Evaluation of Output and Unit Cost of Power Generation Systems Utilizing Solar

Energy Under Various Solar Radiation Conditions Worldwide

TAKANOBU KOSUGI, PYONG SIK PAK, and YUTAKA SUZUKIOsaka University, Japan

SUMMARY

Characteristics and economics of three power gen-

eration systems which utilize solar energy were investi-

gated and compared for systems located in five different

regions. The three systems investigated were a solar thermal

system, a solar photovoltaic system, and a CO2-capturing

hybrid power generation system utilizing solar thermal

energy (referred to as the hybrid system) which has been

proposed by the authors. The net generated power energy

and the net exergetic efficiency of the hybrid system have

been estimated to be larger and higher, respectively, than

those of the others. Economic evaluation reveals that the

unit cost of generated power energy of the solar thermal

system changes most widely corresponding to the change

in solar radiation condition and that the cost of the hybrid

system changes the least. In general, the most economical

system has been estimated to be the solar thermal system in

a location which is superior in solar condition and to be the

hybrid system in a not so good solar condition. The solar

photovoltaic system has the possibility of being the most

economical if its construction cost is greatly improved,

though the hybrid system is still the most economical under

considerably worse solar conditions such as in Osaka.

© 1999 Scripta Technica, Electr Eng Jpn, 127(3): 1�12,

1999

Key words: Solar thermal system; solar photovol-

taic system; CO2-recovering hybrid power generation sys-

tem; exergy.

1. Introduction

In order to reduce carbon dioxide (CO2) emission

from power generation plants, introducing power genera-

tion systems utilizing natural energy such as solar energy

is expected to be effective. As to the power generation

systems utilizing solar energy, solar thermal power genera-

tion and solar photovoltaic power generation are well

known. Solar thermal power generation systems, e.g.

SEGS, have already been operated commercially in loca-

tions where the solar radiation condition is good [1]. With

respect to solar photovoltaic power generation, the produc-

tion capacity of photovoltaic modules has increased rapidly

in the developed nations [2]. However, the solar energy

which can be utilized on the ground depends on the region;

and the worsening of economics due to a decrease in the

capacity cannot be avoided for these solar-energy-utilizing

systems in regions where the solar radiation condition is not

good. Therefore, widespread introduction is difficult as

long as drastic drops in construction costs cannot be

achieved.

The authors and others have previously proposed a

CO2-capturing hybrid solar thermal power generation sys-

tem in which both solar thermal energy and fossil fuel are

utilized [3�5]. In the proposed system, since relatively

low-temperature steam is produced by utilizing the solar

thermal energy and is used as the main working fluid of the

gas turbine to generate electric power, the heat collecting

efficiency can be raised compared to the case of conven-

tional solar thermal power generation; and since fossil fuel

is also used, the capacity can be drastically improved. For

this reason, there is a possibility that the proposed system

is economically feasible even when the proposed system is

installed in places where the solar radiation condition is not

good, such as in Osaka City, Japan [6]. Using the pure

oxygen combustion method, the proposed system can cap-

ture in principle all the CO2 emitted, and it has been

estimated that a high fuel-based power generation effi-

ciency of over 60% can be obtained on a lower heating value

basis even taking into account the electric power energy

consumed in CO2 recovery and liquefaction.

When strict CO2 emission restriction is imposed in

the future, the worldwide introduction of solar-energy-util-

izing power generation systems worldwide may become

necessary. However, the characteristics and economics of

CCC0424-7760/99/030001-12

© 1999 Scripta Technica

Electrical Engineering in Japan, Vol. 127, No. 3, 1999Translated from Denki Gakkai Ronbunshi, Vol. 118-B, No. 3, March 1998, pp. 246�253

1

solar-energy-utilizing power generation systems depend on

the solar radiation condition at the installed location; more-

over, the structure of the installed systems varies. Since the

regional difference in solar radiation conditions is large

worldwide, the introduction must be promoted in such a

way that an appropriate system is selected from the various

solar-energy-utilizing power generation systems according

to the installation location from the viewpoint of charac-

teristics and economics.

