (SMES)
Others
Hybrid Energy Storage
Additionally, Figure 1 shows the categorization of primary energy storage systems.
Figure 1: Categorization of primary energy storage systems. Available at:
https://onlinelibrary.wiley.com/doi/full/10.1002/advs.201700322
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1.3 Significance of Energy Storage in Different Sectors
ESS have garnered increasing importance across diverse sectors due to their capacity to
enhance energy efficiency, improve reliability, and facilitate the seamless integration of
renewable energy sources. These systems occupy a pivotal role in tackling the unique
energy-related challenges faced by various sectors, ultimately contributing to sustainability
and economic viability. This section explores the importance of energy storage across
different sectors, highlighting its transformative influence.
Residential Sector
ESS have brought about a revolution in the residential sector by granting homeowners
increased authority over their energy consumption. Residential ESS empowers households
to store surplus energy generated by solar panels during daylight hours and utilize it during
the evenings or during grid outages. This reduces dependence on the grid and leads to
decreased electricity expenses. This technology fosters energy autonomy and diminishes
the carbon footprint of households.
Commercial and Industrial Sector
In the commercial and industrial sector, ESS plays a pivotal role in managing electricity
costs. A significant portion of energy expenses in these sectors often stems from peak
demand charges. Energy storage assists businesses in mitigating these expenses by
discharging stored energy during peak demand periods, thereby circumventing costly peak-
hour tariffs. Furthermore, it offers backup power to safeguard critical operations during
grid disruptions, averting potential substantial losses.
Utilities and Grid Operators
ESS is reshaping the way utilities and grid operators oversee electricity distribution. It
facilitates the integration of intermittent renewable energy sources such as wind and solar
into the grid. By accumulating surplus renewable energy during periods of high generation
and releasing it during peak demand or low renewable generation, utilities can uphold grid
stability and reduce the necessity for fossil fuel-based backup generation.
Transportation Sector
ESS lie at the core of the EVs transformation. Li-ion batteries, in particular, have rendered
EVs practical and accessible to a broader audience. They offer the requisite range and
performance for electric automobiles and are increasingly employed in buses, trucks, and
even maritime vessels. Energy storage is rendering transportation more environmentally
friendly, curbing greenhouse gas emissions, and reducing reliance on fossil fuels.
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Renewable Energy Sector
The renewable energy sector undergoes a seismic shift due to energy storage. It addresses
the inherent intermittency of wind and solar power by stockpiling surplus energy during
favorable conditions and releasing it as needed. This ensures a dependable energy supply
even when environmental conditions are not conducive to energy generation. Additionally,
ESS permits grid stabilization and mitigates the wastage of surplus renewable energy.
Telecommunications and Remote Applications
In remote locales or for critical telecommunications infrastructure, energy storage is
indispensable. It offers a reliable power source for cell towers, remote monitoring systems,
and emergency communication centers. ESS coupled with renewable energy sources
ensure uninterrupted operation, even in off-grid settings.
Agriculture and Water Management
In agriculture, energy storage can optimize water management systems. ESS can
accumulate energy during low-demand periods, such as nighttime, and dispense it during
the day to power irrigation systems. This enhances water efficiency and aids farmers in
reducing energy expenditures while maintaining crop yields.
ESS are reshaping diverse sectors by confronting energy-related challenges and bolstering
sustainability. Whether it involves curtailing residential power costs, upholding grid
stability, propelling clean transportation, or guaranteeing the reliability of vital
infrastructure, the significance of energy storage across distinct sectors is unequivocal. As
technology progresses and costs decrease, the adoption of ESS is poised to expand,
engendering a more resilient, efficient, and sustainable energy landscape across the
spectrum.
CHAPTER 2: THERMAL ENERGY STORAGE (TES) SYSYEMS
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CHAPTER 2: THERMAL ENERGY STORAGE (TES)
SYSYEMS
Chapter 2 provides an introduction to TES systems, emphasizing their importance in
achieving sustainable energy solutions. It delves into Sensible Heat Storage (SHS), Latent
Heat Storage (LHS), and Thermochemical Energy Storage (TCES) systems, offering an in-
depth look at their principles, mechanisms, and practical applications. The chapter
underscores the potential of these TES technologies to revolutionize sustainable energy
practices and decrease the global dependence on environmentally detrimental energy
sources.
2.1 Introduction to Thermal Energy Storage Systems
TES systems are purposefully designed for the retention of heat energy through processes
such as cooling, heating, melting, condensing, or vaporization of a substance. These
systems, which are insulated repositories, store materials either at elevated or reduced
temperatures, and subsequently, the reclaimed energy from these materials serves various
residential and industrial functions. These functions encompass activities like space
heating or cooling, hot water production, or even electricity generation, contingent on the
specific temperature range for operation.
TES systems find application in a diverse range of scenarios, spanning industrial cooling
at temperatures below -18°C, cooling for buildings falling within the 0 to 12°C range,
heating of structures between 25 and 50°C, and high-temperature industrial heat storage
exceeding 175°C [17]. Categorically, TES systems are classified into two primary groups:
Low-Temperature Energy Storage (LTES) systems and High-Temperature Energy Storage
(HTES) systems, based on the temperature at which the energy storage material operates
concerning the surrounding ambient temperature [17, 23].
Within the domain of LTES, there exist two key components: Aquiferous Low-temperature
TES (ALTES) and cryogenic energy storage. ALTES involves the cooling or freezing of
water during periods of low energy demand, subsequently deploying this stored cooling
capacity during peak energy demand periods. In contrast, cryogenic energy storage entails
the boiling of cryogens, typically liquid nitrogen or liquid air, utilizing heat from the
environment. The resulting high-pressure gas is employed to generate electricity via a
cryogenic heat engine. LTES systems are particularly suited for high-power-density
applications such as load management, industrial cooling, and future grid power
optimization [24]. As shown in Figure 2, there exist three primary types of TES systems
currently in use.
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Figure 2: Categorization of thermal energy storage systems. Available at:
https://www.researchgate.net/publication/272179312
Overall, there are three primary types of TES systems currently in use, each serving distinct
purposes. The following sections will provide a brief overview of these systems.
2.2 Sensible Heat Storage (SHS) System
SHS stands as the most widely adopted TES system. This system operates by accumulating
heat energy through elevating the temperature of either a solid or liquid substance by a
specific temperature change (ΔT) while maintaining its phase unchanged. The heat storage
capacity, the extent of temperature alteration (either increase or decrease), and the quantity
of storage material used are primarily governed by the specific heat of the medium [25].
The categorization of SHS, contingent upon the state of the energy storage materials
employed, is succinctly discussed by Socaciu [26]. As depicted in Figure 3, SHS can be
categorized into two distinct types based on the state of the energy storage material
involved: sensible solid storage and sensible liquid storage.
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Figure 3: Categorization of thermal energy storage systems by the material used for energy
storage. Available at: https://www.sciencedirect.com/science/article/pii/S277268352200022X
Sensible liquid storage encompasses a range of approaches, including Aquifer Thermal
Energy Storage (ATES), hot water thermal energy storage, gravel-water thermal energy
storage, cavern thermal energy storage, and molten-salt thermal energy storage. Sensible
solid storage, on the other hand, comprises borehole thermal energy storage and packed-
bed thermal energy storage. Notably, gravel-water thermal energy storage represents a
hybrid system that combines features of both sensible solid and sensible liquid storage
systems.
Among these, ATES, borehole TES, and cavern TES all fall under the category of
Underground Thermal Energy Storage (UTES) systems as they utilize subterranean spaces
as the primary storage medium. A notable advantage of SHS is that the charging and
discharging processes of the storage material are entirely reversible and entail an unlimited
number of life cycles. However, it is worth mentioning that SHS systems do have
significant drawbacks, namely their considerable spatial requirements for storage and
substantial initial capital investment.
2.2.1 Aquifer thermal energy storage (ATES)
An aquifer, which is a permeable rock formation capable of storing and transporting
groundwater, serves as the foundational component of ATES. ATES is a form of sensible
seasonal thermal storage employed for both heating and cooling buildings during winter
and summer seasons, respectively. Typically, an ATES system comprises at least two
interconnected wells and a heat pump, facilitating the extraction and injection of
groundwater (see Figure 4). One of these wells stores hot water (approximately 14–16°C),
while the other contains cold water (approximately 5–10°C). These wells can be configured
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either horizontally, known as a doublet, or vertically within a single borehole, referred to
as a monowell [27]. To maintain effective separation between the warm and cold storage,
a critical distance is maintained, determined by factors such as well production rates,
aquifer thickness, and hydraulic and thermal properties influencing the storage volume.
Large-scale ATES systems feature multiple wells in a multi-well configuration [28].
Figure 4: Diagram representation of aquifer thermal energy storage system. Available at:
https://blog.softinway.com/heat-balance-analysis-of-thermal-energy-storage/
During the summer, groundwater is extracted from the cold well and utilized for cooling
purposes, while the residual warm water is directed into the warm well to replenish the
warm storage. In the winter, this process is reversed, with groundwater from the warm well,
at 14–16°C, being heated to approximately 40–50°C for heating purposes. After heat
transfer, the cold water is reintroduced into the cold well, renewing the cold storage for
subsequent summer use [29]. Given the bidirectional water flow, both wells are often
equipped with heat pumps. It is essential to note that the amount of energy savings achieved
with ATES is greatly influenced by the geological characteristics of the site [30, 31].
The concept of utilizing aquifers to store thermal energy in the form of heated water traces
its origins back to the mid-1960s [32]. Subsequently, numerous laboratory experiments and
field tests have been conducted to explore this aquifer storage concept. Early pioneers in
the theoretical research of ATES include Kazmann [33], Rabbimov et al. [34], Meyer and
Todd [35], and Sauty et al. [36]. In 1965, the first ATES application was reported in
Shanghai, China, as a response to three interrelated problems: ground subsidence,
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groundwater pollution, and the lack of summer cooling for factories. Industries began using
cold water collected during the winter for summer cooling, leading to an expansion of
ATES applications [37]. By 1984, more than 400 wells were utilized for both injection and
extraction. While Shanghai initially employed ATES primarily for industrial cooling, the
need to store both warm and cold energy throughout the year necessitated further
technological development and research. The first application of combined heating and
cooling ATES began at the Scarborough Centre building of the Government of Canada
[38]. Challenges encountered in storing both warm and cold energy included issues like
corrosion, buoyancy flow, and an imbalance between stored heat and cold. However,
research demonstrated that a well-designed ATES operation could mitigate most of these
challenges [39].
Fleuchaus et al. [40] conducted a technical performance assessment of ATES using data
from 73 Dutch ATES systems. The analysis revealed only minor thermal imbalances and
temperature losses over the storage period. However, operational optimization is still
necessary, as the temperature difference between abstraction and injection temperatures is
3 K to 4 K lower than the ideal design value. Guo et al. [41] conducted a review
encompassing theoretical and numerical modeling studies, along with field testing, to
assess the potential of a burgeoning technology known as compressed air energy storage
in aquifers, gaining attention as a means to address the intermittent nature of solar or wind
energy sources. Matos et al. [42] reviewed specific site screening criteria employed to
assess the feasibility of both the reservoir and the technology, aiding in the identification
of suitable technology-reservoir combinations. Comprehensive summaries of the most
significant developments, major hindrances to ATES development, operational
performance, and economic viability can be found in the existing literature [43–48].
ATES facilities are currently operational in several European countries, including Sweden,
Germany, the Netherlands, Belgium, and others [28, 49, 50]. Worldwide, there are around
3,000 ATES systems installed, with over 90% of them located in the Netherlands alone.
Key technical parameters of various global ATES systems, including the world's largest
ATES at the University of Technology in Eindhoven, which saves approximately 13,000
tonnes of CO2 annually (equivalent to the average annual CO2 footprint of 800 Americans
or 1,300 German citizens) , are summarized in Table 3 [51–65]. An assessment of the
benefits and drawbacks of ATES can be found in Table 4.
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Table 3: Overview of technical parameters for various global aquifer thermal energy storage
systems.
Year
Location
Purpose
Number
of Wells
Depth
of
Well
(m)
Flow
Rate
(m3/h)
Capacity (MW)
1991
[51]
University of
Utrecht,
Netherlands
Heating
and
Cooling
2
210–
260
50–100
2.6–6
1996
[52]
Anova
Verzekering Co.
Building,
Amersfoort,
Netherlands
Heating
and
Cooling
2
240
-
2
1997
[53]
Gas and Steam
Turbine Power
Plant in
Neubrandenburg,
Deutschland
Heating
2
1200
200
77
1998
[54]
Hooge Burch,
Zwammerdam
near Gouda,
Netherlands
Heating
and
Cooling
2
135–
151
200
0.6
1999
[55]
Reichstag, Berlin,
Germany
Heating
and
Cooling
12
60–
320
100–
300
-
1999
[53]
Rostock, Germany
Heating
2
15–20
20
-
1999
[56]
IKEA Store,
Amersfoort,
Netherlands
Heating
and
Cooling
2
-
200
1.4
2000
[57]
Klina Hospital,
Belgium
Heating
and
Cooling
2
65
100
1.2
2001
[58]
West Harbour,
Malmö
Heating
and
Cooling
10
70–80
120
1.3
2001
[59]
Mersin, Türkiye
Cooling
2
100
-
0.194
2002
[60]
Agassiz, British
Columbia, Canada
Heating
and
Cooling
5
60
40
0.563
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2002
[61]
University of
Technology in
Eindhoven,
Netherlands
Heating
and
Cooling
36
28–30
3000
20
2003
[62]
Malle ETAP,
Belgium
Cooling
2
67
90
0.57
2008
[63]
Richard Stockton
College, New
Jersey (US)
Cooling
6
35–60
272
2
2009
[64]
Stockholm
Arlanda Airport,
Sweden
Heating
and
Cooling
11
20
720
10
2009
[65]
IKEA Store in
Malmö, Sweden
Heating
and
Cooling
11
90
180
1.3
2011
[50]
National Maritime
Museum,
Greenwich, UK
Heating
and
Cooling
2
60
45
0.4
Table 4: Advantages and disadvantages of aquifer thermal energy storage systems.
ATES
Description
Advantages
Large storage capacity
Affordable construction expenses
Minimal contamination risk
Adaptability for expanding storage capacity as needed
Disadvantages
Potential blockages
Risk of corrosion
Environmental consequences, including the influence of
temperature fluctuations on local ecosystems around hot/cold
wells, the impact of varying temperatures on soil geological
structures, and alterations in aquifer water chemistry and
properties due to temperature variations.
2.2.2 Hot water thermal energy storage
A hot water TES system typically consists of a concrete structure, which may be partially
or entirely buried in the ground, depending on the required storage volume and available
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space (see Figures 5 and 6). Alternatively, some high-capacity storage tanks are
constructed as freestanding structures on the ground. Water serves as a common storage
medium due to its substantial specific heat capacity and rapid charging and discharging
rates. In contrast, concrete can withstand higher temperatures, reaching up to 1,200°C. The
energy storage capacity of such systems is contingent upon the temperature of the stored
hot water and the volume of the storage tank. The level of thermal losses and the duration
of energy storage are determined by the insulation of the tank.
Figure 5: Different categories of large-scale hot water tanks: (1) above-ground tank, (2) partially
buried tank, and (3) fully buried tank. Available at:
https://www.sciencedirect.com/science/article/pii/S277268352200022X
Figure 6: Schematic diagram of hot water thermal energy storage system. Available at:
https://www.sciencedirect.com/science/article/pii/S277268352200022X
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Hot water TES represents a well-established technology widely used on a large scale for
seasonally storing solar thermal heat, often in conjunction with district heating systems.
These systems typically possess storage volumes in the thousands of cubic meters and
operate with charging temperatures ranging from 80 to 90°C [66, 67].
For instance, in Friedrichshafen-Wiggenhausen, Germany, a hot water TES system has
been in operation since 1996. The hot water storage tank in this case is partially buried to
reduce heat losses during the winter. The storage tank is constructed using reinforced
concrete and is insulated only on the roof and side walls. It is lined with 1.2 mm stainless-
steel sheets to ensure water tightness, protect the heat insulation on the outer side, and
minimize heat losses due to steam diffusion through the concrete walls. The stainless steel
liner was initially a costly component of the tank. However, advances in construction
techniques led to the construction of a heat storage tank in Hannover-Kronsberg, Germany,
without the need for a liner. Instead, high-density reinforced concrete was employed [68].
Recent developments in hot water TES systems include the exploration of Glass Fiber-
Reinforced Polymers (GFRP) as a novel wall material, particularly at the Technical
University of Ilmenau in Germany. Omer et al. [69] conducted a comprehensive
examination encompassing a broad range of thermal insulation materials intended for
application in hot water storage cylinders. These materials encompassed organic foams,
inorganic insulations, composite insulations, and vacuum insulation panels. Furthermore,
several studies [70–72] have reported that the incorporation of Phase Change Materials
(PCMs) within hot water tanks can lead to an augmentation in their energy density and
extended discharge duration. Notably, there have been designs of hot water tanks equipped
with PCMs specifically tailored for household applications [73, 74].
Further research has examined TES technologies for solar water heating systems with
integrated PCMs, including integrated PCM storage vessels, integrated PCM solar
collectors, and integrated PCM units within the solar hot water circuit [75-78]. Several
studies have provided insights into optimizing large-scale hot water TES systems,
considering aspects such as geometrical construction, structural design integration, wall
material specifications, operational parameters, and system performance [47, 79- 81]. From
a thermodynamic perspective, hot water TES has demonstrated superiority over other
systems [26, 82].
Table 5 [52, 68, 80, 82, 83] summarizes the geometrical parameters of hot water TES
systems installed in Germany.
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Table 5: Overview of the geometric characteristics of several hot water thermal energy storage
systems installed in Germany [52, 68, 80, 82, 83].
Year
Location
Storage
Volume
(m3)
Solar
Collector
Area (m2)
Tank
Depth
(m)
Service
Area
Solar
Fraction
(%)
1996
Hamburg-
Bramfeld
4500
1650
25.7
124 row
houses
49
1996
Friedrichshafen-
Wiggenhausen
12000
5600
33
570
apartments
47
1998
Ilmenau
300
262
7.2
-
-
2000
Hanover-
Kronsberg
2750
1350
19
106
apartments
39
2002
Attenkirchen
9850
800
8.9
30 single
family
houses
55
2006
Crailsheim-Hirt
480
362
6.3
260
apartments
50
2007
Munich-Acker
5700
2900
24.6
300
apartments
47
2.2.3 Cavern thermal energy storage
A cavern, essentially an underground cave, serves as the foundation for cavern TES
systems, utilizing caverns that can be either natural or man-made structures. However,
these systems are relatively rare due to the scarcity of suitable caverns for implementation.
Typically, abandoned mines, tunnels, and natural karst structures are explored as potential
sites for cavern TES. In the case of artificial caverns, the construction of substantial
underground water reservoirs is necessary to serve as the TES medium. This requirement
results in higher construction and installation costs, which, in turn, limit the practical
feasibility and usage of cavern TES, making it a less commonly employed technology
today. Figure 7 provides an illustration of a basic cavern TES setup. In this system, thermal
energy is either added to or extracted from an insulated tank or store buried underground
by transferring water into or out of the storage unit. During the charging cycle, excess heat
is utilized to raise the temperature of the water inside the storage tank. Subsequently, hot
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water is withdrawn from the top of the insulated tank and used for heating purposes during
the discharging cycle. When initially storing warm Heat Transfer Fluid (HTF) within the
cavern, significant heat losses to the surrounding rocks occur. However, within one to two
years of installation, the cavern establishes a stable thermal halo around itself, with
temperatures gradually decreasing away from the central warm point. Despite some heat
loss, the extremely low thermal conductivity of dry rock results in limited heat loss,
typically less than 10% during one operational cycle under ideal conditions.
Figure 7: Representation of a cavern thermal energy storage system. Available at:
https://www.researchgate.net/publication/331393399
The advantage of rock cavern TES lies in its exceptionally high injection and extraction
rates, but it comes with the significant drawback of extremely high construction costs [84,
85]. While full-scale heat storages have been demonstrated, the elevated installation
expenses have hindered widespread commercialization. Nevertheless, the economic
viability of cavern TES systems can potentially be improved by repurposing existing
caverns or abandoned mines. There are relatively few instances of operational rock cavern
TES systems worldwide. However, researchers have conducted numerous experimental
studies on cavern TES.
For instance, Park et al. [86, 87] and Böttcher et al. [88] utilized numerical analyses to
assess cavern structural stability and thermal performance. Researchers like Park et al. [89],
Park et al. [90], and Kim et al. [91] have investigated the influence of aspect ratio on
thermal stratification and heat loss within rock caverns for underground TES applications.
In the early 1980s, Sweden pioneered the construction of the first two cavern TESs. The
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Avesta cavern TES system, boasting a capacity of 1.5×104 m3, was established in 1981 to
temporarily store heat from an incineration plant. The Lyckebo TES system, with a storage
volume of 1.15×105 m3 and a maximum temperature of 90°C, has been operational since
1983 [92, 93]. There are a few other instances of cavern TES systems that have been
constructed and utilized for thermal storage in district heating systems [94, 95].
2.2.4 Gravel-water thermal energy storage
Gravel-water TES represents an underground heat storage system that offers an alternative
to the construction of large and expensive hot water storage tanks. Instead, it employs an
excavated pit, buried in the ground at a shallower depth typically ranging from 5 to 15
meters [96]. This storage pit is typically designed to be waterproof and is insulated on the
sidewalls and the top (as depicted in Figure 8). Depending on its shape and size, insulation
may also be applied at the bottom of the storage pit. The storage medium typically consists
of a mixture of gravel and water, although alternatives like sand and water or soil and water
can be used. The maximum storage temperature achievable depends on the insulating
materials employed, with a maximum of approximately 90°C attainable.
Figure 8: Schematic diagram of gravel-water thermal energy storage system. Available at:
https://www.researchgate.net/publication/272179524
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Heat is charged into and discharged from the storage system either through direct water
exchange or via plastic pipes installed at various layers within the storage. Notably, due to
the lower specific heat capacity of the gravel-water mixture compared to water alone, the
storage system must be approximately 50% larger in size to achieve the same heat storage
capacity at equivalent temperature levels as water-based TES systems [26, 97].
