Energy and Power Engineering, 2013, 5, 300-305
doi:10.4236/epe.2013.54B059 Published Online July 2013 (http://www.scirp.org/journal/epe)
Energy and Exergy Analysis of a New Small
Concentr a ti ng Solar Power Plant
Heng-Yi Li, Tsair-Fuh Huang, Meng-Chang Tsai, Yung-Woou Lee, Shing-Lei Yuan,
Ming-Jui Tsai, Chi-Fong Ai
Physics Division, Institute of Nuclear Energy Research, Taoyuan, Chinese Taipei
Received March, 2013
ABSTRACT
A new small concentrating solar power plant which is suitable for urban area is presented, and a theoretical framework
for the energy and exergy analysis in the overall power plant is also constructed. The framework can be used to evaluate
the energy and exergy losses in each component. Furthermore, the energy and exergy efficiencies have also been com-
puted and compared for the individual components as well as for the overall plant.
Keywords: Exergy Analysis; Concentrating Solar Power; Thermal Energy Storage; Stirling Engine
1. Introduction
In the present world, the daily primary energy source is
fossil fuels such as coal, petroleum and natural gas. They
are not only limited in the earth but also release gaseous
or liquid pollutants during operation. Because solar en-
ergy is an inexhaustible, clean and safe source of energy,
it has received much attention as one of the most prom-
ising candidate to substitute for the conventional fuels for
electricity supply. Taiwan is located in subtropical zone,
rich in solar energy resources. However, Taiwan is
mostly mountainous in the east, with gently sloping
plains in the west. Hence, the installation area for solar
power plant is limited.
Recently, rapid development occurred worldwide in
the basic technology and market strategy for the concen-
trating solar power (CSP) technologies, including para-
bolic trough, power tower, and dish/engine. However, the
power generation efficiencies of the CSP systems are
found to be low, which indirectly increases the capital
costs of electricity generation, and great efforts have to
be concentrated on the future research and development
of CSP systems. Dish–Stirling systems have demon-
strated the good efficiency of any solar power generation
system by converting nearly 30% of the direct-normal
incident solar radiation into electricity after accounting
for parasitic power losses. Furthermore, solar-powered
Stirling engines can operate at low, medium, and high
temperatures. Hence, the feasibility of a solar power sys-
tem based on the Stirling dish and current status for
commercial markets are very attractive compared with
other concentrated technology [1-3]. However, the gen-
eral solar dish Stirling engine systems do not contain
thermal energy storage (TES), so their power production
is influenced by the weather.
TES involves the temporary storage of high or low
temperature thermal energy for later use. It is an excel-
lent candidate to offset the mismatch between thermal
energy availability and demand. For example, storage of
solar energy is used for overnight heating. TES systems
achieve benefits by fulfilling one or more of the follow-
ing purposes: increase generation capacity, enable better
operation of cogeneration plants, shift energy purchases
to low cost periods, increase system reliability and inte-
gration with other functions [4]. TES options for CSP
plants are classified to three categories: sensible, latent,
and thermo chemical storage. The only TES that cur-
rently operates with multiple hours of storage is the sen-
sible, two-tank, molten salt system. The system has
demonstrated reliable operation at commercial scale [5].
Generally, the performance of thermal power plants is
evaluated through energetic performance criteria based
on first law of thermodynamics, including electrical
power and thermal efficiency. In recent decades, the ex-
egetic performance based on the second law of thermo-
dynamics has found as useful method in the design,
evaluation, optimization and improvement of thermal
power plants. The exegetic performance analysis can not
only determine magnitudes, location and causes of irre-
versibilities in the plants, but also provides more mean-
ingful assessment of plant individual components effi-
ciency. These points of the exegetic performance analy-
sis are the basic differences from energetic performance
analysis. Therefore, it can be said that performing exe-