For the purpose of clarifying which power generation

system should be introduced in the future under what

conditions, three systems are investigated�a solar thermal

power generation system, a solar photovoltaic power gen-

eration system, and a CO2-capturing solar thermal hybrid

power generation system. Our research has entailed a com-

parative evaluation of the characteristics and economics of

these systems. In this evaluation, we have taken into ac-

count that the characteristics and economics of the systems

differ due to solar radiation conditions, and estimated the

characteristics of the systems based on various solar radia-

tion conditions. In the economic evaluation, we have esti-

mated and evaluated the construction cost and unit cost of

fuel in the future; and we have also studied the conditions

under which condition each system is preferable economi-

cally.

2. Structures of the Evaluated Systems

Figure 1 shows the fundamental structures of the

three systems evaluated. In the solar thermal power genera-

(a) Solar thermal power generation system, STS

(b) Solar photovoltaic power generation system, PVS

Fig. 1. Fundamental structures of the evaluated systems utilizing solar energy.

(c) Solar thermal hybrid power generation system, HBS

2

tion system (STS) shown in Fig. 1(a), by utilizing the

energy obtained in the solar thermal collectors, the super-

heated steam produced by the heat exchanger is used as the

working fluid of the steam-turbine power generation sys-

tem. In the solar photovoltaic power generation system

(PVS) shown in Fig. 1(b), the electric power obtained by

flat-plate photovoltaic arrays is transmitted through an in-

verter. In the CO2-capturing solar thermal hybrid power

generation system (HBS), the steam is produced by utiliz-

ing the energy obtained in the solar thermal collectors and

used as the working fluid of the CO2-capturing H2O-turbine

power generation system. As shown in Fig. 1(c), the steam

accumulator is used as a heat storage devise as was done in

Ref. 6. For details of the configuration of the HBS, see Refs.

3 to 7.

In these systems, the solar thermal collector is of

trough type; and for both solar thermal collectors and

photovoltaic arrays, the single north�south axis method is

adopted the same as in Ref. 4.

3. Evaluation of Hourly Solar Radiation at Various

Locations

As an example of the installation locations of the

systems studied, five sites shown in Table 1 are selected.

The regional difference in solar radiation conditions is seen

to be large; for example, the total horizontal solar radiation

energy in the Great Sandy Desert (Australia) is 1.80 times

that in Osaka. Moreover, the mean value of the total solar

radiation energies worldwide is about 4.7 kWh/m2�day,

which is roughly equivalent to the average value of the total

horizontal solar radiation energies of Ottawa or Miami.

Solar radiation can be divided into direct solar radia-

tion and diffuse solar radiation. In the trough thermal col-

lectors used in the STS and the HBS, only the direct solar

radiation component can be used; however, in the flat-plate

photovoltaic arrays of the solar photovoltaic PVS, the dif-

fuse solar radiation component can also be utilized. Thus,

both direct and diffuse solar radiation data are required in

estimating the characteristics of the systems studied. More-

over, in order to accurately estimate the characteristics of

thermal collection and storage, it is desirable to obtain the

solar radiation data hourly; however, the locations where

the measurement data can be obtained are limited. In this

paper, we have estimated by the following procedures, the

hourly mean direct solar radiation intensity and mean dif-

fuse solar radiation intensity with the daily mean total solar

radiation energy according to month in the locations where

the data are easy to obtain.

(i) From the monthly mean values of daily total

horizontal solar radiation energy, the monthly mean values

of hourly total horizontal solar radiation intensity is esti-

mated with the use of Fig. 2. Figure 2 shows the curves,

obtained on the basis of many measured values, that can be

used to determine the hourly total solar radiation energy

from the daily total solar radiation energy [12].

