Impact of balance-of-system costs on photovoltaic electric power systems

Solar Cells, 8 (1983) 125 - 136 125

IMPACT OF BALANCE-OF-SYSTEM COSTS ON PHOTOVOLTAIC ELECTRIC POWER SYSTEMS

H. SAHA, P. BASU and K. MUKHOPADHYAY

Department of Physics, University of Kalyani, Kalyani, West Bengal, 741235 (India)

(Received October 7, 1981 ;accepted April 2, 1982)

Summary

A simple method based on the balance~f-system (BOS) costs is proposed for computing the allowable solar cell module cost and efficiency for typical applications of photovoltaic electric power systems: microirrigation and a rural electric supply. It is shown that in India the allowable module cost of cell modules with a conversion efficiency of about 5% is about U.S. $2 Wp -1 (1980) for microirrigation and about U.S. $0.8 Wp -1 for a rural electricity centre. It is further observed that relatively low BOS costs in India and similar places tend to make the allowable module cost become invariant with the efficiency, thus permitting solar cell modules of lower efficiency (5%) to become commercially viable for large-scale applications.

1. Introduction

The technical feasibility of photovoltaic electric power systems (PEPSs) has already been demonstrated in a number of varied applications. In developing countries like India, where less than 20% of the small villages have so far been electrified, applications such as microirrigation, village community centres and small-scale industries present good opportunities for the large- scale utilization of photovoltaic solar energy. At present, the relatively high initial cost restricts the use of photovoltaic systems. The overall costs depend both on the cost of the solar cell modules and on the balance-of-system (BOS) costs which can be broadly divided into two components: (a) area- related costs (such as the cost of the array structure, the site preparation, the module interconnections, the installation and other costs, including module testing, sizing and packaging) and (b) non-area-related costs (such as the cost of storage, power conditioning and field wiring). So far, the greatest amount of at tention has been paid to the reduction of solar cell panel costs in an a t tempt to meet the U.S. Depar tment of Energy (DOE) target of U.S. $700 kWp -x by 1986 [1]. Recently, some attention has been paid to the analysis of BOS costs; these may eventually dominate the PEPS costs [2, 3] and de-

0379-6787/83/0000-0000/$03.00 © Elsevier Sequoia/Printed in The Netherlands

126

termine the cell efficiency and the cost of the systems required for photovoltaics to be economically viable. It has been pointed out by Wolf [4] that the variation in the solar cell module cost as a function of efficiency provides a valuable tool for the selection of modules for a given system or for the assessment of the cost effectiveness of solar cell and module fabrica- tion processes. The value-efficiency is again dominated by BOS costs. In the more developed countries BOS costs are rather high; the DOE target for area- related BOS costs is U.S. $60 m -2 [5] (all values of U.S. dollars referred to in the present paper are based on the 1980 rate). Consequently, cells with a conversion efficiency of less than 10% - 14% are considered not to be viable economically in more developed countries [4]. However, the nature of BOS costs is such that they will vary from country to country, depending on a number of socioeconomic factors. It is therefore interesting to analyse BOS costs in some detail to arrive at a realistic estimation of the PEPS cost for a particular application in a particular part of a developing country.

A detailed analysis of the BOS costs for a number of typical Indian applications has already been reported [6, 7]. The absolute BOS cost per peak watt is considerably less in India than that reported by the Lewis Research Center, National Aeronautics and Space Administration {NASA) [2, 6]. It appears that the DOE target of U.S. $60 m -2 for the area-related BOS cost, which is the assessment for countries like the U.S.A., will not apply to developing countries like India. Again, as the BOS costs determine the overall cost effectiveness of the solar-powered system, lower BOS costs should permit the use of solar cell panels with lower conversion efficiencies and higher cell costs.

In the present paper, an at tempt has been made to determine the impact of BOS costs on the use of photovoltaics in developing countries like India. The allowable efficiency and cost of solar cells have been worked out by first determining the break-even cost for particular applications such as (a) a diesel-based pump set for microirrigation and (b) a rural electricity centre. The microirrigation application has been chosen since it is thought to be the application of photovoltaics with the greatest potential for countries like India. A recent survey has indicated that about 10 million pump sets driven by PEPSs may be in operation in developing countries under favourable conditions [8]. It is envisaged that the example of a rural electricity centre will provide a good insight into the impact of BOS costs on PEPSs.

2. Estimation of balance-of-system costs

As stated in Section 1, the BOS costs may be broken down into a number of components; these have been estimated from a survey of the local Indian market for four different applications: (a) microirrigation; (b) a community television centre; (c) domestic lighting; (d) street lighting.