H.-Y. LI ET AL. 301
getic and energetic analyses together can give a complete
depiction of system characteristics. Such a comprehen-
sive analysis will be a more convenient approach for the
performance evaluation and determination of the steps
towards improvement [6].
So far, the energy and exergy analysis of solar power
tower plant, which uses parabolic dish, TES tank, and
stirling engine, has not been reported till now. In this
regard, the objective of this article is to present a new
small concentrated solar power plant which use molten
salt as storage media and is suitable for urban area. Fur-
thermore, a theoretical framework for the energy and
exergy analysis, which can be used to evaluate the en-
ergy and exergy losses in each component and in the
overall power plant, is constructed. The energy and ex-
ergy efficiencies have also been computed and compared
for the individual components as well as for the overall
plant.
2. System and Analysis
2.1. System Description
Figure 1 depicts the schematics of the proposed small
concentrating solar power plant using molten salt as
thermal energy storage media. The solar power plant
under developing is consisted of a parabolic dish concen-
trator, receiver, TES tank, and Stirling engine. The sun
rays fall on the parabolic dish concentrator which has
dual-axis tracking system and are reflected into the aper-
ture area of the receiver. The receiver is a heat pipe with
selective absorbing film on the top and able to downward
transfer the absorbed thermal energy to TES tank without
pump when absorbed thermal energy is enough. Besides,
the receiver stops operation when solar irradiation is
weak so that reversely heat transferring is avoided, The
TES tank uses molten salt as thermal energy storage me-
dia and transfers the energy to Stirling engine. The Stir-
ling engine then converts the thermal energy to me-
chanical power by expanding the gas in a piston cylinder.
In this study, two modes of operation are considered:
solar and dark modes. Solar mode is happened during the
day time and sunny weather, and all the solar energy ab-
sorbed by receiver is stored in TES tank and then used to
drive Stirling engine. Dark mode is happened during the
night time or cloudy weather, and no solar energy is
available and the stored energy in TES tank is used to
drive Stirling engine.
TES system designed here is to guarantee the supply
of energy even in the absence of solar radiation, such as
at nights or on cloudy days. Therefore, the energy pro-
duced by the CSP is not limited solely to hours of sun-
shine. Furthermore, the choice of molten salt is moti-
vated by the fact that this type of material provides an
efficient heat storage system, it is not toxic, eco-com-
patible and cheap. Above all, it is able to keep the tem-
perature at a higher level than hot water storage system.
To reduce the inventory cost relatively expansive molten
salt in TES tank, a low cost filler material compatible
with molten salts, such as quartzite rock, is used to fill
much of the volume in the TES tank and acts as the pri-
mary thermal storage material [7, 8].
2.2. Energy and Exergy Analysis
The analysis of the individual subsystems of the pro-
posed CSP plant in Figure 1 is carried out at steady state
condition by assuming steady state operation, and the
energy flow is illustrated in Figure 2. The solar input
power is transferred through parabolic dish and receiver,
and then buffered in TES tank. Finally, the stored energy
in TES tank is transferred to Stirling engine for electric
power generation. For the first and second law of ther-
modynamics, the energy balance and exergy balance of a
non-flow control volume can be expressed as:
c.v. c.v.j
j
WE Q
 (1)
c.v.c.v.j des
j
WEx ExI 
 (2)
where ..cv , ..cv , and W E
j
Q are the work, the internal
energy accumulation, and transferred thermal energy of
control volume, ..cv and Ex
j
Ex are the exergy of
and
..cv
E
j
Q, des
I
is the exergy consumption.
From equation (1) and equation (2), the energy balance
and exergy balance of parabolic dish subsystem are given
by:
12
P
Dloss
QQQ
 (3)
1
P
DS
QAI


 (4)
Figure 1. Schematic of a new small concentrating solar
power plant.
1
Q

2
Q

3
Q

4
Q

SEloss
Q

SE
W

PDloss
Q

Rloss
Q

TESloss
Q

TES
E

Figure 2. Energy flow diagram of the new small concen-
trated solar power plant.

H.-Y. LI ET AL.
302
12
P
Dloss
Ex I

Ex
 (5)
where is the total solar power inpu
1
Q

he
t to parabolic dish,
2
Q
 is t reflected solar power out from parabolic dish,
P
D
A
is the reflector area of parabolic dish, S
I
is the
ct normal irradiation, XXloss
Q
 and XXloss
I
re the
heat and exergy loss of XX sm.
The total exegetic solar power input
dire a
ubsyste
to parabolic dish
subsystem is given by [9]:
 
00
11
4
41
11 33
1TT
TT
ExQ  


  (6)


22 02
1ExQT T
  (7)
where T0 is the atmosphere temperature1
bo
, T is the sun
temperature, and T2 is the receiver absorber temperature.
The energy efficiency and exergy efficiency of para-
lic dish subsystem are defined as:
_2IPD QQ
 1 (8)

_2II PDEx Ex
 
1
(9)
Similarly, the energy balance and e
re
xergy balance of
ceiver subsystem are given by:
23
R
loss
QQQ
  (10)
20
(11)


44
320RRSR RR
QACFIUTT TT 

 


23
R
loss
ExEx I

(12)


33 03
1ExQT T
  (13)
where is the thermal power transferred to TES tank,
of re-
ce
3
Q

e aAR is thbsorber area of receiver, C is the concentration
ratio, FR is the intercept factor of receiver, T3 is the re-
ceiver bottom temperature, UR is the heat convective co-
efficient of receiver, εR is the absorber emissivity of re-
ceiver, and σR is the Stefan-Boltzmann constant.
The energy efficiency and exergy efficiency
iver subsystem are defined as:
_3IHD QQ
 2

(14)

_3II HDExEx
 
2
(15)
The energy balance balance of TES tank is given by:
)
T (17)
where is the thermal power transferred to Stirling
34TES TESloss
EQQQ
  (16

