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Citation for the original published paper (version of record):
Gong, M., Wall, G. (2014)
Life Cycle Exergy Analysis of Solar Energy Systems.
Journal of Fundamentals of Renewable Energy and Applications, 5(1): 1000146
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Fundamentals of Renewable
Energy and Applications
Gong and Wall, J Fundam Renewable Energy Appl 2014, 5:1
http://dx.doi.org/10.4172/2090-4541.1000146
Research Article
Open Access
Life Cycle Exergy Analysis of Solar Energy Systems
Mei Gong1 and Göran Wall2*
1
2
School of Business and Engineering, Halmstad University, PO Box 823, SE-30118 Halmstad, Sweden
Oxbo gard, SE-43892 Härryda, Sweden
Abstract
Exergy concepts and exergy based methods are applied to energy systems to evaluate their level of sustainability.
Life Cycle Exergy Analysis (LCEA) is a method that combines LCA with exergy, and it is applied to solar energy systems.
It offers an excellent visualization of the exergy flows involved over the complete life cycle of a product or service. The
energy and exergy used in production, operation and destruction must be paid back during life time in order to be
sustainable. The exergy of the material that is being engaged by the system will turn up as a product and available for
recycling in the destruction stage. LCEA shows that solar thermal plants have much longer exergy payback time than
energy payback time, 15.4 and 3.5 years respectively. Energy based analysis may lead to false assumptions in the
evaluation of the sustainability of renewable energy systems. This concludes that LCEA is an effective tool for the design
and evaluation of solar energy systems in order to be more sustainable.
Keywords: Exergy analysis; Sustainable development; Life cycle
analysis
Introduction
With a dependence on finite natural resources, increasing
energy demands and increasing environmental problems, the use of
renewable resources becomes even more important. This is one part
of a sustainable development. The ultimate source of most of our
renewable energy supplies is the sun. The world’s primary energy use
is easily exceeded by the solar energy by a factor of about 10000 [1].
The renewable energies except geothermal and tidal energy depend on
solar radiation. Thus, it is essential for future energy systems to rely
on energy from the sun and presently, the use of direct solar energy is
rapidly increasing. Solar energy systems can be classified into directsolar systems and indirect-solar systems [2]. Here solar energy systems
refer to direct-solar systems.
Exergy is a useful concept in the work towards sustainable
development [3]. Exergy accounting of the use of energy and material
resources provides important knowledge on how effective and balanced
a society is regarding conserving nature’s capital. This knowledge can
identify areas in which technical and other improvements should be
undertaken, and indicate the priorities, which should be assigned to
conservation measures, efficiency improvements and optimizations.
Hepbasli [4] offers a careful review of exergy analysis applied to
renewable energy resources for a sustainable future. Thus, the exergy
concept and exergy tools are essential to the creation of a new
engineering toolbox or paradigm towards sustainable development.
In the literature, a number of studies applies energy and exergy
analyses to the whole or part of solar energy systems, e.g. solar
heater [5,6], solar power plant [7], solar photovoltaic (PV) system [8]
combined solar photovoltaic and thermal (PV/T) plants [9,10]. Exergy
of solar radiation is often regarded as input into the energy systems of
these studies, and the overall energy efficiency is about 25% for solar
power plant, 14-15% for PV/T system and less than 12% for PV system.
Life cycle analysis/assessment (LCA) is a tool used to evaluate the
total environmental impact and total energy resource use of a product
or service during its complete lifetime or from cradle to grave. It covers
three steps – construction, operation and destruction. In previous
studies LCA has been applied to solar energy system, e.g. PV system
[11-14], solar thermal power system [15], solar heating system [16]
and PV/T system [17,18]. However, none of these studies made use
J Fundam Renewable Energy Appl
ISSN: 2090-4541 JFRA, an open access journal
of exergy analysis. Some of the papers [11,13,18] estimated material
recycling with present technology and future developed technology as
well as using recycled material that will reduce the need for energy in
the construction stage.
Two different methods combining LCA and exergy analyses have
been proposed, e.g. Exergetic Life Cycle Analysis (ELCA) [19] or Life
Cycle Exergy Analysis (LCEA) [20]. Cumulative Exergy Consumption
(CExC) was introduced by Szargut et al. [21] to calculate the sum of all
exergy input in all steps of a production process. Ayres with co-authors
[22] stated the advantage of using exergy in the context of LCA, and
concluded that exergy is appropriate for general statistical use, both
as a measure of resource stocks and flows and as a measure of waste
emissions and potential for causing environmental harm, which was
also indicated by Wall in 1977 [3]. However, no detailed comparison
has been made with existing methods, like the LCA. ELCA [19,23]
introduced by Cornelissen is based on the framework of LCA with
exergy applied to the inventory analysis and the impact assessment.