The BOS costs for the array structure, the foundation and the installation etc. were computed on the basis of the actual expenses incurred in

TABLE 1

Percentage breakdown of balance<if-system costs for different systems

127

Microirrigation Community Domestic Street television lighting lighting centre

Lewis Research Center, NASA

Array structure and site preparation

Storage

Electrical (including domestic service conveniences and energy centre electrical wiring)

Installation

Others

Total

21.80 9.60 11.97 11.00 18

32.30 61.40 64.48 66.32 20

36.26 23.02 18.85 17.32 33

2.80 1.70 1.88 2.16 20

6.84 4.28 2.82 3.20 9

100 100 100 100 100

setting up the PEPS in the village of Charsarati, Kalyani, West Bengal (see Appendix A). The storage cost is computed on the assumption of a baseline cost of U.S. $100 kW -1 h -1 and a life of about 10 years. The electrical and electronic costs are estimated from local market data. Table 1 shows some details of these four rural applications, including the percentage breakdown of the BOS costs for each of them. The nature of the variation in the BOS costs for different applications [6, 7] and its comparison with that reported by the Lewis Research Center, NASA, are shown in Fig. 1. It is interesting to note that, whereas the nature of the variation in the BOS cost per peak

I t ~

8 ~

1do 1o000 SIZE OF THE S~rSTEM IN PEAK WATTS

Fig. 1. Variation in the BOS cost per peak watt with the size of the installation for different systems: A, data from NASA; e, Indian data.

128

watt with the size of the installation is similar in both cases, the absolute BOS cost per peak watt is considerably less in India than it is in the U.S.A. This is mainly a result of the cheap labour costs involved in the area-related component of the BOS costs in countries like India. (The cheap cost of labour is also reflected in the cost of some of the other materials used.) It is our intention to study the implication of these low BOS costs and their role in the determination of the allowable cell costs and conversion efficiencies.

3. Break~even cost calculation

An assessment of the economic viability of a solar energy system for a particular application can be made through a break-even cost calculation with the conventional energy system. The break-even cost can be calculated by finding the present value of the average yearly savings that would result from using a solar energy system rather than a conventional energy system. Alternatively, for microirrigation the break-even cost can be determined in terms of the cost of a unit volume of water pumped by the pump set. This approach is followed in the present paper. The costs of the diesel pump set and the diesel generators were calculated following the method of Parikh [9].

3.1. Microirrigation For microirrigation the capacity of the diesel pump set was chosen to

be 5 h.p. It is interesting to note that on the Indian market diesel pump sets with a capacity of less than 5 h.p. are not generally available. Private discussions with the manufacturers and users of diesel pump sets in India indicate that efforts to introduce diesel pumps with a capacity of 3 h.p. or less onto the Indian market have encountered both technical and economic problems and the at tempt has been abandoned. Thus, to irrigate a plot of about 1 ha, which is the average size of land holding for cultivation in India, 5 h.p. diesel pumps are universally used (there has been a tendency, however, to under- utilize the engines). This is why the 5 h.p. diesel pump was taken as the basis for comparison with a solar power system for microirrigation, even though the effective capacity required for this purpose is much less.

Usually a diesel pump set of this capacity can pump 20 m 3 of water per hour. A farm of 1 ha generally requires about 10 000 m 3 of water per year in India for the cultivation of 3 crops in a year. The operating time of the diesel pump set was therefore taken to be 500 h year -1 . The fuel consumption was taken to be 1 1 h -1 and the present price of diesel is taken to be U.S. $400 ton -1 (1980), including the cost of transportation. Pump sets cost approx- imately U.S. $600 each and maintenance costs are about U.S. $50 year -1. The diesel pump is expected to have a life of 10 years and its resale value is assumed to be negligible. The total discounted cost of the diesel pump over a period T of 10 years was calculated (by considering a 10% annual increase in diesel price):

129

C = ~, K + d) t_ 1 d)n- 1 (1) n=l t=l (1 + (1 +

where C is the discounted cost of the installed capacity, K is the capital cost, O = m + f (where m is the maintenance cost and f is the fuel cost), I is the lifetime of the diesel pump set (l = 10 years) and d is the discount rate (d = 10%). The present discounted cost thus calculated for a diesel pump set with a lifetime of 10 years is U.S. $4100. This pump set can discharge l 0 s m 3 of water for 10:years of operation. Thus the cost of pumping water is U.S. $0.041 m -3. It is interesting to note that in a recent United Nations Development Program (UNDP) project on small-scale irrigation the water cost was assumed to be 5 U.S. ~ m -3 (at 1979 prices) [10].

