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energies
Article
Hydrogen Economy Model for Nearly Net-Zero Cities
with Exergy Rationale and Energy-Water Nexus
Birol Kılkı¸s 1, * and ¸Siir Kılkı¸s 2
1Energy Engineering Graduate Program, Ba¸skent University, Ankara 06790, Turkey
2
The Scientific and Technological Research Council of Turkey, Ankara 06100, Turkey; siir.kilkis@tubitak.gov.tr
*Correspondence: birolkilkis@hotmail.com
Received: 15 March 2018; Accepted: 2 May 2018; Published: 10 May 2018
Abstract:
The energy base of urban settlements requires greater integration of renewable energy
sources. This study presents a “hydrogen city” model with two cycles at the district and building
levels. The main cycle comprises of hydrogen gas production, hydrogen storage, and a hydrogen
distribution network. The electrolysis of water is based on surplus power from wind turbines and
third-generation solar photovoltaic thermal panels. Hydrogen is then used in central fuel cells to meet
the power demand of urban infrastructure. Hydrogen-enriched biogas that is generated from city
wastes supplements this approach. The second cycle is the hydrogen flow in each low-exergy building
that is connected to the hydrogen distribution network to supply domestic fuel cells. Make-up water
for fuel cells includes treated wastewater to complete an energy-water nexus. The analyses are
supported by exergy-based evaluation metrics. The Rational Exergy Management Efficiency of
the hydrogen city model can reach 0.80, which is above the value of conventional district energy
systems, and represents related advantages for CO
2
emission reductions. The option of incorporating
low-enthalpy geothermal energy resources at about 80
◦
C to support the model is evaluated.
The hydrogen city model is applied to a new settlement area with an expected 200,000 inhabitants to
find that the proposed model can enable a nearly net-zero exergy district status. The results have
implications for settlements using hydrogen energy towards meeting net-zero targets.
Keywords:
hydrogen; hydrogen economy; renewable energy; photovoltaic thermal; wind turbine;
biogas; geothermal energy; exergy; low-exergy buildings; net-zero targets
1. Introduction
Hydrogen production from renewable energy sources based on options for power-to-gas or
power-to-liquid is one of the essential components of smart energy systems, which require the
integration of smart electricity, thermal, and gas grids [
1
]. Smart energy systems are deemed as
the most feasible approach towards 100% renewable energy solutions [
2
]. In this context, electrolysers
and fuel cells are options to allow energy systems to gain flexibility [
3
]. A hydrogen economy that
encompasses an entire supply chain based on hydrogen energy from production to usage [
4
] is also a
valid option for supporting progress towards cleaner, smarter, and integrated energy systems.
Among related studies, an outlook for hydrogen as an energy storage medium and energy carrier
in renewable energy systems for islands, including water, waste treatment, and wastewater treatment,
was put forth for Porto Santo Island [
5
,
6
]. Future scenarios for the energy system of Denmark [
7
]
were undertaken with the aim of enabling a hydrogen economy. Those for Italy [
8
] involved the
use of hydrogen energy to increase energy system flexibility. In contrast, studies that undertake the
integration of hydrogen-based options at the urban level as a whole for districts and cities are still
limited. One of the examples may be given from the analyses of Sveinbjörnsson et al. [
9
] who evaluated
a smart energy system for Sønderborg in Denmark. As a contribution to these and other studies,
Energies 2018,11, 1226; doi:10.3390/en11051226 www.mdpi.com/journal/energies
Energies 2018,11, 1226 2 of 33
the present research work provides a hydrogen economy based model for districts, including original
metrics and an extended outlook to energy-water relations in the urban context.
At the building level, Singh et al. [
10
] had presented the selection and analysis of a hybrid
energy system for an academic building, including a system configuration that involved a system
of solar photovoltaic (PV) arrays, an electrolyser, a hydrogen fuel cell, and a hydrogen storage tank.
Cao et al. [
11
] analyzed a zero-energy building with a ground-source heat pump (GSHP), solar
PV panels and/or a wind turbine according to the geographical context, and a hydrogen vehicle
in a vehicle-to-building (V2B) scheme. As other related developments in the field of hydrogen
systems, Reuß et al. [
12
] analyzed hydrogen production from electrolysis and its seasonal storage,
transport, and fuelling means, including liquid organic hydrogen carrier tanks, trailers, and stations.
Nabgana et al. [
13
] overviewed developments in hydrogen production from biomass using steam
reforming. In addition, Qolipour et al. [
14
] compared options to produce hydrogen from wind power
plants, PV, and hybrid PV-wind power plants, of which the latter was found to be more feasible.
Tebibel et al. [
15
] proposed an off-grid system with a PV array, an aqueous methanol (CH
3
OH) tank,
an electrolyser that produces hydrogen from CH
3
OH, and a hydrogen tank to supply hydrogen on
demand. The proposed system was found to be more suitable than the selection of an option for
hydrogen production based on water electrolysis at the location of Algiers. In contrast, these studies
did not provide a district energy model with hydrogen, solar, and wind energy utilization.
In the urban transport context, Xu et al. [
16
] calculated the quantity of fuel cell vehicles on the
road and the daily hydrogen demand in Shenzhen, China to the year 2025. The quantities were
estimated based on cautious, moderate, and optimistic scenarios. Mohareb and Kennedy [
17
] used
the Pathways to Urban Reductions in Greenhouse Gas Emissions modeling tool to analyze possible
scenarios for Toronto, including hydrogen fuel cell vehicles. Miranda et al. [
18
] analyzed the energy
management system of a prototype city bus using a hybrid electric-hydrogen fuel cell powertrain
that was demonstrated during the Rio Olympics. In addition, Franzitta et al. [
19
] evaluated the use
of electricity from wind and wave farms as well as solar energy to produce hydrogen for fuel cells
to substitute diesel fuel in the public transport fleets of the city of Trapani and island of Pantelleria
in Italy. Briguglio et al. [
20
] further analyzed possible uses of hydrogen energy for urban mobility
in another Italian city. At the country level, Moreno-Benito et al. [
21
] modeled the required quantity
of hydrogen production to satisfy transport demands in the next 50 years for the United Kingdom.
In contrast, additional recommendations to shift modes of transport from the use of private vehicles to
public mass transit were not given, which could further reduce carbon dioxide (CO2) emissions.
It is possible to evaluate multiple sectors with relevance for urban areas from an urban systems
perspective. Oldenbroek et al. [
22
] analyzed the possibility of a 100% local renewable energy system
to provide for the energy needs of power, heat, and transport in an urban area. The options were
based on solar, wind, and fuel cell options with hydrogen as an energy carrier. The proposed energy
system was applied to a hypothetical smart city area as an average city based on European statistics.
The possibility of eliminating high and medium voltage electricity grids was assessed. This study,
however, did not involve energy self-sufficiency or near-zero targets and exergy-based analyses.
Other studies focused on hydrogen production from available sources at the city or industrial
complex vicinity with a technological focus. For example, Kumar et al. [
23
] evaluated the prospects
of valorizing industrial wastewater for biological hydrogen production and techniques to increase
the hydrogen yield. Nahar et al. [
24
] reviewed the technological options for producing hydrogen
from biogas in India, including industrial wastewater and landfill gas. Khan et al. [
25
] concluded on
the applicability of the use of microbial electrolysis cells in replacing conventional technologies for
municipal wastewater treatment technologies. In contrast, none of these studies addressed the need to
plan for a more closed urban water cycle or compare possibilities to progress in net-zero targets.
Among other necessities, the need to address an energy-water nexus in the water treatment sector
is crucial [
26
]. This need also extends to processes of water desalination when this option may be valid
or required in a given local context. Rather than the use of fossil fuels, solar thermal, solar PV [
27
],
Energies 2018,11, 1226 3 of 33
hybrid solar PV-wind, geothermal, and wave energy [
28
] as well as hybrid wave-solar [
29
] systems can
be used to satisfy the energy intense demands of water desalination. In this respect, Viola et al. [
26
]
used an island as a laboratory to experiment with the use of wave energy to support cleaner energy
options for water desalination. At the same time, studies that span across hydrogen energy, an urban
systems perspective that extends to the water sector, and net-zero targets remain to be addressed.
For example, Sanseverino et al. [
30
] conceptualized a “net zero energy island” based on the use of solar,
wind and geothermal energy while hydrogen energy as an energy carrier was not involved. In contrast,
Da Silva et al. [
31
] analysed prospects for a hydrogen production plant in Brazil based on electricity
from solar, wind and hydropower for export to neighbouring countries. Despite the combined use of
renewable energy sources for hydrogen production, the study focused on a centralized approach at
the country level without considerations of an energy-water nexus.
Most recently, Alanne and Cao [
32
] reviewed small-scale options for hydrogen economy in
buildings and communities and proposed that future research work may be directed to “zero-energy
hydrogen economy” (ZEH
2
E) concepts where hydrogen is the main energy carrier. Based on the most
recent literature, it is therefore evident that there is a knowledge gap for integrating hydrogen economy
models for urban renewable energy systems, especially those that involve net-zero targets.
Moreover, hydrogen economy models for urban systems may be supported with guidance based
on metrics that involve the quality of energy, namely, exergy. Exergy is a measure of the useful
work potential of energy. Unlike energy, exergy is irreversibly destroyed according to the Second
Law of Thermodynamics while temperatures converge to thermal equilibrium with a given reference
environment [
33
]. In this way, this research work seeks to put forth hydrogen economy models in the
urban context based on renewable energy using exergy metrics and net-zero targets. The framework
and the analytical results are expected to be instrumental for engineers and city planners in integrating
a multitude of renewable and waste energy resources at the urban level.
Aims of the Research Work
The main objective of this research is to develop a hydrogen economy model for nearly net-zero
cities with a holistic approach. The metrics involve those from the Rational Exergy Management Model
(REMM), which provides an analytical framework based on exergy in planning for CO
2
mitigation
measures, including those for districts that may seek to reach net-zero targets [
34
]. This necessitates
that energy resources, including renewable energy, are allocated with the priority of ensuring better
compatibility in exergy levels to streamline primary energy spending [
34
]. Among others, REMM has
been applied to districts [
35
,
36
], university campuses [
37
], airports [
38
] and dairy farms [
39
] while
applications that involve hydrogen production based on renewable energy and its utilization within
the urban context remain to be analyzed as a further basis for the present study.
The paper proceeds to the method of the research work and the metrics that are utilized. As an
additional novelty of the research work, net-zero targets for a hydrogen community are combined with
an energy/exergy and water nexus perspective. To achieve the main aim, multiple hydrogen cycles for
the urban context are envisioned and analyzed, including comparisons to conventional district energy
systems. The analyses are extended to an application that involves a new settlement.
2. Method of the Research Work
The large-scale mobilization of renewable and waste energy resources is required for a net-zero
or net-positive concept based on exergy at large, such as at the district and city levels. In addition,
the hybridization of systems with energy conversion and distribution systems that are connected
to respective demand points is necessary. This must be planned at an optimum mix based on local
conditions, constraints as well as options for effective and efficient distribution, energy storage,
and cogeneration. The concept of a hydrogen economy can provide a valid response in several aspects:
•
Hydrogen may be produced by renewable energy resources to provide a suitable energy storage
and distribution system.
Energies 2018,11, 1226 4 of 33
•
Hydrogen may be distributed even with existing natural gas pipelines [
40
] given upgrades
involving hydrogen meters and sensors [41].
•Hydrogen is a suitable fuel for fuel cells, which are in essence a cogeneration system.
•
With optimum design and operation, exergy destruction in a hydrogen economy may be minimal.
•
Hydrogen production may be realized in a closed-cycle energy-water nexus in a district
energy system.
In addition, a hydrogen distribution network based on existing natural gas pipelines can
consume less pumping energy than the district hot and cold-water piping in conventional systems.
Western Europe already has a hydrogen gas pipeline network with a total length of 1500 km [40].
These and other aspects indicate that hydrogen economy can have multiple attributes for a more
efficient energy supply base in districts. This research work acknowledges that the existing unresolved
issues of future net or near net-zero cities and districts based on hydrogen economy is an important
knowledge gap in the literature. To fill such a gap, an exergy-based hydrogen economy model is put
forth with proper evaluation metrics and compared to a baseline district energy system.
The proposed hydrogen economy for nearly net-zero districts based on exergy is coupled with the
hybridization of several systems, such as solar photovoltaic thermal (PVT), wind turbines, fuel cells,
poly-generation systems, organic Rankine cycle (ORC) and heat pumps with biogas and/or geothermal
energy. According to REMM, the level of exergy matches in a district must be improved to minimize
related CO
2
emission responsibilities. This includes comparisons based on the avoidable CO
2
emissions
impact due to exergy destruction that takes place within the boundaries of the district. Improvements
in the level of exergy match are compared based on respective Exergy Flow Bars [42].
In the proposed energy system, two cycles of a hydrogen economy at district and building levels
are analyzed in an exergy-based framework. Comparisons with a geothermal energy option are further
put forth to evaluate integration possibilities. The model is applied to the planning of a new settlement
with 200,000 inhabitants that is conceived as a case study of the research work.
In the first cycle, hydrogen gas is produced by electrolysis of water in the district power
plant based on wind turbines with double-blade arrangement [
43
] and third-generation PVT panels.
PVT panels were designed such that coolant fluid has minimum pumping requirements by extensively
using heat pipes in the PVT modules with internal thermal energy storing capability. The embedded
layer contains phase change material (PCM) to obtain efficiency improvements. Experimental data
on the PVT modules are conducted and integrated into the analyses and the case study. Accordingly,
low-pressure hydrogen is supplied to the district through a network of hydrogen pipelines.
The second cycle is the hydrogen utilization in each low-exergy building based on building scale
fuel cells to satisfy virtually all types of domestic energy demands. Power that is produced by all
energy systems, including the fuel cell unit, is in direct current (DC) electricity form. Buildings are
equipped with low-exergy heat distribution/absorption equipment, such as radiant wall, ceiling and
floor panels, chilled beams, desiccant type of humidity controls, and high-efficiency appliances, faucets,
and drainage systems. In the buildings, fuel cells also produce water and heat. The heat is used in
low-exergy space heating systems and for domestic hot water (DHW) subject to temperature peaking.
Absorption chillers produce cold and their waste heat is collected. Separate large-scale thermal energy
storage systems (TES) with different exergy levels are utilized in the buildings.