Finnveden and Östlund [24] used exergy consumption as an indicator in
LCA. Several metal ores and other natural resources were analyzed with
system boundaries compatible with LCA. Since the cumulative exergy
consumption index is just the sum of the chemical exergy contents of
all original input flows, Valero [25] has introduced “exergetic cost for
replacement of material” into the analysis. Lombardi (2001) performed
an ELCA and a classical environmental LCA for a carbon dioxide low
emission power cycle in which exergy was considered to be an indicator
of resource depletion.
Life cycle exergy analysis (LCEA) includes sustainability aspects
[20,26]. LCEA uses the same framework as LCA, but makes an
important distinction between renewable and non-renewable resources.
In LCEA, renewable resources as solar energy are excluded in the cost
*Corresponding author: Göran Wall, Oxbo gard, SE-43892 Härryda, Sweden, Tel:
46 35 16 71 00; E-mail: [email protected]
Received November 10, 2014; Accepted December 09, 2014; Published
December 16, 2014
Citation: Gong M, Wall G (2014) Life Cycle Exergy Analysis of Solar Energy Systems.
J Fundam Renewable Energy Appl 5: 146. doi: 10.4172/20904541.1000146
Copyright: © 2014 Gong M, et al. This is an open-access article distributed under
the terms of the Creative Commons Attribution License, which permits unrestricted
use, distribution, and reproduction in any medium, provided the original author and
source are credited.
Volume 5 • Issue 1 • 1000146
Citation: Gong M, Wall G (2014) Life Cycle Exergy Analysis of Solar Energy Systems. J Fundam Renewable Energy Appl 5: 146. doi:
10.4172/20904541.1000146
Page 2 of 8
of calculation due to being free of charge, and/or otherwise wasted.
LCEA has also been applied to industrial processes [27,28] and to wind
power systems [29].
Thus, LCEA is a powerful tool in the design of sustainable systems,
especially in the design of renewable energy systems. The application of
LCEA to different solar energy systems offers an excellent visualization
of the exergy flows involved during the life time of the system. The
analyzed plants are net producers of exergy, since the exergy consumed
can be paid back during their life time, however, in a varying degree.
The exergy of material that is part of the system in various components
during its operation will turn up as a product in the destruction phase
and is depicted in the LCEA diagram. The recycling of this material will
considerably reduce the payback time for future energy systems.
In this paper we present the first application of LCEA to solar
systems together with a proposal of how to evaluate the recycled
material in the LCEA method. The aim is to: (1) increase the awareness
of and encourage the use of LCEA, (2) show the advantages of exergy
instead of energy in systems analysis, (3) apply LCEA to solar energy
systems, and (4) introduce a new way to evaluate recycled material in
LCEA.
Method
Concept of Exergy
The exergy concept originates from works of Carnot [30], Gibbs
[31], Rant [32] and Tribus [33] and the history is well documented [34].
Exergy of a system is [3,35]
E =U + PV
− T0 S − ∑ µi 0 ni
0
i
(1)
where U, V, S, and ni denote extensive parameters of the system
(energy, volume, entropy, and the number of moles of different
chemical materials i) and P0, T0, and μi0 are intensive parameters of
the environment (pressure, temperature, and chemical potential).
Analogously, the exergy of a flow can be written as:
E =H − T0 S − ∑ µi 0 ni
i
(2)
where H is the enthalpy.
The exergy of material substances can be calculated by [3]
=
E
∑ (µ
i
i
− µ0 )ni
(3)
where μi is the generalized chemical potential of substance i in its
present state.
The exergy of solar radiation is related to the exergy power per unit
area of black body radiation e, which is [36]
 1  T 4 4 T 
(4)
e=
u 1 +  0  − 0 
 3  T  3 T 
Where u is energy power emission per unit area which can be
calculated according to Stefan-Boltzmann law, T is taken to equal the
solar radiation temperature 6000K.
All real processes involve the conversion and consumption of
exergy, thus high efficiency is of utmost importance. This implies that
the exergy use is well managed and that effective tools are applied.
Presently, an excellent online web tool for calculating exergy of chemical
substance is also available [36,37].
