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On the Necessity to Integrate Power Flexibility
in Cooling Systems
D Vuarnoz1, 3, E-L Niederhäuser1, D. Torregrossa1, D Gabioud2
1 ENERGY Institute, School of Engineering and Architecture Fribourg (HEIA-FR),
University of Applied Sciences of Western Switzerland (HES-SO), Passage du
Cardinal 13b, CH – 1700 Fribourg, Switzerland
2 Institute of Sustainable Energy, School of Engineering, HES-SO Valais-Wallis,
Rawil 64, CH– 1950 Sion, Switzerland
3 Author to whom any correspondence should be addressed, email:
didier.vuarnoz@hefr.ch
Abstract. Today, cooling systems are widely used, notably with the unprecedented growth of
data centres and building space cooling. These thermodynamic systems are powered mainly with
electricity, and their peak loads are generally associated with very high carbon footprints. At the
same time, congestion of the grid due to high load or renewable power injection is becoming an
issue for all actors involved with electricity (producers, providers, consumers, and prosumers).
Actually, both the price and associated carbon footprint of electricity usually fluctuates along
with the charge of the network. This paper discusses the integration of power flexibility (PF) in
new and existing cooling systems to avoid a possible cold crunch in the near future. After
defining PF, several cooling systems archetypes are presented. Three possible ways to integrate
PF are explained: flexibility by thermal inertia and energy storage (thermal and electrochemical).
While PF principally targets the reduction of stress on the electric grid, other benefits can also
be achieved, e.g. mitigation of direct carbon emissions and decrease of costs related to operating
the refrigeration system. We explain how better management of energy transits and possible
imbalance in electricity networks can be achieved by thermal inertia. The choice of integrating
thermal storage or electric battery is discussed, and both solutions are considered in a specific
case study. The study aims at better management of power loads on electricity network caused
by the cooling system and could be useful for anyone involved with grid management and/or
refrigeration systems.
Keywords: cooling, flexibility, electricity peak, energy storage, load management.
1. Introduction
Human production of cold has been a challenge, and today, modern society means, among other things,
refrigeration at every corner. In 2015, 17% of the electricity used worldwide was devoted to
refrigeration, providing employment for nearly 12 million people [1] in a wide range of activity sectors
(see Figure1). Refrigeration has a direct impact on human health through the preservation of foods,
pharmaceuticals and medicines, and with magnetic resonance imaging and low-temperature treatments
(cryosurgery or cryotherapy). In the industry, refrigeration is necessary for numerous manufacturing and
conservation processes and is found in food and drink industries, chemistry, plastic processing, and
mechanical engineering. In the building sector, air conditioning plays a key role in the economic and
social development of warmer countries. One important factor in the booming demand for refrigeration
1800
1000
200
Emission s (Mt CO2-eq)
Domestic r efrigeration
Commerc ial refrigerati on
Industrial r efrigerati on
Transport r efrigeration
Stationary AC
Mobile AC
Foam
is global warming. Projection of the evolution of cooling degree days that measure the impact of the
weather on the overall need for cooling shows an increase of around 25% globally by 2050, with the
most significant increases occurring in already hot places where income and population are rising fastest.
Although refrigeration is becoming affordable for more people, access to cooling is still uneven over
the world and is a major social issue; 90% of the people from the US and Japan have air conditioning
(AC), against 8% of the 2.8 billion people living in the hottest regions of the world [2]. Most of the
energy used for cooling is in the form of electricity. For example, in the United States in 2005 for the
AC sector, 99.9% of all units shipped were based on electricity-driven technology [3].
From the electric grid, this expansion is less appealing and could soon become critical. This possible
upcoming phenomenon is called a cold crunch. Even if the penetration of renewables (RE) has never
been as intense, the mismatch between the erratic production of solar and wind-based electricity and
requires the necessity to anticipate load problem on the electric infrastructure. Peak demand of grid
electricity is usually covered by fuel-based power plants, inducing a very high carbon footprint (at a
world average Global Warming Potential (GWP) of circa 0.5 kgCO2-eq/kWh [2]). As a result,
refrigeration is responsible for a massive amount of greenhouse gas (GHG) emission (see Figure 2) due
to direct emissions (leakage) of refrigerant and indirect emissions originating from electricity
generation.
