Energy and Exergy Analysis of a New Small Concentrating Solar Power Plant

doi:10.4236/epe.2013.54B059

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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

Email: [email protected]

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

WE Q

 (1)

c.v.c.v.j des

WEx ExI 

 (2)

where ..cv , ..cv , and W E

Q are the work, the internal

energy accumulation, and transferred thermal energy of

control volume, ..cv and Ex

Ex are the exergy of

and

..cv

Q, des

is the exergy consumption.

From equation (1) and equation (2), the energy balance

and exergy balance of parabolic dish subsystem are given

by:

Dloss

QQQ

 (3)

QAI





 (4)

Figure 1. Schematic of a new small concentrating solar

power plant.



SEloss



PDloss



Rloss



TESloss



TES



Figure 2. Energy flow diagram of the new small concen-

trated solar power plant.

H.-Y. LI ET AL.

302

Dloss

Ex I



 (5)

where is the total solar power inpu



t to parabolic dish,

 is t reflected solar power out from parabolic dish,

is the reflector area of parabolic dish, S

is the

ct normal irradiation, XXloss

 and XXloss

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]:

 

11 33

1TT

ExQ

 







 (6)





22 02

1ExQT T



 (7)

where T0 is the atmosphere temperature1

, 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





(9)

Similarly, the energy balance and e

xergy balance of

ceiver subsystem are given by:

loss

QQQ



 (10)

(11)



320RRSR RR

QACFIUTT TT





 





loss

ExEx I



(12)





33 03

1ExQT T



 (13)

where is the thermal power transferred to TES tank,

of re-



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





(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



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

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:





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









 (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







The energy and exergy balance of Stir

(25)

where is the output power of St

(24)

ling engine is

ven by:

4SE SEloss

QW Q





4SE SEloss

Ex WI



(26)

 irling engine.



TESloss



TES



(a)



TESloss



TES



(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





 (27)

SEE S



 (28)

HM C

EEK (29)

 





1ln

 (30)

1111





 

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:



 (31)

The exergy efficiency of Stirling engine is defined as:

_4II TESSE

WEx





(32)

3. Results and Discussion

section is validation and

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

200

400

600

800

1000

1200

min

W/m

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)

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

100

150

200

250

300

350

min

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

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

lar mode and falls to initial value in dark mode. Thi

dicates that the internal energy accumulation is enough

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

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

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

efficiency and large loss, the system overall efficiency is

e help of TES. The TES offers great

mode.

ubsys-

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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