Energy is always in balance, however, for real processes exergy is
never in balance due to irreversibilities, i.e. exergy destruction that is
related to the entropy production by
J Fundam Renewable Energy Appl
ISSN: 2090-4541 JFRA, an open access journal
tot
Eintot − Eout
=T0 ∆S tot =∑ (E in − E out ) j 〉 0
j
(5)
where ΔStot is the total entropy increased,
Eintot is the total input energy
tot
Eout
is the total output energy and (Ein-Eout)j is the exergy destruction
in process j.
The exergy loss, i.e. exergy destruction and exergy waste, indicates
possible process improvements. In general “tackle the biggest loss
first” approach is not always appropriate since every part of the system
depends on each other, so that an improvement in one part may cause
increased losses in other parts. As such, the total losses in the modified
process may in fact be equal or even larger, than in the original process
configuration. Also, the use of renewable and non-renewable resources,
as well as recycled resources must be considered. Therefore, the problem
needs a more complete and careful approach.
Exergy factor and the reference state
Exergy factor is defined as the ratio of exergy to energy, and is
sometimes referred to as quality factor, exergy coefficient and exergy
quality factor.
The exergy factor for electricity and solar radiation is 1 and 0.93
respectively according to Eq. 4 with the temperature of the sun and the
earth 6000K and 300K respectively, more detailed calculation can be
found in [38].
When the heat capacity is independent of temperature and
temperature decrease from T to T0, the exergy factor for a heat flow can
be calculated by [38]:
T
E
T
= 1 − 0 ln
Q
T − T0 T0
(6)
The exergy factor depends on the environment or reference state.
The exergy reference state is carefully analyses by Gaudreau [39],
and Dincer and Rosen [40] offer a summary of models of referenceenvironment state. The reference-substance model [40] is applied in
this study, and the local environment temperature is used as reference
temperature.
Exergy analysis
In engineering, Sankey diagrams are often used to describe the
energy or exergy flows through a process. The energy/exergy efficiency
is defined as the ratio of output energy/exergy to input energy/exergy
of the systems.
Figure 1 shows a medium-temperature solar thermal power plant
with solar collector, heat exchanger, turbine, condenser, regenerator
and pump, its main components and roughly the main energy and
exergy flows of the plant. This diagram shows where the main energy
and exergy losses occur in the process, and also whether exergy is
destroyed from irreversibility or whether it is emitted as waste, often
waste heat, to the environment. In the energy flow diagram energy is
always conserved, the waste heat carries the largest amount of energy
into the environment, far more than is extracted as work in the turbine.
However, in the exergy flow diagram the temperature of the waste heat
is close to ambient so the exergy becomes much less than the energy.
In the solar collectors the energy efficiency is assumed to be about
55%. This depends on type of collector, average temperature difference
between absorber and environment, and saturation temperature in
the boiler [41]. The exergy efficiency of the concentrated medium-
Volume 5 • Issue 1 • 1000146
Citation: Gong M, Wall G (2014) Life Cycle Exergy Analysis of Solar Energy Systems. J Fundam Renewable Energy Appl 5: 146. doi:
10.4172/20904541.1000146
Page 3 of 8
solar thermal and hybrid PV/thermal as an example. The produced heat
is used for hot water and/or space heating. In the solar PV systems, the
energy and exergy efficiencies are almost the same or 15% and 16%
since for solar radiation exergy is 93% of the energy [38] and for the
outflow of electricity both energy and exergy is identical. In a PV cell
solar radiation is directly transferred to electricity by means of photons
of light exciting electrons into a higher energy state to act as carriers
of an electric current. The low energy efficiency of a PV cell is partly
due to physical limitations in the photo-electric conversion, and the
energy losses are mainly due to this that instead becomes heat radiation
to the environment. A solar thermal converter has an energy efficiency
of about 75%, however the exergy efficiency is very low or 10% because
the temperature of the heat is close to ambient and thus of low exergy. In
the case of hybrid PV/thermal systems, the energy and exergy efficiency
is about 66% and 16% respectively.
Solar radiation
Steam
turbine
Solar
collectors
Pump
Power
Condenser
Energy flow
Electricity
Motion
Heat loss
Hot steam
Solar energy
Waste heat
Reflection and heat loss
Electricity
Motion
Exergy flow
Hot steam
Waste heat Heat loss
Solar exergy
Reflection and heat loss
Figure 1: Energy and exergy flow of a simple solar thermal power plant.