Figure 1. Share of cooling systems (total of
3.06E9 units) by sectors of use and by system
types, based from data published in [1]
Figure 2. GHG emissions associated to
refrigeration [4]. Dark grey: direct emissions.
Light grey: indirect emissions.
To limit the impact of an unprecedented expansion of cooling demand on the electricity network
infrastructure, different measures are proposed. One consists of increasing the energy efficiency of
cooling systems through enforcement of standard and labels, combined with a reduction of cooling needs
with tighter building codes. Another is the massive implementation of RE generation. However, even
the combination of both will not be sufficient to prevent the installation of more energy and power for
electricity generation [2]. In this paper, we explore the possibility to integrate power flexibility (PF) in
cooling systems. After a semantic definition of PF, the benefits that can be obtainable by PF are reviewed
(Section 2). Section 3 reviews the most frequent cooling systems for understanding how PF can be
practically implemented in new and existing systems (Section 4). Practical implementations of PF are
addressed in Section 5.
Air conditionning
Refrigerators, Freezers
Commercial refrigeration
Refrigerated transports
Mobile Acs systems
Water chillers
Air-cooled systems
Heat pumps
Magnetic Resonance Imaging
Liquefied Natural Gas
Ice rinks
Health
Industy
Sport and leisure
Heat pumps
Food
2. What is Power Flexibility (PF)?
Flexibility in the power system, called power flexibility, can be defined as “the modification of
generation injection and/or consumption patterns, on an individual or aggregated level, often in reaction
to an external signal, in order to provide a service within the energy system or maintain stable grid
operation” [5]. In our context, the electrical consumption for cooling is shifted over time to serve
multiple goals: increase of the self-consumption rate either locally or in a local microgrid, congestion
avoidance on the local distribution grid, GHG mitigation by an optimum use of low-carbon energy
sources, participation to different energy and power markets. The main objective of PF is to temporally
disconnect the electrical energy demand usually linked with the instantaneous cooling demand. The
means by which the temporal disconnection is performed lies in the deployment of technical artefacts
and regulation strategies (see Section 4).
3. Cooling systems archetypes
The majority of deployed cooling systems in use throughout the world are based on vapor compression
cycle technology. The process uses refrigerants, often human-made gases with significant GWP,
releasing or absorbing heat respectively during condensation and evaporation. Food storage, by using,
refrigerators, freezers and cold storage rooms, with AC in residential and commercial buildings,
represents 95 % of cooling systems units (see Figure 1). For these two sectors, compressor-based
refrigeration equipment dominates the market due to its low initial cost, low operating cost by superior
efficiency, and good safety record. Replacing electricity used to drive mechanical compressors by heat
is possible through absorption technology. However, its use represents a share of just over 1% of natural
gas against a total electricity use of 2000 TWh in 2016 for cooling purposes [1]. Other sources of heat,
such as excess industrial heat or solar thermal energy are potentially promising technologies. Today,
absorption systems are large ACs, typically used in the non-residential sector. A less common and
simpler form of cooling is evaporative cooling, which is also used for AC. Other emerging technologies
(e.g. desiccant cooling, thermo-acoustic, magnetic cooling) has developed much slower than predicted
[3]. The most current technologies found in the market for food storage and space cooling are:
3.1. Packaged systems
These refrigeration appliances are unitary systems, containing both the condenser and evaporator in a
single box. The customer buys a ready to install system (e.g. refrigerators, freezers, and ACs). At the
domestic level, ACs often consist of window units that fit into standard window frames. Common in
hotels, packaged terminal ACs consist of the evaporative unit on the inside, generally under a window,
with a grilled opening passing through the wall, connected to the condensing unit on the outside.
Packaged portable units are designed to be easily transported from room to room inside a building, with
a mobile duct to evacuate the hot air from the unit to the exterior. Packaged rooftop units are larger
packaged chiller systems delivering cooled air into the building through ducts.
3.2. Splits systems
Ranging from small room units to very large systems (large building complexes), split systems have an
evaporator placed inside the space to cool, which is connected through piping that carries the refrigerant
to the evaporator, or air handling unit, to the condenser located outside. Split systems – either individual
mini-split or multi-split ACs – represent the vast majority of ACs in use today.