A review article by Pfeil and Koch [98] highlights the technical advancements associated
with the new third-generation gravel-water TES and demonstrates the significant potential
for cost reduction as well as the environmental compatibility of the materials used.
Additionally, Novo et al. [99] conducted a review focusing on technological advancements
and challenges encountered during the construction and operation of gravel-water TES
systems.
Table 6 [80, 98–102] provides a summary of relevant details regarding gravel-water TES
systems, including technical parameters and advancements in the field.
Table 6: Characteristics of thermal energy storage systems using gravel-water and sand/soil-water
configurations [80, 98–102].
Year
Location
Storage
Volume
(m3)
Solar Collector
Area (m2)
Tank
Height
(m)
Heat Storage
Capacity (MWh)
1984
Institute for
Thermodynamics
and Thermal
Engineering of
Stuttgart
University
1050
-
-
-
1995
Chemnitz,
Germany
8000
540
7
-
1997
Eggenhausen,
Germany
300
115
4
-
1999
Steinfurt-B,
Denmark
1500
1305
4
-
2008
Eggenstein,
Germany
4500
1600
-
-
2011
Marstal, Denmark
75000
1850
16
6000
2013
Dronninglund,
Denmark
60000
37500
-
5400
2014
Gram pit storage,
Denmark
125000
44000
15
12100
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2.2.5 Molten salt thermal energy storage
Molten salts are effective for storing sensible heat above 100 °C, created by fusing
inorganic salts into a liquid state. Their advantages include high boiling points, low
viscosity, low vapor pressure, and substantial volumetric heat capacities. The low vapor
pressure allows for non-pressurized storage solutions, optimizing storage tank space [103].
Molten salt is frequently used in concentrated solar facilities utilizing parabolic or sun-
tracking mirrors. Molten salt fluids like Solar Salts, Hitec, and Hitec XL are commonly
used in energy storage. Table 7 [104–106] provides a comparative overview of the key
characteristics of these three molten salt mixtures. Two primary system configurations
exist: direct (using molten salt for both heat transfer and storage) and indirect (employing
a separate medium for storage). Cold storage ranges from 280 °C to 290 °C, and hot storage
from 380 °C to 550 °C. Tanks vary in size based on capacity, often 12–14 m in height and
over 35 m in diameter for commercial use [107].
Table 7: Characteristics of different combinations of molten salt blends [104–106].
Property
Solar Salt
Hitec
Hitec XL
Composition
by Weight
60% NaNO3, 40% KNO3
7% NaNO3,
53% KNO3,
40% NaNO2
45% KNO3, 7% NaNO2, 48%
Ca(NaNO3)2
Maximum
Operating
Temperature
(°C)
585
450–538
480–505
Melting Point
(°C)
220
142
120
Density
(kg·m−3)
1899
1860
1992
Viscosity
(cp)
3.26
3.16
6.37
In the direct molten salt TES system (as illustrated in Figure 9), concentrated sunlight
reflected from heliostats heats the "cold" molten salt (at 290 °C) entering the tower
receiver. The heated salt (at 550 °C) is then pumped downward into a hot storage tank.
When energy is required, the hot salt from the tank flows through a heat exchanger to
generate superheated steam, which in turn powers the turbine and generates electricity.
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This configuration allows for the achievement of higher temperatures, approximately 535
°C. However, molten solar salt has a relatively high freezing point [103, 106, 108].
Figure 9: Schematic diagram of direct molten salt thermal energy storage system. Available at:
https://www.researchgate.net/publication/255250263
In an indirect molten salt storage system (as depicted in Figure 10), the High-Temperature
Fluid (HTF), typically synthetic oil, circulates through the solar field during the charging
cycle, where it is heated to 390 °C. A portion of the synthetic oil from the solar field is
directed to the oil-to-salt heat exchanger, where it cools down from approximately 390 °C
to around 295 °C. Concurrently, molten salt from the cold storage is routed through this
heat exchanger, where it is heated from 290 °C to 380 °C by the high-temperature HTF.
The heated salt is subsequently stored in a hot storage tank. During the discharging cycle,
heat is transferred from the salt to the synthetic oil within the oil-to-salt heat exchanger to
generate the necessary thermal energy for the power block [103, 106].
Several molten salt energy storage demonstration plants are operational worldwide, and
Table 8 [106, 109, 110] provides details regarding the characteristics of some prominent
systems. These demonstration plants have laid the foundation for the commercialization of
both indirect and direct molten salt storage systems. Notably, French scientists have
introduced the concept of PV-plus-Thermal-Storage (PV-TS) for regions characterized by
low direct solar beam radiation but significant levels of global solar radiation. Their model
integrates solar power generation from utility-scale facilities with high-temperature
molten-salt storage, indicating that when paired with molten salt storage, the grid
penetration rate of a large-scale PV facility may increase from roughly 30% to as much as
95% [111]. Additionally, Roper et al. [112] briefly discuss advanced energy applications
of molten salts, while Caraballo et al. [113] assess the performance of various molten salts
using specific performance metrics.
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Figure 10: Schematic diagram of indirect molten salt thermal energy storage system. Available at:
https://www.sciencedirect.com/science/article/pii/S277268352200022X
Table 8: Attributes of some energy storage systems utilizing molten salt [106, 109, 110].
Plant
Location
Heat
Transfer
Fluid
Storage
Material
Tank Volume
(m3)
Capacity (MW)
CESA-1
Spain
Steam
Hitec
200
12
Thermis
France
Hitec
Hitec
310
40
CRTF
USA
Solar salt
Solar salt
53
7
Solar Two
USA
Solar salt
Solar salt
875
105
Archimede
Italy
Solar salt
Solar salt
25
4
TES-PS10
Spain
Synthetic
oil
Solar salt
220
8.1
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2.2.6 Borehole thermal energy storage
The borehole TES system represents an underground structure designed to store substantial
amounts of solar heat accumulated during the summer for subsequent use in the winter.
Figure 11 provides a schematic representation of a U-shaped borehole TES system. In this
configuration, heat is primarily transferred to the underground environment through
conductive flow via a network of closely spaced boreholes. Vertical Borehole Heat
Exchangers (BHE) are responsible for charging and discharging heat and typically possess
diameters of 100–150 mm, being situated at depths ranging from 30 to 200 meters below
the ground surface [85, 114].
Figure 11: Simplified schematic of a borehole thermal energy storage system during (a) summer
heat storage of solar energy (charging) and (b) winter heat extraction (discharging). Available at:
https://www.researchgate.net/publication/292186784
Within the borehole, a heat exchange pipe, often adopting a U-shaped design, is housed.
These pipes are typically constructed from materials like High-Density Polyethylene
(HDPE) or Cross-linked Polyethylene (PEX). A heat transfer fluid, commonly composed
of an antifreeze solution (ethanol, ethylene, or propylene glycol) supplemented with
additives like biocides and corrosion inhibitors, circulates through the BHE via pumping
[115]. To ensure effective contact between the pipe and the surrounding ground, as well as
to minimize heat resistance, the space between the pipe and the borehole is filled with an
appropriate material. In certain applications, PCMs such as a mixture of n-decanoic acid
and lauric acid (DLC), are utilized as grout instead of conventional materials [116].
Additionally, the system's upper surface is adequately insulated to minimize heat losses.
During the charging phase, heat transfer fluid flows from the borehole's center toward the
storage boundaries, resulting in elevated temperatures at the center and lower temperatures
at the boundaries. The flow direction is reversed during the discharging phase. In summer,
the hot Fluid circulates through the BHE, transferring excess heat to the soil for long-term
storage, while in winter, the stored heat is retrieved through reverse HTF circulation and
used for building heating.
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Key factors influencing the performance of borehole TES include the thermal conductivity
of the storage material (typically water), the number of pipes employed, and the pipe
material. Due to the considerable expenses associated with drilling multiple deep
boreholes, borehole TES tends to be the most costly alternative among natural underground
TES applications. In certain borehole TES systems, ambient ground temperature is directly
utilized for natural cooling, referred to as passive systems, as they recharge naturally.
However, most applications necessitate active cold storage in the soil and rock to achieve
the desired temperature levels, leading to actively recharged systems, which represent true
borehole TES applications. This system boasts high adaptability to various ground
conditions, rendering it one of the most prevalent TES forms [26, 96].
Successful large-scale borehole TES installations have been established worldwide.
Several research works in the archival literature have offered concise reviews of borehole
TES. Shah et al. [117] explored potential seasonal TES systems for space heating,
considering parameters like heating demand, climate conditions, solar resource
availability, storage temperature, energy efficiency, and life cycle cost. Their analysis
suggested that the double U-tube borehole TES integrated with Ground-Coupled Heat
Pump (GCHP) and Evacuated Tube Solar Collector (ETSC) systems represented the
optimal choice for space heating in cold climate zones. Amara et al. [45] assessed the
potential and advantages of utilizing borehole TES technology in buildings located in harsh
climates. The literature also provides comprehensive summaries of significant
developments, major barriers to borehole TES development, operational performance,
economic viability, and state-of-the-art insights through various reviews by Rad and Fung
[97], Skarphagen et al. [114], Gao et al. [118], and Welsch et al. [119]. For additional
reference, Table 9 [120–129] compiles key characteristics of prominent borehole TES
systems.
Table 9: Attributes of notable borehole thermal energy storage systems.
Year
Location
Number of
Boreholes
Storage
Volume
(m3)
Solar
Collector
Area (m2)
Storage Capacity
(MWh)
1980
Kungsbacka,
Sunclay,
Sweden [120]
-
85000
1500
86000
1982
Vaulruz
Project,
Switzerland
[121]
Horizontal
tubes
3500
-
90
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1985
Kerava Solar
Village,
Finland [122]
54
11000
1100
320
1997
Neckarsulm,
Germany [123]
528
63400
5670
-
2000
Attenkirchen,
Germany [124]
90
104
836
77
2001
Anneberg,
Stockholm
[125]
99
60000
3000
550
2004
University of
Ontario,
Canada [126]
370
1.4
million
7000
9700
2007
Akershus
University
Hospital,
Norway [127]
228
300000
-
-
2007
Crailsheim,
Germany [128]
80
39000
-
1135
2007
Drake Landing
Solar
Community
(DLSC) in
Okotoks,
Canada [129]
144
34000
2293
780
2.2.7 Packed-bed thermal energy storage
In packed-bed TES system, a loose arrangement of rock materials forms a bed-like
structure designed for heat transfer. Unlike borehole TES, heat exchange within this system
occurs through inlet and outlet tubes incorporated into the storage setup (as depicted in
Figure 12) [130]. During the charging phase, hot air from the solar collector enters the
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upper section of the storage through an inlet tube, imparting thermal energy to the rock
bed. As a consequence of density variations, the hot air releases heat to the packed rock-
bed storage and then returns to the collector for the subsequent charging cycle. Conversely,
in the discharging phase, typically during the nighttime, cool air from the indoor space
enters the packed-bed storage through the bottom tube. It retrieves the stored heat energy
from the rock bed, which, again due to density differences, heats up and serves as a source
of space heating. Once all the stored heat in the packed bed has been extracted, the storage
system is primed for the next charging cycle during the day [131].
Figure 12: Schematic diagram of packed-bed thermal energy storage system. Available at:
https://www.researchgate.net/publication/353690320
The inception of commercial high-temperature packed-bed TES systems can be traced to
Ait Baha, Morocco, where the construction of the first such system was based on pilot-
scale thermal model test results, affirming the viability of using air as a heat transfer
medium [132]. In 2016, Kröger [133] patented a design concept for a thermal packed-bed
storage system. In this approach, hot air was introduced at the top section of the packed
bed, while cold air entered from the bottom, establishing a thermocline with hot air residing
at the upper part and cold air at the lower portion of the packed-bed. The packed rock-bed
is enclosed by a solid structure that insulates it from external environmental conditions.
One drawback of this concept was the associated insulation costs. Gauché and Louw [134]
introduced a second concept primarily aimed at simplifying the previous design and
making it more cost-effective. In this alternative, hot air was introduced at the center of the
rock bed and allowed to flow towards the outer surfaces. Notably, this concept eliminated
the need for thermal insulation. An optional cover was included to protect the rockpile from
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elements like rain and wind. However, this design had a limitation; during extended TES,
the buoyancy effect caused hot air to rise and dissipate into the atmosphere. Consequently,
the discharge of hot air led to flow reversal, pulling the air back into the duct and out
through the pipe that had originally supplied the hot air to the rock bed. Based on this
second concept, a facility was constructed at the Stellenbosch University Renewable
Energy (SUNREC) site in Stellenbosch, Western Cape, South Africa. However, after the
facility's commissioning, it was recommended to add insulation to ensure proper heat
recovery. Numerous experimental studies and Computational Fluid Dynamics (CFD)
simulations are ongoing to validate the adaptability of this concept [135, 136].
Trevisan et al. [137] explored the thermo-economic performance of a packed-bed TES and
introduced a comprehensive methodology for the design of packed-bed TES systems. He
et al. [138] conducted a review of numerical research pertaining to the heat transfer
performance of packed bed TES systems with spherical PCM capsules, along with
highlighting some practical applications. Alami et al. [139] conducted a comprehensive
comparative study of three different types of Moroccan rocks to identify the most
promising candidates for high-temperature thermal storage materials. The literature offers
an array of further reviews encompassing diverse aspects, including design parameters,
economic feasibility, operation, and performance of packed bed TES systems, authored by
several researchers [140–143]. Table 10 [132,144−159] presents an overview of some
characteristics of packed-bed TES systems, including the solid and fluid material, the
diameter of packed-bed, the temperature and the capacity.
Table 10: Attributes of several thermal energy storage systems utilizing packed-bed
configurations.
Plant
Solid
Material
Fluid
Material
Diameter
of Packed-
Bed (m)
Temperature
(°C)
Capacity (MW)
Daggett,
California
Solar-
Thermal
Pilot Plant
[144]
River
gravel and
No. 6
silica sand
Caloria
HT43
3.2
232–316
10
Numerical
Study of
Rock Beds
[145]
Rocks
Air
0.57
-
-
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Full-Scale
Rock-Bed
Experiment
[146]
River
gravel
Air
1.8
20–60
-
Analytical
Study of
Rock-Bed
Thermal
Storage [147]
Crushed
quarry
rocks
Air
1
30–75
-
100 kWh
Pebble-Bed
Thermal
Storage Pilot
Plant [148]
Soda lime
glass
spheres
Air
0.375
25–70
-
High-
Temperature
Rock Bed
Storage
Experimental
[149]
Rocks
Hindustan
Petroleum
Hytherm
500
2.2
230–250
0.1
Pebble-Bed
Thermal
Energy
Storage
Simulation
[150]
Porcelain
spheres
Air
0.15
25–550
-
Molten-Salt
Thermocline
TES
Experimental
[151]
Quartzite
rocks and
silica sand
Hitec XL
nitrate salt
3
290–390
-
Oil-Pebble
Bed TESS
Simulation
[152]
Sandy
stones
Oil
0.29
20–240
-
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Packed-Bed
Thermal
Storage
Pilot-Scale
[132]
Rocks
Air
2.5–4
20–650
6.5
Experimental
TES
Prototype
System [153]
Solid
bricks
Air
0.51×0.20
20–527.8
-
Packed-Bed
Thermal
Storage
Experimental
[154]
Ceramic
pebbles
Flue gas
0.4
25–900
-
Molten Salt
Thermocline
Storage Tank
[155]
Crushed
rocks
Air
0.4
20–350
-
Packed-Bed
Thermal
Storage
Experimental
[156]
Ceramic
spheres
Mixed
nitrate
molten salt
0.263
250–355
-
Laboratory-
Scale
Experiment
at PROMES-
CNRS Lab
[157]
Alumina
beads
Air
0.58
38–239
-
Pilot-Scale
Thermal Oil
Packed Bed
System [158]
Quartzite
rock
Rapeseed
oil
0.4
160–210
-
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Experimental
Pilot-Scale
Thermal Oil
Packed Bed
[159]
Rock and
sand
Therminol
66
1
20–250
-
The efficiency of a packed-bed TES system is influenced by a multitude of factors such as
the shape and size of storage materials, the porosity of the storage system, and the rate of
heat transfer, among others. Notably, the heat storage capacity of a packed-bed storage
system is approximately 60% lower than that of water-based TES systems. Unlike water-
based TES, which allows for simultaneous heat addition and removal from storage, a
packed-bed TES system does not permit concurrent heat addition and removal [160].
However, the installation and operational costs of packed-bed TES systems tend to be
lower compared to those of water-based storage systems. Furthermore, packed-bed systems
can be effectively employed at high temperatures, whereas water storage systems face
limitations associated with temperature differentials.
2.3 Latent Heat Storage (LHS) System
The LHS system harnesses the energy absorbed or released when a storage material
undergoes a phase change. The ability of the storage material to undergo this phase change
at a consistent temperature is a crucial factor affecting the performance of LHS systems.
The phase transition profile for a storage material when heat is added to or removed from
it is illustrated in Figure 13. Notably, as heat is introduced into the material, it results in
either an increase in temperature (referred to as sensible heating) or a change of phase
(known as latent heating). The material begins in the solid phase, denoted as point O. Heat
addition initiates sensible heating of the solid, leading to a temperature increase (region O–
A). Subsequently, further heating causes melting, transitioning the material from a solid
phase to a liquid phase (region A–B). The heat absorbed during this transformation is
termed the latent heat of fusion. Region B–C represents sensible heating of the liquid,
followed by a phase change from liquid to vapor (region C–D). The heat absorbed during
this phase transition is known as the latent heat of vaporization. Finally, regions D–E
signify sensible heating of the vapor [161].
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Figure 13: Temperature-Energy diagram for heating and cooling of a substance. Available at:
https://www.sciencedirect.com/science/article/pii/S277268352200022X
The categorization of LHS systems (as depicted in Figure 14) is based on the specific phase
change type and the type of energy storage material employed. Sharma et al. [25] and
Kalaiselvam and Parameshwaran [162] provide a comprehensive breakdown of these
classifications. LHS systems can assume various forms contingent upon the type of phase
change, which includes solid-solid, solid-liquid, solid-gas, liquid-gas, and their respective
reversals. In the context of solid-solid transitions, energy is stored as the storage material
shifts from one crystalline structure to another. Such phase changes typically entail
minimal latent heat and result in negligible volume alterations. In contrast, solid-gas and
liquid-gas phase transitions involve higher latent heat, but they also yield substantial
volume changes, which may pose containment challenges. Solid-liquid transitions, when
compared to liquid-gas transitions, exhibit lower latent heat. However, these transitions
lead to only minor volume variations, typically around 10% or less. Consequently, solid-
liquid transitions are frequently employed in TES systems [25].
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Figure 14: Categorization of latent heat storage systems by phase change type and storage
material. Available at: https://www.sciencedirect.com/science/article/pii/S277268352200022X
2.3.1 Ice-cool thermal energy storage (ITES)
The utilization of ice or solid water, either in the form of crystals or slurries, as an energy
storage medium is commonly referred to as ITES [162]. Tables 11 [163] and 12 [164]
provide a summary of the primary characteristics of the two media, namely chilled water
and ice, along with a comparison between them. In the context of ITES, the process
involves storing cool thermal energy (charging) during the phase transition from water to
ice, and subsequently releasing the same energy (discharging) by utilizing the latent heat
of fusion during the phase transition from solid ice to water. The charging and discharging
processes are activated by a Heat Transfer Medium (HTM) or refrigerant that circulates
through cooling coils embedded within the storage tank, as illustrated in Figure 15.
Table 11: Key characteristics of two prevalent storage mediums employed in cold thermal energy
storage systems, specifically, chilled water and ice [163].
Feature
Chilled Water
Ice
Specific Heat (kJ·kg−1·K−1)
4.19
2.04
Latent Heat of Fusion (kJ·kg−1)
-
334
Tank Volume (m3·kWh−1)
0.089–0.169
0.019–0.023
Charging Temperature (°C)
4–6
-6 to -3
Discharging Temperature (°C)
5–10
1–3
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Figure 15: Schematic representation of an ice-cool thermal energy storage system. Available at:
https://www.evapco.eu/products/thermal-ice-storage/extra-pakr-ice-coil
Table 12: An evaluation of the two frequently employed storage options in cold thermal energy
storage systems: ice and chilled water [164].
Ice
Water
Provides a substantial cooling capacity
relative to its volume.
Requires less space to deliver an
equivalent cooling load.
Demands lower maintenance.
Operates at a lower storage temperature.
Demonstrates enhanced chiller charging
efficiency.
Utilizes the same refrigeration design as
non-storage systems.
Features a lower initial cost.
During the charging cycle, the HTM flows through the heat exchanger coil. Heat transfer
occurs between the HTM and the water within the tank, leading to the phase transition of
water into ice while extracting cool thermal energy from the flowing HTM. Subsequently,
during the discharging phase, the cool thermal energy stored in the ice is exchanged with
the warm water to meet the cooling requirements of the building. The effectiveness of the
ITES system in fulfilling building cooling and energy redistribution needs relies on factors
such as the operational mode, type of storage medium, and the characteristics of charging
and discharging processes [165]. Yau and Rismanchi [166] provided a comprehensive
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summary of research and analysis concerning TES systems utilized for space cooling
applications, encompassing various storage technologies and operating strategies. Liu et
al. [167] conducted simulations to optimize the operation of an electric-thermal integrated
energy system with different cool storage subsystems, with results demonstrating that the
integrated energy system employing dynamic ice storage technology exhibited superior
energy efficiency compared to other systems. Hunt et al. [168] explored the use of
swimming pools as long-term cold energy storage systems, wherein solar energy is stored
for cooling purposes throughout the year by creating an ice slurry in the pool during winter
and subsequently using that ice for house cooling in the summer. The authors also
conducted a case study in Phoenix, Arizona, USA, and found that the cost of energy storage
utilizing their proposed system was considerably lower than that of batteries.
Furthermore, ITES has emerged as a primary solution to address the electrical power
imbalance between daytime demand and nighttime surplus. During the night, it establishes
a reservoir of cold storage through refrigeration equipment. This stored cold energy can be
harnessed during the day to provide cooling capacity. The lower nighttime temperatures
enable refrigeration equipment to operate more efficiently compared to daytime, resulting
in reduced energy consumption and load. Consequently, this leads to a reduced requirement
for chiller capacity, ultimately lowering capital equipment costs [169].