Electricity
Solar PV
Sunlight
Electricity
Sunlight
ηen = 15%
Solar thermal
Sunlight
Heat
ηen = 75%
ηex = 16%
Sunlight
ηex = 10%
Electricity
Hybrid photoSunlight
voltaic/thermal
Heat
ηen = 66%
Heat
Electricity
Sunlight
Heat
ηex = 16%
In Figure 1, a solar thermal power plant, the exergy efficiency
is about the same as for a PV plant (Figure 2). This can be better
understood from the exergy diagrams. The main exergy loss in the
thermal power plant occurs in the conversion of solar radiation into
high temperature heat for the turbine. The total exergy efficiency
depends on the quality of heat, i.e. the temperature and pressure of the
heat. The exergy efficiency would be higher in a concentrating power
plant (CSP) due to higher temperatures and pressures.
Energy/Exergy payback time
Energy/exergy pay-back time means time to recover primary
energy/exergy use throughout its life cycle by the energy/exergy of the
product, which is calculated as ratio of total energy/exergy input to the
energy/exergy of the annual production.
Stepanov [43] compiles some of the different methodologies
proposed for analyzing solar energy systems. The models developed by
Valero and Lozano [44] for obtaining the chemical exergy of fossil fuels
are applied here. However it must be pointed out that more complex
calculation procedures do not necessarily mean more reliable results.
Both, the experimental error associated to the determination of the
heating values and the error associated to the correlations are in an
interval close to ±2% [45]. Additionally, the chemical exergy of fuels is
approximated to the higher heating value (HHV). The HHV assumes
that water is in liquid state after combustion.
In this paper all primary thermal energy inputs are converted
into primary electrical energy, with an assumed efficiency of 35%,
i.e. 1 MJ (1/3.6 kWh), primary thermal energy becomes 0.097 kWh
primary electrical energy. Thus, primary energy and primary exergy
value becomes the same. Since the efficiency from primary thermal to
primary electric energy varies from country to country local conditions
are recommended.
Life Cycle Exergy Analysis (LCEA)
Life Cycle Analysis (LCA)
temperature solar collectors is much lower or about 30% due to the low
saturation temperature needed in order to drive a steam turbine. The
exergy efficiency of solar collector, PV and hybrid solar collector are
about 4%, 11% and 13% respectively [42].
To estimate the total exergy used in a process, it is necessary to take
all the different inflows of exergy to the process into account. This type
of budgeting is often termed Exergy Analysis [3,35], Exergy Process
Analysis, see Figure 3, or Cumulative Exergy Consumption [46], and
focuses on a particular process or sequence of processes for making a
specific final commodity or service. It evaluates the total exergy use by
summing the contributions from all the individual inputs, in a more or
less detailed description of the production chain.
Figure 2 illustrates the energy and exergy flows of solar PV cells,
Life Cycle Analysis or Assessment (LCA) is common to analyze
Figure 2: Energy and exergy flows through typical some solar energy
systems.
J Fundam Renewable Energy Appl
ISSN: 2090-4541 JFRA, an open access journal
Volume 5 • Issue 1 • 1000146
Citation: Gong M, Wall G (2014) Life Cycle Exergy Analysis of Solar Energy Systems. J Fundam Renewable Energy Appl 5: 146. doi:
10.4172/20904541.1000146
Page 4 of 8
Figure 3: Levels of an exergy process analysis.
Production
Use
The product ”gets life”
The product ”lives”
Disposal or recycling
The product ”dies” or
”reborn”
Figure 4: The life cycle “from cradle to grave”.
Objectives & Boundaries
Inventory
Energy & material balances
Environmental impact
Classifying
Characterization
Evaluation
Measures
Identifying
Evaluation
Conclusions
Recommendations
First Path
Next Path
Figure 5: Main steps of a LCA.
environmental impacts associated with three “life processes”:
production, use and disposal or recycling of products or product
systems, or as it is sometimes named “from cradle to grave”, see Figure 4.
For every “life process” the total inflow and outflow of energy
and material is calculated, thus, LCA is similar to Exergy Analysis.
In general Exergy Analysis and LCA have been developed separately
even though they are very similar. In LCA the environmental burdens
are associated with a product, process, or activity by identifying and
quantifying energy and materials used, and wastes released to the
environment. This inventory of energy and material balances is then
put into a framework in order to assess the impact on the environment,
Figure 5. Four parts in the LCA can be distinguished: (1) objectives
and boundaries, (2) inventory, (3) environmental impact, and (4)
measures. These four main parts of an LCA are indicated by boxes, and
the procedure is shown by arrows. Green arrows show the initial path
and red dashed arrows indicate suitable next paths, in order to further
improve the analysis.