Moreover, rainwater is collected and utilized in the water supply system. In an energy-water
nexus, water is cycled between the plant where it is first electrolyzed to produce hydrogen and then
recovered mostly in the fuel cells at the power plant and the buildings. Make-up water is supplied
by the building fuel cells, treated wastewater from the district grid, and sea (lake) water, if nearby or
feasible to transport. In the latter case, seawater is converted to fresh water by light-assisted catalysis
oxidation where power is received from the plant fuel cells. This is an important aspect of the system
to close the energy-water nexus. The possibilities of directly connecting biogas generation based on
city waste and low-enthalpy geothermal energy resources are also evaluated for further utilization.
Energies 2018,11, 1226 5 of 33
Prior to the application of the method to realize the analyses in this research work, a justification
of an exergy-based framework is put forth based on two examples, particularly those that involve
net-zero buildings and Coefficient of Performance (COP) based on exergy principles. These examples
are used to emphasize the crucial role of the Second Law of Thermodynamics in addressing major
urban challenges, most importantly CO
2
mitigation. Needs for the exergy-based metrics that are used
to evaluate the hydrogen city model are further put forth with discussions.
2.1. Near-Zero Targets for Buildings and Districts
Net-zero energy buildings (NZEB), near zero energy buildings (nZEB) and net positive-energy
buildings (NPEB) [
44
] are gaining importance in the quest of reducing CO
2
emissions towards reaching
the goals of the Paris Agreement. At the same time, there are still issues to be resolved [
45
]. A major
issue that is not addressed in the building and energy sector is the fact that renewable energy resources
and systems in the built environment have or require different energy quality or exergy levels. With
an increasing share of renewable energy resources, differences in exergy levels need to be identified
to ensure an exergy balance between the supply (resource) and the demand points in the built
environment. In addition, the importance of renewable energy resources in optimum and net-positive
solutions has to be acknowledged [
46
]. The First Law of Thermodynamics is necessary but not
sufficient to address these problems as demonstrated in the following contexts.
2.1.1. Necessity for Net-Zero Exergy Targets
In addition to the exchange of electricity, the exchange of heat through NZEBs can support district
networks [
47
]. At the same time, thermal energy at different temperatures means variation in quality.
Several shortcomings of the NZEB definition may be inferred from references [34,48]:
•
Thermal energy exchange definitions must distinguish between different forms of heat with
different exergy levels, such as steam, hot water, service water, and cold water.
•The quality of energy exchange needs to be embedded into the nZEB definition.
•The impact of the exchanged energy quality must be considered when calculating emissions.
Hence, differences in the energy received from and supplied to a district energy system must be
considered. For example, a NZEB may exchange electrical and thermal power with a district energy
system. The building may receive 10,000 kWh of alternating current (AC) electrical energy with an
average power rms of 5% and provide 10,000 kWh AC electrical energy with an average power rms of
10% annually. The building may also receive 15,000 kWh of heat in the form of hot water from the
district at an average supply temperature of 353 K (80
◦
C) and provide 15,000 kWh of thermal energy
to the district at an average temperature of 343 K (70
◦
C). From the ideal Carnot cycle with reference
environment temperature of 283 K, the thermal exergy exchange between the building and district,
namely Exsup as the supplied exergy (Equation (1)) and Exret as the returned exergy (Equation (2)) is:
Exsup =1−283 K
353 K×15, 000 kWh =2974.5 kWh (1)
Exret =1−283 K
343 K×15, 000 kWh =2623.9 kWh (2)
By definition, this building is a net-zero energy building with an exact annual exchange of
15,000 kWh with the district but has a deficit based on the exergy levels of the energy amount that is
exchanged. The qualities of the exchanged electrical energy are also different in terms of power quality
characteristics, possibly due to the electronics involved in the DC to AC power conversion.
Evidently, the building in Equations (1) and (2) is not a building that satisfies the NZEXB
target. In order to account for an exergy balance, a Net Zero Exergy Building (NZEXB) was defined,
which generates energy at the same grade and quality as consumed on an annual basis while involving
Energies 2018,11, 1226 6 of 33
exchanges with the grid [
48
]. Such a definition is important especially when renewable energy systems
become more diversified and coupled to the district at different exergy levels [49–51].
2.1.2. Exergy-Based Coefficient of Performance
Figure 1represents the energy and exergy flow of a GSHP driven by grid electricity [
52
,
53
].
The electrical power input to a GSHP is utilized with a given COP value at given operating conditions
to supply thermal energy. From an exergy perspective, the GSHP needs to have such a COP value that
the exergy of the electrical power supply (
εin
) is at least equal to the exergy of the thermal output (
εout
).
Equation (3) defines a minimum COP as COP
min
that reaches this threshold for a temperature output
(Tout) of 55 ◦C (328 K) and an environment reference temperature (Tref) that is equal to 283 K.
Energies 2018, 11, x FOR PEER REVIEW 6 of 32
2.1.2. Exergy-Based Coefficient of Performance
Figure 1 represents the energy and exergy flow of a GSHP driven by grid electricity [52,53]. The
electrical power input to a GSHP is utilized with a given COP value at given operating conditions to
supply thermal energy. From an exergy perspective, the GSHP needs to have such a COP value that
the exergy of the electrical power supply (εin) is at least equal to the exergy of the thermal output
(εout). Equation (3) defines a minimum COP as COPmin that reaches this threshold for a temperature
output (Tout) of 55 °C (328 K) and an environment reference temperature (Tref) that is equal to 283 K.
Figure 1. Exergy input and output for GSHP.
17.28
283 K
1328 K
min
COP ==
−
(3)
The example shown in Figure 1 indicates that most conventional heat pumps will have an
exergy-based COP value (COPEX) that is less than one according to Equation (4) even if an optimum
Tout is found. In Equation (5), an optimum Tout is based on maximum COPEX for a given reservoir
temperature and TR considering function constants a and b that are linearized for a given heat pump.
Combining Equations (4) and (5), taking a derivative of the product, and equating it to zero gives the
optimum Tout value in Equation (6) as put forth within the method of this research work:
1
ref
out
out
EX
in in
T
T
COP COP COP
ε
εε
−
=×=× (4)
()
out R
COP a b T T=− − (5)
out re f R
a
TTT
b
=+
(6)
New developments are promising in making heat pumps exergetically feasible above the
threshold value in Equation (3). These include water-source heat pumps with heat recovery that has
a heating COP of 8.15 and a cooling Energy Efficiency Ratio (EER) of 5.02 [54]. With technological
advances, heat pumps may perform better in hybridized applications that involve hydrogen energy
(see Section 3). The Primary Energy Ratio (PER) definition can also be advanced with a Primary
Exergy Ratio (PEXR) definition as put forth in Equation (7) that considers a power plant with a First
Law efficiency ηI and a heat pump with COPEX. If ηI is 0.3 for a conventional power plant running on
fossil fuels and COPEX is 0.49 as in Figure 2 (the blue circled point), PEXR is 0.147. This means that a
heat pump uses only 14.7% of the exergy available in the fossil fuel consumed at the power plant. In
contrast, the PER definition would give a result of 0.3 times 2.85, which is 0.86:
I
EX
P
EXR COP
η
=× {Quality flow of energy from the primary resource} (7)
Tout
εin
εout
TR
Figure 1. Exergy input and output for GSHP.
COPmin =1
1−283 K
328 K =7.28 (3)
The example shown in Figure 1indicates that most conventional heat pumps will have an
exergy-based COP value (COP
EX
) that is less than one according to Equation (4) even if an optimum
T
out
is found. In Equation (5), an optimum T
out
is based on maximum COP
EX
for a given reservoir
temperature and TRconsidering function constants aand bthat are linearized for a given heat pump.
Combining Equations (4) and (5), taking a derivative of the product, and equating it to zero gives the
optimum Tout value in Equation (6) as put forth within the method of this research work:
COPEX =COP ×εout
εin =COP ×1−Tref
Tout
εin (4)
COP =a−b(Tout −TR)(5)
Tout =rTre fTR+a
b(6)
New developments are promising in making heat pumps exergetically feasible above the threshold
value in Equation (3). These include water-source heat pumps with heat recovery that has a heating
COP of 8.15 and a cooling Energy Efficiency Ratio (EER) of 5.02 [
54
]. With technological advances,
heat pumps may perform better in hybridized applications that involve hydrogen energy (see Section 3).
The Primary Energy Ratio (PER) definition can also be advanced with a Primary Exergy Ratio (PEXR)
definition as put forth in Equation (7) that considers a power plant with a First Law efficiency
ηI
and a
heat pump with COP
EX
. If
ηI
is 0.3 for a conventional power plant running on fossil fuels and COP
EX
is 0.49 as in Figure 2(the blue circled point), PEXR is 0.147. This means that a heat pump uses only
14.7% of the exergy available in the fossil fuel consumed at the power plant. In contrast, the PER
definition would give a result of 0.3 times 2.85, which is 0.86:
PEXR =ηI×COPEX {Quality flow of energy from the primary resource}(7)
Energies 2018,11, 1226 7 of 33
Evidently, the utilization of exergy-based analyses is necessary to effectively show the quality
flow of energy rather than the quantity flow [55] in related analyses, design, and operation steps.
Energies 2018, 11, x FOR PEER REVIEW 7 of 32
Evidently, the utilization of exergy-based analyses is necessary to effectively show the quality
flow of energy rather than the quantity flow [55] in related analyses, design, and operation steps.
Figure 2. Sample variation of COP and COPEX with a = 5, b = 0.04 K−1, TR = 288 K, Tref = 283K.
2.1.3. Exergy-Based Formulations for a Nexus Approach
Equations (8)–(13) put forth additional formulations that are used in the evaluation of the
hydrogen city model. The unit exergy of each 1 kWh of the supply heat (εsup) according to the ideal
Carnot cycle is given in Equation (8). Here, Tsup is the supply temperature. Similarly, Equation (8) is
adapted for unit destroyed exergy (εdes), unit demand exergy (εdem), and unit returned exergy (εret):
()
11 kWh
ref
sup
sup
T
T
ε
=− ×
{Unit Exergy} (8)
x
sup sup
E
Q
ε
=× {Energy and Exergy} (9)
The basis for establishing the energy, exergy, and environment nexus is provided by the exergy
magnitude Ex, which is based on εsup and magnitude of thermal energy Qsup (Equation (9)), REMM
efficiency (see Equations (10) and (11)) and CO2 emissions (see Equations (12) and (13)), respectively.
The latter formulations are based on REMM in which a mismatch in the supply and demand of exergy
is linked to additional primary energy spending in the energy system and related CO2 emissions [34].
In Equation (10), ψR is the metric for the exergy utilization rationale, namely the Rational Exergy
Management Efficiency [34]. The formulation is for cases that involve power generation. If in any
process, major exergy destruction takes place upstream of the useful application at the absence of
power generation, then Equation (10) is replaced based on a re-arrangement of terms as in Equation
(11) [34]. A weighted mean value is used when multiple energy outputs are involved:
1des
R
sup
ε
ψ
ε
=−
{Rationality of Exergy Use} (10)
dem
R
s
up
ε
ψ
ε
= (11)
By definition, the annual average of ψR must be at least equal to 0.80 for any connected building
in a hydrogen economy district with the aim of obtaining a better exergy match. This is instrumental
for reducing available CO2 emission impacts in the energy supply due to any need to re-supply
primary energy resources. Equation (12) defines the compound CO2 emissions, which includes
avoidable emissions due to exergy destruction in a process as represented by the term (1 − ψR) [34]:
Figure 2. Sample variation of COP and COPEXwith a= 5, b= 0.04 K−1,TR= 288 K, Tref = 283K.
2.1.3. Exergy-Based Formulations for a Nexus Approach
Equations (8)–(13) put forth additional formulations that are used in the evaluation of the
hydrogen city model. The unit exergy of each 1 kWh of the supply heat (
εsup
) according to the
ideal Carnot cycle is given in Equation (8). Here, T
sup
is the supply temperature. Similarly, Equation (8)
is adapted for unit destroyed exergy (εdes), unit demand exergy (εdem), and unit returned exergy (εret):
εsup =1−Tre f
Tsup ×(1 kWh){Unit Exergy}(8)
Ex=εsup ×Qsup {Energy and Exergy}(9)
The basis for establishing the energy, exergy, and environment nexus is provided by the exergy
magnitude E
x
, which is based on
εsup
and magnitude of thermal energy Q
sup
(Equation (9)), REMM
efficiency (see Equations (10) and (11)) and CO
2
emissions (see Equations (12) and (13)), respectively.
The latter formulations are based on REMM in which a mismatch in the supply and demand of exergy
is linked to additional primary energy spending in the energy system and related CO
2
emissions [
34
].
In Equation (10),
ψR
is the metric for the exergy utilization rationale, namely the Rational Exergy
Management Efficiency [
34
]. The formulation is for cases that involve power generation. If in any
process, major exergy destruction takes place upstream of the useful application at the absence of power
generation, then Equation (10) is replaced based on a re-arrangement of terms as in Equation (11) [
34
].
A weighted mean value is used when multiple energy outputs are involved:
ψR=1−∑εdes
εsup {Rationality of Exergy Use}(10)
ψR=εdem
εsup (11)
By definition, the annual average of
ψR
must be at least equal to 0.80 for any connected building in
a hydrogen economy district with the aim of obtaining a better exergy match. This is instrumental for
reducing available CO
2
emission impacts in the energy supply due to any need to re-supply primary
energy resources. Equation (12) defines the compound CO
2
emissions, which includes avoidable
emissions due to exergy destruction in a process as represented by the term (1 −ψR) [34]:
∑CO2=cl
ηl+cm
ηmηT(1−ψR)QH+cm
ηmηTE{Environment}(12)
Energies 2018,11, 1226 8 of 33
Equation (12) as formulated in REMM [
34
] establishes the metric to evaluate the environmental
dimension of the nexus. The first term within the square brackets is the direct CO
2
emissions from
an on-site (local) energy conversion unit, such as a boiler with a thermal efficiency, ηl, which satisfies
a thermal load Q
H
. Here, c
l
is the CO
2
intensity of the energy resource that is used locally on-site.