3.3. Chillers
Almost exclusively in commercial buildings, large residential blocks and district cooling networks,
chillers are large central systems ACs that produce chilled water and distribute it throughout a building
or cooling network through pipes to an indoor system that cools the air. Compression (centrifugal,
reciprocating or screw driven) cycle chillers are electric powered and water or air cooled (35 million
units, representing 60% of total commercial air-conditioning needs), and thermally driven chillers
(absorption) account for another 5.5 million units [2].
4. How to integrate PF in cooling systems
In this paper, three possible ways to integrate PF in cooling systems are investigated. They consist of
thermal inertia, electrochemical batteries, and thermal storages. An overview of the different possible
inclusions of these elements enabling PF is presented in Figure 3.
Basic system
Grid mix
Renewable
Battery
Cold production
Thermal storage
Cold demand
Flexibility by thermal inertia Flexibility by electric batteries Flexibility by thermal storage
FI 1 FB 1
FB 2
FB 3
FT 1
FT 2
LEGEND PICTOGRAM
BS 1
Figure 3. Example of PF integration in cooling systems.
4.1. Flexibility provided by the thermal inertia of the refrigerated mass
Using the refrigerated mass as thermal storage is an a priori interesting option, as it requires no dedicated
storage appliance. Let us assume that a refrigerated mass with a heat capacity C is stored in an envelope
with heat transfer coefficient K. The mass temperature set point is Tm ref) and the outdoor temperature is
To (assumed constant for simplicity). Then, the thermal power Pth required to maintain the mass
temperature constant is given by:
(1)
Assuming efficiency forcold generation, the corresponding electrical power can be calculated:
(2)
If the refrigerated mass temperature may vary in the range , the thermal energy
that can be stored in the mass is given by:
(3)
The corresponding electrical energy is given by:
(4)
Assuming an initial temperature equal to , electrical power can be cut for a maximum duration
equal to:
(5)
where is the maximum permitted temperature deviation for the refrigerated mass. Starting from
the same initial state, electrical power can be increased up to the peak available power for a
duration shorter than or equal to :
(6)
Hence, the refrigerated thermal mass can be modeled as an ideal battery (see Figure 4).
Refrigerated mass
temperature
Energy in “battery”
(50% SoC: 0 Wh)
Battery model
May charge and discharge
power
Figure 4. Refrigerated mass modeled as an ideal battery system.
However, real systems do not behave as described above for the following reasons:
1. Most cooling systems are not variable loads: they work in on – off mode (i.e. is either 0 or
), and they are regulated by a hysteresis controller.
2. Inlets of warm mass and outlets of cold mass may fundamentally change the behaviour.
Unlike other types of storage, storage in the refrigerated mass cannot be considered as reliable, and its
control must account for that fact.
4.2. Flexibility given by electrochemical batteries
When integrating flexibility by an electricity storage, the battery is placed up front the refrigeration
machine. Technically, it involves an electrician, and in practice it can be easily implemented in the
existing standard environments. The storage capacity could range from small to medium size (domestic),
but due to its high cost, the autonomy criteria alone cannot, most of the time, justify the lack of economic
viability for a large capacity. As of today, Lithium-ion (Li-ion) is the benchmark technology due to its
energy density, in and out power and conversion efficiency. Progresses is constant in the field of
electrochemistry, and examples of current achievable characteristics of Li-Ion batteries are presented in
Table 1.One important feature of batteries is the decay of the storage capacity over their use. The end-
of-life of a battery is usually determined when the storage capacity reaches 80% of its initial value, but
there is nothing against using them longer, especially because of their relatively high environmental
impacts (Table 1).
A battery is generally not implemented alone (case FB 1), but could plays a back-up solution in case of
grid black-out. Taking advantage of the dynamic features of the grid electricity (cost, GWP) by
sophisticated energy management systems to supervise the operation of the battery (see e.g. carbon-
based load-levelling [6]), can still not offer operational benefits balancing the grey energy of the storage
system itself [7]. Most of the time, a battery comes with a local RE generation to increase the onsite use
of energy (case FB 2 and FB 3). Also, the battery is usually not only devoted to supplying the cooling
system, but also the building electricity demand. This makes sense when the seasonal variation of cold
=
C
demand occurs, since the viability (cost, primary energy and GHG emissions) of the battery is a function
of its use. A method to define the minimum required batteries specifications for enabling sustainability
are detailed in [6].