Life Cycle Exergy Analysis (LCEA)
The multidimensional approach of LCA causes large problems when
it comes to comparing different substances, and general agreements are
crucial. This problem is avoided if exergy is used as a common quantity,
which is done in a Life Cycle Exergy Analysis (LCEA) [26].
J Fundam Renewable Energy Appl
ISSN: 2090-4541 JFRA, an open access journal
In this method we distinguish between renewable and nonrenewable resources. The total exergy use over time is also considered.
These kinds of analyses are of importance in order to develop sustainable
supply systems of exergy in society. The exergy flow through a supply
system over time, such as a power plant, usually consists of three
separate stages (Figure 6). At first, during the construction stage (0 ≤ t ≤
tstart) exergy is used to build a plant and put it into operation. The exergy
is spent, of which some is accumulated or stored in materials, e.g. in
metals, as well as exergy used for transportation etcetera. Secondly
the system need to be maintained during time of operation (tstart ≤ t ≤
tstop), and finally the cleaning up and disposal stage during destruction
stage (tstop ≤ t ≤ tlife). Eventually, some material, i.e. stored exergy, can be
recycled. These time periods are analogous to the three steps of the life
cycle of a product in an LCA. The exergy input used for construction,
maintenance and destruction are called indirect exergy Eindirect and
it is assumed that this originates from non-renewable resources. By
using recycled material in the production stage, the indirect exergy
may be considerably reduced. If exergy is recovered by recycling in the
destruction stage, this is accounted for as an additional product of the
system, Erec. When a power plant is put into operation, it starts to deliver
a product, e.g. electricity with exergy power Epr , by converting the
direct exergy power input into demanded energy forms, e.g. electricity.
In Figure 6, the direct exergy is a non-renewable resource, e.g. fossil
fuel and in Figure 7 the direct exergy is a renewable resource, e.g. solar
radiation.
In the first case as shown in Figure 6, the system is not sustainable,
since the system use exergy originating from a non-sustainable resource
and it will never reach a situation where the total exergy input will
be paid back, simply because the situation is powered by a depletion
of resources, i.e. Epr+Erec<Ein+Eindirect. In the second case, as shown in
Figure 7, at time t= tpayback the produced exergy that originates from a
natural flow has compensated for the indirect exergy input, i.e.
t payback
∫
tlife
E pr t (dt=
) + Erec
tstart
∫E
=
(t )dt Eindirect
indirect
0
Since the exergy input originates from a renewable resource, we
dE(t )
E (t ) =
dt
Operation
Construction
Destruction
Epr < Eindirect+Ein
tstart
tstop
Erec
tlife
Eindirect
t
Ein
Inflow of non sustainable exergy resources as
uranium, coal, oil and gas
Eindirect+Ein
Figure 6: LCEA of a fossil fueled power plant.
dE(t )
E (t ) =
dt
Operation
Construction
tstart
Eindirect
=Eindirect
tpayback
Destruction
Enet,pr >0
tstop
Inflow of sustainable exergy resources as
solar, wind and ocean currents are free
Erec
tlife
t
Figure 7: LCEA of a solar power plant.
Volume 5 • Issue 1 • 1000146
Citation: Gong M, Wall G (2014) Life Cycle Exergy Analysis of Solar Energy Systems. J Fundam Renewable Energy Appl 5: 146. doi:
10.4172/20904541.1000146
Page 5 of 8
E1
E2
Manufacturing
(Produced
material)
Transport
E3
E4
Installation
(Set up
system
assembly)
Esolar
Running
system
E5
Clean up
landfilled
LCEA diagrams are of particular importance in the planning of large
scale renewable energy systems of multiple plants. Initially, this system
will consume most of its supply within its own constructions phase.
However, sometime after completion it will deliver at full capacity.
Thus, the energy supply over time is heavily affected by internal system
dynamics.
LCEA of solar systems
Electricity/heat
Construction
Operation
recycled
Destruction
Figure 8: LCEA boundary of solar energy system.
do not account for it. By regarding renewable resources as free, then
after t = tpayback there will be a net exergy output from the plant, which
will continue until it is closed down, at t = tstop. Then, exergy has to
be used to clean up and restore the environment, which accounts for
the last part of the indirect exergy input Eindirect. Exergy in recycled
materials Erec now turns up as an additional product of the system.