For the solar photovoltaic pump set to be cost effective the cost of pumping water should be of the order of U.S. $0.041 m -3 if 104 m s of water is to be pumped annually. The average annual cost thus will be U.S. $410. If a very conservative estimate of the average annual maintenance cost of U.S. $50 year -1 is taken for the entire PEPS, the average annual capital cost must not exceed U.S. $360 if the total cost is to be kept within the target. Thus, with a life of 10 years, a negligible resale value and a 10% discount rate the maximum acceptable discounted capital cost of a solar pump set is U.S. $2212 if a delivered water cost of U.S. $0.041 m -3 is to be achieved.

3.2. Rural electricity centre A typical small remote Indian village was taken as an example. Such a

village usually has a low load factor and it is considered to be uneconomic to connect it to the grid distribution line. The village comprises about 100 households of which 60 are electrifiable. The area of land that can be cultivated is 70 ha. There are two small-scale industries and 1 km of road on which street lights may be placed within the village. There are three communi ty centres. There is no scarcity of potable water for which there is no need for electrical energy. A detailed analysis of the energy needs, the load pattern and the storage requirements etc. has already been reported [11]. It was assumed that the village is remote and that because of the low load factor it is not economically viable to obtain power through transmission lines. Therefore, an on-site diesel generator was used for comparison for this village energy centre. It is estimated that for daytime loads like irrigation pumping and small-scale industries which operate for a total of 8 h a 50 kV A diesel generator is required. For night-time loads such as domestic lighting, lighting for the communi ty centre and street lighting operating for a period of 4 h, a 4 kV A diesel generating set is needed. The various parameters are shown in Table 2. The break-even cost for the photovoltaic system, calculated in a manner similar to that in Section 3.1 with the aid of eqn. (1), is U.S. $4351 kW -1 on the assumption of a power factor of 0.8. Thus the break-even discounted capital cost of photovoltaic generation should be U.S. $4351 kW -I for a typical village energy centre in India.

130

TABLE 2

Various parameters for the diesel generators which provide energy for the village

Systems Annual Fuel Average annual Present discounted (kV A) operating consumption maintenance costs (U.S. $)

days (1 h -1) cost (U.S. $)

50 330 14 625 15843 4 365 1.5 100 2430

4. Allowable cell conversion efficiency

To determine the allowable cell conversion efficiency and to study the cost effectiveness of the photovoltaic system, the photovoltaic system cost (PSC) should be estimated, including factors such as storage, power conditioning and installation. PSC may be calculated to be

AC PSC = - - AA + PSO(SGC + PC) (2)

A

where AA is the array area, AC/A is the area-related cost per unit area, PSO is the photovoltaic systems output {kW), SGC is the storage cost (kW) and PC is the power conditioning cost (kW). The power conditioning includes any modified form of load such as the compact motor pump unit that will be used for microirrigation in place of the diesel pump set. The area of the photovoltaic array is given by

PSO AA - (3)

~plant flpeak

where

T~plant ---- ~cT~SG~P C

and

AC CC SC WC INSC CC CAr - + - - + - - + - - - + - - ( 4 )

A A A A A A A

where ~7¢ is the module conversion efficiency, ~?SG is the storage efficiency, 7?pc is the power-conditioning efficiency, Ipeak is the peak insolation (kW per unit area), CC/A is the cell cost per unit area, SC/A is the structure cost per unit area, WC/A is the wiring cost per unit area, INSC/A is the installation cost per unit area and CAr is the structure cost plus the wiring cost plus the installation cost and is basically the area-related BOS cost. The insolation pattern factor f in eqn. (3) is due to the insolation pattern which is assumed to be trapezoidal with an 8 h daytime such that for every peak watt of a solar cell panel about 5 W h day -1 can be obtained, a typical value for the tropical region [6, 7]. The value of f is such that f X 8 = 5, which gives

131

f = 0.625. Combining eqns. (2) - (4), we obtain the power plant cost in U.S. dollars per kilowatt:

PSC CC/A + CArlA - + SGC + PC (5)

PSO ~?c ~TsG~TPcflp~ak

4.1. Microirrigation Let us suppose that a photovoltaic microirrigation system with a pump-

ing capacity of 10 a m a year -1, and with the same cost per unit volume of water pumped as a diesel pump, is required to irrigate a I ha farm.

This size of solar-powered microirrigation system is chosen so that it can irrigate 1 ha of land. The details of the energy requirements, the size of the solar cell panel and the storage battery etc. were calculated on the assumption that the transevaporation rate is 2.5 mm day -1 , the water depth is 5 m, the pumping efficiency is 50% and the distribution efficiency is 70% [11]. The results are presented in Table 3. The actual values of the BOS costs involved in the two sample installations are given in Table 4.