In conventional thermal systems, exergy is usually destroyed upstream of the thermal load. Hence,
the second term within the square brackets derives from the forgone power generation opportunity as
a function of the destroyed exergy (1
−ψR
) while satisfying a thermal load Q
H
. This second term is the
avoidable CO
2
emissions impact, which is associated with a power plant at the energy system level
that in effect has to compensate for the forgone opportunity of generating power on-site. The variable
c
m
is the CO
2
intensity of the energy resource that is used at the power plant and
ηm
is the power
generation efficiency of the power plant. According to an energy system boundary, the variable
ηT
is
the overall efficiency of power transmission and power feeding. The last term in Equation (12) is the
CO2emissions that take place to satisfy the on-site electrical power demand, E.
For a net-zero CO
2
building (NZCB) or district, Equation (12) implies that renewable energy
resources must be used (c
l
and c
m
approach zero) and exergy mismatches must be reduced for
ψR
to approach one. In addition, the Ratio of Emissions Difference (EDR) as given in Equation (13)
must be close to one. Here, the CO
2base
term is the standardized emission rate with unit defaults for
0.5 kWh thermal (Q
H
) and 0.5 kWh electrical power demand (E) with a power to heat ratio (C) of one.
Other default values include 0.2 for
ψR
for an energy system that does not involve any combined heat
and power (CHP) with renewables. In Equation (14), CO
2base
is 0.63 kg CO
2
per 1 kWh total energy
load based on Equation (12). The CO
2base
is compared within EDR for a given hydrogen economy
option:
EDR =1−[CO2/(QH+E)]
CO2base (13)
∑CO2base =h0.2 kg CO2/kWh
0.85 +0.2 kg CO2/kWh
0.35 (1−0.2)i×0.5 kWh +0.2 kg CO2/kWh
0.35 ×0.5 kWh =0.63 kg CO2(14)
2.1.4. Definition of a Composite Rationality Indicator
The efficiency of energy activities can be improved based on at least six major parameters:
1. Type of fuel or renewable energy source
2. Equipment and plant energy efficiency
3. Rational Exergy Management Efficiency (ψR)
4. Thermal loads
5. Plant and grid power transmission efficiency, transformer losses, etc.
6. Power loads
The trend of transitioning to renewable energy is already improving the first parameter.
The second parameter, namely the equipment efficiency, is also improving as CHP, condensing boilers,
and other energy technologies are approaching theoretical limits so that there is limited room for
improvement. Parameters 4, 5, and 6 are also on the right track with smart grids, DC underground
lines, and energy saving measures for thermal and electrical loads. In contrast, the third parameter
ψR
remains unresolved although it has large room for improvement. This parameter is important since
the current average value for most cities is less than 0.3 [
51
]. This value will substantially improve by
addressing more structural issues in the energy system, namely imbalances between the supply and
demand of exergy. Re-thinking exergy aspects can support innovative combinations of technology in a
circular economy approach, improve urban quality, and reduce CO2emissions.
Given both quantity and quality oriented efficiency aspects, a new indicator that combines the
First and Second Law efficiencies is defined as a Composite Rationality Indicator, C
R
. Equation (15) is
valid for the use of energy efficiency values that may also be COP in Equation (16). The defined C
R
is
used to compare proposed options, including possible uses of geothermal energy.
Energies 2018,11, 1226 9 of 33
CR=ηl×ψRor, (15)
CR=COP ×ψR. . . (16)
2.1.5. Exergy-Based Net and Near-Zero Definitions
Net-zero targets based on exergy are valid for buildings and districts as developed in previous
phases of the research work and summarized in Table 1. Prior to these definitions, various applied
definitions for a Low-Exergy Building (LowExB) were present [
56
], which may be considered as a
building that satisfies its heating loads with low-exergy sources at about 40
◦
C and sensible cooling
loads at about 15
◦
C to 18
◦
C [
57
]. All such definitions have been put forth for approval in ASHRAE
Technical Committees, namely Exergy Analysis for Sustainable Buildings and Terminology based
on [
48
–
51
]. In Table 1, related definitions are also harmonized based on above Equation (9) or
Equation (13). Based on Table 1, for example, a nearly Zero Exergy Building (nZEXB) is a building or
building cluster that is connected to the district returning at least 80% of the total exergy of heat and
power to the district as the total exergy of heat and power supplied from the district annually.
Table 1. Building and District Level Net and Near Zero Definitions Based on Exergy.
Building or District Target Acronym Ref. Definition Equation
Net-Zero Exergy Building NZEXB [34,48]Exsup =Exret (9)
Nearly Zero Exergy Building nZEXB [38]aExret ≥Exsup ×0.8 (9)
Net Positive Exergy Building NPEXB [38]aExret ≥Exsup (9)
Net-Zero Exergy District NZEXD [35,36]Exsup =Exret (9)
Near Net-Zero Exergy District nZEXD [35,36]Exret ≥Exsup ×0.8 (9)
Net-Zero CO2Building NZCB [51]aEDR = 1.0 (13)
Near Zero CO2Building nZCB [51]a0.8 ≤EDR < 1.0 (13)
Net-Zero CO2(Emissions) District NZCD/NZCED [36]aEDR = 1.0 (13)
Near Zero CO2District nZCD [36]a0.8 ≤EDR < 1.0 (13)
aExtended in the present manuscript based on the defined Exret or EDR conditions.
A Net Positive Exergy Building (NPEXB) supplies a surplus of total exergy of heat and power to
the local district energy system when compared to the total exergy of heat and power received from
the district energy system on an annual basis.
At a district level, a Net-Zero Exergy District (NZEXD) [
35
,
36
] is a district that has its own local
centralized and/or distributed energy system with any sub-stations in the same district so that the
same total exergy of heat and power is supplied by the local district energy system as the total exergy
of heat and power used in the district on an annual basis. In this context, lower temperature supply
networks [
1
,
2
] that take place in Fourth Generation District Energy Systems (4GDE) can support
the NZEXD target. Figure 3shows the relation between NZEXD and NZEXB targets. By definition,
the parameter ψRmust be equal to or greater than 0.80.
Energies 2018, 11, x FOR PEER REVIEW 9 of 32
is valid for the use of energy efficiency values that may also be COP in Equation (16). The defined CR
is used to compare proposed options, including possible uses of geothermal energy.
RlR
C
ηψ
=× or, (15)
R
R
CCOP
ψ
=×
…. (16)
2.1.5. Exergy-Based Net and Near-Zero Definitions
Net-zero targets based on exergy are valid for buildings and districts as developed in previous
phases of the research work and summarized in Table 1. Prior to these definitions, various applied
definitions for a Low-Exergy Building (LowExB) were present [56], which may be considered as a
building that satisfies its heating loads with low-exergy sources at about 40 °C and sensible cooling
loads at about 15 °C to 18 °C [57]. All such definitions have been put forth for approval in ASHRAE
Technical Committees, namely Exergy Analysis for Sustainable Buildings and Terminology based on
[48–51]. In Table 1, related definitions are also harmonized based on above Equation (9) or Equation
(13). Based on Table 1, for example, a nearly Zero Exergy Building (nZEXB) is a building or building
cluster that is connected to the district returning at least 80% of the total exergy of heat and power to
the district as the total exergy of heat and power supplied from the district annually.
Table 1. Building and District Level Net and Near Zero Definitions Based on Exergy.
Building or District Target Acronym Ref. Definition Equation
Net-Zero Exergy Building NZEXB [34,48] Exsup = Exret (9)
Nearly Zero Exergy Building nZEXB [38] a Exret ≥ Exsup × 0.8 (9)
Net Positive Exergy Building NPEXB [38] a Exret ≥ Exsup (9)
Net-Zero Exergy District NZEXD [35,36] Exsup = Exret (9)
Near Net-Zero Exergy District nZEXD [35,36] Exret ≥ Exsup × 0.8 (9)
Net-Zero CO2 Building NZCB [51] a EDR = 1.0 (13)
Near Zero CO2 Building nZCB [51] a 0.8 ≤ EDR ˂ 1.0 (13)
Net-Zero CO2 (Emissions) District NZCD/NZCED [36] a EDR = 1.0 (13)
Near Zero CO2 District nZCD [36] a 0.8 ≤ EDR ˂ 1.0 (13)
a Extended in the present manuscript based on the defined Exret or EDR conditions.
A Net Positive Exergy Building (NPEXB) supplies a surplus of total exergy of heat and power to
the local district energy system when compared to the total exergy of heat and power received from
the district energy system on an annual basis.
At a district level, a Net-Zero Exergy District (NZEXD) [35,36] is a district that has its own local
centralized and/or distributed energy system with any sub-stations in the same district so that the
same total exergy of heat and power is supplied by the local district energy system as the total exergy
of heat and power used in the district on an annual basis. In this context, lower temperature supply
networks [1,2] that take place in Fourth Generation District Energy Systems (4GDE) can support the
NZEXD target. Figure 3 shows the relation between NZEXD and NZEXB targets. By definition, the
parameter ψR must be equal to or greater than 0.80.
Figure 3. NZEXD and NZEXB Targets.
N
Z
E
XD
NZEXB
Figure 3. NZEXD and NZEXB Targets.
Energies 2018,11, 1226 10 of 33
3. Characterization of the Hydrogen City Model
Based on the method, a hydrogen city model is characterized based on two cycles at the district
and building levels as described subsequently. Both of the cycles support the hybridization of energy
options in the district energy system for the effective use of renewable energy sources.
3.1. Main Cycle of the Hydrogen City at the District Level
The first cycle consists of the central CHP plant (Figure 4). The CHP system runs on locally
produced biogas from city wastes. Wind turbines and solar PVT systems are further combined to
generate on-site electricity. Surplus renewable electricity is utilized in an on-site hydrogen production
facility by the electrolysis of water. The produced hydrogen is stored in high-pressure tanks and upon
demand, de-pressurized below 100 bar and then served to the city-wide grid. The central fuel cell
system generates DC electricity that is supplemented by the DC electricity, which is generated by wind
and solar energy systems. A smart low voltage DC (LVDC) micro-grid serves the district along with all
information and data services. Hydrogen is about 1.5 times more energy dense compared to natural
gas. The higher heating value (HHV) of hydrogen is 142 MJ/kg that favorably compares with natural
gas that has a HHV of 52 MJ/kg [
58
]. This allows hydrogen to be better suited for being distributed
in the district. In addition, the stored hydrogen is partly used to enrich the biogas that is used in
the central CHP plant to generate AC electricity for the city infrastructure, mass transport systems,
and industry. Biogas enriched with hydrogen increases the net reaction rate with higher addition ratios
of hydrogen, thereby improving combustion [
59
]. The reaction of the CO
2
in biogas with hydrogen in
a Sabatier process substitutes conventional upgrading units [60].
Energies 2018, 11, x FOR PEER REVIEW 10 of 32
3. Characterization of the Hydrogen City Model
Based on the method, a hydrogen city model is characterized based on two cycles at the district
and building levels as described subsequently. Both of the cycles support the hybridization of energy
options in the district energy system for the effective use of renewable energy sources.
3.1. Main Cycle of the Hydrogen City at the District Level
The first cycle consists of the central CHP plant (Figure 4). The CHP system runs on locally
produced biogas from city wastes. Wind turbines and solar PVT systems are further combined to
generate on-site electricity. Surplus renewable electricity is utilized in an on-site hydrogen
production facility by the electrolysis of water. The produced hydrogen is stored in high-pressure
tanks and upon demand, de-pressurized below 100 bar and then served to the city-wide grid. The
central fuel cell system generates DC electricity that is supplemented by the DC electricity, which is
generated by wind and solar energy systems. A smart low voltage DC (LVDC) micro-grid serves the
district along with all information and data services. Hydrogen is about 1.5 times more energy dense
compared to natural gas. The higher heating value (HHV) of hydrogen is 142 MJ/kg that favorably
compares with natural gas that has a HHV of 52 MJ/kg [58]. This allows hydrogen to be better suited
for being distributed in the district. In addition, the stored hydrogen is partly used to enrich the
biogas that is used in the central CHP plant to generate AC electricity for the city infrastructure, mass
transport systems, and industry. Biogas enriched with hydrogen increases the net reaction rate with
higher addition ratios of hydrogen, thereby improving combustion [59]. The reaction of the CO2 in
biogas with hydrogen in a Sabatier process substitutes conventional upgrading units [60].
Figure 4. Hydrogen-Solar-Wind District Plant in the Energy-Water-Environment Nexus.
The production of hydrogen from numerous renewable energy sources as given in Figure 4 can
provide the basis for a more stable and sustainable energy supply profile for the district. Among the
renewable energy options, double-blade wind turbines are considered to expand the feasible
operational wind speed range by starting at low speeds and sustaining generating power [43]. These
turbines are located only in and around the district plant due to the relatively high turbine noise.
Figure 5 shows the water cycle in the main cycle of the hydrogen city where water recycling
takes place between the plant where it is first electrolyzed to produce hydrogen and the fuel cells at
the central plant. Additional water input as make-up water includes treated wastewater and any
light-assisted catalysis oxidation from seawater with the partial use of the power that is generated by
Figure 4. Hydrogen-Solar-Wind District Plant in the Energy-Water-Environment Nexus.
The production of hydrogen from numerous renewable energy sources as given in Figure 4
can provide the basis for a more stable and sustainable energy supply profile for the district.
Among the renewable energy options, double-blade wind turbines are considered to expand the
feasible operational wind speed range by starting at low speeds and sustaining generating power [
43
].
These turbines are located only in and around the district plant due to the relatively high turbine noise.
Figure 5shows the water cycle in the main cycle of the hydrogen city where water recycling
takes place between the plant where it is first electrolyzed to produce hydrogen and the fuel cells
at the central plant. Additional water input as make-up water includes treated wastewater and any
Energies 2018,11, 1226 11 of 33
light-assisted catalysis oxidation from seawater with the partial use of the power that is generated by
the fuel cells. The integration of treated wastewater into the hydrogen production plant alongside any
additional fresh water sources provides an opportunity to attain a more closed water cycle.
Energies 2018, 11, x FOR PEER REVIEW 11 of 32
the fuel cells. The integration of treated wastewater into the hydrogen production plant alongside
any additional fresh water sources provides an opportunity to attain a more closed water cycle.
Figure 5. Closing the Water Cycle and Generating Fresh Water from the Sea or a Large Lake.