0TESlossTES TESTES
QAUT

4
Q

AT
engine, ES is the surface area of TES tank wall and
bottom, TTES is the molten salt temperature of TES tank
and UTES is the heat convective coefficient of TES tank,
TES
E is the net internal energy accumulation of TES and
ssed as
TE
E
expre
STES TESavgTESavgTES
VCp T


  (18)
where VTES, 
TESavg and CpTESavg are the volu
 (19)
where ρsalt and Cpsalt are the density and the specifi
(20)
me, average
density and average specific heat of thermal storage ma-
terial, ΔTTES is the temperature rising of TES. The av-
erage property of thermal storage material is expressed
as:


1
TESavgTESavgv saltsaltv qq
CpCp Cp  
c heat
of molten salt, ρq and Cpq are the density and the specific
heat of quartzite rock, εv is the void fraction.
The TES tank is charging and discharging at the same
time in solar mode, and discharging only in dark mode,
as shown in Figure 3 the energy balanced diagram.
The exergy balance of TES tank is given by:
34TES TESloss
ExEx Ex I


 
021
1ln
TESTESTES TESTES
ExETTTT 21TES T




 (21)


44 04
1ExQT T
  (
where TTES is rising from TTES1 to TTES
22)
2, and T4 is the hot
space temperature of Stirling engine. The energy effi-
ciency and exergy efficiency of TES tank are defined as:


_43ITES TES
EQQ  
  (23)


_4II TESTES
ExExEx  

3
The energy and exergy balance of Stir
gi
(25)
where is the output power of St
(24)
ling engine is
ven by:
4SE SEloss
QW Q
 
4SE SEloss
Ex WI

(26)
SE
W
 irling engine.
3
Q

4
Q

TESloss
Q

TES
E

(a)
4
Q

TESloss
Q

TES
E

(a)
Figure 3. Energy balance dim of the TES tank: (a) so-
lar mode; (b) dark mode.
agra

H.-Y. LI ET AL. 303
The energy efficiency of Stirling engine is given by
[10]:
_4ISE SE
WQ  
 (27)
_
I
SEE S
F
E

 (28)
E
HM C
F
EEK (29)
 


1
1ln
V
V
k  (30)
2
1111
S
Ee 

 
where FE is the empirical factor, ES is the thermodynamic
efficiency, k is the air specific heat ratio, and temperature
ratio τ is defined as:
04
TT

 (31)
The exergy efficiency of Stirling engine is defined as:
_4II TESSE
WEx   (32)
3. Results and Discussion
section is validation and
A
The model built in the preceding
used for energy and exergy analysis. In this study, the
new CSP plant with 25 kWe Stirling engine is considered.
Besides, the storage material is molten salt accompanied
with filler material. The molten salt is the mixture of 60
wt% NaNO3 and 40 wt% KNO3, and the filler material is
quartzite rock. The other properties of the new small CSP
plant are shown in Table 1.
For a typical sunny weather in Taiwan, the variation of
direction normal irradiation for length of a day is shown
in Figure 4. The irradiation rises from zero at 06:00,
reaches maximum value 1100 W/m2 at 12:00, and falls to
zero at 18:00. As shown in Figure 5, the thermal power
to TES is varied according to solar irradiation and the
thermal power time is smaller than 12 hours, the Stirling
engine output power still keeps 25 kW constantly all 24
hours. Viewing the temperature of TES for length of a
sunny day shown in Figure 6, the temperature rises in
Table 1. Properties of the new small CSP plant.
PD 445.5 m2 KC 0.7
AR 1.485m2 T0
0200 400 600 800 1000 1200 1400
0
200
400
600
800
1000
1200
min
W/m
2
Direct Normdiational Irra
298 K
ATES 15 m2 T1 6000 K
C 300 T2 873 K
EH 0.9 T3 773 K
EM 0.85 T4 673 K
ES 0.55 UR 8 2)
V
10 2
k 1.4 σR 5.6710-8 W/(K4m2)
W/(Km
e 0.99 UTES 2 W/(Km2)
FR 0.75 V1/ 2 2
IS 00 W/mεR 0.9
06:00 18:00 06:00 of next day
Figure 4. Variation of direct normal irradiation for length
of a sunny day.
0200 4006008001000 1200 1400
0
50
100
150
200
250
300
350
min
kW
Thermal Power Transferred to TES
06:00 Stirling Engine Power Output06:00 of next day
Figure 5. Thermal power transferred to TES and Stirling
engine output power for length of a sunny day.
0200 400 600 800100012001400
690
700
710
720
730
740
750
760
770
Temperature of TES
min
K
Internal energy accumulation of
TES is decreasing.
Internal energy accumulation of
TES is increasing.
06:00
06:00 of next day
Figure 6. Temperature of TES for length of a sunny day.
so s
n
lar mode and falls to initial value in dark mode. Thi
dicates that the internal energy accumulation is enough i
to supply heat loss and Stirling engine operation all day
long.
The results of energy and exergy analysis of the sys-
tem are shown in Figures 7-10. From the energy analysis,
it is found that the solar input energy of 445.5 kW can
generate net output electricity of 25 kW and thermal sto-
rage power of 184.157 kW for the new CSP plant in solar
mode. The overall energy efficiency of the whole system