(ii) By utilizing the direct�diffuse separation

method based on IEA [8], the hourly mean direct normal

solar radiation intensity and the hourly mean diffuse hori-

zontal solar radiation intensity are estimated from the

hourly mean total horizontal solar radiation intensity ob-

tained in (i).

The solar radiation intensity incident on the light-ab-

sorbing surface of the solar thermal collectors and photo-

voltaic arrays is estimated as follows. Let IH, ID, and IS be

respectively the total horizontal solar radiation intensity,

Table 1. Estimated annual mean values of daily direct

and diffuse solar radiation energy (units: kWh/m2�day)

Location of

installation

Total

horizontal

solar

radiation

energy*

Direct

normal

solar

radiation

energy

Diffuse

solar

radiation

energy

Osaka 3.63 2.50 1.99

New York 3.84 3.54 1.63

Ottawa 3.91 4.42 1.38

Miami 5.31 5.60 1.55

Great Sandy Desert 6.53 7.68 1.22Fig. 2. Graph for estimating hourly total solar radiation

from daily total solar radiation.

3

direct normal solar radiation intensity, and diffuse horizon-

tal solar radiation intensity; and let HD be the direct com-

ponent of solar radiation intensity, and HS the diffuse

component of solar radiation intensity among the solar

radiation incident on the light-absorbing surface in the

one-axis tracking scheme with N�S axis fixed. Then, HD

can be expressed by the following equation [4, 13]

where T is the angle of incidence on the light-absorbing

surface of the sunlight and can be calculated from

Here, T, G, and Z are, respectively, the latitude of the

location of installation, solar declination, and hour angle.

The diffuse solar radiation component HS can be

estimated by using the following equation with an accuracy

which is practically no problem [8, 13]:

In Eq. (3), the first term on the right-hand side is the sky

solar radiation component and the second term is the re-

flected solar radiation component; E expresses the tracking

angle of sunlight on the light-absorbing surface calculated

using Eq. (4); and U expresses the reflection factor of the

ground surface.

U takes different values depending on the state of the ground

surface; however, we use 0.15 as the value of U in this paper.

From the measured values of the monthly mean total

horizontal solar radiation energy at various locations, the

hourly mean direct solar radiation intensity and mean dif-

fuse solar radiation intensity according to month are esti-

mated by the above-mentioned procedures. The estimated

annual mean values of the daily direct and diffuse solar

radiation energies are also shown in Table 1. It can be seen

that the difference in direct solar radiation energy becomes

even larger than for total solar radiation energy, and it is

estimated that the direct solar radiation energy of the Great

Sandy Desert is 3.07 times that of Osaka.

4. Characteristics Evaluation

4.1 Assumed conditions

Using the hourly solar radiation data estimated in

section 3, the power generation characteristics of the sytems

are estimated with simulation models constructed by the

authors [3�7]. In these simulation models, the thermody-

namic relational equations on the state quantities of the

fluid on the inlet side and outlet side in the apparatus

comprising the power generation system, such as solar

thermal collector, steam turbine, and gas turbine, are de-

scribed respectively in block implementation by means of

the object-oriented programming language C++. The simu-

lation programs of the power generation systems can be

easily constructed by combining the blocks according to the

system configurations.

Table 2 shows the main conditions assumed in the

characteristics estimation.

(a) Solar thermal power generation system, STS

For the solar thermal collectors with a total aperture

area of 10 ha having the characteristics shown in Table 2(a),

superheated steam of 346 °C, 14 kg/cm2a�which is the

same as the pilot plant constructed in Nio-cho of Kagawa

in Japan�is produced by the heat exchanger. In the steam-

turbine power generation system, it is assumed that the

water-cooling-type condenser can be adopted and the outlet

pressure of the condenser is set at 0.1 kg/cm2a. For simplic-

ity, the heat storage tank is not installed.