Integration with Solar PVT-3 System
In solar PV systems, the electrical efficiency is a function of temperature with higher panel
temperatures resulting in lower efficiency. PVT systems can stabilize the PV efficiency despite hotter
panel surfaces. In many cases, water is circulated through heat exchanging pipes on the backside of
PV panels. However, proper control of the circulation pump flow rate is essential to minimize motor
power consumption. The water output temperature needs to be low if the PV is to be cooled
effectively and vice versa. It is also important to maximize the total exergetic efficiency by controlling
the coolant flow rate by recognizing that power and heat have different exergy levels.
PVT systems become more feasible in warmer and hot climates in which PV systems need to be
cooled frequently and the temperature of the heated water can satisfy useful applications on-site.
Figure 6 provides the feasibility contours of PVT systems based on average solar radiation on a flat
surface in Europe. The plant size also makes a difference since unit costs reduce with total surface
area of solar radiation, including costs for automation software, hardware, and equipment (e.g.,
pyranometers).
Figure 6. Solar PVT Feasibility Map for Different Levels of Solar Irradiation in Europe.
The simple payback periods are evaluated based on PVT area in Figure 7. Based on Figure 7,
even smaller systems in residential applications become more feasible and can payback the initial
financial investment in a shorter time if the annual solar insolation level, I is high as in Southern
Figure 5. Closing the Water Cycle and Generating Fresh Water from the Sea or a Large Lake.
Integration with Solar PVT-3 System
In solar PV systems, the electrical efficiency is a function of temperature with higher panel
temperatures resulting in lower efficiency. PVT systems can stabilize the PV efficiency despite hotter
panel surfaces. In many cases, water is circulated through heat exchanging pipes on the backside of
PV panels. However, proper control of the circulation pump flow rate is essential to minimize motor
power consumption. The water output temperature needs to be low if the PV is to be cooled effectively
and vice versa. It is also important to maximize the total exergetic efficiency by controlling the coolant
flow rate by recognizing that power and heat have different exergy levels.
PVT systems become more feasible in warmer and hot climates in which PV systems need to
be cooled frequently and the temperature of the heated water can satisfy useful applications on-site.
Figure 6provides the feasibility contours of PVT systems based on average solar radiation on a flat
surface in Europe. The plant size also makes a difference since unit costs reduce with total surface
area of solar radiation, including costs for automation software, hardware, and equipment (e.g.,
pyranometers).
Energies 2018, 11, x FOR PEER REVIEW 11 of 32
the fuel cells. The integration of treated wastewater into the hydrogen production plant alongside
any additional fresh water sources provides an opportunity to attain a more closed water cycle.
Figure 5. Closing the Water Cycle and Generating Fresh Water from the Sea or a Large Lake.
Integration with Solar PVT-3 System
In solar PV systems, the electrical efficiency is a function of temperature with higher panel
temperatures resulting in lower efficiency. PVT systems can stabilize the PV efficiency despite hotter
panel surfaces. In many cases, water is circulated through heat exchanging pipes on the backside of
PV panels. However, proper control of the circulation pump flow rate is essential to minimize motor
power consumption. The water output temperature needs to be low if the PV is to be cooled
effectively and vice versa. It is also important to maximize the total exergetic efficiency by controlling
the coolant flow rate by recognizing that power and heat have different exergy levels.
PVT systems become more feasible in warmer and hot climates in which PV systems need to be
cooled frequently and the temperature of the heated water can satisfy useful applications on-site.
Figure 6 provides the feasibility contours of PVT systems based on average solar radiation on a flat
surface in Europe. The plant size also makes a difference since unit costs reduce with total surface
area of solar radiation, including costs for automation software, hardware, and equipment (e.g.,
pyranometers).
Figure 6. Solar PVT Feasibility Map for Different Levels of Solar Irradiation in Europe.
The simple payback periods are evaluated based on PVT area in Figure 7. Based on Figure 7,
even smaller systems in residential applications become more feasible and can payback the initial
financial investment in a shorter time if the annual solar insolation level, I is high as in Southern
Figure 6. Solar PVT Feasibility Map for Different Levels of Solar Irradiation in Europe.
The simple payback periods are evaluated based on PVT area in Figure 7. Based on Figure 7,
even smaller systems in residential applications become more feasible and can payback the initial
Energies 2018,11, 1226 12 of 33
financial investment in a shorter time if the annual solar insolation level, Iis high as in Southern
Europe and the Mediterranean. The payback period is three years if Iis 1800 kWh/m
2
-year and the
PVT area is 200 m
2
as denoted in the marking in Figure 7. In contrast, the same-sized PVT plant will
have a payback period of 5.2 years in a climatic region with Iequal to 1200 kWh/m2-year.
Energies 2018, 11, x FOR PEER REVIEW 12 of 32
Europe and the Mediterranean. The payback period is three years if I is 1800 kWh/m2-year and the
PVT area is 200 m2 as denoted in the marking in Figure 7. In contrast, the same-sized PVT plant will
have a payback period of 5.2 years in a climatic region with I equal to 1200 kWh/m2-year.
Figure 7. Solar PVT Feasibility Diagram for Europe with Different PVT Plant Size.
The proposed hydrogen city includes a novel third-generation solar PVT system, namely PVT-
3 that involves multiple layers as shown in Figure 8, including a thermoelectric generator (TEG) layer
[61,62]. Thermal energy storage is achieved with an embedded layer of PCM. The circulation pump
is eliminated by using heat pipes (HP), which transfer the heat when there is thermal demand and
according to the level of solar insolation at the site. The glass cover (GC) and the air gap (AG) over
the PV surfaces form a flat plate collector, which is optimized to maximize PVT performance.
After sunrise, solar irradiation enables the generation of power while the undesired heating of
the PV panel surfaces takes place. Cooling is effectively achieved by transferring the additional solar
heat to the backside of the TEG modules with a heat-conducting nano-sheet (NS). While the packed-
bed type PCM layer is thermally charging at a relatively cool temperature, a temperature difference
across the TEG units takes place. This temperature difference generates additional DC power.
Depending on the thermal demand, heat may be transferred to the external manifold via the heat
pipes. After sunset, the PVT-3 module starts to back radiate to the cooler atmosphere from the top
surface. This generates a reverse heat flow starting from the bottom of the TEG units via the heat
conducting sheet. In turn, additional electrical power with a reverse polarity is generated. A polarity
switch corrects the DC output sign. Power generation can be extended after sunset depending on the
total PCM mass, temperature distribution, thermal mass, and the material of the module.
Figure 8. Photo-Heat-Voltaic-Thermal (PVT-3) Module (not to scale) [61,62].
Figure 9 shows the PVT-3 test set-up in a horizontal position with packets of PCM material that
eliminates the gravity effect of molten PCM in operation. The PVT-3 unit may also be positioned
vertically for integration to building façades. In practice, it is difficult to control the flow in a heat
pipe. For this reason, a device to control the heat pipes was developed, which eliminates this problem
mechanically that is depicted in Figure 10. Figure 11 shows a power output performance curve of the
Figure 7. Solar PVT Feasibility Diagram for Europe with Different PVT Plant Size.
The proposed hydrogen city includes a novel third-generation solar PVT system, namely PVT-3
that involves multiple layers as shown in Figure 8, including a thermoelectric generator (TEG)
layer
[61,62]
. Thermal energy storage is achieved with an embedded layer of PCM. The circulation
pump is eliminated by using heat pipes (HP), which transfer the heat when there is thermal demand
and according to the level of solar insolation at the site. The glass cover (GC) and the air gap (AG)
over the PV surfaces form a flat plate collector, which is optimized to maximize PVT performance.
After sunrise, solar irradiation enables the generation of power while the undesired heating
of the PV panel surfaces takes place. Cooling is effectively achieved by transferring the additional
solar heat to the backside of the TEG modules with a heat-conducting nano-sheet (NS). While the
packed-bed type PCM layer is thermally charging at a relatively cool temperature, a temperature
difference across the TEG units takes place. This temperature difference generates additional DC
power. Depending on the thermal demand, heat may be transferred to the external manifold via the
heat pipes. After sunset, the PVT-3 module starts to back radiate to the cooler atmosphere from the
top surface. This generates a reverse heat flow starting from the bottom of the TEG units via the heat
conducting sheet. In turn, additional electrical power with a reverse polarity is generated. A polarity
switch corrects the DC output sign. Power generation can be extended after sunset depending on the
total PCM mass, temperature distribution, thermal mass, and the material of the module.
Energies 2018, 11, x FOR PEER REVIEW 12 of 32
Europe and the Mediterranean. The payback period is three years if I is 1800 kWh/m2-year and the
PVT area is 200 m2 as denoted in the marking in Figure 7. In contrast, the same-sized PVT plant will
have a payback period of 5.2 years in a climatic region with I equal to 1200 kWh/m2-year.
Figure 7. Solar PVT Feasibility Diagram for Europe with Different PVT Plant Size.
The proposed hydrogen city includes a novel third-generation solar PVT system, namely PVT-
3 that involves multiple layers as shown in Figure 8, including a thermoelectric generator (TEG) layer
[61,62]. Thermal energy storage is achieved with an embedded layer of PCM. The circulation pump
is eliminated by using heat pipes (HP), which transfer the heat when there is thermal demand and
according to the level of solar insolation at the site. The glass cover (GC) and the air gap (AG) over
the PV surfaces form a flat plate collector, which is optimized to maximize PVT performance.
After sunrise, solar irradiation enables the generation of power while the undesired heating of
the PV panel surfaces takes place. Cooling is effectively achieved by transferring the additional solar
heat to the backside of the TEG modules with a heat-conducting nano-sheet (NS). While the packed-
bed type PCM layer is thermally charging at a relatively cool temperature, a temperature difference
across the TEG units takes place. This temperature difference generates additional DC power.
Depending on the thermal demand, heat may be transferred to the external manifold via the heat
pipes. After sunset, the PVT-3 module starts to back radiate to the cooler atmosphere from the top
surface. This generates a reverse heat flow starting from the bottom of the TEG units via the heat
conducting sheet. In turn, additional electrical power with a reverse polarity is generated. A polarity
switch corrects the DC output sign. Power generation can be extended after sunset depending on the
total PCM mass, temperature distribution, thermal mass, and the material of the module.
Figure 8. Photo-Heat-Voltaic-Thermal (PVT-3) Module (not to scale) [61,62].
Figure 9 shows the PVT-3 test set-up in a horizontal position with packets of PCM material that
eliminates the gravity effect of molten PCM in operation. The PVT-3 unit may also be positioned
vertically for integration to building façades. In practice, it is difficult to control the flow in a heat
pipe. For this reason, a device to control the heat pipes was developed, which eliminates this problem
mechanically that is depicted in Figure 10. Figure 11 shows a power output performance curve of the
Figure 8. Photo-Heat-Voltaic-Thermal (PVT-3) Module (not to scale) [61,62].
Figure 9shows the PVT-3 test set-up in a horizontal position with packets of PCM material that
eliminates the gravity effect of molten PCM in operation. The PVT-3 unit may also be positioned
vertically for integration to building façades. In practice, it is difficult to control the flow in a heat
pipe. For this reason, a device to control the heat pipes was developed, which eliminates this problem
Energies 2018,11, 1226 13 of 33
mechanically that is depicted in Figure 10. Figure 11 shows a power output performance curve of the
PVT-3 prototype on a typical summer day on a flat surface with 1 m
2
area and I
n
at 750 W/m
2
where
I
n
is the net solar insolation intensity reaching perpendicular to the solar PV surface. Here, E
1
and E
2
are the power generated by the PV layer and TEG elements of the PVT-3 module.
Energies 2018, 11, x FOR PEER REVIEW 13 of 32
PVT-3 prototype on a typical summer day on a flat surface with 1 m2 area and In at 750 W/m2 where
In is the net solar insolation intensity reaching perpendicular to the solar PV surface. Here, E1 and E2
are the power generated by the PV layer and TEG elements of the PVT-3 module.
Figure 9. Experimental Set-up. (Photo courtesy of Varışlı and Aydoğan).
Figure 10. Heat Pipe Controls of the PVT-3.
Figure 11. Combined power performance of PVT-3 on a typical summer day [61,62].
3.2. Second Cycle of the Hydrogen City at the Building Level
A hydrogen city may be considered either for retrofit cities or new green cities in brownfield
area developments. One of the first essential steps, however, is to retrofit buildings accordingly or to
construct new buildings of a plug-in type that are ready for innovative hydrogen energy systems.
Figure 12 represents a transition to a hydrogen city through the phased introduction of net-
positive exergy buildings that are connected to the hydrogen pipeline. A domestic fuel cell is the
centerpiece of the building system with close to or even higher than 60% energy efficiency for power
generation. The distributed power and heat system in Figure 12 enables a downsizing of the central
fuel cell system and eliminates a thermal grid previously servicing buildings. Rather, the central fuel
cell system is dedicated to other city infrastructure, mass transit, and industrial applications.
Total
Figure 9. Experimental Set-up. (Photo courtesy of Varı¸slı and Aydo ˘gan).
Energies 2018, 11, x FOR PEER REVIEW 13 of 32
PVT-3 prototype on a typical summer day on a flat surface with 1 m2 area and In at 750 W/m2 where
In is the net solar insolation intensity reaching perpendicular to the solar PV surface. Here, E1 and E2
are the power generated by the PV layer and TEG elements of the PVT-3 module.
Figure 9. Experimental Set-up. (Photo courtesy of Varışlı and Aydoğan).
Figure 10. Heat Pipe Controls of the PVT-3.
Figure 11. Combined power performance of PVT-3 on a typical summer day [61,62].
3.2. Second Cycle of the Hydrogen City at the Building Level
A hydrogen city may be considered either for retrofit cities or new green cities in brownfield
area developments. One of the first essential steps, however, is to retrofit buildings accordingly or to
construct new buildings of a plug-in type that are ready for innovative hydrogen energy systems.
Figure 12 represents a transition to a hydrogen city through the phased introduction of net-
positive exergy buildings that are connected to the hydrogen pipeline. A domestic fuel cell is the
centerpiece of the building system with close to or even higher than 60% energy efficiency for power
generation. The distributed power and heat system in Figure 12 enables a downsizing of the central
fuel cell system and eliminates a thermal grid previously servicing buildings. Rather, the central fuel
cell system is dedicated to other city infrastructure, mass transit, and industrial applications.
Total
Figure 10. Heat Pipe Controls of the PVT-3.