By considering the total life cycle of the plant the net produced exergy
becomes Enet,pr =Epr - Eindirect + Erec. These areas representing exergies are
indicated in Figure 7. Assume that, at time t=0, the building of a solar
PV/power plant starts and at time t = tstart the plant is completed and
put into operation. At that time, a large amount of exergy has been
used in the construction of the plant, which is indicated by the area of
Eindirect between t = 0 and t = tstart. Then the plant starts to produce
electricity, which is indicated in Figure 7 by the upper curve Epr. At t
= tpayback the exergy used for construction, maintenance and destruction
has been paid back. The payback time will be further reduced if exergy
is recycled from the destruction. For solar PV/power plants this time is
only some months. Then the system has a net output of exergy until it
is closed down, which for a solar energy system usually last for 20-25
years. Thus, LCEA diagrams could be used to show if a power supply
system is sustainable.
LCEA is very important in the design of sustainable systems,
especially in the design of renewable energy systems. Take a solar panel,
made of mainly aluminum and glass that is used for the production of
hot water for household use, i.e. about 50°C. Then, it is not obvious that
the exergy being spent in the production of this unit ever will be paid
back during its use, i.e., it might be a misuse of resources rather than
a sustainable resource use. The production of silicon, aluminum and
glass require a lot of exergy as electricity and high temperature heat
or several hundred degrees Celsius, whereas the solar panel delivers
small amounts of exergy as low temperature heat. LCEA must therefore
be carried out as a natural part of the design of sustainable systems in
order to avoid this kind of misuse. Another case to investigate is the
production of biofuels in order to replace fossil fuels in the transport
sector. This may not necessarily be sustainable since the production
process uses a large amount of fossil fuels, directly for machinery or
indirectly as fertilizers, irrigation and pesticides. This would be well
described by a LCEA.
In order to be sustainable, energy supply systems must be based
on a use of renewable resources in such a way that the input of nonrenewable resources will be paid back during its life time, i.e. Epr > Ein
+ Eindirect - Erec. In order to be truly sustainable, the used deposits must
also be completely restored or, even better, not used at all. Thus, by
using LCEA and distinguishing between renewable and non-renewable
resources we have an operational method to estimate the sustainability
of energy systems.
J Fundam Renewable Energy Appl
ISSN: 2090-4541 JFRA, an open access journal
LCEA of a solar energy system
Figure 8 indicates a LCEA of a solar energy system where the red
dashed box indicates the system boundary and blue dotted lines indicate
the three steps of a life cycle. Construction includes manufacturing,
transport and installation in order to set up the system.
The indirect exergy E1 can be exergy of used electricity, fuels and
material from natural resources. The produced materials for solar
systems contain the PV module, metal, and electrical equipment.
For large systems there are also electrical substations, fence and
land. Fabrication of PV modules includes silicon production, PV
cell manufacturing and supporting structures. Electrical equipment
has inverters, transformers, cables and low and medium voltage
switchboards, charge regulations (control panel) and bank of batteries
(only for standard-alone-system). In the case of solar thermal system
the produced materials are solar collector, storage tank, pipes and so
on. The electricity used for manufacturing material during indirect
use can be from both non-renewable and renewable energy. In this
study electricity is, for practical reasons, assumed to be only from nonrenewable resources.
The indirect exergy E2 is exergy used for transportation of material
from the manufactures to the installation site, and E3 is exergy
consumption during installation.
During operation phase the indirect exergy E4 can be material
used for maintenance. The direct exergy from solar is not accounted
for during LCEA analyses since it is renewable energy and would other
vice most probably be wasted. The product is electricity and/or heat.
Part of the electricity production may be used for the control system.
The destruction phase restores the process to its original state. The
indirect exergy E5 is the exergy consumption for cleaning up; land filled
and recycled material, such as aluminum and the PV module. Land
filled and recycling depends on technology applied.
The total indirect exergy is E1+E2+E3+E4+E5 and recycled exergy is
an additional product of the system from its destruction.
LCEA applied to PV solar energy systems
There are numerous publications on LCA of different solar
energy systems [11-18]. The energy use for construction, operation
and destruction are often presented. However, some do not consider
recycling at the destruction phase. The use of recycled material will
often reduce the amount of energy needed from using fresh resources.
Two studies use recycled material in the construction [11,15].