2.3.2 Phase change material thermal energy storage (PCM-TES)
PCM-TES involves the use of different phase change materials based on the required
temperature range. It serves as an efficient method for storing thermal energy, offering
advantages such as a high thermal energy storage capacity and an isothermal storage
process. A wide variety of PCMs exist, each melting and solidifying at specific temperature
ranges, making them suitable for a broad range of applications [25]. For PCMs to be
considered suitable for commercial use, they must meet specific criteria outlined in Table
13 [16, 162, 170].
PCMs can be broadly categorized into two groups, as depicted in Figure 16: organic PCMs
and inorganic PCMs. Organic PCMs, exemplified by paraffin wax and fatty acids, offer
high storage density with minimal temperature differences between melting and freezing.
Paraffin wax consists of hydrogen and carbon molecules (CnH2n+2), where an increase in
chain length corresponds to higher melting points and latent heat of fusion, resulting in
greater energy storage capacity [171]. Fatty acids, which are carboxylic acids with
extended aliphatic tails with formula CH3(CH2)2nCOOH, typically have even-numbered
chain lengths between 12 and 28. Fatty acids exhibit superior phase transition properties
compared to paraffin wax but are generally more expensive [172].
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Table 13: Key characteristics of phase change materials [16, 162, 170].
Thermal
Physical
Chemical
Fusion temperature
within the preferred
operational range.
Significant latent
heat of fusion
concerning unit
volume.
Elevated thermal
conductivity in both
the solid and liquid
phases.
Operates at a lower
storage
temperature.
Limited alteration in
volumetric capacity
during phase transition.
Low vapor pressure of
the substance.
Substantial material
density.
Fully reversible.
Capability for charging and
discharging cycles.
Chemical and thermal stability
to endure high temperatures.
Absence of phase separation.
Not flammable, non-toxic, and
environmentally friendly.
Figure 16: Categorization of phase change materials. Available at:
https://www.sciencedirect.com/science/article/pii/S277268352200022X
In contrast, inorganic PCMs offer sharp melting points, high heat of fusion, substantial
latent heat storage per unit mass, and high thermal conductivity. They are classified into
salt hydrates and metallic [173]. Salt hydrates consist of inorganic salts containing water
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of crystallization. In these systems, thermal energy is stored after drying the salt hydrate,
with the dry salt and water stored separately. Salt hydrates are renowned for their high
storage density and minimal heat loss during storage [174]. Metallic PCMs encompass
metals with low melting points and metal eutectics. Metallic PCMs possess a high latent
heat of fusion, excellent thermal conductivity, low specific heat, and low vapor pressure.
Eutectic materials have recently gained attention due to their fixed freezing/melting points,
which result from specific proportions of two or more components that prevent
crystallization, ultimately leading to a lower melting temperature than the individual
constituents [25]. Table 14 [16, 25, 31, 173, 175–177] provides melting points and latent
heat of fusion data for selected organic and inorganic PCMs. These include paraffins,
metallics, inorganic substances, and inorganic eutectics.
Table 14: Melting temperatures and latent heat of fusion for various organic and inorganic phase
change materials [16, 25, 31, 173, 175–177].
Compound
Melting Point (°C)
Heat of Fusion (kJ·kg−1)
Paraffins
Paraffin 14-carbons
5.5
228
Paraffin 15-carbons
10
205
Paraffin 16-carbons
18.2
237.1
Paraffin 17-carbons
21.7
213
Paraffin 18-carbons
27.7
243.5
Paraffin 19-carbons
32
222
Paraffin 20-carbons
36.7
246
Paraffin 21-carbons
40.2
215
Paraffin 22-carbons
44
249
Paraffin 23-carbons
47.5
232
Paraffin 24-carbons
50.6
255
Paraffin 25-carbons
54
238
Paraffin 26-carbons
56.3
256
Paraffin 27-carbons
58.8
236
Paraffin 28-carbons
61.6
253
Paraffin 29-carbons
64
240
Paraffin 30-carbons
65.4
251
Paraffin 31-carbons
68
242
Paraffin 32-carbons
69.5
170
Paraffin 33-carbons
73.9
268
Paraffin 34-carbons
75.9
269
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Metallics
Mercury
38.87
11.4
Cesium
28.65
16.4
Gallium
30
80.3
Rubidium
38.85
25.74
Cerrolow Eutectic
59
91
Inorganic Substances
H2O (Water)
0
333
LiClO3· 3H2O
8.3
253
KF· 4H2O
18.7
231
Mn(NO3)2· 6H2O
26
125.9
CaCl2· 6H2O
29.2
190.8
LiNO3· 3H2O
30.2
296
Na2SO4· 10H2O
32.6
254
Zn(NO3)2· 6H2O
36.2
246.5
Na2CO3· 10H2O
34.2
146.9
CaBr2· 6H2O
30.2
115.5
Na2HPO4· 12H2O
35.7
265
Na2S2O3· 5H2O
48.2
201
Na(CH3COO)· 3H2O
58.2
264
Na2P2O7· 10H2O
70.2
184
Ba(OH)2· 8H2O
78.2
265.7
Mg(NO3)2· 6H2O
89.2
162.8
(NH4)Al(SO4)· 6H2O
95.2
269
MgCl2· 6H2O
117.2
168.6
NaNO3 (Sodium Nitrate)
307.2
172
KNO3 (Potassium Nitrate)
333.2
266
KOH (Potassium Hydroxide)
380.2
149.7
MgCl2 (Magnesium Chloride)
714.2
452
NaCl (Sodium Chloride)
800.2
492
Na2CO3 (Sodium Carbonate)
854.2
275.7
KF (Potassium Fluoride)
857.2
452
K2CO3 (Potassium Carbonate)
897.2
235.8
Inorganic Eutectics
58.7% Mg(NO3)·6H2O + 41.3%
MgCl2·6H2O
59.2
132.2
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66.6% CaCl2·6H2O + 33.3%
MgCl2·6H2O
25.2
127
48% CaCl2 + 4.3% NaCl + 0.4%
KCl + 47.3% H2O
27
188
47% Ca(NO3)2·4H2O + 33%
Mg(NO3)2·6H2O
29.2
190.8
60% Na(CH3COO)·3H2O + 40%
CO(NH2)2
30.2
136
66.6% Urea + 33.4% NH4Br
76.2
161
14% LiNO3 + 86% Mg(NO2)2
·6H2O
72
180
In PCM-TES, when heat is supplied during the charging phase, the PCM absorbs heat
without experiencing a significant temperature increase until it undergoes a phase transition
from solid to liquid. During the discharging phase, as the surrounding ambient temperature
decreases, the liquid PCM undergoes a phase change from liquid to solid, thereby releasing
the stored latent heat [178]. PCMs have the capacity to store 5–14 times more energy per
unit volume compared to traditional sensible storage materials like water and rocks.
Additionally, PCMs can store and release heat energy at nearly constant temperatures,
which is another notable advantage. However, the low thermal conductivities of PCMs
present a significant challenge for their use in large-scale applications. Extensive research
is underway to address this limitation, with the development of innovative methodologies.
For instance, Xu et al. [179] reviewed methods for enhancing the thermal conductivity of
PCMs, with carbon-based and metal-based materials demonstrating superior thermal
properties. Mishra et al. [180] conducted a review of various carbon-based PCMs and their
applications in textiles, heat therapy packs, electronics, and packaging. Hu [181] provided
an overview of recent advances, challenges, and emerging applications for high-
performance polymeric phase change composites in thermal energy storage. Several other
review articles, such as those by Nie et al. [182] and Zhang et al. [183], summarized recent
progress in PCM applications for cold thermal energy storage. Additionally, Panchal et al.
[184] summarized the development of multiple-phase change materials in cascaded latent
heat TES, primarily used for low-grade thermal energy storage applications (cold: below
20 °C and low-temperature heat: between 20 and 100 °C). The authors reviewed criteria
for PCM selection, stacking, and the operating and geometric parameters that influence the
performance of cascaded latent heat TES. Shoeibi et al. [185] provided a review of recent
studies on nano-enhanced PCMs in various solar technologies, highlighting their improved
thermal conductivities and lower melting points, which facilitate the charging and
discharging processes of heat storage units.
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2.4 Thermochemical Energy Storage (TCES) System
TCES represents an indirect method of preserving heat energy, distinct from the direct heat
storage mechanisms seen in SHS or LHS. Instead, TCES relies on the absorption and
release of heat energy during the dissociation and association of molecular bonds, a fully
reversible chemical reaction [5]. This process harnesses the enthalpy of reaction, denoted
as ΔH, to store heat energy. The amount of heat retained hinges on various factors,
including the type and quantity of storage material, the enthalpy of the reaction, and the
extent of conversion [25].
In the context of an endothermic reaction marked by a positive change in ΔH, heat is stored
as reactive components disintegrate into individual constituents. Subsequently, the stored
energy is liberated through exothermic reactions (ΔH < 0) as these individual components
recombine, as illustrated in Figure 17.
Figure 17: Illustration of a thermochemical energy storage system utilizing reversible chemical
reactions. Available at: https://www.sciencedirect.com/science/article/pii/S277268352200022X
The selection of a suitable thermochemical material for TES demands consideration of
several parameters, given their substantial impact on the TES system's effectiveness. Table
15 [186] compiles potential thermochemical storage materials, including their energy
density and reaction temperature values—two pivotal factors for their suitability in
thermochemical TES systems.
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Table 15: Materials with significant potential for use in thermochemical energy storage systems
[186].
Thermochemical
Material
Solid
Reactant
Working Fluid
Energy
Storage
Density
(GJ·m−3)
Charging
Reaction
Temperature
(°C)
Epsomite
(MgSO4·7H2O)
Magnesium
sulfate
(MgSO4)
-
2.8
122
Ferrous Carbonate
Ferrous
oxide
(FeO)
Carbon dioxide (CO2)
2.6
180
Calcium Hydroxide
Calcium
oxide
(CaO)
Water (H2O)
1.9
479
Ferrous Hydroxide
Ferrous
oxide
(FeO)
Water (H2O)
2.2
150
Calcium Carbonate
Calcium
oxide
(CaO)
Carbon dioxide (CO2)
3.3
837
Calcium Sulfate
Dihydrate
Calcium
sulfate
(CaSO4)
Water (H2O)
1.4
89
Numerous review articles [187–189] have sought to evaluate and summarize the realistic
potential applications of TCES, often highlighting challenges impeding the maturation of
this technology. Han et al. [190] conducted a comprehensive assessment of the technical
properties of TCES systems based on cobalt, manganese, and copper oxide, with a focus
on energy density and cycle life. Additionally, recent developments and applications of
TCES systems, particularly those involving hydrated salts, are elaborated upon in
references [191–193].
TCES systems are categorized into two primary types, as depicted in Figure 18: open and
closed systems. In open systems, the working fluid, primarily gaseous, is directly
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discharged into the environment, thus releasing entropy. In contrast, closed systems do not
release the working fluid directly but release entropy into the environment via a heat
exchanger interface [194, 195].
Figure 18: Categorization of thermochemical energy storage systems. Available at:
https://www.sciencedirect.com/science/article/pii/S277268352200022X
TCES systems hold immense potential in the future energy landscape. Among the available
TES systems, TCES stands out due to several promising advantages, including (i) superior
energy densities compared to sensible or phase change materials storage, (ii) versatility in
accommodating a broad operating temperature range, (iii) minimal heat leakage, and (iv)
long-term storage capabilities. However, it is essential to acknowledge that this system's
increased complexity necessitates the development of innovative integration strategies to
address these intricacies [196].
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CHAPTER 3: MECHANICAL ENERGY STORAGE (MES)
SYSTEMS
Chapter 3 focuses on MES Systems, essential for balancing energy supply and demand in
the context of renewables. It introduces MES principles and then examines Pumped Hydro
Energy Storage (PHES) in detail, highlighting its widespread use and efficiency. Gravity
Energy Storage (GES) is introduced as a gravity-based alternative, especially useful in
challenging terrains. Compressed Air Energy Storage (CAES) Systems are discussed,
emphasizing versatility and sustainability. The chapter also explores Flywheel Energy
Storage (FES) Systems, known for their rapid response. Throughout, it emphasizes the
significance of these MES systems in shaping sustainable energy storage solutions.
3.1 Introduction to Mechanical Energy Storage Systems
In the MES system, energy is preserved by converting it between mechanical and electrical
forms [24]. During periods of low demand, typically off-peak hours, the electrical energy
drawn from the power source undergoes a transformation into mechanical energy, which
takes the shape of either potential or kinetic energy. When demand surges during peak
hours, this stored mechanical energy is reconverted back into electrical power. Figure 19
shows the classification of mechanical energy storage systems.
Figure 19: Categorization of mechanical energy storage systems. Available at:
https://www.sciencedirect.com/science/article/pii/S277268352200022X
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As depicted in Figure 19, MES systems are primarily divided into three distinct categories:
Pumped Hydro Energy Storage (PHES), Gravity Energy Storage (GES), Compressed Air
Energy Storage (CAES), and Flywheel Energy Storage (FES). PHES, GES, and CAES
systems store potential energy, while FES systems store kinetic energy [11]. One notable
advantage of the MES system is its ability to rapidly convert and release stored mechanical
energy [5].
3.2 Pumped Hydro Energy Storage (PHES) System
The PHES system stands as one of the most widely adopted MES systems, known for its
vast energy capacity, extended storage duration, and commendable efficiency [197]. A
typical PHES configuration (as depicted in Figure 20) comprises three key components: (i)
two sizable reservoirs positioned at varying elevations, (ii) a mechanism to transfer water
from the lower reservoir to the higher one, and (iii) a turbine that generates electricity as
water descends from the upper reservoir to the lower one [198]. During periods of low
demand, typically off-peak hours (as illustrated in Figure 21), electrical energy sourced
from the power supply undergoes a transformation into mechanical energy. Subsequently,
this mechanical energy is converted into potential energy by pumping water from the lower
reservoir to the higher reservoir. Conversely, during peak hours (during discharge, Figure
21), the stored water from the upper reservoir is released back into the lower reservoir,
initiating the rotation of turbines and thereby generating electricity via generators [198].
Figure 20: Schematic representation of pumped hydro energy storage system. Available at:
https://www.researchgate.net/publication/320755664
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Figure 21: The working principle of pumped hydro energy storage system. Available at:
https://www.researchgate.net/publication/335987186
The amount of energy stored in PHES systems is contingent upon the volume of water
housed in the reservoirs and the disparity in elevation between them [17]. There are
innovative variations of PHES technology such as Underground PHES (UPHES) and
Seawater PHES (SPHES). These adaptations share working principles akin to traditional
PHES systems, with the primary distinction being the type of lower reservoir utilized. In
UPHES, abandoned quarries or mines serve as the lower reservoirs, while in SPHES, the
sea functions as the lower reservoir, offering cost and time-saving benefits in construction
and minimal environmental impact compared to conventional PHES systems.
Additionally, variable speed PHES, a technology already operational in several Japanese
plants, employs asynchronous motor-generators, enabling control of the pump/turbine
unit's rotational speed for energy absorption regulation [199, 200].
The genesis of PHES can be traced back to the Alpine regions of Switzerland, Austria, and
Italy in the 1890s. Initially, these systems employed separate pump impellers and turbine
generators. However, in the 1950s, a more efficient design was introduced, featuring a
single reversible pump-turbine unit, which has since become the preferred standard for
PHES plants [201]. While the development of PHES was somewhat sluggish until the
1960s, it experienced a substantial upswing from the 1960s to the late 1980s. This surge
can be attributed mainly to the deployment of nuclear power plants, which were effectively
complemented by the flexibility of PHES. However, during the 1990s, global PHES
development decelerated considerably due to the scarcity of suitable geographical locations
and a saturation of cost-effective sites, as well as a slowdown in nuclear development
growth [199]. From the year 2000 onward, numerous PHES plants have been established
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across Europe, particularly in Germany and Austria, serving not only as a tool for
addressing fluctuating electricity demand but also as an integrator for variable wind power
generation [202].
Currently, the United States and Japan possess the highest installed PHES capacities on a
global scale. In India, the Nagarjunasagar Pumped Storage Plant was operational between
1980 and 1985, and the country currently hosts 11 active PHES plants with a total installed
capacity of 4,804 MW [203]. Furthermore, the United Arab Emirates (UAE) is in the
process of constructing a plant in the Hatta region, featuring a water reservoir suitable for
PHES purposes. Upon completion in 2024, this project is projected to supply 2.06 TWh
per year, significantly aiding the UAE in achieving its goal of relying on 25% renewable
energy sources in its energy mix by 2030 [204]. A comprehensive examination of PHES,
including its technology, costs, environmental impacts, siting opportunities, and market
prospects, has been conducted by Evans et al. [205], Vilanova et al. [206], and Blakers et
al. [207]. Numerous off-river pumped hydro sites have been identified across the world,
known for their minimal environmental impact and low water consumption [208]. The
viability and components of low-head and seawater PHES systems have been reviewed by
Ruiz et al. [208] and Hoffstaedt et al. [209], particularly focusing on design, grid
integration, control, and modeling. Additionally, Vasudevan et al. [210] have provided a
critical review of varying levels of control and energy management strategies for variable
speed PHES, which is emerging as a feasible alternative for PHES with existing
infrastructure. It is important to note that PHES systems require suitable geological
locations and an ample water supply as limiting factors [199].
3.3 Gravity Energy Storage (GES) System
Due to the inherent geological constraints and substantial water prerequisites associated
with Pumped Hydro Energy Storage (PHES), there has been a notable shift towards a novel
energy storage concept that relies on gravitational forces, known as the Gravity Energy
Storage (GES) system [211]. This innovative system stores energy by utilizing water to
raise a piston or any suitable mass and subsequently releases the piston to propel the water
through hydroelectric generators when power demand arises. The concept, termed the
"gravity power module," was initially conceptualized by Gravity Power, LLC [212]. A
schematic representation of this storage technology is depicted in Figure 22.
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Figure 22: Gravity power module. Available at:
https://www.sciencedirect.com/science/article/pii/S277268352200022X
The GES system consists of a sizable piston suspended within a deep shaft filled with
water, connected to a return pipe that facilitates water flow. The powerhouse component
comprises a pump, turbine, and motor/generator. During the charging phase, off-peak
electricity is employed to drive the motor/generator, which sets the pump in motion,
converting electrical energy into mechanical energy. By expelling water into the shaft via
the return pipe, the pump transforms mechanical energy into kinetic flow energy, thereby
raising the piston to a high position, where it remains until power is required. During the
discharging phase, energy is generated as the piston descends. When the piston is released,
it imparts high pressure to the clockwise-flowing water through the return pipe. The kinetic
energy of this flow is then converted back into mechanical energy by the turbine, which
drives the generator to produce electrical energy [213, 214].
The energy storage capacity of this technology varies with the dimensions of the piston
and the depth of the shaft. Considered sizes typically range from piston diameters of 30–
100 meters, shaft depths of 500–1,000 meters, and piston lengths that are precisely half the
shaft length. The primary aim of the gravity power system is to provide power within the
range of 40 MW to 1.6 GW [212]. Researchers have extensively explored various aspects
of the GES system. Berrada et al. [215] proposed a dynamic model for the GES system,
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capable of estimating container size and piston movement during both charging and
discharging cycles. They also conducted an economic analysis to calculate the system's
levelized cost of energy, demonstrating its superior operational and economic performance
compared to alternative storage technologies like PHES and Compressed Air Energy
Storage (CAES) [216]. Additionally, Berrada and Loudiyi [217] evaluated suitable
materials for the system's various components.
Botha and Kamper [214] conducted a comprehensive review of current storage strategies
rooted in the principle of gravitational potential energy. Furthermore, Botha et al. [218]
explored a novel GES system that leverages the inherent ropeless operation of linear
electric machines to vertically transport multiple solid masses for energy storage and
discharge purposes. When deployed in applications characterized by high power demands
and a high number of annual cycles, this proposed storage technology has demonstrated
cost competitiveness.
3.4 Compressed Air Energy Storage (CAES) System
CAES represents an energy storage technology that accomplishes energy retention by
compressing air. The stored energy hinges on factors such as the volume of the storage
container, along with the pressure and temperature at which the air is stored [219]. The
constraints posed by specific geological formations required for PHES, in conjunction with
associated environmental concerns, have stimulated interest in alternative solutions. CAES
has emerged as a promising energy storage method, boasting attributes like high reliability,
economic feasibility, and minimal environmental impact [220]. Figure 23 shows a
schematic representation for CAES.
A typical CAES system includes the compression and storage of atmospheric air. When
generating power, the compressed air is combined with fuel in a combustion turbine to
generate electricity, with efficiency improved through waste heat recovery. The air
expansion stage drives an air expander connected to an electric generator, converting
mechanical energy to electrical power. This electricity is subsequently integrated into the
power grid. CAES systems are available in adiabatic and diabatic variations, employing
distinct methods to regulate air temperature during expansion, enhancing energy
efficiency. During periods of low demand, surplus electricity drives the motor, converting
it into mechanical energy and powering the multi-stage compressor. The compressor
elevates atmospheric air pressure, which is subsequently stored in the underground cavity.
During peak hours, the compressed air held in the cavity is employed to drive the pressure
turbines. These turbines convert compressed air energy into mechanical energy, which is
then used to drive a generator generating electricity [221].
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Figure 23: Schematic representation of compressed air energy storage system for a wind turbine.
Available at: https://contest.techbriefs.com/2015/entries/sustainable-technologies/6164
CAES systems can be categorized into different types based on the method used to manage
the heat generated during air compression. These categories encompass diabatic (D-
CAES), adiabatic (A-CAES), isothermal (I-CAES), and liquid air energy storage (LAES).
In the D-CAES system, air is compressed and heated during the compression process. An
air cooler is employed to remove the generated heat, allowing the compressed air to be
stored in the cavern. During discharge, the cooled compressed air is heated in a combustion
chamber, and the heated air propels the pressure turbine, ultimately driving the generator
to produce electricity [222]. Figure 24 provides a schematic representation of D-CAES.
A-CAES represents a variant of CAES with a distinctive thermal management mechanism.