4.3. Flexibility given by thermal storage
Not easily implementable everywhere, especially in existing cooling systems, thermal storages are
particularly adapted when chillers (see section 3.3) are involved. Therefore, it concern for the most part,
application scaling from medium to big size. Thermal storage is placed downstream of the refrigeration
machine sot that it is particularly interesting to limit the refrigeration capacity (and the harmful
refrigerant amount) when encountering high peaks of cold demand (see case study in section 5). Heat
losses are responsible for relatively low efficiency (see Table 1), which is why insulation for thermal
storage and piping systems is particularly important. Practically, implementing a thermal storage into a
cooling system requires hydraulic works.
Two categories of thermal storages exist. The first is on sensible heat so the energy stored is directly
proportional to the fluid temperature. The second involves the phase change of materials that are spread
into a carrier fluid (see e.g.[8]). The resulting energy medium is called phase change material slurries,
and the associated storage is based therefore mainly on latent heat. These man-made fluids are tailorable
so their physical properties (e.g. melting point) are adjusted for their specific applications and have the
specificity of very high energy density and could allow the release of cold at nearly constant temperature
(see e.g. [9, 10]).
Table 1. Comparison between thermal and electricity storage technologies
Characteristics
Thermal storage
Electricity storage
Sensible
Latent
Li-Ion
Efficiency
50-90 (%)[11]
75-90 (%)[11]
90 (%)[12]
Lifetime
25 years[13]
25 years[13]
5.65E-4 (%/kWh)[12]
GWP
32*b (kg CO2-eq) [13]
32*b (kg CO2-eq) [13]
0.021(kg CO2-eq/kwh el) [12]
CEDnr
106*b (kWhoil-eq) [13]
106*b (kWhoil-eq) [13]
0.319(kWhoil-eq/kwh el) [12]
Costs
0.1-10(Euro/kWhth)[11]
10-50(Euro/kWh th)[11]
176(Euro/kWh el)[14]
Storage capacity
1.16 (kWhth /K*m3)
30-120*a(kWh th /m3)[10]
114 (Wh el /kg) [7]
*a: heat of fusion only *b: storage tank of 600l. without fluid
4.4. The choice between the different types of PF
The typology and technical specificities of the concerned refrigeration system often at least indicates
which solution cannot be applied (e.g. battery in absorption chillers powered by gas as a fuel). If
tolerated, flexibility through thermal inertia is the first strategy to consider, especially in the existing
system, because it requires nothing other than some regulation thermostats enabling switching
hysteresis. Often performance targets drive investments, and vice-versa. Financial aspects also guide the
choice of adopting technologies which enable PF. Current research efforts tend to establish the
advantages and drawbacks of each strategy. For example, researchers used a model to assess the cost
performance of a district cooling system in Indian cities. Rather than for batteries, the choice of thermal
storage enables larger profit (30%) and lower peak load at the same time [2].
5. Practical assessment of power flexibility in cooling systems
5.1. Power flexibility for minimization of the balancing energy
In European countries, electrical connection points are grouped in logically defined sets called balance
groups. Each balance group must elaborate ahead of a global schedule made up of bought/sold energy
volumes per quarter hour and must buy/sell energy accordingly. Day-after measurement results from
energy meters are consolidated, and the difference between planned and measured energy volumes is
computed. Balancing groups must pay penalties proportional to that difference named balancing energy.
As precise forecasting is difficult, especially with intermittent renewables, means to limit balancing
energy must be deployed. At any given time, a flexibility manager controls flexible processes to 1)
minimise the energy balance within the current 15-minute period, and 2) correct the forecasted
imbalance over the (typically 4) coming 15-minute periods.
A warehouse storing deep-frozen goods can be one of such flexible processes. Offering flexibility is a
side function that must not jeopardize the main function (refrigeration). Hence, chillers are always
controlled by the warehouse automation system. The flexibility management system may only give
recommendations to the controller through a simple interface, as illustrated by Figure 5.
Figure 5. Interface between a
deep-frozen warehouse
automation system and a
flexibility management
system.