Solar exergy is often used to produce heat and/or electricity. The
amount of solar exergy captured and converted by solar collectors
and/or solar PV cells depend on the location, type of solar PV cell
and collector, and working conditions. For a solar thermal plant, the
produced heat often has low temperature which means low exergy
values. For a solar thermal power plant, e.g. concentrate solar power
(CSP) plant, the heat usually have high temperature, e.g. 500-1000ºC
for solar power tower, as steam with high exergy to drive a turbine and
Volume 5 • Issue 1 • 1000146
Citation: Gong M, Wall G (2014) Life Cycle Exergy Analysis of Solar Energy Systems. J Fundam Renewable Energy Appl 5: 146. doi:
10.4172/20904541.1000146
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Exergy power
[kW]
4.2 kWp stand-alone PV plant
Electricity: 362 GJ
1
Recycled material: 5 GJ
Time [yr]
5
-1
10
Material: 118 GJ
Transport: 0.3 GJ
-2
-3
15
20
25
End-of-life: 7 GJ
Battery: 40 GJ
-4
Exergy power
[kW]
1
2.7 kWp grid-connected PV plant
Recycled material: 13 GJ
Electricity: 234 GJ
Time [yr]
5
-1
-2
10
15
20
Material: 120 GJ
Transport: 0.3 GJ
-3
25
End-of-life: 5 GJ
-4
Figure 9: LCEA of a stand-alone and a grid-connected PV plant.
Energy power
[kW]
0.1
Electricity: 12 GJ
Heat:
66 GJ
Time [yr]
5
-0.1
-0.2
-0.3
10
15
20
25
End-of-life: 0.07 GJ
Material: 11 GJ
Transport: 2 GJ
-0.4
Exergy power
[kW]
0.1
Electricity: 12 GJ
Heat:
6 GJ
Recycled material: 0,6 GJ
Time [yr]
-0.1
-0.2
-0.3
5
10
15
Material: 11 GJ
Transport: 2 GJ
20
25
End-of-life: 0.07 GJ
-0.4
Figure 10: LCA and LCEA of a solar Photovoltaic-Thermal (PV/T) plant.
generator to produce electricity. The efficiency of PV cells is usually
less than 20%, and the rest is mostly converted into waste heat to the
environment. It is also very sensible to the working temperature, e.g.,
the efficiency of crystalline silicon (c-Si) cells typically decreases 0.4%
when temperature rise 1 degree [47]. A PV/solar thermal (PV/T)
system directly converts solar exergy to electricity and thermal exergy.
In such systems the PV cells also work as thermal collectors in which
the cell temperature can be controlled in order to prevent the decrease
of efficiency.
Garcia-Valverde et al. [11] analyzed a 4.2 kWp stand–alone
photovoltaic system in Spain. The system consists of 40 monocrystalline modules (24V, 106Wp) mounted on a building rooftop.
In the construction phase the exergy requirements for production is
divided into two parts: manufacturing with and without recycling.
Recycled materials use less exergy in its production. It is assumed that
35% of aluminum, 50% of the lead-acid batteries, 90% of the steel, and
43% of the copper came from recycled materials in the original. The
highest exergy requirements in the production relates to PV modules
and lead-acid batteries. In the operation phase the only exergy needed
is to replace the lead-acid batteries after 10 years. The electricity for
regulators and inverters come from the system itself. In the destruction
phase exergy is required in recycling and transport to the recycling
plant, and then PV modules will become landfill.
J Fundam Renewable Energy Appl
ISSN: 2090-4541 JFRA, an open access journal
Kannan et al. [12] studied a 2.7 kWp grid-connected monocrystalline solar PV system in Singapore. The system consists of 36
mono-crystalline modules (12V, 75Wp) mounted on a building rooftop.
Most primary exergy was used in the production of PV modules which
is about 81.4% of the life cycle primary exergy use. In the operation
phase, no external exergy is used since produced electricity is
transferred to the electric grid; however, this implies special conditions
and consequences. In the destruction phase, it is assumed that the solar
PV module would be landfill and 90% of aluminum frames are recycled
with 90% recovery rate.
LCEAs of these two PV plants are depicted in Figure 9. By use of
some recycled material during construction phase, the indirect exergy
is about 118 GJ [11] for the stand-alone plant and if we assume no use
of recycled material this would instead become about 130 GJ. For the
grid-connected plant that does not make use of recycled material in
the construction about 120 GJ is used. In addition about 40 GJ are
used to replace batteries after 10 years in the stand-alone plant. The
exergy of the metal will show up as a product as we have indicated in
the diagrams, 5 GJ for the stand-alone and 13 GJ for the grid-connected
plant. This material and exergy is available for recycling. In the gridconnected plant the authors assumed that 90% of the metal is able to
recycle [12]. Probably, in the future more material will be recycled.
The exergy pay-back time becomes about 7 years for the stand-alone
plant and about 9 years for the grid-connected plant. With improved PV
module production technology and increased use of recycled material
the pay-back time could be further reduced. These results relate to a
thermoelectric conversion efficiency of 35% as indicated in Section 2.4
above. However, if the thermoelectric conversion efficiency varies from
30 to 40% the energy and exergy payback time will vary from 6 to 11
years and 8 to about 11 years, respectively.