The economics of the system is expected to be im-

proved when the generated energy is increased and the

construction cost is cheaper. Here, since the area of the solar

thermal collectors is fixed, the utilizable total energy of the

solar radiation is constant. However, since the solar radia-

tion energy changes with time, when the capacity (net

power output) of the steam-turbine power generation sys-

tem is small, the energy that cannot be effectively utilized

will occur in time zones where the solar radiation condition

is good. In order to increase the generated energy, it is

necessary to make the net power output large. However,

since the increase of the net power output increases the

construction cost, there exists an appropriate scale from the

viewpoint of economics. Accordingly, for the rated output,

we search for the value at which the unit cost of power

generation is the lowest at the respective locations of instal-

lation.

(b) Solar photovoltaic power generation system,

PVS

The array area is set as 10 ha, the same as for the solar

thermal collectors in the STS. Polycrystalline silicon pho-

tovoltaic cells are used and their efficiency is set at 15%

(array efficiency) taking into account the drop of efficiency

due to array implementation [14]. The energy loss factor is

set as 10% summarizing the loss due to inverter, etc.

(c) Hybrid power generation system, HBS

The solar thermal collector and heat exchanger hav-

ing the same characteristics and scales as in the STS are

used. The temperature of the saturated steam produced is

set as 220 °C. The inside volume of the steam accumulator

and the steam pressure at the inlet of the CO2-captured H2O

(1)

(2)

(3)

(4)

4

turbine power generation system are assumed to be 2000

m3 and 10 kg/cm2a, respectively, the same as in Ref. 6. The

inlet temperature of the turbine is set at 1200 °C and the

outlet pressure of the condenser is set at 0.1 kg/cm2a. As to

scale capacity (net power output), the value has been sought

at which the unit cost of generated power energy becomes

the lowest at the respective installed locations, as is done in

the solar thermal power generation system. In addition to

the exogenous variables listed in Table 2(c), as to the values

of exogenous parameters required in simulation, such as

Table 2. Assumed conditions for characteristics estimation

(a) Solar thermal power generation system, STS

Total aperture area of solar thermal collector 10 ha (100,000 m2)

Optical efficiency of solar thermal collector 70%

Concentration ratio of solar thermal collector 30

Effective emittance of solar thermal collector 0.4

Temperature difference on high-temperature side and low-temperature side of heat ex-

changer

both 20 °C

Energy loss rate in thermal transportation and heat exchanger 10%

Temperature of steam produced by utilizing solar thermal energy 346 °C

Net power output of steam-turbine power generation system searching for an optimal value

such that the unit cost of

generated power energy becomes

minimum

Inlet steam pressure of steam turbine 14 kg/cm2a

Adiabatic efficiency of steam turbine 80%

Generator efficiency 95%

Outlet pressure of condenser 0.1 kg/cm2a

(b) Solar photovoltaic power generation system, PVS

Total area of photovoltaic-cell array 10 ha (100,000 m2)

Conversion efficiency of photovoltaic array 15%

Energy loss rate due to inverter, etc. 10%

(c) Hybrid power generation system, HBS

Characteristics and capacities of solar thermal collector and heat exchanger same as for solar thermal power

generation system

Temperature of saturated steam produced by utilizing solar thermal energy 200 °C

Internal volume of steam accumulator 2000 m3

Rated power output of CO2-capturing H2O turbine power generation system searching for an optimal value

such that the unit cost of

generated power energy is a

minimum

Inlet steam pressure of CO2-capturing H2O turbine power generation system 10 kg/cm2a

Outlet pressure of condenser 0.1 kg/cm2a

Inlet temperature of turbine 1200 °C

Temperature of return water 100 °C

5

temperature efficiency of regenerator, adiabatic efficiency

of turbine, and generator efficiency, the same values are

used as those found in Ref. 6.

4.2 Estimated characteristics and comparison

For convenience of explanation, the cost data of the

power generation systems used in estimating the unit cost

of power generated energy will be described in section 5.