In the phase of charging or compression, the heat produced is captured and stored within a
TES system. This accumulated thermal energy is subsequently injected back into the
compressed air flow during the discharge or expansion phase, resulting in an enhanced
overall system efficiency. Through the utilization of stored heat to elevate the temperature
of the compressed air prior to its entry into the turbine, A-CAES achieves superior
efficiency in comparison to conventional CAES configurations [223]. Figure 25 delineates
a schematic diagram that elucidates the operational concept of A-CAES.
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Figure 24: Schematic diagram of diabatic compressed air energy storage system. Available at:
https://www.sciencedirect.com/science/article/pii/S277268352200022X
Figure 25: Schematic diagram of adiabatic compressed air energy storage system. Available at:
https://www.sciencedirect.com/science/article/pii/S277268352200022X
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I-CAES represents an emerging energy storage system that simplifies the compression,
cooling, heating, and expansion stages of air compared to conventional CAES. I-CAES
eliminates the need for compressors and turbines, instead employing liquid piston
machinery to compress the air. This approach increases compression efficiency by up to
95%. I-CAES maintains a nearly constant temperature throughout the entire charging and
discharging cycle, reducing losses due to temperature variations. For lower power
requirements, isothermal and adiabatic storage systems are typically employed. Diabatic
storage systems find commercial application to facilitate flexible energy storage and
regeneration [224]. LAES, often referred to as cryogenic energy storage, stores air in a
liquefied state within specialized cryogenic containers. This differs from conventional
CAES, where air is stored in a compressed gaseous state within underground caverns.
During charging, excess electrical energy is utilized to liquefy air or nitrogen, which is then
stored in an insulated tank. During discharge, the liquefied air or nitrogen from the storage
tank is reheated and expanded to generate a high-pressure gas stream, driving the turbine
and generator to produce electricity [225, 226]. The schematic diagram for LAES is
demonstrated in Figure 26.
Figure 26: Schematic diagram of liquid air energy storage system. Available at:
https://sinovoltaics.com/learning-center/storage/liquid-air-energy-storage/
However, CAES systems have certain limitations, including the requirement for favorable
geographical conditions, the need for fossil fuels for combustion, and the emission of
pollutants. As of now, CAES technology remains relatively limited in deployment, with
only 11 operational CAES projects worldwide, totaling an installed capacity of 406.690
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MW. The Kraftwerk Huntorf plant, Germany's inaugural commercial D-CAES facility,
was established on January 12, 1978, boasting a capacity of 290 MW, serving as a peaking
power provider for Nordwestdeutsche Kraftwerke AG [227]. The second facility, featuring
a capacity of 110 MW, was constructed in McIntosh, Alabama, in 1991, incorporating
recuperators to harness heat from exhaust. Nevertheless, no commercial CAES facilities
were erected in the two decades following McIntosh [228]. Subsequently, in 2012, the third
CAES project globally, a 2 MW near I-CAES facility, was constructed in Gaines, Texas
[229]. A 1.5 MW SustainX I-CAES facility was installed in Seabrook, New Hampshire,
employing constant-temperature pressure vessels and staged hydraulic compression and
expansion to enhance system efficiency [230]. The first commercial A-CAES system,
known as Hydrostor, became operational in Goderich, Ontario, in 2019, providing services
to the Ontario Grid [231]. Additionally, several smaller-capacity plants, each under 1 MW,
have been established in Canada, Switzerland, China, the UK, and the USA [232].
A comprehensive review of CAES systems, encompassing thermodynamic analysis,
modeling and simulation analysis, experimental investigations, various control strategies,
case studies, and economic evaluations of CAES system roles in smart grids, has been
conducted by Venkataramani et al. [233]. Other comprehensive reviews on CAES systems
have been conducted by Saidur et al. [234], Budt et al. [235], Zhou et al. [236], and Olabi
et al. [237], encompassing evaluations of components, operational parameters, selection
criteria, benefits, drawbacks, economic impacts, and current research status for various
CAES systems. Additionally, Gouda et al. [238] have reviewed the current state-of-the-art
in Liquid Piston (LP) CAES systems, providing insights into their functionality and
advancement.
3.5 Flywheel Energy Storage (FES) System
The FES system represents a mechanical energy storage apparatus designed to store energy
in the form of mechanical kinetic energy, primarily through the rotational energy of a large
rotating cylinder [11]. A modern flywheel system comprises five fundamental components,
as depicted in Figure 27: a flywheel, magnetic bearings, an electrical motor/generator, a
power conditioning unit, and a vacuum chamber. The integrated reversible electrical
machine serves as a motor during the charging phase, drawing power from the grid to spin
the flywheel system at high velocities, thus storing kinetic energy. During the discharging
phase, as the rotor decelerates, the reversible machine functions as a generator, converting
the stored kinetic energy within the flywheel back into electrical energy [239]. Hence, in
the FES system, electricity is utilized to accelerate or decelerate the flywheel, with the
stored kinetic energy transferred to or from the flywheel via the integrated motor/generator
[24]. The classification of FES typically encompasses two categories: (a) low-speed FES,
employing steel flywheels rotating at less than 6×103 revolutions per minute (rpm), and (b)
high-speed FES, characterized by advanced composite materials for the flywheel and often
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rotating at speeds up to 105 rpm. Table 16 [232, 240, 241] provides an overview of the
advantages and disadvantages of the FES system.
Figure 27: Schematic diagram of flywheel energy storage system. Available at:
https://www.researchgate.net/publication/320755664
Table 16: Benefits and drawbacks of the flywheel energy storage system [232, 240, 241].
Benefits
Drawbacks
Elevated energy and power density
Extensive cycle lifespan
Exceptionally rapid response time
Excellent cycle efficiency
(90%–95%)
Extremely brief discharge duration
Considerable self-discharge rate
Limited storage capacity
High initial investment
The term "flywheel" made its initial appearance in print in 1784, marking the onset of the
industrial revolution. In those early days, flywheels found significant use in steam engine
boats, trains, and as energy accumulators in factories [242]. Subsequently, the decline in
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the cost of cast iron and cast steel facilitated the production of single-piece flywheels.
During the mid-19th century, notably large flywheels with curved spokes were constructed.
The application of flywheels in transportation became evident when they were employed
in a Gyrobus manufactured in Switzerland during the 1950s. These flywheels weighed
approximately 1,500 kg and boasted a diameter of 1.6 meters [243]. The concept of the
FES system was introduced in the 1960s and 1970s, primarily for applications in electric
vehicles, stationary power backup systems, and space missions. A comprehensive account
of the latest technologies, materials used in FES system production, and an overview of its
applications across various sectors can be found in the work of Olabi et al. [240]. Zhang et
al. [244] conducted a review of control strategies, power converters, and machine types
applied in FES systems. Further insights into the advances, components, characteristics,
applications, and potential future directions of FES systems can be found in the works of
Dragoni [245], Choudhury [246], and Li and Palazzolo [247]. Table 17 [248−255]
compiles information about some of the prominent FES systems worldwide. Although
substantial progress was made in the early stages of development, the commercialization
of flywheels remained limited. Nevertheless, recent advancements in materials, magnetic
bearings, and the introduction of high-speed electric machines have solidified the FES
system as a viable alternative for various energy storage applications [248].
Table 17: Overview of significant flywheel energy storage systems worldwide.
Year
Project/Company
Location
Storage
Capacity
(MW)
Power
(MWh)
Application
1987
Max Planck
Institute – FESS
[248]
Garching,
Bavaria,
Germany
387
-
Onsite power
2001
New Energy
Development
Organization
(NEDO) [249]
Japan
-
10
-
2005
PowerStore [250]
Flores
Island,
Portugal
0.5
-
Frequency
regulation, voltage
support
2006
ABB Ltd. [250]
Graciosa
Island
0.5
-
Frequency
regulation
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2006
Utsira project [251]
North Sea,
outside
Haugesund
0.2
0.005
Short-term storage
2008
Railway Technical
Research Institute
(RTRI) [252]
Japan
0.3
0.1
Stabilize power
supplies
2015
Beacon Power
Corporation [253]
Massachu-
setts, USA
20
0.1
Grid-scale frequency
regulation and
power backup for
telecommunication
-
Piller Flywheel
Technology [254]
Dresden,
Germany
5
-
UPS solution and
load leveling in the
local grid
-
Flores Power
System [255]
Island of
Flores,
Azores
0.35
0.005
Improving system
frequency stability
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CHAPTER 4: CHEMICAL ENERGY STORAGE (CES)
SYSTEMS
Chapter 4 delves into the domain of CES systems, investigating three essential aspects:
Hydrogen Energy Storage, Synthetic Natural Gas (SNG), and Solar Fuels. It examines
hydrogen's capacity as a clean energy transporter, the techniques for its generation, and the
constituents of a hydrogen energy setup. Furthermore, the chapter introduces SNG as an
intermediary between conventional fuels and renewable sources, elucidating the
transformation process and emphasizing noteworthy SNG facilities. Lastly, it explores the
notion of Solar Fuels, encompassing both natural and artificial photosynthesis, alongside
the thermochemical route, showcasing advancements in the upscaling of solar fuel
technologies and worldwide research initiatives.
4.1 Introduction to Chemical Energy Storage Systems
ESS are most suitable for the long-term retention of chemical energy. In these systems,
energy becomes stored within the chemical bonds between atoms and molecules of
materials, and this stored chemical energy is released during chemical reactions. As energy
is released, the composition of the materials undergoes changes, involving the breaking of
original chemical bonds and the formation of new ones [256]. Currently, chemical fuels
play a dominant role in both global electricity generation and the transportation industry.
These fuels encompass a range of substances, including coal, gasoline, diesel fuel, natural
gas, liquefied petroleum gas (LPG), propane, butane, ethanol, and hydrogen. Initially, these
chemicals are converted into mechanical energy, which is then further transformed into
electrical energy for electricity generation [257]. CES systems primarily encompass
hydrogen, synthetic natural gas, and solar fuel storage systems [258].
4.2 Hydrogen Energy Storage System
Hydrogen is widely recognized as an optimal energy carrier due to its cleanliness and its
status as a carbon-neutral or emission-free chemical energy medium [256, 257]. It can be
generated from water through electrolysis or directly from sunlight using photocatalytic
water splitting. A standard hydrogen energy system consists of three primary components,
as illustrated in Figure 28: (i) a hydrogen generation unit, such as an electrolyzer, which
converts electrical energy input into hydrogen, (ii) a hydrogen storage system, and (iii) a
hydrogen energy conversion unit, such as a Fuel Cell (FC) or regenerative FC, which
transforms the stored chemical energy in hydrogen back into electrical energy.
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Figure 28: Schematic representation of hydrogen energy storage system. Available at;
http://www.chiangmaisolar.com/hydrogen-storage-systems/
During surplus power periods in the charging phase, hydrogen is produced via electrolysis
from water and stored in a dedicated tank. Conversely, during peak hours when power
availability is constrained (during discharging cycles), electricity is generated from stored
hydrogen employing fuel cells. The electrolyzer utilizes electrolysis to split water into
hydrogen and oxygen. The released oxygen is released into the atmosphere, while the
hydrogen is retained in the storage tank [258, 259]. Moreover, a fuel cell comprises four
essential components: an anode, a cathode, an electrolyte, and an external circuit. At the
anode, hydrogen undergoes oxidation, resulting in the generation of protons (positively
charged hydrogen ions) and electrons that power the fuel cell. Positively charged hydrogen
ions from the anode are transported to the oxygen electrode through the electrolyte. The
reaction involving oxygen, hydrogen ions, and electrons at the cathode generates water and
heat. Throughout this process, electrons flow from the anode to the cathode via an external
circuit, resulting in the flow of current and, ultimately, the production of electricity.
Fuel cells for hydrogen are categorized into six types [24], and the specific characteristics
of each type are outlined in Table 18 [260]. These types are Proton Exchange Membrane
FC (PEMFC), Alkaline FC (AFC), Phosphoric Acid FC (PAFC), Molten Carbonate FC
(MCFC), Solid Oxide FC (SOFC), and Direct Methanol FC (DMFC).
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Table 18: Attributes of different fuel cell types [260].
Fuel Cell
Type
Fuel
Operating
Temperature
(°C)
Efficiency
(%)
Application
PEMFC
Pure Hydrogen (H2)
60–140
55
Backup Power
AFC
Pure Hydrogen (H2)
150–200
60
Spacecrafts,
Military
Applications
PAFC
Pure Hydrogen (H2)
150–200
>40
Distributed
Generation
MCFC
Hydrogen (H2), Carbonate (CO3)
600–700
45
Power Utility
Storage
SOFC
Pure Hydrogen (H2)
200–700
40
Power Utility
Storage
DMFC
Methanol (CH3OH)
30–80
30
Electric
Transportation
Vehicles
Abe et al. [261], Yue et al. [262], Tarhan and Çil [263], Hassan et al. [264], and Grimaldo
et al. [265] have provided concise reviews concerning the applications, limitations, and the
current state of solid-state hydrogen storage in metal hydrides, as well as recent innovative
advancements towards realizing a hydrogen-based economy. Additional insight into the
advancements in materials for hydrogen energy storage and strategies for the safe long-
term storage of hydrogen fuel is presented in the studies [266, 267]. Thiyagarajan et al.
[268] have conducted a comprehensive review of the mechanisms involved in underground
hydrogen storage systems, including monitoring techniques and optimization of injection-
withdrawal procedures. Various challenges encountered during subsurface storage, such as
microbial growth, well integrity, and geochemical reactions, are also briefly addressed.
Recent progress in solid-state hydrogen storage systems involving Mg-Li-Al and Mg-Na-
Al systems has been examined by researchers [269, 270]. Since 2000, practical projects
and demonstrations of hydrogen energy storage have started to showcase its viability
beyond laboratory research. For instance, in 2015, the Energiepark Mainz project was
initiated in Mainz, Germany, in collaboration with Stadtwerke Mainz, Linde, Siemens, and
the RheinMain University of Applied Sciences. This energy park converts surplus wind
power from nearby wind parks into hydrogen fuel, subsequently utilized for energy
generation. In 2018, Enbridge Gas Distribution and FCHEA member Hydrogenics
collaborated to inaugurate the Markham Energy Storage Facility, a 2.5 MW multi-
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megawatt power-to-gas facility located in Ontario, Canada. This facility employs
renewably-sourced hydrogen and presently provides grid regulation services to Ontario's
Independent Electricity System Operator (IESO). Additionally, Orsted, Denmark's largest
energy company, has outlined plans to construct a 700 MW facility using excess energy
from its North Sea wind farms to power electrolysis and generate renewable hydrogen
energy. Mitsubishi Hitachi Power Systems and Magnum Developer are also considering
the construction of a 1000 MW power plant in Millard County, Utah, for renewable
hydrogen storage. Xcel Energy, a major utility provider, is partnering with the National
Renewable Energy Laboratory (NREL) on a 110 kW wind-to-hydrogen project, which
would leverage surplus wind energy to produce hydrogen. This hydrogen could be stored
for future use or converted back into the electricity grid during peak demand periods. These
initiatives highlight the growing adoption of hydrogen energy storage on a larger scale to
address energy supply and demand fluctuations [271, 272].
In conclusion, various types of fuel cells have been developed, including PEMFC, AFC,
PAFC, MCFC, SOFC, and DMFC.
4.3 Synthetic Natural Gas (SNG)
Natural gas stands out as the most extensively utilized and environmentally friendly fossil
fuel within the current landscape of energy provision. The notion of transforming coal into
SNG has been empirically validated as a viable alternative for energy production [17].
Another captivating concept is harnessing biomass to produce SNG, given its carbon-
neutral attributes. The technique of methanation was previously employed to produce SNG
from coal during the 1960s and 1970s, particularly in Germany and the USA. More
recently, novel thermal gasification procedures have emerged for the production of SNG
from both coal and dry biomass. The production of SNG through a thermochemical process
involving coal and solid, desiccated biomass, such as wood and straw, necessitates several
sequential conversion stages, encompassing drying, gasification followed by gas
purification, methanation of the producer gas, and subsequent gas enhancement, as
delineated in Figure 29 [273]. SNG can be retained in high-pressure tanks, underground
storage caverns, or directly injected into the natural gas distribution network. The Great
Plains Synfuels Plant (GPSP), situated near Beulah, North Dakota, USA, represents the
world's exclusive SNG gasification facility, operational since 1984 and churning out
approximately 4.1 million cubic meters of SNG daily, derived from lignite coal [274].
Table 19 [275−279] provides a consolidated overview of some of the largest SNG
technology-based plants ever established across the globe.
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Figure 29: Standard steps for producing SNG using coal and dehydrated solid biomass. Available
at: https://www.sciencedirect.com/science/article/pii/S277268352200022X
Table 19: A compilation of significant facilities constructed globally using synthetic natural gas
technology [275–279].
Year
Project
Location
Storage
Capacity
(MW)
Fuel
Status
2009
Biomass
CHP Plant
Güssing, Austria
1
Wood
Chips
Demonstration
2010
Gaya
-
-
Wood,
Straw
Demonstration
2011
Dong
Pyroneer
Plant
Denmark
6
Wood Mass
Demonstration
2012
Waste-to-
Energy
Gasifier
Lahti, Finland
160
Shredded
Textiles,
Wood,
Paper,
Card, and
Plastics
Industrial
Scale
2013
GoBiGas
Sweden
20
Forestry
Feedstock
Industrial
Scale
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2013
Birmingham
Bio Power
Plant
Tyseley,
Birmingham, UK
10.3
Wood
Waste
Commercial
2016
WoodRoll
Test Plant
Sweden
6
Forestry
Feedstock,
Waste from
Industry,
Agricultural
Waste
Commercial
4.4 Solar Fuels
The primary objective of solar energy storage is to harness the abundant energy provided
by the sun, convert it into usable forms, store it within the chemical bonds of fuel, and
subsequently utilize it as needed. Solar fuels represent chemical fuels designed to store
energy acquired from the sun. Researchers are presently exploring three key approaches to
generate solar fuels: natural photosynthesis, artificial photosynthesis, and thermochemical
production [280].
Nature has developed an intricate system for converting solar energy into fuel, a process
known as photosynthesis. It is a biochemical mechanism that allows plants, algae, and
certain bacteria to capture solar energy and store it in the form of carbohydrates, serving as
fuels used for their growth and maintenance. Natural photosynthesis relies on water and
CO2 as the essential raw materials [281]. Conversely, the term "artificial photosynthesis,"
often referred to as the "artificial leaf," signifies a process explicitly designed to mimic
natural photosynthesis. It involves capturing sunlight and utilizing the stored energy to
chemically transform water and carbon dioxide into fuels, resulting in the production of
solar fuels (Figure 30). The thermochemical approach employs sunlight to heat materials
to extremely high temperatures, prompting them to react with steam or CO2, ultimately
yielding carbon monoxide (CO) or hydrogen (H2) [282].
Researchers and engineers have collaboratively worked on upscaling laboratory prototypes
of solar fuel production to a commercial scale since the 1950s. Comprehensive reviews by
Mustafa et al. [283] and Ezugwu et al. [284] provide an overview of both historic and
contemporary thermal technologies utilized in CO2 conversion, encompassing detailed
information on their operational principles, types, various methodologies, advancements,
conversion efficiencies, resulting products, catalysts, and operational parameters.
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Figure 30: Artificial photosynthesis: pathway from sunlight to fuels. Available at:
https://tsarabarbero.wordpress.com/author/tsarabarbero/
Xiao et al. [285] reviewed recent advancements, achievements, and operational principles
of three typical hybrid systems, including photovoltaic-driven biological inorganic
systems, microbial photoelectrochemical systems, and photosensitized biological
inorganic systems, for the production of organic fuels and chemicals from CO2. Gong et
al. [286] offered an extensive review highlighting recent progress in CO2 photo-reduction,
addressing critical challenges such as light harvesting, charge separation, and CO2
molecule activation, while proposing potential solutions to enhance photocatalytic activity.
Recent advancements in the synthesis and regulation of photocatalysts based on Few-Layer
Carbon Nitride (FLCN) and their applications in converting sunlight into fuels and
chemicals are briefly summarized in the work of Chen et al. [287]. Numerous dedicated
research programs on solar fuels have been initiated globally, including efforts in the USA,
Australia, and South Korea, along with the establishment of renewable energy research
centers in India, China, and Japan. Table 20 [281, 288] provides an overview of the various
research and innovation programs worldwide in the field of solar fuels.
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Table 20: Global Initiatives for research and innovation in solar fuels [281, 288].
Country
Research Programs
USA
In 2010, the Joint Centre for Artificial Photosynthesis (JCAP), a
Department of Energy Innovation Hub, was established in collaboration
between the California Institute of Technology and the Lawrence
Berkeley National Laboratory in California. Its primary focus is on
developing a cost-effective method for producing fuels using sunlight,
water, and carbon dioxide. In 2012, the Solar Fuels Institute (SOFI) was
launched at Northwestern University to support the development of an
efficient and cost-effective system that uses sunlight to produce liquid
fuel. SOFI is a research collaboration involving universities, government
labs, and industry with the goal of developing and commercializing liquid
solar fuels within 10 years.
Europe
The Dutch government, together with six universities, has initiated the
largest single national investment in solar fuels research. Collaborative
research is being conducted to produce solar fuels through both natural
and artificial photosynthesis.
Australia
Australia is at the forefront of solar research, with researchers in Canberra,
Sydney, and Melbourne working on novel methods, applications, and
products for the future of solar energy. The University of New South
Wales is known for its advanced solar research and is developing
hydrogen trapping technology to create a new type of wafer that could
contribute to low-cost solar cells. Monash University in Melbourne is
developing highly efficient, semi-transparent Photovoltaic (PV) that
could be used in the construction of Integrated Photovoltaic (BIPV)
windows. The Commonwealth Scientific and Industrial Research
Organization (CSIRO) in Melbourne aims to scale up laboratory-efficient
perovskite cells to larger commercial-scale PV modules.
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India
The Solar PV Hub, a joint initiative by the Department of Science &
Technology (DST), Government of India, and the Centre of Excellence
for Green Energy and Sensor Systems (CEGESS) at the Indian Institute
of Engineering Science and Technology (IIEST), was established to
develop a state-of-the-art facility for the fabrication and characterization
of industry-compatible solar cells and solar photovoltaic systems.