Energies 2018, 11, x FOR PEER REVIEW 13 of 32
PVT-3 prototype on a typical summer day on a flat surface with 1 m2 area and In at 750 W/m2 where
In is the net solar insolation intensity reaching perpendicular to the solar PV surface. Here, E1 and E2
are the power generated by the PV layer and TEG elements of the PVT-3 module.
Figure 9. Experimental Set-up. (Photo courtesy of Varışlı and Aydoğan).
Figure 10. Heat Pipe Controls of the PVT-3.
Figure 11. Combined power performance of PVT-3 on a typical summer day [61,62].
3.2. Second Cycle of the Hydrogen City at the Building Level
A hydrogen city may be considered either for retrofit cities or new green cities in brownfield
area developments. One of the first essential steps, however, is to retrofit buildings accordingly or to
construct new buildings of a plug-in type that are ready for innovative hydrogen energy systems.
Figure 12 represents a transition to a hydrogen city through the phased introduction of net-
positive exergy buildings that are connected to the hydrogen pipeline. A domestic fuel cell is the
centerpiece of the building system with close to or even higher than 60% energy efficiency for power
generation. The distributed power and heat system in Figure 12 enables a downsizing of the central
fuel cell system and eliminates a thermal grid previously servicing buildings. Rather, the central fuel
cell system is dedicated to other city infrastructure, mass transit, and industrial applications.
Total
Figure 11. Combined power performance of PVT-3 on a typical summer day [61,62].
3.2. Second Cycle of the Hydrogen City at the Building Level
A hydrogen city may be considered either for retrofit cities or new green cities in brownfield
area developments. One of the first essential steps, however, is to retrofit buildings accordingly or to
construct new buildings of a plug-in type that are ready for innovative hydrogen energy systems.
Figure 12 represents a transition to a hydrogen city through the phased introduction of net-positive
exergy buildings that are connected to the hydrogen pipeline. A domestic fuel cell is the centerpiece
of the building system with close to or even higher than 60% energy efficiency for power generation.
The distributed power and heat system in Figure 12 enables a downsizing of the central fuel cell system
and eliminates a thermal grid previously servicing buildings. Rather, the central fuel cell system is
dedicated to other city infrastructure, mass transit, and industrial applications.
Energies 2018,11, 1226 14 of 33
Energies 2018, 11, x FOR PEER REVIEW 14 of 32
Figure 12. All DC-Solar-Central Hydrogen Hybrid Net-Zero/Positive Exergy Building.
The building fuel cell system primarily satisfies the base loads. Other renewables are assisted by
domestic daily or weekly TES and are used mainly for satisfying the peak loads, such as cooling in
the summer months. Separate TES units at different exergy levels serve the building heating and
cooling system. During the summer season, part of the heat is used to temperature peak the reject
heat from the absorption cooling system (ABS) for DHW supply to avoid the risk of Legionella
bacteria. Cold energy is used in fan-coils for peak loads and in-wall cooling panels are used for the
base loads. In the winter season, if there is a cooling load present in the building, the reject heat of the
ABS is used for low-temperature space heating through radiant floor systems. If thermal loads are
too high, then a GSHP is installed that also serves for seasonal thermal storage in the ground.
The energy supply is complemented by roof-top and façade integrated PVT-3 that generates
both power and warm water. The warm water charges the desiccant dehumidification system. In the
net-positive exergy building, rainwater, fuel cell water condensate, and wastewater are domestically
treated and returned to the plant in a separate water line to close an energy-water nexus (Figure 12).
Hydrogen Building to Hydrogen Car Interaction
There can be four power inputs to the net-positive exergy building of Figure 12, namely the
domestic fuel cell, the solar PVT, the grid electricity provided by the central fuel cell system in DC
current as well as power inputs from private vehicles, including those from any hydrogen cars.
Private vehicles spend almost 95% of their time in a parked position in or around the buildings
[63]. Hourly electrical energy storage is possible by connecting the hydrogen and electric cars to the
building power system. In addition, any gasoline-engine car may be a part of the hourly/nightly
electrical energy storage system based on car batteries. In total, three types of cars may be docked to
the building, namely those with a conventional gasoline engine, an electric car, or a hydrogen car.
The source of supply to electric cars depends on the context of the energy system in which they
operate. If an electric car is parked in the building of Figure 12, then car batteries may be charged by
the fuel cell system at a much higher efficiency of power conversion using hydrogen gas and with
almost zero emissions due to the fact that hydrogen is produced by renewable energy. Even in the
Figure 12. All DC-Solar-Central Hydrogen Hybrid Net-Zero/Positive Exergy Building.
The building fuel cell system primarily satisfies the base loads. Other renewables are assisted
by domestic daily or weekly TES and are used mainly for satisfying the peak loads, such as cooling
in the summer months. Separate TES units at different exergy levels serve the building heating and
cooling system. During the summer season, part of the heat is used to temperature peak the reject heat
from the absorption cooling system (ABS) for DHW supply to avoid the risk of Legionella bacteria.
Cold energy is used in fan-coils for peak loads and in-wall cooling panels are used for the base loads.
In the winter season, if there is a cooling load present in the building, the reject heat of the ABS is used
for low-temperature space heating through radiant floor systems. If thermal loads are too high, then a
GSHP is installed that also serves for seasonal thermal storage in the ground.
The energy supply is complemented by roof-top and façade integrated PVT-3 that generates
both power and warm water. The warm water charges the desiccant dehumidification system. In the
net-positive exergy building, rainwater, fuel cell water condensate, and wastewater are domestically
treated and returned to the plant in a separate water line to close an energy-water nexus (Figure 12).
Hydrogen Building to Hydrogen Car Interaction
There can be four power inputs to the net-positive exergy building of Figure 12, namely the
domestic fuel cell, the solar PVT, the grid electricity provided by the central fuel cell system in DC
current as well as power inputs from private vehicles, including those from any hydrogen cars.
Private vehicles spend almost 95% of their time in a parked position in or around the buildings [
63
].
Hourly electrical energy storage is possible by connecting the hydrogen and electric cars to the building
power system. In addition, any gasoline-engine car may be a part of the hourly/nightly electrical
energy storage system based on car batteries. In total, three types of cars may be docked to the building,
namely those with a conventional gasoline engine, an electric car, or a hydrogen car.
The source of supply to electric cars depends on the context of the energy system in which they
operate. If an electric car is parked in the building of Figure 12, then car batteries may be charged by
the fuel cell system at a much higher efficiency of power conversion using hydrogen gas and with
almost zero emissions due to the fact that hydrogen is produced by renewable energy. Even in the
case of the conventional gasoline engine car, the car may be connected to the electrical system of the
building to provide electricity from its battery that is charged during the daytime while driving.
Energies 2018,11, 1226 15 of 33
In the presence of a parked hydrogen car, hydrogen fuel may be received at the building site
after pressurization. In turn, the hydrogen car may provide power, water, and heat to the building.
Figure 13 depicts a “hydrogen building to hydrogen car” interaction. In this case, a micro hydrogen
generating system may be added to the hydrogen building, driven by the excess power generated
by the PVT-3 system, possibly during the daytime when the hydrogen car is away off-site and the
power load of the building may be at a minimal level according to lower occupancy. This additional
generation supplements the main hydrogen grid supply. When the car docks back to the building,
the hydrogen stored in the dedicated storage tank is pressurized more and supplied to the hydrogen
car. In the meantime, electrical energy derived from the building may charge the backup battery of the
car or vice versa to enable nightly electrical energy exchange between the car and the building.
Energies 2018, 11, x FOR PEER REVIEW 15 of 32
case of the conventional gasoline engine car, the car may be connected to the electrical system of the
building to provide electricity from its battery that is charged during the daytime while driving.
In the presence of a parked hydrogen car, hydrogen fuel may be received at the building site
after pressurization. In turn, the hydrogen car may provide power, water, and heat to the building.
Figure 13 depicts a “hydrogen building to hydrogen car” interaction. In this case, a micro hydrogen
generating system may be added to the hydrogen building, driven by the excess power generated by
the PVT-3 system, possibly during the daytime when the hydrogen car is away off-site and the power
load of the building may be at a minimal level according to lower occupancy. This additional
generation supplements the main hydrogen grid supply. When the car docks back to the building,
the hydrogen stored in the dedicated storage tank is pressurized more and supplied to the hydrogen
car. In the meantime, electrical energy derived from the building may charge the backup battery of
the car or vice versa to enable nightly electrical energy exchange between the car and the building.
Figure 13. Schematic for Hydrogen Building to Hydrogen Car Interaction.
4. Results for the Exergy Rationale of the Hydrogen City Model
Figure 14 shows the Exergy Flow Bars [42] for a baseline district energy (DE) system to which
the hydrogen city model is compared. The baseline DE system uses natural gas with a combustion
temperature of 2000 K while the reference environment temperature is 283 K. The available unit
exergy (blue bars) is initially equal to εsup that reduces at each proceeding application and point of
exergy destruction. The first application is electricity production after which an alternating order of
exergy destruction and applications at lower temperature levels take place for heat and cold
production. The REMM Efficiency ψR of the baseline DE system is calculated from Equation (10) as
0.25 based on two temperature intervals that represent points of exergy destruction. The values of
the temperature intervals that determine εdes(1) and εdes(2) are given in Table 2 that represent un-used
temperature intervals between demanded applications. If the same system involves an additional
steam generation process starting from 600 K and ending at 450 K, then the temperature interval for
εdes(1) will be split and reduce to (1 − 600 K/700 K) plus (1 − 365 K/450 K), which would increase the
REMM efficiency only to 0.42. Exergy losses due to the pumping of any steam if generated and the
hot and cold-water circulation in separate circuits in the district are not included in this value.
Table 2. Un-Utilized Temperature Intervals between Demanded Applications.
Case εdes Temperature Intervals (K)
DE Baseline εdes(1) Tdem(1)out 700 Tdem(2)in 365
εdes(2) Tdem(2)out 345 Tdem(3)in 288
Figure 13. Schematic for Hydrogen Building to Hydrogen Car Interaction.
4. Results for the Exergy Rationale of the Hydrogen City Model
Figure 14 shows the Exergy Flow Bars [
42
] for a baseline district energy (DE) system to which
the hydrogen city model is compared. The baseline DE system uses natural gas with a combustion
temperature of 2000 K while the reference environment temperature is 283 K. The available unit exergy
(blue bars) is initially equal to
εsup
that reduces at each proceeding application and point of exergy
destruction. The first application is electricity production after which an alternating order of exergy
destruction and applications at lower temperature levels take place for heat and cold production.
The REMM Efficiency
ψR
of the baseline DE system is calculated from Equation (10) as 0.25 based on
two temperature intervals that represent points of exergy destruction. The values of the temperature
intervals that determine
εdes(1)
and
εdes(2)
are given in Table 2that represent un-used temperature
intervals between demanded applications. If the same system involves an additional steam generation
process starting from 600 K and ending at 450 K, then the temperature interval for
εdes(1)
will be split
and reduce to (1
−
600 K/700 K) plus (1
−
365 K/450 K), which would increase the REMM efficiency
only to 0.42. Exergy losses due to the pumping of any steam if generated and the hot and cold-water
circulation in separate circuits in the district are not included in this value.
Table 2. Un-Utilized Temperature Intervals between Demanded Applications.
Case εdes Temperature Intervals (K)
DE Baseline εdes(1)
T
dem(1)out 700 Tdem (2)in 365
εdes(2)
T
dem(2)out 345 Tdem (3)in 288
Energies 2018,11, 1226 16 of 33
Energies 2018, 11, x FOR PEER REVIEW 16 of 32
Figure 14. Exergy Flow Bars for a Conventional DE System with Power, Heat, and Cold Supply.
The baseline DE system that provides a basis of comparison is to be upgraded to a hydrogen
city. Figure 15 puts forth the linkages between the various components in providing exergy supply
and useful applications. In the hydrogen city model, the circulation of hot and cold water and any
steam is eliminated through the circulation of hydrogen gas, which is less energy intensive. In the
upper left component of Figure 15, solar energy is utilized in the PVT-3 system in the plant. First, DC
electric power is generated in the PV modules. The heat that is absorbed by the PV coolant is utilized
in the thermal charging of the biogas reactor. The exergy flow bar for a typical PVT-3 application at
a solar insolation level of 600 W/m2 is depicted in the form of the upper left bar in Figure 15. Here,
the Carnot cycle equivalent temperature for solar energy Tfs is by definition the mapped equivalent
source temperature for solar energy at a given insolation level In as given by Equation (17) [64]. This
enables an exergy accounting with a more consistent boundary other than a Sun-Earth boundary.
4
16.9610
ref
fs
n
T
T
I
−
=−××
(17)
Figure 15. Exergy Flow Bars for the District Energy Plant of the Main Cycle Generating Hydrogen
from Renewables for Power, Heat, and Cold Supply.
Figure 14. Exergy Flow Bars for a Conventional DE System with Power, Heat, and Cold Supply.
The baseline DE system that provides a basis of comparison is to be upgraded to a hydrogen city.
Figure 15 puts forth the linkages between the various components in providing exergy supply and
useful applications. In the hydrogen city model, the circulation of hot and cold water and any steam is
eliminated through the circulation of hydrogen gas, which is less energy intensive. In the upper left
component of Figure 15, solar energy is utilized in the PVT-3 system in the plant. First, DC electric
power is generated in the PV modules. The heat that is absorbed by the PV coolant is utilized in
the thermal charging of the biogas reactor. The exergy flow bar for a typical PVT-3 application at a
solar insolation level of 600 W/m
2
is depicted in the form of the upper left bar in Figure 15. Here,
the Carnot cycle equivalent temperature for solar energy T
fs
is by definition the mapped equivalent
source temperature for solar energy at a given insolation level I
n
as given by Equation (17) [
64
].
This enables an exergy accounting with a more consistent boundary other than a Sun-Earth boundary.
Tf s =Tre f
1−6.96 ×10−4×In(17)
Energies 2018, 11, x FOR PEER REVIEW 16 of 32
Figure 14. Exergy Flow Bars for a Conventional DE System with Power, Heat, and Cold Supply.