Table 3 and Fig. 3 show the net power output of the systems

at the respective locations of installation and the estimated

results of the annual net generated energy, respectively. In

Fig. 3, the horizontal axis expresses the average direct

normal solar radiation energy per day in a year, and the

trend of the dependence of the generated energy on the solar

radiation condition can be shown by connecting the esti-

mated values at the five locations. As seen in Table 3, the

net power outputs of the STS and HBS when the unit cost

of generated power energy is estimated to be minimum are

all estimated to become larger as the direct solar radiation

energy at the location of installation is larger. As seen in

Fig. 3, the same relation is also applicable for the generated

energy. The net power output of the PVS is constant regard-

less of location of installation; however, it is also seen that

larger electrical energy can be obtained at the locations

where the direct solar radiation energy is abundant, similar

to the other two systems. When the generated energies of

the systems are compared, we can see that they become

larger in increasing order of the STS, PVS, and HBS at all

locations.

The estimated capacity factors determined from the

estimated net power output and generated energy are also

shown in Table 3. As seen, the capacity factor is estimated

to be 24% to 36% in the STS, 16% to 34% in the PVS, and

70% to 79% in the HBS. Also, the capacity factor of the

PVS is most heavily dependent on the direct solar radiation

energy at the location of installation.

The capacity factor of the STS is estimated to be

higher than that of the PVS. The reason is as follows, taking

the case of Osaka as an example. The maximum possible

net power output of the STS is estimated to be 13.6 MW,

about the same as that of the PVS. When the net power

output of the STS is set at 13.6 MW, the capacity factor of

the system becomes 7.09%, considerably lower than that of

Table 3. Estimated maximum net power output and capacity factor

Solar thermal power

generation system, STS

Solar photovoltaic power

generation system, PVS

Solar thermal hybrid power

generation system, HBS

Net power

output

Capacity

factor

Net power

output

Capacity

factor

Net power

output

Capacity

factor

Location of Installation (MW) (%) (MW) (%) (MW) (%)

Osaka 3.80 24.2 13.5 16.3 3.96 75.2

New York 5.49 25.4 13.5 18.6 4.68 74.3

Ottawa 6.34 27.6 13.5 20.3 5.20 74.7

Miami 7.21 35.0 13.5 26.9 7.02 78.7

Great Sandy Desert 10.1 36.3 13.5 34.3 9.52 70.4

Fig. 3. Estimated annual generated power energy.

6

the PVS. When the net power output of the STS is smaller,

the capacity factor increases. Here, since the net power

output of the STS is set at the small value of 3.80 MW from

the viewpoint of economics, the capacity factor of the STS

is estimated to be higher (24.2%) than for the PVS (the net

power output for the same capacity factor as the PVS is

estimated to be 5.88 MW).

The capacity factor of the HBS is drastically higher

than for the STS. This is because not only solar energy but

also fossil fuel are used as primary energy in the HBS.

Figure 4 shows the estimated heat-collecting effi-

ciency of the solar thermal collector in the STS and HBS.

When the estimated heat-collecting efficiency of the solar

thermal collector in the HBS is 2.8% to 9.4% higher than

that of the STS, the difference in efficiency is seen to be

larger at locations where the direct solar radiation energy is

smaller. This is because the average temperature of the solar

thermal collector in the STS is 216 °C, whereas that of the

HBS is lower (180 °C), and, in general, the heat-collecting

efficiency of the solar thermal collector becomes higher

when the average temperature of the solar thermal collector

is lower [3].