Additionally, the Solar Energy Harnessing Centre, a collaboration
between DST and the Indian Institute of Technology, Madras, is a first-
of-its-kind center for research and technology development in a
multidisciplinary domain, including highly efficient solar PV, cutting-
edge research for solar fuel production, and solar energy storage using
indigenous resources.
South Korea
The Korean Centre for Artificial Photosynthesis (KCAP) was established
in 2009 at Sogang University, and in 2011, it signed a cooperation
agreement with POSCO, a steel company, to conduct joint research on the
commercialization of artificial photosynthesis.
China
In 2020, the world's first thousand-ton scale demonstration project for
direct solar fuel synthesis began operations in Lanzhou, China. This
project converts carbon dioxide, water, and solar energy into transportable
liquid fuels such as methanol using three technological units: solar PV to
generate electricity, an electrolyzer to produce hydrogen by splitting
water, and CO2 hydrogenation to produce methanol.
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CHAPTER 5: ELECTROCHEMICAL ENERGY STORAGE
(ECES) SYSTEMS
Chapter 5 delves into the world of EcES systems. It consists of four main sections, each
exploring different aspects of EcES systems. It begins by examining Battery Energy
Storage (BES) systems, tracing their historical development and explaining how they work.
Then, it delves into Flow Battery Energy Storage (FBES) systems, highlighting the
distinctions between redox flow batteries and hybrid flow batteries. The chapter also covers
Paper Batteries, emphasizing their thin, flexible nature and various applications. Lastly, it
explores Flexible Batteries, designed to meet the needs of modern, adaptable electronic
devices. These sections collectively provide a comprehensive overview of EcES systems,
presenting their historical context, operational principles, and potential applications in
today's evolving energy landscape.
5.1 Introduction to Electrochemical Energy Storage Systems
The most commonly employed energy storage system, known as EcES, relies primarily on
three major processes, as depicted in Figure 31. This system is primarily divided into two
categories: (a) BES systems, where electrical charge is stored within the electrodes, and
(b) FBES systems, where charge is initially stored within the fuel and then transferred
externally onto the surface of the electrodes, as shown in Figure 32 [7]. In addition to these
two conventional energy storage methods, substantial research efforts are underway in the
field of electrochemical storage capabilities to address the increasing demand for
lightweight, compact, and adaptable electronic devices.
Figure 31: The core operational processes in an electrochemical energy storage system. Available
at: https://www.sciencedirect.com/science/article/pii/S277268352200022X
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Figure 32: Categorization of electrochemical energy storage systems. Available at:
https://www.sciencedirect.com/science/article/pii/S277268352200022X
5.2 Battery Energy Storage (BES) System
Batteries function as electrochemical devices that transform chemical energy into electrical
energy, as demonstrated in Figure 33. They consist of multiple cells, each comprising three
fundamental components: two electrodes, referred to as an anode and a cathode, along with
an electrolyte. These batteries are broadly divided into two primary categories: primary
and secondary. Primary batteries are designed for one-time use, and once their chemical
contents are depleted, they cannot be recharged. In contrast, secondary batteries are
engineered for rechargeability [257]. Secondary batteries can be further classified into
various types based on the materials used for their electrodes and electrolytes, including
Lead-Acid (LA), Lithium-ion (Li-ion), Nickel-Cadmium (Ni-Cd), Sodium-Sulfur (NaS),
Sodium-ion (Na-ion), and metal-air batteries. Detailed information about each of these
battery types is provided in the subsequent sections.
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Figure 33: Schematic diagram of battery energy storage system. Available at:
https://www.sunstore.co.uk/wp-content/uploads/2017/05/battery-storage-diagram.jpg
5.2.1 Lead-Acid (LA) Batteries
LA batteries represent the most widely utilized and oldest electrochemical energy storage
technology, with their inception dating back to 1859. These batteries consist of two
electrodes: a metallic sponge lead anode and a lead dioxide cathode, as depicted in Figure
34. They are immersed in an electrolyte composed of 37% sulfuric acid and 63% water,
with a porous separator in between to prevent direct electron flow from the anode to the
cathode [289]. During the discharge phase, both electrodes become covered with lead
sulfate, while the electrolyte transforms into water. Charging reverses this process,
restoring both electrodes to their initial state. LA batteries are typically classified into two
types: Flooded Lead-Acid (FLA) and Valve-Regulated Lead-Acid (VRLA), both operating
on the same principle. VRLA batteries differ in construction, featuring a sealed design with
a pressure-regulating valve to prevent air infiltration into the cells [259].
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Figure 34: Schematic diagram of a lead-acid battery. Available at:
https://iu.pressbooks.pub/openstaxchemistry/chapter/17-5-batteries-and-fuel-cells/
The foundations of lead-acid battery technology were established over a century and a half
ago when Gaston Plante, a French physicist, demonstrated the world's first rechargeable
LA battery in 1859, consisting of nine parallel-connected cells [290]. Subsequently, in
1880, Camille Alphonse Faure advocated coating lead sheets with a paste composed of
lead oxides, sulfuric acid, and water. This innovation led to the development of the modern
pasted-plate battery, which remains the most commonly used LA battery design today
[291].
In the late 19th century, LA batteries were employed to power electric vehicles and served
as standby batteries for providing emergency power to critical equipment in electricity
generation facilities and other essential installations. By the early 1920s, a more acid-
resistant hard rubber casing was introduced, alongside significant advancements in plate
design and production techniques, resulting in more efficient LA batteries with higher
specific power. In the late 1960s, injection-molded polypropylene cases and covers were
introduced, yielding durable, compact, and lightweight containers for LA batteries. Later
on, VRLA batteries were developed, with commercial units being manufactured by
German company Sonnenschein in the late 1960s and by the Gates Corporation in the USA
in the early 1970s [292–294].
Presently, LA batteries find extensive applications in various projects worldwide.
Numerous industrial and academic research initiatives have been ongoing for decades to
enhance LA battery performance. Zhang et al. [295] reviewed the prevailing research
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limitations in LA batteries, focusing on grid composition, structure, lead paste formulation,
additives, curing, and formation processes. Pradhan and Chakraborty [296] conducted a
review of recent advances and historical innovations in bipolar LA batteries, considering
substrate materials, designs, fabrication methods, and sealing techniques. Additionally,
researchers have explored the use of different forms of elemental carbon in LA batteries,
as documented in studies such as Hao et al. [297], Lach et al. [298], and Mandal et al.
[299]. The incorporation of carbon materials as additives to the negative active mass has
been shown to improve battery cycle life and charge acceptance. Table 21 [300–307]
summarizes various LA battery storage systems employed worldwide, while Table 22 [308,
309] discusses the advantages and disadvantages associated with LA batteries.
Table 21: Summary of global deployments of lead-acid battery energy storage systems.
Location
Year
Service Area
Reference
Indonesia
2013
Small village in
western Indonesia
[300]
Turkey
2013
50 residential houses
[301]
Saudi
Arabia
2015
Remote area
[302]
Tamil
Nadu,
India
2016
Household load
[303]
Estrella,
Colombia
2016
Off-grid village in
Colombia
[304]
Kohmak
Island,
Thailand
2017
Hotel
[305]
Bangladesh
2017
Village
[306]
New Delhi,
India
2017
Electrical faculty
load, New Delhi
[307]
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Table 22: Advantages and disadvantages of lead-acid battery-based energy storage systems
[308, 309].
Advantages
Disadvantages
Economical initial investment
Reliable performance in various
temperatures
Low maintenance requirements
High specific power
Extremely brief discharge duration
Limited cycle life
Slow charging
High self-discharge rate
Environmental concerns
Limited energy density
5.2.2 Lithium-ion (Li-ion) Batteries
Li-ion batteries are widely utilized in the electronics and transportation sectors, particularly
in applications related to power grids and plug-in hybrid electric vehicles. This popularity
stems from their superior charge density compared to other rechargeable battery types [11,
310]. Li-ion batteries are composed of key components: a cathode made of lithium metal
oxide, an anode comprising graphitic carbon, and an electrolyte containing dissolved
inorganic lithium salt. These batteries function by enabling the flow of current as lithium
ions move between the anode and cathode. During the charging process, lithium cations
migrate to the carbon anode through the electrolyte, where they combine with external
electrons and become deposited as lithium atoms within the carbon layers. This process
reverses during discharge [23]. The operational principle of Li-ion batteries is illustrated
in Figure 35.
The concept of Li-ion batteries was initially proposed by Stanley Whittingham, an English
chemist at ExxonMobil, in the 1970s. Whittingham introduced the idea of intercalation
electrodes, employing titanium disulfide as the cathode and lithium metal as the anode
[311]. In 1981, Goodenough experimented with lithium cobalt oxide as the cathode, a
significant development that doubled the battery's energy potential [312]. Subsequently, in
1987, Japanese researchers, Yoshino et al. [313], introduced a novel cell design using
petroleum coke, a carbonaceous material, which substantially enhanced Li-ion battery
performance. Commercial production of Li-ion cells commenced in 1991 under the
leadership of Yoshio et al. [314–315] and Hideto et al. [316] at Sony Energytec Inc. The
rapid adoption of Li-ion battery storage systems followed their commercialization due to
their high-energy density and superior performance. The majority of Li-ion battery
manufacturing industries are now located in China, the USA, Asia, and Europe.
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Figure 35: Schematic diagram of a lithium-ion battery. Available at:
https://www.researchgate.net/publication/270909577
While Li-ion batteries have dominated the market, emerging applications demand even
higher energy density, increased capacity, enhanced safety, improved performance, and
reduced costs. Gao et al. [317] have reviewed cutting-edge solutions aimed at enhancing
Li-ion battery performance while lowering costs. Additionally, Shen et al. [318] and Zhang
et al. [319] have provided reviews of developments in electrode materials for next-
generation high-energy-density and low-temperature Li-ion batteries. Safety concerns
related to key materials and cell design techniques, as well as the impact of carbon materials
as electrodes on battery safety and electrochemical properties, have been addressed in
review articles [320–322]. For a comprehensive overview of Li-ion battery energy storage
technologies and battery energy systems worldwide, refer to Table 23 [323−329]. Most
analysts predict that Li-ion batteries will continue to dominate energy storage markets in
the next decade [330]. Table 24 [308, 309, 331] presents the advantages and disadvantages
associated with Li-ion batteries.
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Table 23: Overview of global lithium-ion battery energy storage technologies and battery energy
systems.
Project
Location
Storage Capacity (MW)
Status
Reference
AES Alamitos
Energy Storage
Array
Long Beach,
California,
USA
100
Contracted
[323]
Magdeberg-SK
Innovation –
BESS
Magdeberg,
Saxony-
Anhalt,
Germany
30
Announced in
2014
[324]
Grand Ridge
Energy Storage
Marseilles,
Illinois, USA
31.5
Commissioned
in 2015
[325]
Nishi-Sendai
Substation -
Tohoku
Electric/Toshiba
Sendai,
Miyagi
Prefecture,
Japan
40
Operational
since 2015
[326]
Kingfisher
Project
Roxby
Downs,
South
Australia,
Australia
100
Announced in
2016
[327]
Gyeongsan
Substation
South Korea
49
Operational
since 2016
[328]
Minami-Soma
Substation -
Tohoku
Electric/Toshiba
Minamisoma,
Fukushima
Prefecture,
Japan
40
Operational
since 2016
[329]
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Table 24: Benefits and drawbacks of lithium-ion battery-based energy storage systems
[308, 309, 331].
Benefits
Drawbacks
Lightweight
High energy density
High efficiency
Long lifespan
High specific power
High production cost
Requires a specialized charging
circuit
Sensitive to high temperatures
Risk of bursting in extreme
conditions
Complete discharge can damage the
battery
5.2.3 Nickel-Cadmium (Ni-Cd) Batteries
Ni-Cd batteries, invented by Waldemar Jungner of Sweden in 1899, represent one of the
most established and commonly employed technologies in the field. Jungner established a
factory in Sweden in 1906 specifically for the production of industrial Ni-Cd batteries [12].
The first Ni-Cd batteries were manufactured in the United States in 1946. These initial
batteries were of the "pocket type," containing active materials of nickel and cadmium
[332]. Around the mid-twentieth century, sintered plate Ni-Cd batteries gained popularity
due to their thinner sintered plates compared to pocket batteries, enabling higher current
flow. These plates are manufactured by infusing nickel powder under high pressure at
temperatures below the melting point, resulting in highly porous plates with an
approximate 80% pore volume [333]. Since the early 2000s, all Ni-Cd batteries have been
available in a gel-like form. The evolutionary history of the Ni-Cd battery energy storage
system is summarized in Table 25 [332, 334−340].
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Table 25: Progress in nickel-cadmium battery-based energy storage systems.
Year
Advancement
1899 [332]
Pocket-plate technology
• Demonstrated reliability, durability, and extended
lifespan
• Effective operation across a wide temperature
range
• Excellent charge retention
• Resilient against severe mechanical and
electrical stress, including overcharging, reversal,
and short circuits
1908 [334, 335]
Tubular-plate batteries
• Developed to mitigate deformation caused by
positive active mass swelling in pocket-plate
batteries
• Extended battery cycle life for deep-discharge
cycling applications
1940 [335]
Sintered-plate Ni-Cd batteries
• Thinner profile compared to pocket-plate batteries
• Lower internal resistance
• Superior performance at high discharge rates and
in low-temperature conditions
• Ideal for high-power applications
1983 [336]
Fibre-structured Ni-Cd cells
• Utilized two opposing negative electrodes
separated by a porous fibre-nickel structure,
positioned between two positive plates
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1989 [337]
Super Ni-Cd cells
• Implemented an electrochemical loading process
for positive and negative active materials into a
porous sintered nickel structure
• Replaced polyamide separators with inert polymer-
impregnated Zircar cloth
• Achieved an extended battery lifespan
1991 [338]
Sealed Ni-Cd cells
• Engineered to endure extreme environmental
conditions
• Required no maintenance
• Cost-effective solution
• Enhanced discharge capabilities
1993 [339]
Ultra-low-maintenance Ni-Cd batteries
• Utilized thermally welded cells to ensure leak
resistance
• Employed seam-welded plate tabs, copper cell
links, and terminals to reduce resistance and
minimize voltage drop
• Incorporated a gas barrier membrane to prevent
recombination and material migration
2000s [340]
Jelly–roll Ni-Cd battery
• Introduced a cylindrical form factor where cathode
and anode layers are rolled together
• Primarily applied in portable devices like mobile
phones, digital cameras, and power tools
• Employed cadmium metal as the anode, nickel
oxide hydroxide or nickel oxide as the cathode, and
potassium hydroxide as the electrolyte solution
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The positive electrode in a Ni-Cd battery comprises spongy nickel oxyhydroxide, the
negative electrode consists of metallic cadmium, and the electrolyte used is potassium
hydroxide (as depicted in Figure 36). These electrodes are separated by a nylon divider,
which prevents direct charge transfer between them [289]. During the discharge cycle, the
positive plate's nickel oxyhydroxide reacts with water to form nickel hydroxide, while the
negative plate's cadmium undergoes a reaction to produce cadmium hydroxide [23]. This
entire process is reversed during the charging phase. Ni-Cd batteries find extensive
applications in electrical power-related uses due to their longer life cycle compared to LA
batteries. For a detailed assessment of the advantages and disadvantages associated with
Ni-Cd batteries, the reader is directed to Table 26 [198, 308, 309, 341].
Figure 36: Schematic diagram of a nickel-cadmium battery. Available at: https://www.electricity-
magnetism.org/electrostatics/electric-charge/
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Table 26: Benefits and drawbacks of nickel-cadmium batteries.
Benefits
Drawbacks
Extended Shelf Life
Excellent Low-Temperature
Performance
Cost-Effective
Ultra-Fast Charging
Good Load Performance
Low Specific Energy
High Self-Discharge Rates
Toxic Cadmium
5.2.4 Sodium-Sulfur (NaS) Batteries
NaS batteries typically consist of cylindrical electrochemical cells, wherein the positive
electrode contains molten sulfur, the negative electrode houses molten sodium, and a solid
beta alumina electrolyte separates them, as depicted in Figure 37.
Figure 37: Schematic diagram of a sodium-sulfur battery. Available at:
https://www.researchgate.net/publication/322864295
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In the operation of NaS batteries, molten sodium within the core contributes electrons to
the external circuit, prompting the migration of sodium ions through the electrolyte toward
the positive electrode compartment. Meanwhile, the liberated electrons are captured by the
molten sulfur, converting them into polysulfide. This entire process reverses during the
charging cycle, during which sodium polysulfides decompose, releasing positive sodium
ions back through the electrolyte to recombine as elemental sodium [17]. To maintain the
molten state of sodium and sulfur and ensure sufficient conductivity within the electrolyte,
all components are enclosed within a thermally insulated container maintained at a
temperature exceeding 270 °C [342, 343].
NaS batteries offer a significantly higher energy density than LA batteries, approximately
three times greater. Additionally, they exhibit an extended operational lifespan and require
reduced maintenance. While the development of NaS batteries traces back to the 1960s
with early work by the Ford Motor Company, modern sodium-sulfur technology was
further developed and commercialized in Japan, notably by the Tokyo Electric Power and
NGK Insulators [344]. Table 27 [2, 250, 345–348] provides a summary of significant NaS
BESS demonstration projects and plants. However, it is worth noting that due to the
complexity of this technology, small-scale NaS batteries may be less economically viable
[349]. To delve into the advantages and disadvantages associated with NaS batteries, the
reader is directed to Table 28 [308, 309, 350].
Table 27: Overview of significant pilot initiatives and facilities for sodium-sulfur battery energy
storage systems [2, 250, 345–348].
Year
Location/Company
Power
Capacity
(MW)
Storage Capacity (MWh)
Type/Application
1992
Kawasaki Electric
Energy Storage Test
Facilities, Japan
0.05
4
Prototype
1995
Tokyo Electric
Power Co., Tokyo,
Japan
0.05
4
Load Leveling
1997
Tsunashima
Substation
6
48
Load Leveling
1998
Kinugawa Power
Station
0.193
0.772
Load Leveling
and Emergency
Power Supply
1999
Ohito Substation
6
48
Load Leveling
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2003
Shinagawa
Substation
2
14.4
Load Leveling
2006
Long Island Bus’s
BES System, New
York, USA
1
72
Refueling Fixed-
Route Vehicles
2008
Xcel Energy’s
Wind-to-Battery
(W2B) Storage
Project, Minnesota,
USA
1
-
Peak Shaving,
Frequency
Regulation, Wind
Smoothing, and
Wind Leveling
2017
Graciosa Island,
Younicos, Germany
3
18
Wind and Solar
Power Energy
Storage for
Islands
2019
Abu Dhabi Island,
UAE
108
648
Load Leveling
Table 28: Benefits and drawbacks of sodium-sulfur batteries [308, 309, 350].
Benefits
Drawbacks
Exceptionally high power and energy
density
Superior efficiency
Extended cycle life
Potential for cost-effectiveness
Resilience to ambient conditions
Considerably high production cost
Safety concerns
Strict operational and maintenance
demands
Requires operation above 300 °C
5.2.5 Sodium-ion (Na-ion) Batteries
Among various battery technologies, Li-ion batteries have gained widespread adoption in
numerous energy storage systems. Since their initial commercialization by Sony in
Energytec Inc. back in 1991, Li-ion technology has undergone extensive research and
development. Nonetheless, due to concerns related to low power density, limited cycle life,
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and safety issues, scientists have been actively exploring alternative materials that can store
and release energy in a manner akin to lithium. Among the myriad of potential candidates
for anodes, sodium-based (Na) anodes have garnered significant attention from
researchers. The development of Na-ion batteries commenced alongside that of Li-ion
batteries in the early 1980s. However, after the successful commercialization of Li-ion
batteries in 1991, interest in Na-ion batteries dwindled. Nevertheless, in the early 2010s,
there was a resurgence of interest in Na-ion batteries, driven by the rising demand for Li-
ion battery raw materials and their escalating costs [351]. Sodium, owing to its abundance
and cost-effectiveness, has the potential to address the limitations of Li-ion batteries,
offering a lower-cost alternative that is less susceptible to raw material supply challenges
[352, 353].
A typical Na-ion battery comprises an anode, a cathode, an electrolyte (which can be non-
aqueous or aqueous), and a separator, as illustrated in Figure 38. The operation of Na-ion
batteries closely parallels that of Li-ion batteries. During the charging process, sodium ions
are extracted from the cathode and inserted into the anode, with the reverse process
occurring during discharge [354, 355].
Figure 38: The working principle of a sodium-ion battery. Available at:
https://phys.org/news/2015-09-sodium-ion-batteries-potential-power-technology.html
Significant progress has been achieved in recent advancements concerning the materials
used for the anodes and cathodes in Na-ion batteries. Various cathode materials have been
developed, including sodiated layer transition metal oxides, phosphates, and organic
compounds, as well as anode materials encompassing carbonaceous substances, transition
metal oxides (or sulfides), intermetallic compounds, and organic substances. Furthermore,
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research is actively underway on appropriate electrolytes, additives, and binders [356].
Additionally, similar to Li-ion batteries, aqueous Na-ion batteries have shown promise as
stationary power sources for sustainable energy applications such as wind and solar power.
Over the past few decades, numerous efforts have been undertaken to enhance the
performance of aqueous Na-ion batteries [357].
5.2.6 Metal-Air Batteries
The increasing demand for advanced battery storage systems, driven by the proliferation
of portable electronic devices and the electrification of transportation, has prompted the
search for solutions capable of meeting the elevated energy storage requirements. Metal-
air batteries have emerged as promising contenders for future battery needs due to their
superior specific capacity and energy density compared to conventional batteries [358,
359]. The exploration of metal-air battery technology dates back to the 1960s and early
1970s for various applications [360]. Nevertheless, significant advancements have been
achieved in these battery systems in recent years.