Compare between LCA and LCEA with a PV/T system
A photovoltaic/thermal (PV/T) system in Hong Kong is investigated
by Chow and Jie [18]. PV/T is a combination of photovoltaic and
solar thermal system that produces both electricity and heat. In the
construction phase, energy is used for the collector panels and PV
module. For the solar thermal system, a water tank is needed. The
yearly production thermal energy and electricity energy is 2650 MJ
and 473 MJ respectively. The year average temperature in Hong Kong
is 23ºC, the temperature from solar collector is 85ºC. Thus, the yearly
thermal exergy production becomes only 244 MJ due to the relatively
low temperature of the heat. Figure 10 shows an energy based LCA, the
upper energy diagram, and an LCEA, the lower diagram, of this plant.
In both cases the input of solar energy is excluded. Clearly, the LCA and
LCEA show a large difference during operation phrase. The output of
electricity and heat amounts to 78 GJ in the energy case but only 18 GJ
in the exergy case, due to the low exergy value of the heat. In addition,
the exergy of the metal of the equipment used turn up as a product in
the destruction phase, i.e. about 0.6 GJ is available for recycling. The
total input of non-renewable resources amounts to 12.8 GJ in both
cases. The total energy output becomes more than 6 times the input.
However, this is misleading since the value of heat does not reflect its
true physical value. Instead, from the LCEA the output and input is
more or less the same. If materials used at construction stage came
from non-renewable mostly, this implies an inefficient energy usage.
Such system can hardly be regarded sustainable. From a pure resource
conservation perspective, it may be better to use the input of nonrenewable resources directly for other purposes with less conversion
energy loss instead. Considering that energy is also used for transport
Volume 5 • Issue 1 • 1000146
Citation: Gong M, Wall G (2014) Life Cycle Exergy Analysis of Solar Energy Systems. J Fundam Renewable Energy Appl 5: 146. doi:
10.4172/20904541.1000146
Page 7 of 8
and destruction or end-of-life, the energy and exergy payback time is
3.5 and 15.4 years respectively.
Thus, the exergy payback time is more than four times longer than
the energy payback time since the production consist of both electricity
and thermal energy. Thus, the LCEA offers a better tool than LCA in the
evaluation of energy systems for a sustainable future.
Conclusion
In a solar thermal plant the energy payback time is much shorter
than the exergy payback time since the exergy of the output of heat is
much lower than its energy value. This may lead to false assumptions in
the evaluation of the sustainability of renewable energy systems. From
Figures 6 and 7 we see the advantage of LCEA when applied to systems
based on non-renewable and/or renewable energy resources.
Solar energy systems producing electricity have less exergy back
time than system for heat production. The exergy payback time of PV/T
system is much longer than the energy payback time or about 4 times
longer. This indicates that LCEA gives a completely different view of
these systems that is of essential importance in scientific evaluations.
Among the three solar thermal power plants the PV/T plant has the
shortest energy payback time or 3.5 years. However, by applying exergy
the 4.2 kWp stand-alone PV plant has the shortest payback time or 7
years, since the product in this case has a higher exergy value, i.e., more
of electricity. This system is also the larger of the two pure PV systems.
LCEA is shown to be advantages in the study of solar based energy
systems and is recommended as a suitable tool for the design and
evaluation of renewable energy systems.
Nomenclature
E: Exergy, J
e: Exergy power per unit area, W m-2
E*: Exergy power, W
H: Enthalpy, J
n: The number of moles of substance, mol
P: Pressure, Pa
Q: Heat or thermal energy, J
S: Entropy, J K-1
t: Time, s
T: Temperature, K
u: Energy power per unit area, W m-2
U: Internal energy, J
V: Volume, m3
μ: Chemical potential, J mol-1
η: Efficiency
Superscripts
tot
:Total system, i.e. the system and the environment
Subscripts
0
: Reference or state time
en
: Energy
J Fundam Renewable Energy Appl
ISSN: 2090-4541 JFRA, an open access journal
: Exergy
ex
i
: Unit for different chemical materials
: Input
in
indirect
: Indirect input
: Unit for different process
j
: When a process or product exist
life
net,pr
: Net of product
: Output
out
payback
pr
: All input is paid back
: Product
: Recycled material
rec
: Operation starts
start
stop
tot
: Operation stops
:Total
waste
: Not used products
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10.4172/20904541.1000146
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