Thanks to two binary control signals (Force ON enabled, Force OFF enabled), the warehouse automation
system indicates whether its chiller may be asked to consume full power or to be switched off. Only if
the corresponding Force enabled signal is active, the flexibility management system may request cold
generation with full power (Force ON) or request to disable cold generation (Force OFF). First tests
have been performed on a Swiss warehouse where the refrigerated mass temperature may vary between
-26 °C and -20 °C. Cold generation could be interrupted during significant periods while keeping the
refrigerated mass temperature within the allowed range, as illustrated in Figure 6.
Figure 6. Test results on a
Swiss deep-frozen
warehouse.
The Force OFF signal was active (i.e. request to not generate cold) from 10:00 to 10:45. When the Force
OFF signal stopped being active, the warehouse automation controller decided to let the mass
temperature go up to the upper limit, before starting to generate cold again. During the Force OFF time,
the saved power is about 30 kW. Hence, the saved energy during the 45 minutes Force OFF period is
about 22.5 kWh, but it would have been possible to extend the Force OFF time up to 18:15, which would
have saved 244.5 kWh. When the chiller restarted cold generation at 18:15, the electrical consumption
became larger than average for one hour to compensate for the cold loss during the off time. This
rebound effect must be accounted for in the flexibility management strategy. First results show that
flexibility is usable in practice. It can be forecast reliably only if mass inlets and outlets are known. This
can be achieved either by feeding the flexibility management system with logistics data or by applying
artificial intelligence to discover mass inlets and outlets patterns.
5.2. Electrochemical batteries Vs. thermal storage
The second case study considers the project of a new hospital located in an urban area of Fribourg,
central Switzerland. The architectural feasibility considers a building with a reference energy surface
area of 4355 m2. According to Swiss standards [15], the annual electricity needs for refrigeration are of
350 MWh/yr of thermal energy, concentrated from the middle of May until the middle of September. In
this peculiar case, although no thermal tolerance can be expected from the users (the majority are sick
people, and the professionals need to perform their job in good conditions). Electricity needs (heat pump
for heating and domestic hot water, appliances, lighting, and other technical installation) of the building
account for 321 MWh/yr. A photovoltaic (PV) system (multi-Si n: 15.1%) covers 82% of the roof’s
surface (22x29m). The question is about the choice of the integration of an energy storage technology,
especially the dilemma between electrochemical battery and/or thermal storage.
As the case study considers new construction with large instantaneous cooling needs, the priority is to
cut the electrical power peaks of the cooling demand. This can’t be achieved with an electrochemical
battery placed before the cold production appliance. Consequently, we first proceed with the assessment
of thermal storage, followed by the integration of electrochemical batteries. In both assessments and for
considering the different interaction between the onsite RE production, energy storage and cold demand,
dynamic simulations performed at the hourly time step are used to quantify the autonomy of the energy
system (building and its refrigeration system) and the GHG emission balance related to the system’s
electricity use (See Table 2). The data used for the GWP of the grid mix are those provided in [16] and
those of the weather recorder from the Posieux weather station for that same period (Year 2015).
5.2.1. Integration of thermal storage. The volume of the thermal energy storage is sized based on the
daily thermal energy needs for the hottest day of the year (See Figure 7, Eth = 45.5E9 J) and the power
of cold production, this last inversely proportional to the possible time of operation during a full day of
service. Also, increasing cold power production reduces the necessity of a storage tank. The upper limit
is described as the first alternative in Table 2.
In this case study, the different periods of operation are prioritized as a function of two indicators at
building-owner and society level. They are, respectively, the cost of electricity (1st priority) and the
global warming potential of electricity (2nd priority). Therefore, we limit the time of cold production
operation at 18 hours during the day of the most important demand of the year (see Figure 8). On that
basis, the cold power production is set at nominal capacity 608 kW with an Energy Efficiency Ratio
(EER) of 4.2 value, reflecting current performance of the practice in Switzerland [17].
According to [8], the necessary volume of sensible storage would be of 620 m3 with a secondary loop
operating at 8-12°C, and a safety factor for the tank of 1.2. An important reduction of the storage volume
can be achieved by using phase change material slurries (from 90 to 260 m3, depending which PCMS is
chosen [10]). With such a solution, the annual final energy spent on cooling power represents circa 20%
of the final electricity consumption. This amount roughly corresponds to the same amount of yearly PV
production placed on the maximum possible surface area of the roof (82 MWh/yr.).