Metal-air batteries distinguish themselves from conventional batteries by their connection
to the atmosphere, as they operate using oxygen. In contrast to conventional batteries that
contain both anode and cathode within the battery case, a metal-air battery features a solid
anode within the case (similar to a traditional battery), while the cathode fuel is sourced
from the external atmosphere [361]. The key internal components of a metal-air battery
include oxygen, a metal anode, an air cathode, a separator, and an electrolyte, as depicted
in Figure 39. The metal anodes engage in electrochemical reactions with oxygen from the
atmospheric air. Depending on the anode's nature, the electrolyte can be either an alkaline
aqueous solution such as potassium hydroxide, a neutral aqueous solution of a salt (like
sodium chloride), or a non-aqueous solution of a base metal salt, chosen based on the
specific anode characteristics. During the charging cycle, the metal on the anode undergoes
transformation into ions, while oxygen on the cathode converts into hydroxide ions.
Electrons are generated during this metal-to-metallic ion transition, and the metallic ions
subsequently dissolve into the electrolyte. In the discharge cycle, atmospheric oxygen
combines with the metallic ions to form metal hydroxide, initiating the electrolysis process
and generating current [362].
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Figure 39: Key elements and functioning of a metal-air battery. Available at:
https://www.researchgate.net/profile/Yafei-Zhang/publication/323801858
The chemical equations representing the reactions in a metal-air battery with a metal anode
M are as follows:
Cathode:
Anode:
Overall chemical reaction:
Oxygen, introduced into the battery through an air cathode, serves as an essentially
unlimited source. Consequently, while oxygen is an inexhaustible cathode reactant, the
capacity of metal-air batteries is constrained by the metal anode. Each metal exhibits
distinct properties, yielding varying voltages with the cathode and differing energy content
[361]. Promising metal anode candidates encompass lithium, aluminum, magnesium, zinc,
potassium, sodium, and iron. Lithium and sodium metal anodes are compatible with non-
aqueous electrolytes, while magnesium, aluminum, iron, and zinc are suitable for aqueous
electrolytes. Generally, metal-air batteries offer the highest energy density among all
battery systems [363]. Table 29 [362, 364–371] provides an overview of the characteristics
of various metal-air batteries. The primary advantage of metal-air batteries lies in their
exceptionally high theoretical specific energy. However, they exhibit some drawbacks,
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including sensitivity to environmental conditions and occasional hydrogen gas leakage
from aqueous cells due to anode corrosion.
Table 29: Attributes of several metal-air batteries [362, 364–371].
Battery
Type
Practical
Operating
Voltage
(V)
Theoretical Specific
Energy (Wh/kg)
Actual Specific
Energy (Wh/kg)
Actual Energy
Density (Wh/kg)
Aluminium-
air
1.35
8140
350
350
Lithium-air
2.7–2.8
1.14×104
150–6,000
30–1,000
Magnesium-
air
2.9
6800
700
200
Zinc-air
1.65
1353
400
1300
Iron-air
1.28
1000
75
50–75
Sodium-air
2.27
-
–
1600
5.2.7 Solid-State Batteries (SSBs)
The evolution of portable smart devices and electric vehicles has raised the bar in terms of
energy density and safety requirements for rechargeable secondary batteries. Over the past
three decades, conventional Li-ion batteries have garnered unparalleled attention from both
the research community and industry. These batteries have been instrumental in powering
portable electronic devices. However, commercial Li-ion batteries featuring liquid organic
electrolytes fall short of meeting the scalability demands for various battery applications,
especially in terms of safety and power density [372].
To address these limitations, researchers have developed Solid-State Batteries (SSBs).
SSBs have the potential to not only mitigate safety concerns but also enable operation in
challenging environments, including temperature extremes ranging from -50 to 200°C or
higher. In such conditions, organic electrolytes are unsuitable due to freezing, boiling, or
decomposition issues. Solid electrolytes, with their enhanced thermal stability, energy
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density, and power density, position SSBs as one of the most promising options for next-
generation batteries [373, 374].
Solid electrolytes were initially discovered in the 19th century, but the rise of electric
vehicles in the late 20th and early 21st centuries has reignited interest in solid-state battery
technologies. Leading automakers such as Toyota, Volkswagen, General Motors, Hyundai,
and Ford have made substantial investments in SSB technology companies, with the aim
of achieving full commercial deployment in the first half of the 21st century [375].
In essence, an SSB is a type of battery that employs a solid electrolyte in place of a liquid
one, as illustrated in Figure 40. The cell chemistry of an SSB is akin to that of batteries
using liquid electrolytes. The pivotal component in an SSB is the solid electrolyte, which
can be composed of ceramic, glass, polymer, or a combination thereof [376]. Table 30
[377–381] provides a summary of some anode, cathode, and electrolyte materials used in
SSBs, along with their optimal conductivity values.
Figure 40: Schematic diagram of conventional liquid battery and solid-state battery. Available at:
https://www.researchgate.net/publication/273326665
Approximately 20 companies worldwide have successfully prototyped SSBs. Due to their
inherent lack of cooling requirements, solid systems are lighter and occupy less space
compared to Li-ion batteries used in electric vehicles. Despite these numerous advantages,
commercializing SSBs remains a challenge due to underlying material and cell-level issues
that require resolution [382]. Table 31 outlines the advantages and disadvantages of SSBs.
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Table 30: A summary of anode, cathode, and electrolyte materials employed in solid-state
batteries along with their respective optimal conductivity values [377–381].
Anode
Cathode
Electrolyte
Conductivity (S.cm−1)
Lithium
metal
Carbon
composite
Solid polymer electrolyte with
lithium doping
-
Potassium
Carbon
paper
doped with
iodine
Polyethylene oxide (PEO)/KBrO3
7.74 × 10−8 at 30 ◦C
Graphite-
based
anode
Lithium
cobalt
oxide
(LiCoO2)
Polystyrene with varying quantities
of aluminum oxide (Al2O3)
9.78 × 10−5 for 10 wt.%
Al2O3 at room temperature
Silver
Graphite
coupled
with iodine
Glass ceramic composites
containing silver components
(Ag2O-P2O5-LiCl)
2.88 × 10−2 for 25 wt.%
Al2O3
Lithium
Lithium
Tri-ethyl sulfonium bisimide,
LiTFSI (Lithium
bis(trifluoromethanesulfonyl)imide),
polyethylene oxide, and
tetrahydrofuran
10−2 at 45 ◦C
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Table 31: Benefits and drawbacks of solid-state lithium-ion batteries.
Benefits
Drawbacks
Exceptional thermal stability
High ionic conductivity
Use of non-flammable electrolytes
High energy density
Minimal self-discharge
Capability to withstand a high number
of cycles
Small physical dimensions
Considerably high production cost
Occurrence of lithium dendrite growth in
solid-state lithium-based batteries
Absence of Solid Electrolyte Interphase
(SEI) layer formation
Relatively low specific power density
5.3 Flow Battery Energy Storage (FBES) system
Flow batteries utilize a distinctive approach, employing two separate electrolytes stored
within distinct external tanks (as depicted in Figure 41). A microporous membrane is
interposed between these electrolytes, facilitating the passage of selected ions while
generating an electric current [219]. FBES systems are further categorized into two distinct
types, as illustrated in Figure 42: the reduction-oxidation (redox) flow battery and the
hybrid flow battery. In redox flow batteries, all electroactive materials are dissolved within
a liquid electrolyte, whereas in hybrid flow batteries, one or more electroactive materials
are incorporated or deposited within the electrolyte itself [198]. Large-scale FBES
predominantly falls into three primary categories, which are elaborated upon in the
subsequent sections.
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Figure 41: Schematic representation of flow battery energy storage system. Available at:
https://www.solarchoice.net.au/blog/news/an-introduction-to-flow-batteries-030315/
Figure 42: Categorization of flow battery energy storage system. Available at:
https://www.sciencedirect.com/science/article/pii/S277268352200022X
5.3.1 Vanadium Redox Batteries (VRBs)
VRBs store electrochemical energy through electron transfer between various ionic
vanadium materials. These two electrolytes are separated by a Proton Exchange Membrane
(PEM), as depicted in the schematic diagram in Figure 43. During the charging phase, V3+
ions at the anode undergo conversion to V2+ by accepting an electron. Conversely, during
discharge, V2+ ions are converted back to V3+, releasing one electron in the process. A
similar mechanism of electron transfer occurs between V5+ and V4+ ions at the cathode [23].
VRBs are renowned for their ability to store energy over extended durations, making them
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particularly well-suited for utility-scale applications such as load leveling and peak
shaving.
Figure 43: Schematic diagram of vanadium redox battery. Available at:
https://www.researchgate.net/publication/245106793_NafionSiO_2_hybrid_membrane_for_vana
dium_redox_flow_battery/figures?lo=1
The concept of the all-Vanadium Redox Battery, abbreviated as "all-VRB," was introduced
in 1986 by Maria Skyllas-Kazacos, a chemical engineer at the University of New South
Wales [383]. Since then, the all-VRB has emerged as the most promising option for large-
scale energy storage applications, offering capacities spanning from 1 MW to 100 MW
[384]. The developmental history of VRBs is outlined in Table 32 [385−391]. Notably,
starting from the 1990s, various vanadium battery field trials have been conducted in
countries like Thailand and Japan [392, 393]. More recently, VRBs have found installation
in diverse applications, including Uninterruptible Power Supply (UPS), frequency
regulation, and load shifting applications.
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Table 32: Development and progression of vanadium redox flow battery technology.
Year
Battery Type
Electrodes
Electrolyte
1986
[385]
Laboratory
Test Cell
Graphite Plates
V3+ (0.1 M) and V4+ (0.1 M) in H2SO4
(2 M)
1987
[386]
Redox Flow
Cell
Graphite (Cathode) and
Iridium Oxide-coated
(Anode)
Vanadyl Sulfate in Sulfuric Acid
1989
[387]
Redox Flow
Cell
Graphite Felt/Carbon
Plastic
Vanadium Sulfate in Sulfuric Acid
1991
[388]
Redox Flow
Cell
Graphite Fiber
Vanadium Sulfate in Sulfuric Acid
(2 M)
2002
[392]
Membraneless
Redox Fuel
Cell
Carbon-on-Gold
VOSO4 (1 M) in H2SO4 (25%)
2008
[389]
Redox Flow
Cell
Graphite Felt
V4+ (2 M) in H2SO4 (2.5 M) (Catholyte)
and V3+ (2 M) in H2SO4 (2.5 M)
(Anolyte)
2011
[390]
Single Redox
Flow Cell
Graphite/Graphite Oxide
(GO) Composite
H2SO4 (2.0 M) + VOSO4 (0.1 M)
Solution
2019
[391]
Three-
Electrode
Redox Flow
Cell System
Carbon Nanofiber (CNF)
and ZrO2/CNF Coated
Glassy Carbon (Working
Electrode), Saturated
Calomel Electrode,
Platinum (Pt) Sheet
(Reference and Counter
Electrode)
V3+ (1.6 M) + H2SO4 (3 M) Electrolyte
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Researchers have delved into optimizing the electrode architecture in vanadium redox flow
batteries using interdigitated flow fields, with findings suggesting that electrodes
composed of finer fibers can significantly enhance cell performance [394]. Numerical
models have been developed to analyze different flow fields and patterns, aiding in the
enhancement of VRB performance and lifespan [395]. Additionally, studies have detailed
the factors influencing VRB development, elucidating the working principle,
electrochemical reaction process, and system model [396]. Hybrid polymer composite
membranes incorporating inorganic TiO2 nanofillers for VRB applications have also been
explored [397]. Researchers have reviewed the pivotal role of ion exchange membranes
and advancements in nanocomposite membranes aimed at reducing vanadium ion
permeability and improving proton conductivity to achieve superior performance and
prolonged VRB system life [398, 399].
Key plants built on VRB technology worldwide are listed in Table 33 [400–403]. Despite
VRBs being among the most advanced flow battery technologies, the capital expenditure
associated with these systems poses a barrier to broader commercialization. Table 34 [309,
404] provides an overview of the benefits and drawbacks of VRBs [15].
Table 33: Compilation of significant facilities constructed worldwide utilizing vanadium redox
battery technology [400–403].
Year
Location
Storage Capacity (MWh)
Applications
1996
Kashima-
Kita Electric
Power, Japan
0.8
Load-Leveling
1996
Tatsumi
Substation,
Kansai
Electric,
Japan
0.9
Peak Shaving
2000
Kansai
Electric,
Japan
1.6
Peak Shaving
2001
Hokkaido
Electric
Power Wind
Farm, Japan
1
Stabilization of Wind Turbine
Output Power
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2001
Tottori
Sanyo
Electric,
Japan
1.5
Peak Shaving and Emergency
Backup Power
2000
Stellenbosch
University,
South Africa
0.5
Peak Shaving and UPS Backup
Power
2001
Gwansei
Gakuin
University,
Japan
5
Peak Shaving
2001
Milan, Italy
0.09
Distributed Power Systems
2003
High-Tech
Factory in
Japan
2
UPS and Peak Shaving
2003
Huxley Hill
Wind Farm
on King
Island
1
Wind Energy Storage and Diesel
Fuel Replacement
2004
Castle
Valley,
Moab, USA
2
Voltage Support and Rural Feeder
Augmentation
2005
Electric
Power
Development
Co., Ltd. in
Tomamae,
Hokkaido,
Japan
6
Wind Energy Storage and Wind
Power Stabilization
2010
Vierakker,
Netherlands
0.1
-
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Table 34: Benefits and drawbacks associated with vanadium redox batteries [309, 404].
Benefits
Drawbacks
High power
Extended life cycle
Quick charge and discharge
Fast response times
Considerably high production cost
Wide area
Lowe energy density
5.3.2 Polysulfide Bromide (PSB) Flow Batteries
The PSB battery, also referred to as Regenesys, represents a regenerative fuel cell type
where a reversible electrochemical reaction unfolds between two electrolytes, sodium
bromide, and sodium polysulfide. A polymer membrane serves as a separator, enabling the
transfer of sodium cations between the electrodes. In the charging cycle, three bromide
ions undergo oxidation and combine to form a tribromide ion at the positive electrode.
Meanwhile, at the negative electrode, dissolved sodium particles in the polysulfide
electrolyte transform into sulfide ions. This entire process is reversed during the discharge
phase [405]. The schematic diagram of PSB batteries is illustrated in Figure 44.
Figure 44: Schematic diagram and working principle of polysulfide bromide battery system.
Available at: https://www.researchgate.net/publication/228907220
Technology Ltd. (formerly Innogy Technology Ventures Ltd.) conducted significant
research on PSB from 1993 to 2006 [406, 407]. Reviews encompassing material
developments for membrane separators, electrolyte solutions, and electrodes for PSB flow
batteries can be found in references [405, 408, 409]. Zhang et al. [20] conducted a
comprehensive review of the various polysulfide flow battery systems developed thus far,
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along with their future prospects. Chai et al. [410] delved into recent advancements in
organic redox-active materials in both aqueous and non-aqueous settings, ranging from
small molecules to polymers. Table 35 [405, 406, 411−413] summarizes the developmental
timeline of PSB flow batteries. In 2002, a 15MW Regenesys PSB flow battery was
constructed at Little Barford in the UK. Although the facility's construction was finalized,
it was never fully commissioned due to certain engineering challenges associated with
scaling up the technology [414]. PSB flow batteries come with specific advantages and
disadvantages outlined in Table 36 [415]. Due to their rapid response time, they find
particular utility in applications related to frequency response and voltage control. So far,
three series of PSB systems have been developed, including 5, 20, and 100 kW class
systems. Nevertheless, certain technical challenges continue to impede the
commercialization of PSB batteries, such as the cost of carbon felt-based electrode
preparation and the intricate synthesis methods of sodium polysulfide from molten sodium
[15, 393].
Table 35: Advancement in the field of flow batteries utilizing polysulfide bromide chemistry.
Year
Battery
Type
Electrodes
Electrolyte
Membrane
1984
[405]
Single PSB
Flow
Battery
Solid
Graphite
(Cathode)
and Porous
Sulfided
Nickel
(Anode)
1 M Sodium Bromide Saturated with
Bromine (Catholyte) and 2 M
Sodium Sulfide (Anolyte)
Nafion 125
2001
[406]
S-, L-, and
XL-Series
Cell Stacks
Carbon-
Polyolefin
Composite
Electrodes
Tribromide/Bromide and
Polysulfide/Sulfide Sodium Cation-
Exchange Membrane
Sodium Cation
Exchange
Membrane
2004
[411]
Single Flow
Cell
Ni/C
(Positive
Electrode)
and Pt/C
(Negative
Electrode)
2.0 M Na2S2 Solution (Catholyte)
and 1.0 M Br2 in 2.0 M NaBr
Solution (Anolyte)
Nafion
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2006
[412]
Single Flow
Cell
Carbon
Felt (CF)
and
Activated
Carbon
(AC)
1.3 M Na2S4 Aqueous Solution
(Anolyte) and 4.0 M NaBr Aqueous
Solution (Catholyte)
Nafion-117
2018
[413]
Na-
Aqueous-
Redox-
Flow
Battery
(NaAqRFB)
Cell
Na-Metal
Anode
Organic Electrolyte (1 M NaCF3SO3
in TEGDME)
NASICON
(Na3Zr2Si2PO12)
Table 36: Benefits and drawbacks of energy storage using the polysulfide bromide flow battery
system [415].
Benefits
Drawbacks
Negligible self-discharge rate
Remarkably quick response
(20 ms)
High energy efficiency
Rapid charging capability
Sufficient power density
Limited environmental impact
Contains toxic bromine gas
Costly preparation
5.3.3 Zinc‐Bromine (ZnBr) Flow Batteries
The ZnBr flow battery stands apart from VRB and PSB flow batteries as it belongs to the
category of hybrid flow batteries, a concept that emerged in the 1970s through
collaborative efforts by Exxon, Gould, and the National Aeronautics and Space
Administration (NASA) of the USA. It was first patented by Thaller [416] and Butler et al.
[417]. Prototypes of 1 MW, 4 MWh batteries were constructed in 1991 after years of
research and development. Today, ZnBr flow batteries have reached a mature stage of
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development and are being adopted by various companies [418]. The progression of ZnBr
flow batteries is summarized in Table 37 [419−425].
Table 37: Advancement in the field of flow batteries utilizing zinc-bromine chemistry.
Year
Battery Type
Electrodes
Electrolyte
1991
[419]
Single Zinc-
Bromine
Battery
Carbon/Polyvinylidene
Fluoride (PVDF) Bipolar
Polybromide/Aqueous
2001
[420]
Non-Aqueous
Zinc-Bromine
Battery
Glassy Carbon
Electrodes
0.5 M N-Methyl-Ethyl-Morpholinium
(MEM)-Br, 0.5 M N-Methyl-Ethyl-
Pyrrolidinium (MEP)-Br, and 3 M ZnBr2
2013
[421]
Single Flow
Zinc-Bromine
Battery
Semi-Solid Carbon Felt
Positive Electrode
2 M ZnBr2
2014
[422]
Rechargeable
Zinc-Bromine
Battery
Single-Walled Carbon
Nanotube (SWCNT) and
Multiwalled Carbon
Nanotube (MWCNT)
ZnBr2
2017
[423]
Single Zinc-
Bromine
Redox Flow
Battery
Carbon Felt Electrodes
with Ultrathin Nafion-
Filled Porous Membrane
2.25 M ZnBr2, 0.5 M ZnCl2, 0.8 M MEP,
and 5 M Br2
2018
[424]
Single Zinc-
Bromine
Redox Flow
Battery
Boron-Doped Graphene
(BDG)
0.05 M ZnBr2 and 1 M HClO4
2019
[425]
Aqueous
Zinc-Bromine
Battery
Zinc Anode and Liquid
Br2 Cathode
Alkaline-Acid Hybrid Electrolytes
This technology employs two separate containers, each holding an aqueous electrolyte
solution of zinc and bromine, as depicted in Figure 45. These electrolytes circulate through
the electrolytic cells during both charging and discharging cycles, facilitating reversible
electrochemical reactions. During charging, solid zinc is deposited on the negative
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electrode, while bromine is deposited on the positive electrode [426]. In the discharging
phase, the zinc deposited on the negative electrode dissolves in the electrolyte, making it
available for subsequent charging cycles. The advantages and disadvantages of ZnBr
batteries are outlined in Table 38 [308].
Figure 45: Schematic representation and working principle of zinc-bromine battery system.
Available at: https://www.researchgate.net/publication/320755664
Table 38: Benefits and drawbacks associated with zinc-bromine battery technology [308].
Benefits
Drawbacks
High specific energy
Good energy efficiency
Rapid charging capability
Sufficient power density
Low environmental impact
Initially high self-discharge when
charging is stopped
Requires auxiliary systems for
circulation and temperature control
Safety concerns
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Currently, integrated ZnBr flow battery systems designed for utility-scale applications
have undergone testing on transportable trailers, with capacities of up to 1 MW.
Additionally, a novel type of ZnBr battery is under development in Australia, promising
attributes such as lighter weight, flexibility, enhanced safety, and faster charging
capabilities [15, 198, 259]. Lin et al. [427] conducted a review focusing on the technical
aspects and materials used in the electrolyte, membrane, and electrode of ZnBr batteries,
as well as the potential for their further development. Khor et al. [426] and Wang et al.
[428] provided comprehensive reviews of various zinc-based redox flow batteries,
considering their electrolyte environments and emphasizing advancements in electrode and
membrane materials. Yuan et al. [429] explored the functions, structures, and chemistries
of different materials used in zinc-based flow batteries and their impact on battery
performance. Jiang et al. [430] discussed recent progress in carbon-based material
electrodes and carbon nanotube-modified electrodes, aiming to improve ZnBr flow battery
performance. Lee et al. [431] enhanced the electrochemical performance of ZnBr flow
batteries by improving reaction kinetics and mitigating the growth of zinc dendrites
through the replacement of conventional polymer mesh with a titanium-based mesh
interlayer.
5.4 Paper Batteries
Paper batteries represent a distinctive category of batteries primarily composed of paper or
cellulose and carbon nanotubes. These batteries are exceptionally thin and flexible in
nature. They function in a manner similar to traditional batteries, but stand out due to their
non-corrosive properties and minimal requirement for housing and casing [432]. Their
remarkable flexibility allows them to be bent, twisted, or wrapped around objects,
providing an alternative to rigid setup constraints. Furthermore, paper batteries can be
conveniently accommodated in tight or confined spaces, reducing the overall size and
weight of the system [24]. The fundamental assembly of a paper battery is illustrated in
Figure 46. It consists of a sheet of paper coated with an ionic solution, which is then layered
with a specially formulated carbon nanotube ink. On the opposite side of the paper, a thin
lithium coating functions as the anode. To facilitate the flow of current between the
electrodes, two aluminum rods are connected to both sides [433].