The power generation efficiencies of the systems are

compared on the exergy basis, since the quality of the

energy differs between fossil fuel and solar energy. For the

calculation the exergy, the reference environmental tem-

perature for the calculation of the exergy is set as 25 °C and

the brightness temperature of the sun is set at 4930 °C,

which is the value when the air mass is 1 [13]. Figure 5

shows the estimated results of the total net exergetic effi-

ciencies of the systems. It is seen that the exergetic effi-

ciency of the STS is estimated to become higher at locations

where direct solar radiation energy is abundant. As shown

in Fig. 4, this is because the heat-collecting efficiency of the

solar thermal collector becomes higher with increasing

direct solar radiation intensity. On the other hand, the

exergetic efficiency of the HBS becomes lower at higher

solar radiation locations because the ratio occupied by the

exergy of the solar radiation among the total input exergy

becomes higher. In the PVS, the exergetic efficiency is

estimated to be constant at 14.4% regardless of the solar

radiation condition.

The exergetic efficiency becomes higher in increas-

ing order of the STS, PVS, and HBS, and the exergetic

efficiency of the HBS is estimated to be 1.33 to 2.08 times

that of the STS and 22.1% to 60.3% higher than that of the

PVS. The exergetic efficiency of the HBS is relatively

higher because the HBS uses fossil fuel in addition to solar

thermal energy.

It should be noted, however, the fact that the exergetic

efficiency of a power generation system is high does not

necessarily mean that the economics of that system is high.

5. Economic Evaluation

5.1 Assumed cost data

Except for some regions where the solar radiation

condition is good, the evaluated systems present difficulty

Fig. 4. Estimated heat-collecting efficiency of solar

thermal collector.

Fig. 5. Estimated net total exergetic efficiency.

7

for large-scale introduction because of economic or tech-

nological problems at the present time. Here, as the period

under consideration, we will deal with a future time when

CO2-capturing technology based on the pure oxygen com-

bustion method required for realizing the HBS and the

technology of deep-ocean dumping of captured CO2 may

reach the practical stage. We will carry out the economic

evaluation by assuming the reduction of construction costs

due to technological progress up to that point along with

the possible rise of fuel prices.

Table 4 shows the cost data assumed in the evaluation.

These future cost data are based on Refs. 11 to 19. As to the

solar thermal collector, taking into account an example

where the construction cost is expected to decrease about

40% in the future [11], we have set the low, medium, and

high levels of the unit construction cost to be 14,000,

18,000, and 22,000 yen/m2, respectively. The construction

cost of the PVS is 1 u 106 to 2.5 u 106 yen/kW at the present

time; however, the future uncertainty is considered to be

relatively large, there being a report of the possibility of

dropping to one-fifth along with the increase in the annual

production in the future [15]. In the PVS of large-scale

ground installation type for electric utilities as discussed in

this paper, taking into account also that specialized mount-

ing structures are required (different from the case of in-

stalling a small-scale power generation system on a roof),

and that the cost of the entire system will not drop much,

we assumed here the construction costs of the medium level

as 350 u 103 yen/kW as shown in Table 4, and set the

construction costs of the low level and high level as 250 u

103 and 450 u 103 yen/kW, respectively.*

The unit cost of natural gas at present is about 0.5

yen/MJ (about 2 yen/Mcal); although the outlook is that it

will not change much in the short term, there is a possibility

that it may rise sharply along with the resource exhaustion

in the long term. Here, we will investigate the economics

Table 4. Assumed cost data for economic evaluation

Facilities, etc. Assumed values

Trough type solar thermal collector 14 u 103 yen/m2 (low level)

18 u 103 yen/m2 (medium level)

22 u 103 yen/m2 (high level)

Steam-turbine power generation system 150 u 103 yen/kW

Solar photovoltaic power generation system, PVS 250 u 103 yen/kW (low level)

350 u 103 yen/kW (medium level)

450 u 103 yen/kW (high level)

H2O turbine power generation system 200 u 103 yen/kW

Oxygen production compressor 440 u 106 yen/(t C/h)

CO2 liquefying equipment 310 u 106 yen/(t C/h)

Steam accumulator 47 u 103 yen/m2

Unit cost of fuel Case A: 0.8 yen/MJ

Case B: 1.2 yen/MJ

Case C: 1.6 yen/MJ

Recovered CO2 processing 8000 yen/t C

Annual expense factor 0.173,* 0.143�

*Solar thermal power generation system, STS and solar thermal hybrid power generation system,

HBS.