TABLE 3

Solar photovoltaic system for microirrigation

Working hours Energy Panel size Array o f the day (kWh day -1 ) (Wp) structure area

(m 2)

Storage capacity

08.00 - 16.00 0.96 330 10 30 A h, 48 V

With the aid of eqn. (5) and Tables 3 and 4, we can calculate the allowable module cost for different C~/A costs. This is shown in Fig. 2 for microirrigation. It appears from the graph that for low area-related component costs {structure costs, wiring costs and installation costs), and hence for low overall BOS costs, solar modules with a relatively low efficiency (about 5%) are economically viable. The graph also reflects the invariance of the allowable module cost with the efficiency of the solar cell modules for low BOS costs. This conforms with the observations made by Wolf [4]. In contrast, Fig. 2 indicates that for higher area-related BOS costs modules with a lower efficiency are difficult to manufacture because of the extremely low allowable cost. However, in the particular case of microirrigation, even for an area- related BOS cost as high as U.S. $80 m -2, the allowable module cost is about U.S. $1390 kW -1 with an efficiency of 10%, which is well within the DOE target. The actual area-related BOS costs experienced during the installation of the PEPS at the village of Charsarati (Fig. 3) were U.S. $14.4 m -2. From Fig. 2 it can be seen that with a module efficiency of 5% the allowable module cost for this case is U.S. $1.90 Wv -1 . This is a target which can certainly be achieved more easily than the DOE target of U.S. $0:7 Wp -1 with a module efficiency exceeding 10%.

132

TABLE 4

Balance-of-system cost breakdown

Microirrigation Village community system television centre

Array peak power 330 Wp 100 Wp Array area A 10 m 2 4 m 2

Area-related costs (1980 U.S. $) Array structure and site preparation 97 45 Module interconnection 5 2 Installation 12 8 Other (testing, inspection, 30 20

packaging etc.) Total area-related cost CAr 144 75 CAr/A 14.4 18.75

Non-area-related costs (1980 U.S. $) Storage 144 288 Field wiring 25 24 Motor-pump 170 Control 130 Power conditioning 82 Total non-area-related cost 469 394

BOS costs (1980 U.S. $) Total BOS cost 613 469 BOS cost per peak watt 1.86 4.69

4.2. Rural electricity centre F o r the rural e lec t r ic i ty cent re , it was shown in Sec t ion 3.2 t ha t the

d i scoun ted capi ta l cos t o f the so la r -powered sys t em should be U.S. $4351 kW -1 in order fo r this to be cos t e f fec t ive in c o m p a r i s o n wi th the diesel genera t ing set.

The a l lowable m o d u l e cos t fo r this sys tem can be ca lcula ted fo l lowing the p r o c e d u r e of Sec t ion 4.1 for d i f fe ren t CArlA costs. T h e resul ts o f the c o m p u t a t i o n s are shown in Fig. 4.

Figure 4 has b road ly the same character is t ics as Fig. 2 fo r microirr iga- t ion. The invar iance o f the a l lowable cell cos t wi th the e f f ic iency o f the solar cell modu le s fo r low BOS costs and the e c o n o m i c viabi l i ty o f us ing modu le s wi th a lower convers ion e f f ic iency ( a b o u t 5%) are m o r e p r o n o u n c e d in Fig. 4. In cont ras t , fo r h igher area-re la ted BOS costs, and hence fo r higher BOS costs, i t is seen f r o m Fig. 4 t ha t m o d u l e s wi th a lower e f f ic iency (less than 10%) are n o t economica l ly viable fo r appl ica t ions such as a village energy cent re . The ac tual area-rela ted BOS cost in this case is n o t available bu t exper ience with a village c o m m u n i t y television cen t re p o w e r e d b y a PEPS installed in the same village provides suff ic ient ind ica t ion t h a t the area- re la ted BOS cost wou ld be o f the same order as t ha t fo r microi r r iga t ion . T h e

133

rL= 16%

n~ "-. ~ . ~ . i0o/o

8°/°

N 10

o,

0 14.4 25 50 7J5

A-R-BOS COST IN $/rn 2

Fig. 2. Effect of the area-related BOS cost on the allowable module cost for a microirrigation system.