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Figure 46: Schematic representation of paper battery. Available at:
https://www.slideshare.net/asertseminar/paper-battery-31964910
Various types of paper batteries have been developed for diverse applications. Table 39
[434−444] provides an overview of the advancements in paper batteries. In 2011, it was
observed that when a sheet of cellulose paper is coated with thin layers of materials with
opposing electrochemical potentials on both sides, it generates a voltage across the paper
battery's electrodes. The magnitude of the generated voltage depends on the materials used
for the anode and cathode. For instance, a circuit voltage of 0.5 V was achieved with a
simple Cu/paper/Al combination [445]. In 2020, an innovative approach leveraged paper-
based batteries as sensors for measuring ionic conductivity [446, 447]. More recently, a
cost-effective disposable water-activated paper-based battery was developed, utilizing
activated carbon powder loaded on a carbon sheet as the anode. The power output of this
battery was found to be proportional to the amount of activated carbon loaded on the anode
[446].
Table 39: Progress in the development of paper-based batteries.
Year
Technological Advancement in Paper Batteries
1992 [434]
Solid Rechargeable Paper Batteries
Utilizes polyaniline film as the electrode and non-aqueous gel as
the electrolyte
2005
[435, 436]
Urine-Activated Laminated Paper Batteries
Activation by saliva, urine, and tap water; achieves a maximum
voltage of 1.56 V within 10 seconds
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2007 [437]
Organic-Based Paper Batteries
Introduction of cathode-active thin film through photo-crosslinking
a nitroxide radical functional polymer
2007 [438]
Paper-Based Supercapacitors
Integration of all supercapacitor components into nanocomposite
papers, including multiwalled nanotubes as cathode and lithium as
anode
2009 [439]
Lithium-ion Paper Batteries
Enhanced paper used as electrode and separator; employs carbon
nanotubes for cathode and anode
Utilization of paper as separator enhances adhesion, simplifies
coating process, and reduces cost
2009 [440]
Paper-Based Electrochemical Batteries
Deposition of metal electrodes onto paper or introduction of
electrolytes into paper
2013 [441]
Folded Paper-Based Batteries
Folding technique enhances compactness and 3D morphology;
folded cells offer higher areal energy density
2013 [442]
Paper-Based Electrochemical Batteries
Deposition of metal electrodes onto paper or introduction of
electrolytes into paper
2014 [443]
Aqueous Paper Batteries
Casting of LiMn2O4 and carbon-coated TiP2O7 slurries onto
conductive paper substrates; use of carbon nanotubes as current
collectors
Demonstrates excellent rate capability and reasonable cycle life
2018 [444]
Saliva-Powered Paper Battery
Suitable for extreme conditions where normal batteries fail;
capable of generating required power from a single drop of saliva
Yao et al. [448] conducted a review focusing on recent breakthroughs in the synthesis of
paper-based electrodes, including their structural characteristics, electrochemical
performance, and application as electrodes in flexible energy storage devices. Sharifi et al.
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[449] and Thakur and Devi [450] provided concise overviews covering the mechanics,
emerging materials, technological trends, challenges, current status, and future possibilities
of paper-based devices in the energy sector. Additionally, Hou et al. [451] discussed the
challenges and future development prospects of microfluidic paper-based analysis devices
(µPads) for Point-Of-Care Testing (POCT), along with their applications in disease
analysis, environmental monitoring, and food control.
Table 40 [435, 436, 440, 452−458] catalogs different varieties of paper batteries, outlining
their unique characteristics and performances. It is evident that paper batteries hold
significant potential to power the next generation of portable and compact electronic
devices, medical equipment, and hybrid vehicles.
Table 40: Overview of paper batteries.
Year
Battery Type
Electrolyte
Anode
Cathode
Voltage (V)
2005
[435,
436]
Biofluid- and
Water-Activated
Paper Batteries
Saliva, Urine,
and Tap
Water
Magnesium
Copper
1.56
2009
[440]
Paper-Based,
Printed Zinc-Air
Battery
Lithium
Chloride
Zinc/Carbon/
Polymer
Composite
Poly (3,4-
Ethylenedioxy-
thiophene)
(PEDOT)
12
2009
[452]
All-Polymer
Paper-Based
Batteries
Sodium
Chloride
Platinum Foil
Platinum Foil
-
2011
[453]
Solid-State Paper
Batteries
Water
Copper
Aluminium
3
2012
[454]
Fluidic Batteries
Silver Nitrate
and
Aluminium
Chloride
Aluminium
Silver
1.3
2012
[455]
Paper-Based
Electrochemical
Sensing Platform
Artificial
Urine
Indium Tin
Oxide (ITO)
Indium Tin
Oxide (ITO)
1.1
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2013
[456]
Galvanic Cell
Paper Battery
Water
Aluminium
Carbon
1.53
2019
[457]
Paper-Based Al-
Air Battery
Cellulose
Paper
Low-Grade
Aluminium
Foil
Carbon Paper
7.7
2020
[458]
Nano-Based
Paper Battery
-
Aluminium
Foil
Cellulose-
Based Paper
3.38
5.5 Flexible Batteries
The rapid advancement of flexible, wearable, and multifunctional electronic devices, such
as smart watches, fitness bands, and medical implants, has created a pressing need for
energy storage systems that offer both high performance and the requisite flexibility [459,
460]. Traditional electronic devices are typically heavy and bulky, lacking the flexibility
demanded by these new applications. Consequently, there is a concerted effort to develop
energy storage systems capable of enduring continuous and prolonged mechanical
deformations, including bending, twisting, and stretching, all while delivering high power
and energy output over extended operational cycles. A central challenge in this endeavor
is the development and production of electrode materials possessing high mechanical
flexibility, alongside high energy density, power density, and robust cycling stability.
Additionally, these materials must be compatible with suitable electrolytes and separators.
Notably, safety is of paramount importance, particularly for wearable devices, where solid-
state electrolytes are favored for flexible batteries to enhance safety by eliminating the risk
of flammable liquid electrolyte leakage [461, 462].
Numerous endeavors have been undertaken to adapt conventional batteries like lithium-
ion, lithium-sulfur, sodium-ion, and zinc-ion batteries to meet the requirements of flexible
applications. Simultaneously, researchers are exploring novel materials for flexible battery
electrodes and electrolytes. For instance, Li et al. [463] devised a graphene-based lithium-
ion battery characterized by thinness, lightness, and flexibility. This battery utilized a
conductive interconnected network of graphene foam as a current collector, lithium titanate
(Li4Ti5O12) for the anode, and lithium iron phosphate (LiFePO4) as the cathode.
Remarkably, it exhibited superior performance, energy density, and the ability to bend
repeatedly to a radius of 5 mm without structural failure. Gaikwad et al. [464] explored the
material challenges and mechanical limitations of flexible printed batteries, elucidating the
various printing techniques employed in their production. Hu et al. [465] crafted flexible,
liquid-free Li-CO2 batteries using poly(methacrylate)/poly(ethylene glycol)-LiClO4 with 3
wt.% SiO2 Composite Polymer Electrolyte (CPE) and multiwall Carbon Nanotube (CNT)
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cathodes. These pouch-type batteries demonstrated exceptional electrochemical
performance, maintaining a controlled capacity of 1,000 mA·h·g−1 even while undergoing
extensive bending. Zamarayeva et al. [466] developed a flexible Zn/MnO2 battery, utilizing
a polyvinyl alcohol (PVA)/polyacrylic acid (PAA) gel as a flexible binder for the Zn metal
anode and manganese oxide (MnO2) cathode. This battery exhibited superior capacity
retention and mechanical resilience when subjected to high-rate discharges compared to
conventional batteries. Fu et al. [467], Shi et al. [468], and Xiang et al. [469] conducted
concise reviews, covering the mechanics, design principles, component requirements,
emerging materials, ongoing challenges, current status, practical applications, and future
prospects of flexible batteries. Despite considerable progress, designing, packaging, and
assembling electrolytes and electrodes that meet market demands for flexible batteries
remain ongoing objectives [379, 470].
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CHAPTER 6: ELECTRICAL ENERGY STORAGE (EES)
SYSTEMS
Chapter 6 delves into EES systems, with a focus on Capacitors, Supercapacitors, and
Superconducting Magnetic Energy Storage (SMES). Capacitors, as fundamental electrical
components, store energy through electrostatic fields and find applications in power
systems. Supercapacitors, known for their enhanced energy density and rapid
charge/discharge capabilities, play a vital role in industries like electric vehicles and
consumer electronics. The chapter culminates with an exploration of SMES, a cutting-edge
technology rooted in superconductivity, detailing its components and applications in power
grids and systems. This chapter provides insights into the diverse landscape of electrical
energy storage technologies, spanning from foundational principles to advanced, high-
performance solutions.
6.1 Introduction to Electrical Energy Storage Systems
EES systems retain energy within an electric field, preserving it in its original electrical
form without any conversion into alternative energy forms. EES systems are categorized
into two primary types, as illustrated in Figure 47: electrostatic energy storage systems and
magnetic energy storage systems. Electrostatic energy storage includes capacitors and
supercapacitors, whereas magnetic energy storage is exemplified by SMES.
Figure 47: Categorization of electrical energy storage systems. Available at:
https://www.sciencedirect.com/science/article/pii/S277268352200022X
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6.2 Capacitors
When a capacitor is charged, it stores electrical energy by utilizing an electrostatic field.
This device consists of two closely positioned metal plates separated by a dielectric layer
composed of a non-conductive material (as shown in Figure 48) [471].
Figure 48: Different capacitors utilized in electronic equipment. Available at:
https://www.pinterest.com/pin/693695148843963865/
In its operation, a voltage source is applied across these metal plates, resulting in the
charging of one plate with electricity while inducing an opposite charge in the other plate.
The specifications of several commonly used capacitors are detailed in Table 41 [472, 473],
while Table 42 [474] provides insights into capacitor characteristics and performance
metrics. Due to their inherently low energy density, capacitors are capable of handling high
currents, but only for very brief durations [5, 258]. Researchers such as Han et al. [475],
Ramakrishnan et al. [476], Su et al. [477], and Han et al. [478] have extensively reviewed
the challenges and recent advancements in employing 2D materials for high-energy and
high-power lithium-ion capacitors, emphasizing the crucial advantages and roles of these
materials in constructing capacitors for various applications. Yuan et al. [479] and Zhang
et al. [480, 481] have provided concise overviews of the challenges and opportunities in
developing sodium-ion hybrid capacitors (SICs) and recent progress in electrode materials
for SICs, particularly focusing on material design strategies and their impact on
electrochemical performance. Liu et al. [482] have conducted a comprehensive review of
recent advancements and future directions in the development of next-generation
electrochemical capacitors with high energy and power performance. As the world
undergoes a shift towards electrification, especially in the transportation sector
transitioning to hybrid and fully electric vehicles, there is a growing need for more efficient
and compact portable electronic devices for communication, medical applications, and
high-power electronics, necessitating advanced low-voltage capacitors. Similarly,
advanced high-voltage capacitors are becoming increasingly vital for energy storage,
renewable energy integration with the power grid, and applications involving pulsed power
systems, such as electromagnetic-based pulse power systems [473].
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Table 41: Overview of common types of capacitors [472, 473].
Types of Capacitors
Dielectric
Material
Electrode
Material
Voltage
(kV)
Capacitance
(F)
Applications
Paper Capacitor (1876)
Wax-
impregnated
paper
Aluminium
sheet
0.001 -
2.000
Radio
receivers
Silver Mica Capacitor
(1909)
Mica
Silver
0.100 -
1.000
4.7×10-3
High-
frequency
tuned circuits
like filters
and
oscillators
Ceramic Capacitor
Ceramic
Metal
0.016 -
15.000
1×10-5 - 100
Transmitter
stations,
induction
furnaces,
power circuit
breakers,
printed
circuit boards
Aluminium Electrolytic
Capacitor (1892)
Aluminium
oxide
Pure
aluminium
foil
0.004 -
0.630
0.1 - 2.7×106
Flash
capacitors for
camera
flashes,
motor start
capacitors for
AC motors,
energy
storage for
airbags,
photoflash
devices
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Film Capacitor (1944)
Thin plastic
film
Pure
aluminium
foil
0.050 -
2.000
0.001 -
30.000
Power
electronics
devices,
phase
shifters, X-
ray flashes,
decoupling
capacitors,
and filters
Glass Capacitor
Glass
Aluminium
0.025 -
0.050
2.7×10-4 -
0.1
Critical
military and
space
programs
Tantalum Capacitor
(1930)
Tantalum
oxide
Tantalum
metal
0.002 -
0.500
0.001 -
72.000
Power supply
filtering on
computer
motherboards
and cell
phones
Table 42: Attributes and operational metrics of capacitor-based energy storage system [474].
Parameter
Value
Energy Storage
W-sec of energy
Charging Method
Voltage across terminals
Charging Time
Less than 10-3 seconds
Cell Voltage
6 to 800 V per cell
Specific Energy (Wh/kg)
0.01 to 0.05
Specific Power (W/kg)
Less than 105
Efficiency
Greater than 95%
Operating Temperature
-20 to 100 degrees Celsius
Lifetime
More than 105 cycles
Weight
1 gram to 10 kilograms
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6.3 Supercapacitors
Supercapacitors, also known as electric double-layer capacitors (EDLC) or ultracapacitors,
consist of two conductor electrodes, an electrolyte, and a separator (as depicted in Figure
49). They store energy by creating an electrostatic field through the application of a
continuous direct current voltage between two electrodes, separated by a thin insulating or
dielectric layer [2, 483].
Figure 49: Schematic representation and the working principle of a supercapacitor. Available at:
http://large.stanford.edu/courses/2012/ph240/aslani1/
These electrodes, typically composed of activated carbon, possess a larger surface area,
resulting in enhanced energy density. A porous membrane divides the electrodes,
permitting the movement of charged ions while preventing electronic contact. When an
applied Direct Current (DC) voltage charges the electrodes, ions in the electrolyte infiltrate
the pores of the opposite-charged electrode, forming charged double layers with polarity
opposite to that of the electrode. For instance, a positively charged electrode exhibits two
charged layers: a layer of negative ions at the interface and a second layer of positive ions,
balancing the first negative layer. The reverse is true for a negatively charged electrode.
The electrolyte supplies and conducts ions from one electrode to the other. Structurally,
supercapacitors lie between batteries and conventional capacitors, as they comprise two
electrodes separated by a porous medium and store energy as an electrostatic field, akin to
typical capacitors [198, 484]. Performance parameters and characteristics of
supercapacitors are detailed in Table 43 [474]. While research on EDLC began in the
1950s, commercialization of supercapacitors commenced in the 1980s [485]. The Standard
Oil Company Sohio pioneered the method of producing EDLC from high-specific-surface-
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area carbon materials in 1968, which was later transferred to Nippon Electric Company
(NEC), where supercapacitor production started in 1979, primarily for electric vehicle
starting systems. In parallel, Panasonic developed supercapacitors employing activated
carbon as the electrode material and an organic solution as the electrolyte. Since then,
numerous supercapacitor designs have emerged, as summarized in Table 44 [486, 487],
which highlights global research and development efforts in this field. Over the past few
decades, supercapacitors have evolved into high-performance energy storage devices
widely utilized in electric vehicle energy management, high-power military applications,
consumer electronics, telecommunication applications, and other sectors [488, 489].
Advancements in supercapacitor energy storage technology are documented in Table 45
[490–501]. The expansion of research and development in supercapacitors is driven by the
demand for effective and eco-friendly energy options. Their notable attributes, such as a
high power density, quick charging abilities, and extended cycle durability, are making
them more appealing for a wide array of uses. These range from improving regenerative
braking mechanisms in vehicles to ensuring stability in the power grid and mitigating
irregularities in energy from sustainable sources. Scientists persistently strive for
advancements, delving into novel materials and designs to elevate the capabilities and
adaptability of supercapacitors across diverse fields.
Table 43: Attributes and operational metrics of an energy storage system based on
supercapacitors [474].
Parameter
Value
Energy Storage
W-sec of energy
Charging Method
Voltage across terminals
Charging Time
1 to 10 seconds
Cell Voltage
2.3 to 2.75 V per cell
Specific Energy
1 to 5 Wh/kg
Specific Power
Up to 104 Wh/kg
Efficiency
85% to 98%
Cycle Life
3 × 104 hours
Operating Temperature
-40 to 85 degrees Celsius
Lifetime
Greater than 105 cycles
Weight
1 to 2 grams
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Table 44: Global research and innovation efforts related to supercapacitors [486, 487].
Country
Company/Lab
Year
Capacitance
(F)
Voltage
(V)
Energy
Density
(Wh/kg)
Power
Density
(W/kg)
Japan
Panasonic
1978
800 - 2,000
3
3 – 4
200 –
400
Japan
NEC
1975
1 – 2
5 – 11
0.5
5 – 10
USA
Maxwell
1991
1,000 -
2,700
3
3 – 5
400 –
600
USA
PowerStor
-
7.5
3
0.4
250
USA
Los Alamos
National Lab
1999
0.8
2.8
1.2
2,000
USA
Pinnacle
Research
Institute
1982
125
15
0.5 – 0.6
200
USA
US Army, Fort
Monmouth
-
1
5
1.5
4,000
USA
Evans
-
0.02
28
0.1
30,000
Russia
ELIT
1988
0.5
450
1
900 -
1,000
Russia
ESMA
1993
50,000
17
8 – 10
80 –
100
Australia
Cap XX
1994
120
3
6
300
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Table 45: Progress in the field of supercapacitor technology.
Year
Technological Advancement
1996 [490]
Introduction of supercapacitors with activated carbon/carbon composite
electrodes.
2001[491]
Development of hybrid supercapacitors utilizing p-doped poly (3-
methylthiophene) (pMeT) as the positive electrode and activated carbon as
the negative electrode, resulting in improved specific power and cyclability
performance.
2005 [492]
Innovation of supercapacitors with nanocomposite hybrid molecular
materials, combining organic and inorganic components to enhance charge
storage and release in solid-state supercapacitors, leading to high specific
capacitance and cycle stability.
2006 [493]
Introduction of supercapacitors with multiwalled nanotubes (MWNT) thin
film electrodes, providing superior frequency response and serving as an
effective coating layer for current collectors to enhance electrode
performance.
2010 [494]
Development of all-solid-state polymer supercapacitors with polyaniline-
based electrodes solidified in H2SO4-polyvinyl alcohol gel electrolyte,
offering high specific capacitance, good cycle stability, and flexibility.
2010 [495]
Development of ultra-high-power micro-supercapacitors with onion-like
carbon electrodes, achieving high specific capacitance, energy density, and
discharge rates suitable for applications like wireless sensor networks,
biomedical implants, radio frequency identification tags, and embedded
microsensors.
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2013 [496]
Introduction of fibre-like supercapacitors utilizing MWNT as cathodes,
lithium as anodes, and electrolytes embedded within nanoporous cellulose
paper.
2015
[497, 498]
Innovation of 3D porous supercapacitors with electrodes featuring 3D
architectures, maximizing active sites, promoting effective contact between
active materials and electrolyte ions, facilitating electron transfer between
current collectors and active materials, and enhancing electrochemical
activity.
2016 [499]
Development of paper-like supercapacitors as flexible and ultra-thin energy
storage systems that can be shaped to suit specific applications.
2018 [500]
Creation of flower-like supercapacitors, combining a flower-like MnO2
nanocomposite with nitrogen-doped graphene (NG-MnO2) to achieve better
specific capacitance, high-rate capability, and long-term cycling stability.
2018 [501]
Introduction of ultra-thin and stackable supercapacitors with MnO2/CNT-
web paper electrodes, offering ultra-thin, lightweight, and all-solid-state
symmetric supercapacitors with high areal capacitance.
The integration of energy storage on board railway vehicles, introduced in 2005,
demonstrated energy savings of up to 30% in a prototype light rail vehicle, which has been
in passenger service since September 2003 [502]. The application of supercapacitors in
wind energy involves integrating short-term energy storage devices with a doubly fed
induction generator design to mitigate rapid wind-induced power fluctuations [503]. In
2009, a reversibly stretchable supercapacitor was proposed, utilizing buckled single-walled
carbon-nanotube (SWNT) macrofilms as electrodes. These stretchable supercapacitors
exhibited remarkably stable capacitance under cyclic stretching and releasing, showcasing
controlled wavy shapes [504]. However, it is worth noting that supercapacitors come with
limitations such as low energy density, high operational costs, and substantial voltage
variation during operation, which require further research attention [505]. Several reviews
in archival literature [488, 506–510] assess recent breakthroughs in supercapacitor research
and technology, encompassing charge storage mechanisms, electrode materials,
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electrolytes, and manufacturing methods. Additionally, researchers [511–515] have
examined recent progress in micro-supercapacitors (MSCs), focusing on fabrication
strategies, structural design, electrode materials, substrates, electrochemical properties, and
integrated applications. Other reviews by Uke et al. [516], Wu et al. [517], Sakib et al.
[518], and Das et al. [519] delve into recent advancements in electrode materials for
supercapacitors, including cobalt oxides, cerium oxide, manganese oxides, manganese
sulfide, and their composites. Supercapacitors are predominantly employed in applications
requiring a high number of rapid charge/discharge cycles rather than extended energy
storage. They find utility in regenerative braking, short-term energy storage, and burst-
mode power delivery in vehicles, buses, trains, cranes, and elevators [520].
6.4 Superconducting Magnetic Energy Storage (SMES) System
While the concept of superconductivity was initially proposed in 1911, the first suggestion
for a SMES system came in 1969 from Ferrier, who advocated the installation of a large
toroidal coil capable of providing daily energy storage for France. However, the actual
research and development of SMES began at the University of Wisconsin in the USA in
1971, eventually leading to the construction of the first SMES system [521]. The first
commercialized application of SMES in power grids took place in 1981, specifically along
the 500 kV Pacific Intertie, connecting California and the Northwest [522]. SMES operates
by storing energy in the magnetic field generated by a direct current flowing through a coil
made of superconducting material. The SMES system comprises three key components: a
superconducting coil, a control and power conditioning system, and a cryogenic
refrigerator (as illustrated in Figure 50).