�Solar photovoltaic power generation system, PVS.

*The construction cost is not necessarily the same due to the difference in

installation condition at the respective location of installation. However,

since the objective of this paper is evaluation of the effect of the difference

in solar radiation conditions on the characteristics and economics of the

systems, we have assumed that the construction cost is constant regardless

of the location of installation.

8

by assuming three cases: Cases A, B, and C as shown in

Table 4.

As to the disposal methods of the captured CO2, we

have assumed the case of transporting it from the location

of installation to a location 100 km away and disposing it

into deep-ocean dumping or underground processing; its

cost is set as 8000 yen/t C [16].

The general electric power utilities are assumed to

construct and operate the systems, and the construction cost

is assumed to be depreciated in 10 years. In the systems that

will emit no CO2, the relatively low fixed rate of interest is

regarded as utilizable, and the rate of interest of the capital

is set as 4%/year. The maintenance cost rate of the STS and

HBS is set as 5%/year, and the maintenance cost rate of the

PVS without using the mechanical power system is set as

2%/year. Therefore, the annual cost rate of the facilities is

set as 0.173 for the STS and HBS and 0.143 for the PVS.

5.2 Economic evaluation based on unit cost of

generated power energy

In this paper, the economics of the power generation

systems are evaluated on the basis of the unit cost of

generated power energy. CP, the unit cost of generated

power energy, can be determined as

(c) Case C (unit cost of fuel 1.6 yen/MJ)

Fig. 6. Estimated unit cost of generated power energy where low and high construction costs are assumed.

(5)

(b) Case B (unit cost of fuel 1.2 yen/MJ)(a) Case A (unit cost of fuel 0.8 yen/MJ)

9

where, WN is the annual net generated energy and CG is the

annual cost of generated power energy expressed as*

The estimated results for the case when the unit cost

of fuel has risen are shown in Fig. 6. In the future, the

low-level and high-level assumed values are used as the

construction costs of the solar thermal collector and solar

photovoltaic power generation system. As seen from Fig.

6, the uncertainty of the unit cost of power generated energy

caused by the uncertainty of the assumed costs is estimated

to become larger in the increasing order of the HBS, the

STS, and the PVS. Let us explain the estimated results of

the unit cost of generated energy for the case when the

construction cost is reduced drastically, namely, when the

low-level assumed values are used as the construction costs.

As seen from Fig. 6(a), for Case A, in locations where the

solar radiation condition is not very good (such as Osaka,

New York, and Ottawa), the unit cost of generated power

energy of the HBS is the lowest (that is, most economical).

However, in other locations, the PVS is estimated to be the

most economical. Moreover, as in Cases B and C, when the

unit cost of fuel rises, the economical superiority of the PVS

is estimated to become higher. On the contrary, according

to the estimated unit cost of generated power energy, when

using the high-level values as the construction costs, the

unit cost of the PVS becomes drastically high, and it is seen

that the STS becomes the most economical in locations

where direct solar radiation energy is abundant and that the

HBS becomes the most economical in locations with a

lower solar radiation condition.

As seen from Fig. 6, in the STS and the PVS, the unit

cost of generated power energy drops considerably when

solar radiation energy is abundant; however, it decreases

slightly for the HBS. This is because, as shown in Fig. 5,

the exergetic efficiency (performance of the system) of the

HBS becomes worse with a decrease in the direct solar

radiation energy.

Figure 7 shows the estimated values of CP when the

medium-level values are used as the construction costs and

the unit cost of fuel is that of Case B. As seen, when the unit

costs of generated power energy at the various locations of

installation are compared, the STS is the most economical

at locations where the solar radiation condition is good, and

the HBS is the most economical at locations where the solar

radiation condition is not very good. The PVS of large-scale

ground installation type for electric utilities is estimated to

be of not much advantage economically in either location.