The baseline DE system that provides a basis of comparison is to be upgraded to a hydrogen
city. Figure 15 puts forth the linkages between the various components in providing exergy supply
and useful applications. In the hydrogen city model, the circulation of hot and cold water and any
steam is eliminated through the circulation of hydrogen gas, which is less energy intensive. In the
upper left component of Figure 15, solar energy is utilized in the PVT-3 system in the plant. First, DC
electric power is generated in the PV modules. The heat that is absorbed by the PV coolant is utilized
in the thermal charging of the biogas reactor. The exergy flow bar for a typical PVT-3 application at
a solar insolation level of 600 W/m2 is depicted in the form of the upper left bar in Figure 15. Here,
the Carnot cycle equivalent temperature for solar energy Tfs is by definition the mapped equivalent
source temperature for solar energy at a given insolation level In as given by Equation (17) [64]. This
enables an exergy accounting with a more consistent boundary other than a Sun-Earth boundary.
4
16.9610
ref
fs
n
T
T
I
−
=−××
(17)
Figure 15. Exergy Flow Bars for the District Energy Plant of the Main Cycle Generating Hydrogen
from Renewables for Power, Heat, and Cold Supply.
Figure 15.
Exergy Flow Bars for the District Energy Plant of the Main Cycle Generating Hydrogen
from Renewables for Power, Heat, and Cold Supply.
Energies 2018,11, 1226 17 of 33
According to Equation (17), if I
n
is 600 W/m
2
, then T
fs
is 486 K. From Equation (10), the value of
ψR
for the PVT-3 will then be established as given below. This involves typical operating temperature
values of T
E
as the PV layer average temperature with cooling at 313 K, T
out
as the average supply
water temperature from PVT-3 to the biogas reactor at 303 K, and return temperature Tin at 298 K.
ψRs−PV =1−1−303 K
313 K +1−283 K
298 K
1−283 K
486 K =0.803 (18)
In addition to the PVT-3 system, the Carnot cycle equivalent temperature T
f
for wind energy at a
given mechanical energy of
ηW
, namely T
fw
, is given by Equation (19) [
42
]. Accordingly, if the wind
turbine efficiency ηWis 0.4, then Tfw will be 471.6 K.
Tf w =Tre f
1−ηW(19)
The DC power generated by the wind turbine system is combined with the PVT-3 DC output.
This power combination is further supported by the CHP system, which is driven by biogas.
Cumulative DC power is utilized mainly in the electrolysis process. The generated hydrogen drives
the central fuel cell system to generate electricity as needed. As a means of utilizing residual heat to a
maximum extent, the residual heat of the CHP system mainly goes to the biogas reactor (see arrows
from the red component in Figure 15). Part of the residual heat may be also utilized within the plant
such as in the form of DHW. In contrast, the CHP power output may be partially inverted to AC for
district infrastructure that is not shown. Based on an analysis of the district energy plant, Figure 15
provides the Exergy Flow Bars in each step and marks the respective exergy components.
4.1. Partial REMM Efficiencies and CO2Emissions Avoidance
Partial REMM Efficiencies,
ψRi−j
between any node iand jis calculated by using Equation (10)
or Equation (11) as provided in the method of Section 2. These nodes depend on the locations of the
points of exergy destruction either upstream or downstream of the major application. As an overall
summation, the averaged REMM Efficiency is found as given in Equation (20) [64]:
ψR=
u
∑
i=1
v
∑
j=1ψRi−jExi−j/ηi−j
u
∑
i=1
v
∑
j=1Exi−j/ηi−j
(20)
In Equation (20), E
xi−j
is the exergy flow between nodes iand j. The term E
xi−j
depends on
the energy flow between the same nodes iand jmultiplied by the temperature factor (1
−
T
ref
/T
out
)
and divided by the First Law energy efficiency. After applying typical operational temperatures and
efficiencies for the hydrogen economy cycle, the value of
ψR
bar varies between 0.75 and 0.85. Such a
range represents typical daily variation of the efficiency as well as the overall performance. The value
of
ψR
bar is higher than district energy systems using fossil fuels. Comparing a value of
ψR
bar at
0.80 for the hydrogen economy cycle and 0.42 for the baseline DE system, the hydrogen city model has
a potential of 32% more reduction in CO
2
emissions from the built environment stock ({[(2
−
0.42)/
(2 −0.80)] −1} ×100 = 31.67%).
4.2. Additional Exergy Benefit of the Hydrogen City
In conventional district energy systems, thermal energy in the form of heating, cooling, DHW,
and sometimes steam is distributed in the district in separate circuits, each having different flow rated
pumping requirements. In particular, water circulation that requires pumping power and a piping
network is energy/exergy intensive both in embedded and operational terms. These parasitic losses
Energies 2018,11, 1226 18 of 33
may be up to 10% of the load and even 15% during cooling. The parasitic pumping energy demand for
district energy systems and the parasitic losses will be much less in a hydrogen economy as another
important advantage. In hydrogen piping, the circuit length practically has no limit. In contrary,
depending on the amount of thermal power of different forms to be distributed, there are economical
and technical limits on the maximum piping length as given in Equation (21) for heating.
Lmax =ao+QH
1000n
×∆T
20 1.3
{QH>1000 kWH,∆T≤◦C}(21)
In Equation (21), Q
H
is the useful thermal power to be transmitted and L
max
is the farthest point
that a closed thermal circuit may feasibly reach. Here, a
o
is an empirical constant, which is generally
taken as 0.6 km. The power ndepends on the temperature, thus the exergy of the heat supplied,
as provided in Equation (22). T
ref
is 283.15 K while 333.15 K is the traditional supply temperature.
For cooling circuits, a similar formulation is applicable.
n=0.6 ×
1−Tref
Tf
1−Tref
333.15 K
0.33
{For heating}(22)
4.3. Comparison with a Circular Geothermal Option
Low-enthalpy geothermal energy sources provide another option for the hydrogen city model
if such resources exist in the vicinity (see previous Figure 4). This option emerges from the fact that
low-enthalpy geothermal energy sources have about a 30% share among different heat sources that
drive ORC systems for electricity generation [
65
]. The ORC market is rapidly increasing but their
expansion is dependent on economic incentives, subsidies, and special tariffs [
65
,
66
]. For this reason,
the ORC industry is reliant on the economic benefits of producing and selling electrical energy based on
favorable conditions without considering the existing possibilities of improving exergy efficiency and
acting upon the additional benefits of utilizing the available waste heat [67].
Exergy analysis mainly focuses on the ORC operation and design without a holistic approach
based on its connection between the energy source and demand points in the built environment.
For example, Rowshanzadeh [
68
] underlined the wide-ranging applications of ORC technology while
pointing out the need for exergy analysis. Sun et al. [
69
] investigated the suitable application conditions
of ORC-Absorption Refrigeration Cycle (ARC) and ORC-Ejector Refrigeration Cycle (ERC) and
compared results based on exergy analyses. Marini et al. [
70
] analyzed an ORC system driven by solar
energy with vacuum-tube collectors that provided electrical power for a building. The performance of
different working fluids was simulated based on the objective of minimizing exergy destruction to
conclude that ORC can be exergetically feasible given careful optimization.
Other studies that evaluated the benefits, risks, and potential disadvantages of ORC systems
from a sustainability perspective indicate that ORC units may not be ecologically sound if used in
a stand-alone format to generate only electric power [
67
]. ORC systems need to be bundled with
other renewable energy resources, systems, and energy storage units to be acceptable from an exergy
point of view [
67
]. From this perspective, exergy analyses can be used to quantify the advantages
and disadvantages of using stand-alone ORC units versus different bundling alternatives with other
renewable energy systems. Kılkı¸s et al. [
53
] indicated that the First Law of Thermodynamics is not
sufficient to evaluate ORC systems for maximum performance and environmental sustainability.
Different renewable energy systems and energy storage need to be bundled to form a hybrid system.
In this context, ground heat and geothermal energy is combined in a circular exergy flow to
support the hydrogen city model. The option in heating mode is shown in Figure 16 in which each
unit power of geothermal energy at 80
◦
C is utilized in an ORC unit, which produces 0.08 kW
E
that
is used in a GSHP. The GSHP can generate 0.32 kW
H
given that the average COP is 4.0 at an output
Energies 2018,11, 1226 19 of 33
temperature of 55
◦
C in heating mode. This is coupled with the waste heat of the ORC at the same
output temperature and directed to the district buildings in a local sub-district heating network.
If needed, the saved natural gas from the buildings’ previous on-site thermal systems is utilized in
a poly-generation unit based on fuel cells. TES that are suited to two different levels of exergy are used
to match the loads and shave-off peak loads. Electricity and additional high-exergy heat is generated
at 90
◦
C for high-temperature applications in the district. In the cooling season, this heat may be used
in absorption chillers for cold generation. From the geothermal production well to the re-injection well,
the overall performance results are obtained as the total output. The three thermal power terms that
include the later term at 35 ◦C for the preheating of DHW provides 1 kWH.
Total Output =(0.62 kWH@ 55 ◦C+0.34 kWH@ 90 ◦C+0.04 kWH@ 35 ◦C)+0.348 kWE(23)
In the case that the displaced natural gas, which was originally used in the district, is consumed
internally in the fuel cell unit, then the gross COP of the Circular Geothermal option becomes
1.348 based on 1 kW
H
of geothermal thermal power input (Equation (24) and Figure 16). COP is greater
than a value of one, since ground heat is utilized in the GSHP in addition to the geothermal energy.
COP =(1 kWH+0.348 kWE)/1 kWH=1.348 {First Law}(24)
Starting from a unit geothermal power at 80
◦
C (353 K), the Circular Geothermal option provides
0.348 kW
E
and 1 kW
H
at different supply temperatures. This output compares favorably with the
0.08 kW
E
supplied by the ORC unit without reject heat recovery (see Table 3) and 1 kW
H
at 80
◦
C
supply if the geothermal power is utilized in the district in the form of heating only. In contrast,
the above Equation (24) algebraically combines heat and power although their exergy values are quite
different. While COP is greater than one, this definition is misleading and requires the use of the
COPEx definition that considers the quality of the outputs as put forth in Equation (4).
COPEx =0.62 ×1−283 K
328 K +0.34 ×1−283 K
363 K +0.04 ×1−283 K
308 K +0.348 ×(1)
1×1−283 K
353 K +0.62
0.80 ×1−283 K
2000 K =0.59 (25)
Table 3. Comparison of the Circular Geothermal Option with Conventional Options.
System Output
Electricity Heat at 90 ◦C Heat at 55 ◦C Heat at 35 ◦C
Circular Geothermal 0.348 kWE0.34 kWH0.62 kWH0.04 kWH
DH with NG - - 0.775 kWH-
ORC 0.08 kWE- - -
In the above application of Equation (4), COP
EX
is the exergy-based COP for the entire cycle.
All systems operate at constant base load. Optional solar and wind energy systems in the district
contribute to peak loads with thermal storage. The grid is also acting for electrical energy storage at
large. The entire collection of systems operates in a cascaded form, similar to a single, large-scale heat
pump. If only an ORC unit would be used, then COP
EX
would be 0.092 and only 0.08 kW
E
would be
generated. For this reason, the bundling of renewable energy systems can be warranted.
In some countries, including Italy, New Zealand, and Turkey, geothermal reservoirs are located
in carbonate-rich rock grabens that contain calcium carbonate (CaCO
3
). This means that geothermal
wells extract CO
2
that needs to be recaptured, which is a rather expensive process. Consequently, most
of the applications release CO
2
emissions into the atmosphere, nearly at a rate of 0.5 kg CO
2
/kWh.
In extreme cases, such as in the Menderes and Gediz grabens with high-enthalpy geothermal energy
sources, the CO
2
emissions per kWh as cis 0.9 to 1.3 kg/kWh that can be much higher than coal-based
thermal plants [
71
,
72
]. If, however, a pumped binary power plant is used, then the emission factor
Energies 2018,11, 1226 20 of 33
is zero [
72
]. An effective CO
2
capture and selection of the right technology are necessary if such
reservoirs are to contribute favorably towards the aims of the Paris Agreement.
Energies 2018, 11, x FOR PEER REVIEW 20 of 32
Figure 16. Combined Heat and Power in Circular Geothermal Model in Heating Mode.
The Circular Geothermal option as shown in Figure 16 comprises of geothermal production and
re-injection wells, an ORC system, GSHP or water-source heat pumps (WSHP), a district energy
distribution and collection system, a poly-generation system, and TES at different exergy levels. Such
an option couples and mobilizes ground thermal energy and geothermal energy, including with the
aim of displacing the use of natural gas in boilers for space heating in buildings. Instead, the displaced
natural gas is used in the poly-generation plant with the generated electric power fed into the local
grid. In distributed applications, the evaporator side of the heat pumps may be coupled to PV systems
if this option is used in buildings to absorb the heat collected by the panels to improve the COP of
the GSHP. The flow rate needs to be dynamically optimized according to instantaneous solar
insolation, heat demand, and other operating conditions to maximize the total exergy output (both
power and heat) of the PVT system [61,62]. A biogas system that could be mixed with natural gas
that is saved from the boilers and also provide organic fertilizer for local farming is optional.
The multiple outputs of the Circular Geothermal option surpass the outputs of conventional
options. In this respect, Figure 16 can be contrasted to options that have singular outputs, namely a
DE system based on natural gas and the use of geothermal energy in ORC to produce only electricity.
The integration that enables the multiplicity of outputs in Figure 16 also provides for improvements
in the value of ψR. This is further valid if the Circular Geothermal option is compared to the direct
use of geothermal energy for separate heat and power production. The direct use of geothermal
energy for district heating (Figure 17) and the direct use of geothermal energy for ORC power
generation (Figure 18) are compared based on Exergy Flow Bars as provided below.
In Figure 17 for the direct use of geothermal energy for district heating, exergy destruction takes
place both upstream (εdes(1)) and downstream (εdes(2)) of energy usage. Since exergy is also destroyed
upstream, Equation (11) is used for the REMM Efficiency ψR based on an ideal Carnot cycle [42]:
323 K
1343 K 0.294
283 K
1353 K
dem
R
sup
ε
ψ
ε
−
== =
−
(26)
Figure 16. Combined Heat and Power in Circular Geothermal Model in Heating Mode.
The Circular Geothermal option as shown in Figure 16 comprises of geothermal production
and re-injection wells, an ORC system, GSHP or water-source heat pumps (WSHP), a district energy
distribution and collection system, a poly-generation system, and TES at different exergy levels.