5.2.2. Integration of battery. Cooling demand appears during the period May-September. During the
other period, the battery, previously charged with excess PV production, can contribute to other needs
of electricity of the building. The idea to size the battery storage capacity to erase the cooling demand
from the grid supply actually does not work in this particular case study, at least during the day of the
year presenting the highest cooling load (see Figure. 7). On that specific, the entire PV production is
instantaneously and entirely self-consumed.
Implanting an electrochemical battery of 50 kWh of capacity (Li-Ion NMC technology) allows the onsite
use of 6517 kWh/yr. As presented in Table 2, the low gain in autonomy combined with the investment
cost [14], and the extra GHG emission related to the electricity used during the operational phase of the
building, cannot justify the choice of implementing batteries in the building energy system. This
statement becomes truer if the export of excess electricity to the surrounding grid is possible. An
electrochemical battery would rather be appropriate if RE electricity generation would not be consumed
instantaneously onsite. It is important to highlight that for specific buildings, having a specific time
mismatch between PV generation and load demand could possibly lead to the profitability of battery
energy storage installation [18].
Figure 7. Cooling and electricity needs of the
hospital, and PV production during the hottest
day in the year 2015
Figure 8. One-day operation of the cold
production, associated with the GWP and costs
characteristics of the grid mix
5.2.3. Comparison of simulated performances. Table 2 summarises the different alternatives discussed.
Additionally, we address a scenario consisting of dispatched compressor-based split systems that would
respond instantaneously to the cooling needs. This way of designing a cooling system would consist of
installing extremely high cold-power production with no energy storage capacity at all.
Table 2: Comparison of the different investigated systems
Characteristics
Direct system
Thermal storage
Li-Ion Battery
Storage Volume (m3)
0
90PCMS – 620sensible
440 (kg)
Electrical power cold production (kW)
340
200
200
Energy autonomy (%)
0
18.8
20.4
Indirect GHG emission (kg CO2-eq/yr)
82’620
70’152
70’857
6. Conclusion
The unprecedented growth of cooling demand and its dramatic trend over the globe is driven by both
global warming and the affordability of AC. The situation will soon become very problematic for
electricity networks if no action is taken on the energy supply side (e.g. blackout of the power grid, extra
GHG emissions for ensuring peak cold demand). The main objective of PF is to maximise the local use
intermittent renewable that is planned to become the backbone of electricity supply, and to prevent high
loads in the grid infrastructure. Among other obtainable benefits (i.e. lower primary energy and
associated GHG emissions), the one of prior importance for the user is the possible financial gain since
Electricity high tariff
Low tariff
the grid operator manages the charge and energy fluxes in its energy network by applying time-
dependent tariffs. The main possibilities for implementing flexibly into cooling systems are of three
kinds: thermal tolerance at the customer side; implementing a thermal storage; or integrating an
electrochemical battery that often comes when implementing onsite renewable energy production. The
most affordable and, technically, the simplest way to integrate PF is unquestionably through thermal
tolerance at the user side. But such an integration is not always possible. This is particularly the case in
medicine and food preservation. For such applications, it could be particularly advantageous to
implement latent cold storage tank with phase change slurries that deliver cold at a close-to-constant
temperature. Moreover, compared to sensible storage, dedicated storage volume by PCMS can be
drastically reduced. Electrochemical batteries are disadvantaged mainly due to their inability to cut down
the necessary cooling power. Specific case studies address each of these three possible ways to integrate
PF in cooling systems by quantitative assessments. The first case study considers thermal inertia of a
deep-frozen warehouse (between -26 °C and -20 °C). Experimental measurements show the possible
time during which the cooling process can be stopped. A second case study treats the management of
onsite renewable energy generation, mainly in the service of the refrigeration of a hospital. Based on
dynamic simulations, the autonomy and indirect GHG emissions are assessed and help the
implementation of thermal storage and/or batteries.
Acknowledgments
The work presented in this paper has been found by the Energy District 2050 research programme of
the University of Applied Sciences of Western Switzerland (Project 62’053), and by the Swiss Federal
Office of Energy (Project SI/501689). The authors would like to thanks Aurora’s grid for the ageing and
LCA computations of BES presented in section 5.
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