It is seen from Fig. 7 that the ratio of the change of the unit

cost of generated power energy with respect to the change

of direct solar radiation energy is estimated to become

higher in the increasing order of the HBS, the PVS, and

the STS.

6. Conclusions

The characteristics and economics of three systems

of power generation utilizing solar energy�solar thermal

power generation system (STS), solar photovoltaic power

generation system (PVS), and hybrid power generation

system (HBS)�have been evaluated by choosing five

places where the solar radiation condition is different as the

locations of installation.

Among the systems utilizing the solar thermal collec-

tors or photovoltaic arrays of the same area (10 ha), the net

generated energy and the total net exergetic efficiency of

the HBS are estimated to be the largest in all the locations.

The economics of the systems have also been evalu-

ated on the basis of the unit cost of generated power energy

under several conditions at a future time. The estimated unit

cost of power generation differs depending on the values of

the construction cost and unit cost of fuel. In general, the

unit cost of the STS is the lowest at locations where the solar

CG = facility depreciation cost + facility maintenance

cost + fuel cost + captured CO2 disposal cost(6)

*Here, since it is assumed that the solar thermal collectors and photovoltaic

arrays of the same area are operated by the same tracking scheme for the

three evaluated systems, the land price is not considered explicitly. For

the other costs such as personnel expenses except the maintenance cost,

they are also not considered because the difference due to the system is

considered small.

Fig. 7. Estimated unit cost of generated power energy

where medium construction cost is assumed.

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radiation condition is good, and the unit cost of the HBS is

the lowest at locations where the solar radiation condition

is not very good. The ratio of the change of the unit cost of

power generation with respect to the change of the solar

radiation condition is estimated to be smaller in the decreas-

ing order of the STS, the PVS, and the HBS. When drastic

reduction of the construction costs can be achieved, the

economics of the PVS becomes the highest at many loca-

tions. However, when the construction costs are the me-

dium-level assumed values, the STS is estimated to be the

most economical at locations where the solar radiation

condition is good and the HBS at locations where the solar

radiation condition is not very good.

Under the restriction of CO2 emission in the future,

the worldwide introduction of power generation systems

utilizing solar energy may become necessary; however,

since the regional differences in solar radiation conditions

is large, such introduction must be promoted by selecting

appropriate systems according to the location of installation

from among the power generation systems utilizing solar

energy. We hope that obtained results will prove instructive

on the selection of appropriate systems at various locations

of installation.

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236.

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AUTHORS (from left to right)

Takanobu Kosugi (member) graduated from the Department of Information System Engineering of Osaka University in

1993. He completed the master�s degree and has been a research associate at the Cooperative Research Center for Advanced

Science and Technology, Osaka University, since 1995. He is engaged in research on evaluation of CO, mitigating energy

systems. He is a member of the Japan Society of Energy and Resources.

Pyong Sik Pak (member) graduated from the Department of Electrical Engineering of Osaka University in 1968 and

obtained a Dr. of Eng. degree in 1973. He became a research associate in 1973 and an associate professor in 1988 in the

Department of Electrical Engineering and has been with the Department of Information Systems Engineering, Osaka University

since 1992. He is engaged in research on modeling and analysis of various systems. He is a member of the Japan Society of

Energy and Resources, the Gas Turbine Society of Japan, and the Society of Instrument and Control Engineers.

Yutaka Suzuki (member) graduated from the Department of Electrical Engineering of Osaka University in 1958 and

completed the master�s course in 1960 and later a Dr. of Eng. degree. After serving as a research associate and associate professor,

he became a professor in 1972 in the Department of Electrical Engineering, and has been with the Department of Information

Systems Engineering, Osaka University, since 1989. He is engaged in research on modeling and simulation as well as urban

planning. He is a member of the Japan Society of Energy and Resources, the Operations Research Society of Japan, and IMACS.

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