Such an option couples and mobilizes ground thermal energy and geothermal energy, including
with the aim of displacing the use of natural gas in boilers for space heating in buildings. Instead,
the displaced natural gas is used in the poly-generation plant with the generated electric power fed
into the local grid. In distributed applications, the evaporator side of the heat pumps may be coupled
to PV systems if this option is used in buildings to absorb the heat collected by the panels to improve
the COP of the GSHP. The flow rate needs to be dynamically optimized according to instantaneous
solar insolation, heat demand, and other operating conditions to maximize the total exergy output
(both power and heat) of the PVT system [
61
,
62
]. A biogas system that could be mixed with natural
gas that is saved from the boilers and also provide organic fertilizer for local farming is optional.
The multiple outputs of the Circular Geothermal option surpass the outputs of conventional
options. In this respect, Figure 16 can be contrasted to options that have singular outputs, namely a
DE system based on natural gas and the use of geothermal energy in ORC to produce only electricity.
The integration that enables the multiplicity of outputs in Figure 16 also provides for improvements in
the value of
ψR
. This is further valid if the Circular Geothermal option is compared to the direct use
of geothermal energy for separate heat and power production. The direct use of geothermal energy
for district heating (Figure 17) and the direct use of geothermal energy for ORC power generation
(Figure 18) are compared based on Exergy Flow Bars as provided below.
In Figure 17 for the direct use of geothermal energy for district heating, exergy destruction takes
place both upstream (
εdes(1)
) and downstream (
εdes(2)
) of energy usage. Since exergy is also destroyed
upstream, Equation (11) is used for the REMM Efficiency ψRbased on an ideal Carnot cycle [42]:
ψR=εdem
εsup =1−323 K
343 K
1−283 K
353 K =0.294 (26)
Energies 2018,11, 1226 21 of 33
Energies 2018, 11, x FOR PEER REVIEW 21 of 32
Figure 17. Exergy Flow Bars for Geothermal District Heating.
Here, εdem represents the demand exergy of the district heating system application between 70
°C (343 K) and 50 °C (323 K) for buildings that are connected to the system (see also Equation (11)).
Another feature of the analysis is the ability to identify the exergetic match of the exergy supply with
the final application. The final application is comfort heating at 20 °C indoor air temperature in
buildings so that the εdem term is replaced by (1 − 283 K/293 K). In this case, ψR reduces to 0.172.
Figure 18 provides the Exergy Flow Bars for the ORC power generation case. The un-utilized
thermal output of the ORC is taken at temperatures about 60 °C (333 K) onwards. Since practically
no exergy destruction takes place upstream, Equation (10) is applied as given in Equation (27):
283 K
1333 K
1 1 0.243
283 K
1353 K
des
R
sup
ε
ψ
ε
−
=− =− =
−
(27)
The resource temperature Tf is the geothermal fluid temperature at the wellhead that is taken as
353 K also in Figures 17–19. If any fuel like biogas or natural gas is used, then this temperature is
equal to the Adiabatic Flame Temperature (AFT). As previously defined, an equivalent temperature
is put forth for solar, wind, and any other renewable energy resource without a direct Tf value.
Figure 18. Exergy Flow Bars for Direct Geothermal Power with ORC.
Figure 17. Exergy Flow Bars for Geothermal District Heating.
Here,
εdem
represents the demand exergy of the district heating system application between 70
◦
C
(343 K) and 50
◦
C (323 K) for buildings that are connected to the system (see also Equation (11)).
Another feature of the analysis is the ability to identify the exergetic match of the exergy supply
with the final application. The final application is comfort heating at 20
◦
C indoor air temperature in
buildings so that the εdem term is replaced by (1 −283 K/293 K). In this case, ψRreduces to 0.172.
Figure 18 provides the Exergy Flow Bars for the ORC power generation case. The un-utilized
thermal output of the ORC is taken at temperatures about 60
◦
C (333 K) onwards. Since practically no
exergy destruction takes place upstream, Equation (10) is applied as given in Equation (27):
ψR=1−εdes
εsup =1−1−283 K
333 K
1−283 K
353 K =0.243 (27)
The resource temperature Tfis the geothermal fluid temperature at the wellhead that is taken as
353 K also in Figures 17–19. If any fuel like biogas or natural gas is used, then this temperature is equal
to the Adiabatic Flame Temperature (AFT). As previously defined, an equivalent temperature is put
forth for solar, wind, and any other renewable energy resource without a direct Tfvalue.
Energies 2018, 11, x FOR PEER REVIEW 21 of 32
Figure 17. Exergy Flow Bars for Geothermal District Heating.
Here, εdem represents the demand exergy of the district heating system application between 70
°C (343 K) and 50 °C (323 K) for buildings that are connected to the system (see also Equation (11)).
Another feature of the analysis is the ability to identify the exergetic match of the exergy supply with
the final application. The final application is comfort heating at 20 °C indoor air temperature in
buildings so that the εdem term is replaced by (1 − 283 K/293 K). In this case, ψR reduces to 0.172.
Figure 18 provides the Exergy Flow Bars for the ORC power generation case. The un-utilized
thermal output of the ORC is taken at temperatures about 60 °C (333 K) onwards. Since practically
no exergy destruction takes place upstream, Equation (10) is applied as given in Equation (27):
283 K
1333 K
1 1 0.243
283 K
1353 K
des
R
sup
ε
ψ
ε
−
=− =− =
−
(27)
The resource temperature Tf is the geothermal fluid temperature at the wellhead that is taken as
353 K also in Figures 17–19. If any fuel like biogas or natural gas is used, then this temperature is
equal to the Adiabatic Flame Temperature (AFT). As previously defined, an equivalent temperature
is put forth for solar, wind, and any other renewable energy resource without a direct Tf value.
Figure 18. Exergy Flow Bars for Direct Geothermal Power with ORC.
Figure 18. Exergy Flow Bars for Direct Geothermal Power with ORC.
Energies 2018,11, 1226 22 of 33
The primary characteristic of the Circular Geothermal option is that it represents an integrated,
compound power and heat system at large. The option may be also applied to single buildings and
scaled up to large district energy systems with possible integration with a hydrogen economy cycle.
While most suitable for 4GDE, the Circular Geothermal option can be further applicable to district
cooling applications. In this case, cold storage and absorption/adsorption units may be used.
The two options in Figures 17 and 18 are further compared based on the C
R
indicator as defined
in Equations (15) and (16). From Equation (15), the values of C
R
for geothermal district heating and
power-only ORC options are 0.19 and 0.019, respectively. Here, the value of the net energy efficiency
after parasitic losses ηIis taken as 0.65 for district heating while it is taken as 0.08 for ORC.
The approach of C
R
further reveals advantages when applied to the Circular Geothermal option
for which the Exergy Flow Bars are provided below in Figure 19. Other minor exergy destructions in
heating are neglected. The respective values based on the application of Equation (10) are:
ψR=1−εdes
εsup =1−1−283 K
293 K
1−283 K
353 K =0.827 (28)
and from Equation (16):
CR=COP ×ψR=1.348 ×0.827 =1.114 (29)
Energies 2018, 11, x FOR PEER REVIEW 22 of 32
The primary characteristic of the Circular Geothermal option is that it represents an integrated,
compound power and heat system at large. The option may be also applied to single buildings and
scaled up to large district energy systems with possible integration with a hydrogen economy cycle.
While most suitable for 4GDE, the Circular Geothermal option can be further applicable to district
cooling applications. In this case, cold storage and absorption/adsorption units may be used.
The two options in Figures 17 and 18 are further compared based on the CR indicator as defined
in Equations (15) and (16). From Equation (15), the values of CR for geothermal district heating and
power-only ORC options are 0.19 and 0.019, respectively. Here, the value of the net energy efficiency
after parasitic losses ηI is taken as 0.65 for district heating while it is taken as 0.08 for ORC.
The approach of CR further reveals advantages when applied to the Circular Geothermal option
for which the Exergy Flow Bars are provided below in Figure 19. Other minor exergy destructions in
heating are neglected. The respective values based on the application of Equation (10) are:
283 K
1293 K
1 1 0.827
283 K
1353 K
des
R
sup
ε
ψ
ε
−
=− =− =
−
(28)
and from Equation (16):
1.348 0.827 1.114
RR
CCOP
ψ
=×= × = (29)
Figure 19. Exergy Flow Bars for the Circular Geothermal Option.
After comparing the above results, the CR values are further used to evaluate a CO2 reduction
potential ratio R according to REMM [38]. Equation (30) compares the geothermal district heating
only case with CR = 0.19 and the Circular Geothermal option with CR = 1.114. A similar comparison
with the power-only ORC case with CR = 0.09 indicates 2.15 times higher CO2 reduction potential. The
degree of improvement increases with the geothermal reservoir temperature and applications.
()
()
220.19 2.04
2 2 1.114
Rdistrict heati ng
Rgeotherm
C
RC
−−
===
−−
(30)
4.4. Comparison of Options for the Hydrogen City Model
Table 4 compares each option based on ψR values and the CO2 avoidance capacity based on unit
CO2 emissions, including an avoidable CO2 emissions impact due to exergy mismatches in the energy
Figure 19. Exergy Flow Bars for the Circular Geothermal Option.
After comparing the above results, the C
R
values are further used to evaluate a CO
2
reduction
potential ratio Raccording to REMM [
38
]. Equation (30) compares the geothermal district heating
only case with C
R
= 0.19 and the Circular Geothermal option with C
R
= 1.114. A similar comparison
with the power-only ORC case with C
R
= 0.09 indicates 2.15 times higher CO
2
reduction potential.
The degree of improvement increases with the geothermal reservoir temperature and applications.
R=(2−CR)district heating
(2−CR)geotherm
=2−0.19
2−1.114 =2.04 (30)
4.4. Comparison of Options for the Hydrogen City Model
Table 4compares each option based on
ψR
values and the CO
2
avoidance capacity based on unit
CO
2
emissions, including an avoidable CO
2
emissions impact due to exergy mismatches in the energy
system according to REMM [
34
]. The reference values represent separate heat and power production.
Energies 2018,11, 1226 23 of 33
The comparisons in Table 4indicate that the hydrogen city model with renewables has important
advantages over the reference and conventional DE systems. The inclusion of a Circular Geothermal
option can further improve the values of the proposed model. For example, the hydrogen city model
with all renewable energy and a contribution from the geothermal energy option has a
ψR
value of
0.83 as given in Table 4, which takes place above the targeted value of 0.80.
Such improvements are further represented in the EDR values based on Equation (13) so that the
exergetic advantages of these options are compared to reductions in CO
2
emissions over the reference
case. In particular, while the reference case has no reduction (EDR = 0), the evaluated options represent
incremental or significant improvements in EDR values when compared with CO
2base
. The greatest
improvements take place for the hydrogen city model with all renewable energy sources involving the
inclusion of a Circular Geothermal option. The EDR values of these options are calculated as 0.91 and
0.92, respectively, which indicates greater CO
2
savings over CO
2base
. These EDR values can qualify the
district as a nZCD based on the net-zero definitions that were provided in Table 1. In contrast, none of
the options have EDR values equal to 1 for a strictly NZCD status although these values have closely
approached to 1 already with the respective values in Table 4.
Table 4. Overall Comparison of Conventional and Proposed District Energy System Options.
Compared Options ψRCO2per kWh (kg CO2/kWh) EDR (Equation (13))
Reference Values 0.20 0.63 (CO2base ) 0.00
Basic DE System without Steam Generation a0.25 0.62 0.02
DE System with Steam Generation b0.42 0.27 0.58
Hydrogen City Model (All Renewables) 0.80 0.06 0.91
With Circular Geothermal Option (All Renewables) 0.83 0.05 0.92
aThe energy source is natural gas; bSupported with renewable energy.
5. Discussions on an Application of the Hydrogen City Model to a New Settlement
About half of the carbon budget that remains to have a chance of limiting global warming to at
most 1.5
◦
C by the end of this century could be consumed with the emissions impact from new urban
development alone unless prompt action is taken to avoid lock-in to incumbent technologies [
73
].
One implication of the present research work requires that new settlements, most preferably at
brownfield sites to reduce land use changes, are equipped with an energy system that maximizes the
rational use of renewable energy sources based on exergy matches between the supply and demand.
For this reason, an application of the hydrogen city model was considered for a new district
development in the province of Ankara, Turkey (40.13
◦
N, 33.00
◦
E) that has a population target
of 200,000 inhabitants. Degree days for seasonal heating (less than or equal to 15
◦
C) and cooling
(greater than 22
◦
C) are 2493 and 289 degree-days, respectively [
74
]. Although Ankara appears to
have dominance of the heating season, practices in other recent urban development projects that are
in operation indicate considerable increases in comfort cooling loads. Figure 20 shows the annual
variation of monthly average outdoor dry-bulb temperatures [
75
]. The local climate is dry and latent
cooling loads may be negligible except in gathering places. Design outdoor dry-bulb temperatures are
−12 ◦C for winter and 35 ◦C for summer with a cooling season that is less than three months.
The new district development is expected to consist of residential areas, social service buildings,
offices, shopping plazas, and mixed mode (office, residential, commercial) buildings. Consequently,
low-rise residential buildings, single story homes, and higher-rise office buildings will be common.
The total required floor area of all buildings, AFin the new settlement is obtained by Equation (31):
AF=q·Py(31)
Here, Pis the population and qis the floor area per person averaged for the settlement based on
different building functions, typology, and their relative mix in the settlement plan.
Energies 2018,11, 1226 24 of 33
Energies 2018, 11, x FOR PEER REVIEW 24 of 32
Figure 20. Annual Variation of Monthly Average Outdoor Dry-Bulb Temperatures in Ankara.
In Equation 31, the multiplier q generally decreases with P. Conversely, density per given floor
area increases with higher population [76]. This relation is represented by the power of P, namely y
that depends on climate, population, culture, average affluence, geographic location, population of
neighboring built environments, and the degree of daily or weekly commuting, among other factors.
Therefore, y is a number smaller than 1 (0.8 in this case) while q is taken to be 10 m
2
per person for
the purposes of the case study. The multiplier q also varies based on the development of the region
or country and building function(s). About 65% of the world population has less than 20 m
2
per
person [77]. Moreover, the average number of floors in the buildings for minimum CO
2
emissions
responsibility is optimized. The optimum number of floors in the buildings with conventional walls
(no glass façade) that are partially integrated with PVT-3 panels on south facing walls was
determined to be 25 floors for the present case study based on the optimization approach as
developed in related research [76]. The evaluations are used to find the number of buildings and to
determine energy loads using the tool available for the Building Energy Performance of Turkey,
namely BEP-TR [78].