H.-Y. LI ET AL.
304
is 46.95%. The subsystem energy efficiencies are 75%,
84.96%, 94.98% and 29.25% for parabolic dish subsys-
tem, receiver subsystem, TES tank and Stirling engine,
respectively. The largest percentage energy loss is
47.12% occurred in parabolic dish subsystem, followed
by 25.59% in Stirling engine, 21.26% in receiver sub-
system and 6.03 % in TES tank. However, the results of
the exergy analysis show a different behavior. The over-
all exergy efficiency of the whole system is 30.53%,
while the subsystem energy efficiencies are 52.9%,
79.26%, 88.58% and 47.60% for parabolic dish subsys-
tem, receiver subsystem, TES tank and Stirling engine,
respectively. The largest percentage exergy loss is
67.80% occurred in parabolic dish subsystem, followed
by 15.79% in receiver subsystem, 9.52% in Stirling en-
gine and 6.89% in TES tank.
Thermal and Exergy Eff. for Solar Mode
0.00
10. 00
20. 00
30. 00
40. 00
50. 00
60. 00
70. 00
80. 00
90. 00
1
00. 00
Para. DishReceiverTESSEoverall
Th . Ef f ( %)
Ex. Eff (%)
Figure 7. Comparison of energy and exergy efficiency for
solar mode.
Thermal and Exergy Loss for Solar Mode
80. 00
0. 00
10. 00
20. 00
30. 00
40. 00
50. 00
60. 00
70. 00
Para. DishReceiverTESSE
%
Th . L o ss ( %)
Ex. Loss (%)
Figure 8. Comparison of energy and exergy loss for solar
mode.
Thermal and Exergy Eff. for Dark Mode
10
Thermal and Exergy Loss for Dark Mode
0.00
10.00
20.00
30.00
40.00
50.00
60.00
70.00
80.00
90.00
100.00
Para. DishReceiverTESSE
%
Th. Loss (% )
Ex. Loss (% )
Figure 10. Comparison of energy and exergy loss for dark
It is found that the TES thermal storage power of
99.718 kW can generate net output electricity of 25 kW
for the new CSP plant in dark mode. The overall energy
efficiency of the whole system is 25.07%. The subsystem
energy efficiencies are 85.71% and 29.25% for TES tank
and Stirling engine, respectively. The largest percentage
energy loss is 80.93% occurred in Stirling engine, fol-
lowed by 19.07% in TES tank. The overall exergy effi-
ciency of the whole system is 45.26%, while the s
tem energy efficiencies are 95.08% and 47.60% foTES
exergy loss is 91.02% occurred in Stirling engine,
filler as thermal energy
st
efficiency and large loss, the system overall efficiency is
e help of TES. The TES offers great
mode.
ubsys-
r
tank and Stirling engine, respectively. The largest per-
entagec
followed by 8.98% in TES tank.
It can be seen that although Stirling engine has low
energy efficiency and large loss, the system overall effi-
ciency is not lessened with the help of TES. The TES
offers great contribution in both energy and exergy effi-
ciency. Furthermore, for high overall efficiency the most
significant components requiring careful design and selec-
tion are the parabolic dish subsystem and Stirling engine.
4. Conclusions
In this study, the exergy and energy analysis of the new
CSP plant is considering two modes: solar and dark. The
CSP plant using molten salt and
orage material is under developing. It is consisted of a
parabolic dish concentrator, receiver, TES tank, and Stir-
ling engine. The theoretical model is built and verified.
The validation reveals that the Stirling engine of the new
CSP using TES as an energy buffer can continue to run at
full capacity all day long. Furthermore, the analysis re-
sults show that although Stirling engine has low energy
0.00
10. 00
20. 00
30. 00
40. 00
50. 00
60. 00
70. 00
80. 00
90. 00
0.00
Para. DishReceiverTESSEoverall
Th . E f f ( %)
Ex. Eff (%)
not lessened with th
contribution in both energy and exergy efficiency. Fur-
thermore, in CSP design the most significant components
requiring careful design and selection are the parabolic
dish subsystem and Stirling engine.
5. Acknowledgements
Financial support from the budget of Executive Yuan,
Figure 9. Comparison of energy and exergy efficiency for
dark mode.

H.-Y. LI ET AL.
305
Taiwan, is greatly appreciated.
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