Figure 50: Schematic representation of a superconducting magnetic energy storage system.
Available at: https://www.researchgate.net/publication/329768714
To maintain its superconducting state, the superconducting coil is cryogenically cooled to
extremely low temperatures using a refrigeration system. During the charging phase, the
current in the superconducting coil increases, and it decreases during the discharging cycle.
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The control and power conditioning system manages the electrical energy within the SMES
system, aligning it with the output power requirements and converting between AC and
DC as necessary [2, 198]. SMES systems exhibit excellent performance characteristics for
power applications, including rapid response (in milliseconds), high power output (multi-
megawatts), and high efficiency. Reviews by Xue et al. [523], Mazurenko et al. [524],
Vulusala and Madichetty [525], and Mukherjee and Rao [526] have explored the
applications of SMES systems in electric power grids, power systems, and energy systems.
Mukherjee and Rao [527] have discussed advancements in superconducting coils and the
design of power electronic converters for SMES systems used in the power sector. Table
46 [523, 528-534] provides a summary of SMES applications in power systems. Initially
conceived for large-scale plants primarily intended for daily load leveling, SMES systems
have evolved with technological progress and are now employed in smaller-scale
applications as well. Table 47 [528−533, 535-539] outlines advancements in SMES
technology. Despite being commercially available, the adoption of SMES systems remains
relatively low, primarily due to their high initial cost [540].
Table 46: Overview of applications of superconducting magnetic energy storage systems
[523, 528-534].
Summary
Description
Enhancing Power Quality
Balancing fluctuating loads
Providing spinning reserve
Enabling load leveling
Safeguarding critical loads
Providing backup power supply
Addressing voltage and current imbalances
Enhancing Flexible Alternating Current
Transmission Systems (FACTS) performance
Enhancing Power System Stability
Improving voltage stability
Reducing system oscillations
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Table 47: Progress in the development of superconducting magnetic energy storage technology
[528−533, 535-539].
Country
Company/Lab
Year
Energy
Power
Coil Material
Tmp.
(K)
USA
[528, 529]
Bonneville Power
Administration
1982
30 MJ
11.16 MW
(charge)
Niobium-
titanium
4.2
USA
[528, 529]
Bonneville Power
Administration
1982
30 MJ
9.80 MW
(discharge)
Niobium-
titanium
4.2
USA
[530]
Superconductivity,
Inc.
1988
1 MJ
730 kW
Niobium-
titanium alloy
4.5
Germany
[531]
Forschungszentr-
um Karlsruhe
1995
188 kJ
1 MW
NbTi/CdCuNi
4.2
USA
[532]
American
Superconductor
Corporation
(ASC)
1997
5 kJ
-
Silver-sheathed
BiPb2Sr2Ca2Cu3O
(BSCCO-2223)
25
Germany
[533]
ACCEL
Instruments
2003
150 kJ
-
Silver-sheathed
BiPb2Sr2Ca2Cu3O
(BSCCO-2223)
20
Korea
[535]
Seoul National
University
2007
600 kJ
-
BSCCO-2223
wires
20
France
[536]
French National
Centre for
Scientific
Research
2008
814 kJ
175 kW
BSCCO 2212
PIT tape
20
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China
[537]
China Electric
Power Research
Institute
2011
6 kJ
-
BSCCO and
Yttrium Barium
Copper Oxide
(YBCO) tapes
65–77
UK [538]
University of Bath
2013
60 kJ
10 kW
YBCO tape
65
Italy [539]
-
2018
500 kJ
200 kW
Cryogen-free
magnesium
diboride (MgB2)
test coil
21
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CHAPTER 7: HYBRID ENERGY STORAGE (HES)
SYSTEMS
The concept of HES involves amalgamating advantageous attributes from various ESS to
meet specific performance requirements. ESS can be broadly categorized into two groups:
high-power storage systems, characterized by rapid energy delivery at higher rates but for
shorter durations, and high-energy density storage systems, which offer slower response
times but can sustain power delivery for extended periods [541, 542]. HES systems are
typically constructed by integrating two complementary storage systems, one with high
energy density and the other with high power density [543]. Table 48 provides a catalog of
various HES system combinations based on this principle, although it is essential to
acknowledge that not all combinations may be feasible due to physical and technical
limitations. For instance, recent research has concentrated on power-density solutions like
adiabatic compressed air energy storage (A-CAES) and Pumped Thermal Energy Storage
(PTES) systems. The fundamental operation of A-CAES is elaborated upon in Section 3.4,
while PTES is briefly outlined in the subsequent paragraphs. Table 49 [544-548] presents
a summary of worldwide HES primarily employed in the power, transportation, and
renewable energy fields. Further, Table 49 reports the capacity of each HES system. Recent
research efforts have concentrated on A-CAES and PTES systems as solutions for
enhancing power density.
Table 48: Energy storage systems with high power density and high energy density.
Systems with high power density (fast
response systems)
Systems with high energy density (slow
response systems).
Vanadium redox battery
Lithium-ion
Nickel-cadmium and Lead-acid
Flywheel energy storage
Superconducting magnetic energy
storage
Capacitor
Supercapacitor
Zinc-bromine
Vanadium redox battery
Lithium-ion
Sodium-sulfur
Nickel-cadmium and Lead-acid
Hydrogen
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Table 49: Overview of global hybrid energy storage systems in real-time.
Year
Location
Combination
Capacity
(MW)
Applications
2014 [544]
Japan
Lead-acid
batteries and
lithium-ion
capacitors
1.5
Optimal control of
current input/output
2016 [545]
Rankin
Substation in
Gaston County,
North Carolina,
USA
Supercapacitor
+ battery
1.2
Load shifting, extended
operational life, real-
time solar smoothing,
and extended shelf-life
2017 [546]
Cerro Pabellon
geothermal
power plant,
Chile
Lithium-ion
battery and
hydrogen
storage
-
Grid stability
2018 [547]
Varel, Germany
Lithium-ion and
sodium-sulfur
batteries
11.5
To balance out
frequency fluctuations
in the regional
electricity network
2018 [548]
Monash
University in
Melbourne,
Australia
Vanadium flow
and lithium-ion
batteries
1
The hybrid system acts
as a flexible platform,
integrating with
building management
systems and EV
charging stations while
enabling cutting-edge
“peer-to-pool” energy
trading
At present, PHES stands as the predominant choice for large-scale energy storage,
responsible for over 99% of such endeavors. PHES functions by transferring water between
two reservoirs positioned at different elevations utilizing either pumps or turbines.
However, the applicability of PHES is hampered by geographical constraints. In response
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to these challenges, PTES emerges as a promising alternative solution, a relatively recent
innovation currently undergoing pilot plant evaluations [549, 550]. Table 50 [551-554]
provides a concise overview of the technical characteristics of different PTES systems.
PTES operates on the principle of storing energy in the form of heat, which can exist as
sensible or latent heat. This ingenious technology relies on a high-temperature heat pump
cycle to convert electrical energy into thermal energy, storing it within two substantial
reservoirs. Subsequently, a thermal engine cycle orchestrates the conversion of this stored
thermal energy back into electrical energy [551]. The PTES system comprises a high-
temperature vessel, a low-temperature vessel, four turbomachines (with one
compressor/turbine pair engaged during charging and another during discharging), and two
heat exchangers (one designated as hot and the other as cold). Notably, the reservoirs
function as regenerators crafted from refractory materials, adept at storing or delivering
heat. During the charging cycle, the working fluid circulates in a clockwise direction,
whereas during discharge, it follows a counterclockwise path. The fundamental concept
revolves around leveraging surplus grid electricity during off-peak hours to transfer heat
from the cold vessel to the hot vessel. This process involves the propagation of a cold
thermal front through the low-temperature vessel and a hot thermal front through the high-
temperature vessel during charging, akin to a heat pump. Conversely, the discharge phase
mirrors the workings of a heat engine, harnessing the temperature differential between the
hot and cold storage tanks to generate electrical power. Consequently, during discharge, a
warm front advances through the cold storage while a cold front progresses through the hot
storage, thereby reducing the temperature gap between the two storage units [549]. Figure
51 shows a schematic representation for PTES.
Table 50: Technical specifications of diverse pumped thermal energy storage systems [551-554].
Variant
Discharge
Power
Range
(MW)
Charge
Power
Range
(MW)
Discharge
Time (h)
Charge
Time (h)
Energy
Density
(kWh/m³)
Efficiency
(%)
Brayton PTES
10–150
10–150
6–20
6–72
20–50
50–75
Transcritical
Rankine
10–100
10–100
2–5
3–10
10–15
50–65
Compressed Heat
Energy Storage
(CHEST)
10–100
10–150
6–72
6–72
40–100
60–70
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Figure 51: Schematic diagram of pumped thermal energy storage system. Available at:
https://clearpath.org/tech-101/intro-to-energy-storage/
The classification of PTES systems hinges on their thermodynamic cycles and the choice
of working fluids, encompassing three main categories: (i) Brayton PTES systems,
characterized by closed/reversible Brayton cycles utilizing air or argon as the working
fluid, complete with low and high-pressure, high-temperature reservoirs; (ii) transcritical
Rankine PTES systems featuring ice and pressurized water storage tanks and employing
CO2 as the working fluid; (iii) Compressed Heat Energy Storage (CHEST), a technology
rooted in the traditional steam Rankine cycle [550]. Several PTES concepts have surfaced
in the literature, offering diverse approaches. For instance, Weissenbach proposed an open-
cycle PTES concept back in 1979, involving the compression of air to temperatures ranging
from 800 to 900°C and storing heat in ceramic balls enclosed within heat-resistant steel
tubes. SAIPEM advocated for utilizing temperatures as high as 1,000 to 1,500°C and
storing thermal energy in refractory bricks enriched with alumina or magnesia content.
Isentropic Ltd. put forth a PTES design leveraging argon as the working fluid, grounded in
the closed-cycle principle, while utilizing magnetite pebbles to store heat at temperatures
approximately 500°C. This system exhibited an estimated rated discharge energy capacity
of around 16 MWh, with a rated discharge duration approaching 8 hours [555-557].
Newcastle University marked a significant milestone by commissioning the world's maiden
grid-scale demonstration plant in 2017, based on Isentropic Ltd.'s conceptual framework.
This PHES research facility employs surplus grid electricity amounting to 150 kW to drive
a compression and expansion engine, responsible for heating (500°C) and cooling (160°C)
argon working fluid streams. These streams, in turn, facilitate the heating and cooling of
two thermal storage tanks with a cumulative energy storage capacity of 600 kWh. When
the need arises, this process is reversed to generate 120 kW of electricity for the grid.
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Additional technical specifications of this demonstration plant are outlined in Table 51
[558].
Table 51: Technical specifications of the suggested commercial pumped thermal energy storage
system [558].
Parameter
Value
Rated power during discharge
1.6 MW
Rated power during charge
2.0 MW
Capacity
16 MWh
Life time
20 years
Dimensions
17 m x 7 m
Moreover, researchers have proposed various PTES configurations, including a
transcritical CO2 cycle where water serves as the hot storage medium, and an ice slurry
functions as the cold storage medium [559]. Koen et al. [560] conducted a study that
explored a wide array of working fluids as potential candidates for transcritical PTES
cycles, scrutinizing over 150 working fluids for thermodynamic, environmental, and safety
suitability, ultimately identifying trifluoroiodomethane (R13I1) as a promising working
fluid boasting a high round-trip efficiency of 57%. Frate et al. [561] delved into examining
PTES systems centered on heat pumps and organic Rankine cycles from a thermo-
economic perspective, concluding that this system is well-suited for large-scale and long-
duration energy storage. Zhao et al. [562] carried out thermo-economic assessments
encompassing three distinct PTES system variants, considering various system
configurations, working fluids, and storage media.
PTES presents notable advantages such as flexibility in location, high energy density
surpassing PHES, and durable components with a long lifespan. However, its efficiency
remains a significant challenge, allowing only 40% to 50% of input electricity to be
effectively retrieved during the discharge phase [563, 564].
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CHAPTER 8: COMPARISON AMONG THE ENERGY
STORAGE SYSTEMS
Due to the wide-ranging properties and attributes exhibited by the aforementioned ESS,
they are exceptionally well-suited for a multitude of applications. As depicted in Table 52
[17, 23, 24, 198], a comprehensive overview of the typical characteristics of various ESS
is presented, encompassing aspects such as power range, discharge time, energy density,
efficiency, and longevity in temporal years. Figures 52–54 offer comparative visual
representations of the lifespan, efficiency, and power capacity of the selected ESS, drawing
upon the average values delineated in Table 52.
Table 52: Characteristics of diverse energy storage systems [17, 23, 24, 198].
ESS Type
Power
Range
(MW)
Discharge
Time
Energy
Density
(Wh/kg)
Efficiency
(%)
Lifetime
(years)
SHS
250
-
-
50-90
10-30
LHES
5
-
-
75-90
10-30
PHES
10-5,000
1-24 h
0.5-1.5
70-85
30-60
GES
40-1,600
1-4 h
-
75-80
-
CAES
3-300
1-24 h
30-60
40-80
20-50
FES
0.1-20
Sec-min
5-80
70-95
15-20
Hydrogen
0.1-50
Secs-24 h
600-1,200
20-66
10-20
Lead-acid
0-20
Secs-
hours
30-75
70-90
5-15
Ni-Cd
0-40
Secs-
hours
40-90
60-90
10-20
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NaS
0.05-8
Secs-
hours
150-240
75-90
10-15
Li-ion
0-0.1
Mins-
hours
100-200
70-85
5-15
VRB
<3
<10 h
35-60
70-85
10
PSB
<15
<20 h
15-30
60-75
-
Zn-Br
-
Secs-10 h
75-85
65-75
5-10
Capacitor
0-0.05
Millisecs-
1 h
0.05-5
60-90
5
Supercapacitor
0-0.3
Millisecs-
1 h
1.5-2.5
75-95
>20
SMES
1-110
Millisecs-
8 s
0.5-5
>95
>20
Figure 52: Analyzing the lifespan of various energy storage systems using the mean data gathered
from Table 52. Available at:
https://www.sciencedirect.com/science/article/pii/S277268352200022X
CHAPTER 8: COMPARISON AMONG THE ENERGY STORAGE SYSTEMS
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Figure 53: Analyzing the efficiency of various energy storage systems using the mean data
gathered from Table 52. Available at:
https://www.sciencedirect.com/science/article/pii/S277268352200022X
Figure 54: Analyzing the power range of various energy storage systems using the mean data
gathered from Table 52. Available at:
https://www.sciencedirect.com/science/article/pii/S277268352200022X
CHAPTER 9: CURRENT STATUS OF ENERGY STORAGE SYSTEMS
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CHAPTER 9: CURRENT STATUS OF ENERGY
STORAGE SYSTEMS
The Technology Readiness Level (TRL) scale, established by NASA in 1974 and
subsequently embraced by the Research Framework Programme (2014–2020), serves as a
standardized measure for assessing the developmental stage of a technology. This scale
encompasses nine levels, commencing with the most rudimentary technological state and
progressing through stages involving practical real-world demonstrations and assessments.
Table 53 provides a comprehensive delineation of these levels.
Table 53: Assessment of technology readiness level.
Stage
TRL
Description
Deployment
9
System is deployed and operational in a real
environment
8
System is completely validated and certified
in a real environment
7
Prototype is validated in a real environment
Research
6
Technology is demonstrated in a relevant
environment
5
Technology is validated in a relevant
environment
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Development
4
Technology is validated in a lab
3
Concept is tested
2
Concept/technology is formulated
1
Basic idea/concept
However, it is important to note that not all of the aforementioned energy storage
technologies have advanced to the same level of readiness. Table 54 [108, 551, 565, 566]
offers an overview of the present status of energy storage technologies, based on an
evaluation of their commercial maturity and developmental phases. TRL assessments for
each storage technology are presented herein, drawing upon insights garnered from surveys
conducted within the literature. Notably, technologies such as lead-acid batteries, Li-ion
batteries, Ni-Cd batteries, VRB flow batteries, PHES, and FES have already reached a
mature stage, as illustrated in Table 54, even though substantial ongoing research efforts
continue to refine these concepts. Conversely, EDLC and I-CAES find themselves in early
developmental stages. Additionally, hydrogen fuel cells, phase change material-based
thermal storage systems, and thermochemical materials are anticipated to play an
increasingly prominent role in the energy storage landscape in the near future.
Table 54: The present state of energy storage technologies [108, 551, 565, 566].
Storage System
Current Scenario
TRL
Lead-acid batteries
Mature technology,
commercially available
9
Lithium-ion batteries
Commercial technology
9
Nickel-cadmium batteries
Mature technology
9
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Sodium-Sulfur
Large-scale demonstration
8
Vanadium redox batteries
Mature technology
9
Polysulfide bromide batteries
-
4-5
Zinc bromine batteries
Demonstration
6
Electric double-layer capacitors
Early commercial
technology
8-9
Hybrid energy storage system
-
7
Synthetic natural gas
Prototype testing to large
scale demonstration
4-8
Pumped hydro energy storage
system
Mature technology,
commercially available
9
Compressed air energy storage
system
-
7-8
Low-speed flywheel energy
storage system
-
9
High-speed flywheel energy
storage system
Prototype testing to small
scale demonstration
5-7
Supercapacitors
-
6
Superconducting magnetic
energy storage system
-
5-6
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CHAPTER 10: FUTURE TRENDS AND CHALLENGES
In an era characterized by significant transformations in the global energy landscape, ESS
have become pivotal components, driven by evolving energy needs, technological
advancements, and environmental concerns. These systems play a vital role in fostering
grid stability, integrating renewable energy sources, and ensuring a dependable energy
supply. Looking forward, the future of energy storage is marked by a combination of
promising trends and substantial challenges. This chapter delves into the noteworthy trends
shaping the energy storage arena and the enduring obstacles that demand attention for
harnessing the full potential of these technologies. From grid modernization initiatives to
breakthroughs in battery technology and sustainability endeavors, the energy storage sector
stands poised to redefine the way we generate, store, and distribute energy. With this
introduction, one can continue to outline and provide details about the forthcoming trends
and challenges in ESS.
1. Increasing Integration of Renewable Energy:
Trend: The growing emphasis on reducing carbon emissions and transitioning to cleaner
energy sources has led to a surge in renewable energy adoption, particularly solar and wind.
Challenge: Effectively managing the variable nature of renewables necessitates advanced
ESS capable of storing excess energy during periods of surplus generation and discharging
it as needed. Grid operators and utilities must adapt to these dynamic energy patterns.
2. Ongoing Battery Technology Advancements:
Trend: While lithium-ion batteries have been the go-to choice for energy storage,
continuous research and development efforts are dedicated to enhancing their performance.
Challenge: The persistent challenge revolves around improving energy density, longevity,
and safety while simultaneously reducing costs. Promising technologies like solid-state
batteries, offering higher energy density and extended lifespans, are still in the
experimental phase and face scalability issues.
3. Modernizing the Grid:
Trend: Smart grids and microgrids are being deployed to enhance the efficiency and
resilience of power distribution.
Challenge: Integrating ESS into these modern grids necessitates advanced control systems
and adherence to standards. Grid operators must navigate the complexities arising from
decentralized energy generation, bidirectional power flows, and demand response.
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4. Embracing HES:
Trend: The adoption of hybrid energy storage systems, which combine multiple storage
technologies like batteries and supercapacitors, is gaining momentum.
Challenge: The key challenge lies in designing and optimizing HES configurations that
deliver increased power and energy density, extended cycle life, and heightened efficiency,
all of which require sophisticated control algorithms and seamless system integration.
5. Harnessing Second-Life Batteries:
Trend: With the proliferation of electric vehicles, there is an escalating supply of retired
batteries that retain residual capacity.
Challenge: Establishing systems and standards for repurposing these batteries for
stationary storage, while ensuring safety and reliability, is imperative for resource
efficiency and cost reduction.
6. Addressing Energy Storage for Electric Mobility:
Trend: The growth in electric vehicle adoption underscores the need for a robust charging
infrastructure and innovative ESS solutions for high-power charging stations.
Challenge: Meeting the high-power demands of rapid charging stations and effectively
managing grid impact during peak charging times necessitate advanced ESS technologies
and comprehensive grid planning.
7. Focusing on Material Recycling:
Trend: The sustainability of ESS technologies depends on efficient recycling and
responsible resource management.
Challenge: Developing cost-effective and environmentally friendly recycling processes,
particularly for lithium-ion batteries, is vital to reduce resource depletion and minimize
waste.
8. Pursuing Long-Duration Energy Storage:
Trend: There is a growing emphasis on developing solutions for long-duration energy
storage (e.g., storing energy for days or weeks).
Challenge: Scaling up advanced technologies like pumped thermal energy storage,
adiabatic compressed air energy storage, and long-duration flow batteries demands
substantial investments and research efforts.
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9. Navigating Regulatory Frameworks:
Trend: Governments and regulatory bodies are adapting to accommodate the deployment
and grid integration of ESS.
Challenge: Establishing clear standards, safety regulations, and incentive mechanisms that
promote ESS adoption while safeguarding grid stability and reliability constitutes a
complex undertaking.
10. Achieving Cost Efficiency and Scalability:
Trend: Attaining cost competitiveness with conventional power generation and
distribution is pivotal for ESS adoption.
Challenge: Reducing manufacturing costs through economies of scale, standardized
production methods, and material innovations is critical for enhancing the affordability and
accessibility of ESS.
11. Minimizing Environmental Impact:
Trend: The quest to reduce the environmental footprint of ESS technologies is a mounting
concern.
Challenge: Performing comprehensive life-cycle assessments and embracing sustainable
practices throughout ESS component production, utilization, and disposal are essential
steps in mitigating environmental consequences.
Addressing these challenges while embracing emerging trends will be crucial for unlocking
the full potential of ESS, ushering in a cleaner, more reliable, and resilient energy
landscape. Collaboration among researchers, policymakers, and industry stakeholders will
play a pivotal role in surmounting these obstacles and accelerating the adoption of
advanced ESS solutions.
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