Table 5 provides typical data for the hydrogen city model at the building level for a building
with total area of 250 m
2
based on loads and equipment differentiated for the heating and cooling
seasons. Peak thermal and power loads of the city were predicted for nominally well-insulated
hydrogen homes. The two TES options and cold storage are further integrated into the building.
Electrical loads of the typical building increase in the summer months since the COP values of heat
pumps decrease in the cooling season. More power is required even if the cooling capacity is the same
as the heating capacity. In addition, cooling appliances require more power in the summer months.
However, this increase in individual power demands can level-off at the district scale since the
diversity factor is greater in the summer months with inhabitants locating to coastal areas. Such a
load balancing factor that is typical for the context of Ankara makes it possible to size the system with
respect to the winter season without any redundancy for the cooling season. The same holds true for
the fuel cells, which are also sized for the heating season. The only seasonal redundancy will be the
absorption chillers, which are not used in the heating season. However, this capacity may be directed
to the industry for such applications as cold warehouses. Moreover, the PVT-3 system produces more
power and heat in the summer months due to the increase in solar insolation. This helps to overcome
the increase in the power demand in the summer months and at the same time, charges the desiccant
system. Absorption cooling is activated by hydrogen heat upon demand. The heat output of the PVT-
3 in the summer goes to DHW demands. Therefore, load amounts and type variations in the summer
months are largely compensated at the building scale. This enables the central power system to
operate with the same overall demand loads year-round.
Figure 20. Annual Variation of Monthly Average Outdoor Dry-Bulb Temperatures in Ankara.
In Equation (31), the multiplier qgenerally decreases with P. Conversely, density per given floor
area increases with higher population [
76
]. This relation is represented by the power of P, namely y
that depends on climate, population, culture, average affluence, geographic location, population of
neighboring built environments, and the degree of daily or weekly commuting, among other factors.
Therefore, yis a number smaller than 1 (0.8 in this case) while qis taken to be 10 m
2
per person
for the purposes of the case study. The multiplier qalso varies based on the development of the
region or country and building function(s). About 65% of the world population has less than 20 m
2
per person [
77
]. Moreover, the average number of floors in the buildings for minimum CO
2
emissions
responsibility is optimized. The optimum number of floors in the buildings with conventional walls
(no glass façade) that are partially integrated with PVT-3 panels on south facing walls was determined
to be 25 floors for the present case study based on the optimization approach as developed in related
research [
76
]. The evaluations are used to find the number of buildings and to determine energy loads
using the tool available for the Building Energy Performance of Turkey, namely BEP-TR [78].
Table 5provides typical data for the hydrogen city model at the building level for a building with
total area of 250 m
2
based on loads and equipment differentiated for the heating and cooling seasons.
Peak thermal and power loads of the city were predicted for nominally well-insulated hydrogen homes.
The two TES options and cold storage are further integrated into the building. Electrical loads of the
typical building increase in the summer months since the COP values of heat pumps decrease in the
cooling season. More power is required even if the cooling capacity is the same as the heating capacity.
In addition, cooling appliances require more power in the summer months. However, this increase in
individual power demands can level-off at the district scale since the diversity factor is greater in the
summer months with inhabitants locating to coastal areas. Such a load balancing factor that is typical
for the context of Ankara makes it possible to size the system with respect to the winter season without
any redundancy for the cooling season. The same holds true for the fuel cells, which are also sized for
the heating season. The only seasonal redundancy will be the absorption chillers, which are not used
in the heating season. However, this capacity may be directed to the industry for such applications as
cold warehouses. Moreover, the PVT-3 system produces more power and heat in the summer months
due to the increase in solar insolation. This helps to overcome the increase in the power demand
in the summer months and at the same time, charges the desiccant system. Absorption cooling is
activated by hydrogen heat upon demand. The heat output of the PVT-3 in the summer goes to DHW
demands. Therefore, load amounts and type variations in the summer months are largely compensated
at the building scale. This enables the central power system to operate with the same overall demand
loads year-round.
Energies 2018,11, 1226 25 of 33
Table 5. Typical Building Level Data for the Hydrogen City Model.
Loads and Equipment Demand or Capacity
Heating Season Cooling Season
Loads
Electrical Load 7 kWE10 kWE
Heating Load 16 kWH2 kWH(DHW only)
Cooling Load (Sensible, Latent) - 5 kWC, 3 kWC
Equipment
Fuel Cell (Building Level) 6 kWE, 8 kWHat 40 ◦C 6 kWE, 8 kWHat 40 ◦C
TES 1, TES 2, Cold Storage a10 kWh, 7 kWh 4 kWh
Radiant Floor Panels 2 kWH(Heating) 1.5 kWC(Cooling)
Radiant Wall b2 kWH5.5 kWC
GSHP c2 kWH2 kWC
PVT-3 (Power, Thermal at 40 ◦C) 1 kWE2 kWH4 kWE, 4kWH
Absorption Cooling System - 5 kWC(Hydrogen activated)
Desiccant System c- 1 kWC(Latent)
a
Enables the storage of energy that is not concurrent with the exergy demands.
b
Optimized for low-exergy circuit
temperature for maximum exergetic efficiency [
79
].
c
The cooling component of the equipment satisfies the latent
cooling loads.
At the district level, Table 6tabulates the power and thermal loads for the main cycle of the
hydrogen city model. The power and thermal loads include those for mass transport, city lighting,
and miscellaneous service loads. The base loads are taken to be about 50% of the peak loads, including
diversity factors, while peak loads are to be satisfied by various energy storage. Overall, the district
is evaluated to have electrical loads of 10,000 kW
E
, heating loads of 20,000 kW
H
and cooling loads
for sensible and latent cooling needs at 14,000 kW
C
and 10,000 kW
C
, respectively. As implied by the
loads that can be provided by equipment and energy systems in the hydrogen city, including hydrogen
storage, the district produces sufficient energy at the required exergy levels to qualify in becoming a
nearly self-sufficient district towards reaching a possible NZEXD status. The region is nominally rich
in low-exergy geothermal energy resources with wellhead temperatures in the range of 80
◦
C to 70
◦
C,
which can also permit integrating a Circular Geothermal option.
Table 6. Power and Thermal Loads for the District Infrastructure.
Loads and Equipment Demand or Capacity
Loads
Electrical Loads 10,000 kWE
Heating Loads 20,000 kWH
Cooling Loads (Sensible, Latent) 14,000 kWC, 10,000 kWC
Equipment and Energy Systems
Fuel Cells (Central Plant) a,b 8000 kWE, 12,000 kWHat 40 ◦C
Hydrogen Storage c50,000 kWh
PVT-3 (Power, Thermal at 40 ◦C) 400 kWE, 700 kWH
Cogeneration (Hydrogen Enriched Biogas) d
1500 kW
E
, 2500 kW
H
or 1000 kW
C
Biogas Production Plant 20,000 kW-equivalent fuel
Wind Turbines e20,000 kWE
a
Thermal energy is not distributed from the central plant to the district. Hydrogen distribution has lower pumping
needs.
b
The thermal energy is used for local demands, e.g., greenhouses and agricultural drying.
c
Hydrogen is
mainly used for hydrogen-enriched biogas for the district gas supply.
d
The 1000 kW
c
includes an absorption cycle.
eSurplus from the electrical load is utilized for hydrogen production.
Energies 2018,11, 1226 26 of 33
6. Conclusions
The rapidly emerging penetration of renewable energy systems makes it more feasible to
establish a hydrogen economy, which needs to be optimized by both the First and Second Laws of
Thermodynamics. In this research work, third-generation PVT modules, double-blade wind turbines,
high-efficiency fuel cells at the central plant and building levels, as well as cogeneration based
on hydrogen-enriched biogas, are gathered around a district level hydrogen economy approach.
The renewable energy-based hydrogen city model was found to provide an end result that is
particularly rewarding for DE systems. The analysis of the renewable energy-based hydrogen city
model with PVT-3 and high-efficiency wind energy indicated the possibility of obtaining values
of the REMM efficiency reaching 0.80 and savings in CO
2
emission impacts when compared to a
conventional DE system based on lower primary energy spending. Values of the parameter
ψR
at
or beyond 0.80 were targeted as a criterion for a net or nearly net-zero exergy status for the district.
In addition to exergy benefits, a more closed-loop water cycle is proposed to provide a way in which
hydrogen economy can support a more self-sustaining urban water cycle in an energy-water nexus.
The two cycles of a hydrogen city model and the Circular Geothermal option are found to provide
a useful approach in contributing to the success of future hydrogen economy cities as well as a more
rational utilization and storage of renewable energy sources. Dedicated metrics to evaluate related
improvements, including an assessment of COP values for heat pumps based on exergy, namely COP
EX
were also integrated into the analyses. The dedicated metrics have enabled an effective comparison
with conventional DE systems, including natural gas based DE systems as well as those in which there
may be only district heating or only electricity generation. These metrics underlined the importance of
utilizing renewable energy resources in combined energy systems.
The results for the hydrogen city model and Circular Geothermal option have addressed a gap
in the literature for analyzing renewable energy oriented hydrogen economy solutions for urban
energy systems. The two hydrogen cycles at the district and building levels that are analyzed from
an exergy framework can provide districts with new opportunities to reach near-zero exergy targets.
The comparative analyses have shown that this target is achievable and the level of match between
the supply and demand of exergy can be effectively increased in comparison to the basic DE system.
The effective utilization of renewable energy sources also increased the emissions difference with the
reference values, including avoidable CO
2
emissions. Indeed, utilization of renewables in hydrogen
economy can have a key role in the success of urban energy systems. For example, the Athlete’s
Village of the Tokyo 2020 Olympics will be the first hydrogen city example relying on renewables [
80
].
The possible case study of a 200,000 inhabitant hydrogen city in a new settlement in Ankara, Turkey
as analyzed in this research work is important to diversify the options that may be adapted to reach
net-zero targets in the urban context with an outlook for the more rational use of exergy. The integration
of hydrogen economy principles, considerations of an energy-water nexus and net-zero targets can
provide an effective option for the future of urban settlements and districts.
Author Contributions:
B.K. conceived the Hydrogen City model that provided the basis for the research work and
also designed the experiments for the PVT-3 system. ¸S.K. contributed to analyses in formulations and comparisons
of the energy system options based on REMM metrics and net-zero targets.
Acknowledgments:
The manuscript is a revised and expanded version of an original scientific contribution that
was presented at the 12th Conference on Sustainable Development of Energy, Water and Environment Systems
(SDEWES) held during 4–8 October 2017 in Dubrovnik, Croatia entitled “Hydrogen Economy-Based Net-Zero
Exergy Cities of the Future with Water-Energy Nexus.” A case study of a new settlement is added to the present
version among other original elaborations. Funding has not been received to undertake the research work.
Conflicts of Interest: The authors declare no conflict of interest.
Nomenclature
AF Total floor area of buildings to be occupied by population P, m2
a,bHeat pump COP versus heat output temperature function constants
Energies 2018,11, 1226 27 of 33
aoConstant of Lmax
CPower to heat ratio, dimensionless
cEmissions ratio (Factor), kg CO2/kWh
CO2Compound carbon dioxide emissions, kg CO2
COP Coefficient of Performance (First Law)
COPEX Exergy-Based Coefficient of Performance (Second Law)
CRComposite Rationality Indicator
EElectrical energy (load), kWh
EDR Ratio of Emissions Difference to the base case CO2emissions, dimensionless
ExExergy, kW or kWh
IAnnual solar insolation on horizontal surface, kWh/m2/year
InNet solar insulation intensity reaching perpendicular to the solar PV surface
Lmax Maximum length of the district circuit (one way)
PER Primary energy ratio
PEXR Exergy-based primary energy ratio
Q, QHThermal energy (load), kWh
QFloor area per person, m2/person
RCO2reduction potential ratio
TTemperature, K
Greek Symbols
ηEX Second Law Efficiency, dimensionless
ηTPower transmission and distribution efficiency
ψRRational exergy management efficiency, rationality ratio
εUnit exergy, kW/kW
ηIFirst Law Efficiency
∆Difference
Subscripts
bBiogas
base Base
cCold
dem Demand
des Destroyed
EElectric
fResource temperature, or Adiabatic Flame Temperature (Real or virtual), K
gGenerator
HThermal (Heat)
hHydrogen
i,jNode indexes for partial REMM efficiencies between two nodal connections
in,out Inlet and outlet connections of a hydronic circuit
l,mLocal power plant, distant power plant, respectively
min,max Minimum, maximum
opt Optimum
RRational or Reservoir
ref Reference
ret Return
sSolar
sup Supply
ref Reference
TPower transmission
wWind
X,EX Exergy, exergetic
Energies 2018,11, 1226 28 of 33
Superscripts
nMaximum district circuit length coefficient
yPopulation coefficient
Chemical Symbols
CaCO3Calcium carbonate
CH3OH Methanol
CO2Carbon dioxide
H2Hydrogen
H2O2Hydrogen peroxide
Abbreviations
4GDE Fourth generation district energy system
ABS Absorption chiller
AC Alternating current
AG Air gap
AFT Adiabatic flame temperature, K
ARC Absorption Refrigeration Cycle
CR Composite Rationality Index
CHP Combined heat and power
CWT Cold water tank
DC Direct current
DE District energy
DHC District heating and cooling
DHW Domestic hot water
E1Power generated by PV layer of the PVT-3 module
E2Power generated by the TEG elements in the PVT-3
module
EER Energy Efficiency Ratio
ERC Ejector Refrigeration Cycle
F Frame of the PVT-3 module
FC Fuel cell, Fan-coil
GC Glass cover
GSHP Ground-source heat pump
HHV Higher heating value
HP Heat pipe
HWT Hot water tank
IN Insulator
LowEx Low-exergy
LVDC Low-voltage DC power
NPEB Net Positive Energy Building
NPEXB Net Positive Exergy Building
NS Heat-conducting nano sheet
NZCB Net-Zero CO2Building
nZCB Near Zero CO2Building
NZCD Net-Zero CO2(Emissions) District
nZCD Near Zero CO2District
NZEXB Net-Zero Exergy Building
nZEXB Nearly Zero Exergy Building
NZEXD Net-Zero Exergy District