Fig. 3. Solar-powered village community centre at the village of Charsarati, Kalyani, West Bengal.

details of this installation are described in Appendix A and the actual BOS costs of the various components are indicated in Table 4. It can be seen that the area-related BOS cost for the village community centre is U.S. $18 .75 m -2, well below the DOE target of U.S. $60 m -2 in the U.S.A. If the size of

134

1200

? \

~ 800

u

0

8 ~

I

215 50 A-R-BOS COST IN ~/m 2

Fig. 4. Effec t o f the area-related BOS cost on the allowable module cost for a rural elec- t r ici ty centre .

the PEPS was scaled up to provide electricity for the entire village, i.e. to act as a rural electricity centre, this would bring down the area-related BOS cost further, as evidenced from Fig. 1.

5. Conclusions

The BOS costs of photovoltaic systems determine the allowable cell cost and the required conversion efficiency of the solar cell modules. The lower area-related BOS costs in developing countries like India allow the use of relatively low efficiency (about 5%) solar cell modules. This permits the development of low cost solar cells which stand a better chance of commercial realization than highly efficient and simultaneously low cost solar cell modules. The higher BOS costs in the developed countries restrict the commercial exploitation of solar cell modules with efficiencies lower than 10%- 12%. An interesting result of low BOS costs is that the allowable

135

module cost becomes almost independent of the efficiency indicating that there is no advantage in developing high efficiency cells when the BOS costs are relatively low. Thus, for terrestrial applications of solar photovoltaic systems the lower BOS costs lead to a much higher allowable module cost (about U.S. $2 Wp -1) and a lower efficiency {about 5%) for solar cell modules in the developing countries and reflect the technical feasibility and viability of photovoltaics in this part of the world, particularly for rural applications.

Acknowledgments

The present work has been carried out under the Depar tment of Science and Technology, Government of India, research grant for the project entit led "Photovoltaic electric power systems for rural development" .

The authors also wish to thank Dr. T. K. Bhat tacharya of Central Elec- tronics Ltd. and Mr. N. Barat and Mr. A. Dasgupta of Chloride India Ltd., Calcutta, for many helpful discussions.

References

1 National Photovoltaic Program Multiyear Program Plan, DOE/ET-OIO5-D (draft), June 6, 1979 (U.S. Department of Energy).

2 G. F. Hien, J. P. Cusick and W. A. Poley, Proc. 13th Photovoltaic Specialists' Conf , Washington, DC, June 5 - 8, 1978, IEEE, New York, 1978.

3 E. A. DeMeo and D. F. Spencer, Proc. Int. Workshop on CdS Solar Cells and other Abrupt Heterojunctions, Delaware, April 30 - May 2, 1975, University of Delaware, NE, p. 109.

4 M. Wolf, Proc. 3rd Commission of the European Communities Conf. on Photovoltaic Solar Energy, Cannes, October 27 - 31, 1980, Reidel, Dordrecht, 1980, p. 204.

5 R. Ross, 14th Project Integration Meet. o f the U.S. Department o f Energy-Jet Propulsion Laboratory Low Cost Solar Array Program, December 5 - 6, 1979, an- nouncement.

6 H. Saha, P. Basu and K. Mukhopadhyay, Proc. 3rd Commission o f the European Com- munities Conf. on Photovoltaic Solar Energy, Cannes, October 27 - 31, 1980, Reidel, Dordrecht, 1980, p. 541.

7 P. Basu, K. Mukhopadhyay and H. Saha, Proc. Natl. Solar Energy Convention, Anna- malainagar, 1980, Allied Publishers, New Delhi, p. 400.

8 D. V. Smith and S. V. Allison, Micro-irrigation with photovoltaics, MIT Rep. (draft), 1978 (Energy Laboratory, Massachusetts Institute of Technology).

9 J. K. Parikh, Sol. Energy, 21 (1978) 99. 10 Testing and demonstration of small scale solar powered pumping systems, UNDP

Rep., December 1979 (United Nations Development Program) (Project GLO/78/004). 11 H. Saha, Sol. Energy, 27 (1981) 103.

Appendix A

A village communi ty centre (Fig. 3) at the village of Charsarati, about 5 km from the University o f Kalyani, West Bengal, was set up in December

136

1980 under the project entitled "Photovoltaic electric power systems for rural development" sponsored by the Department of Science and Technol- ogy, Government of India. A television is used to entertain the villagers and lights are used to light the adult education classes, which are held regularly at the centre.

A solar-powered d.c. pump with a power of 300 W has recently been installed to irrigate 1 ha of land adjacent to the centre. The panels were placed on the roof of the centre by extending the array structure. The module connections were made with 7/20 in gauge wire. The details of this community centre and of the pump set are given in Sections A.1 and A.2.

A.1. Village community television centre

Array output: 100 W Number of modules: 8 Array area: 40 ft 2 Voltage: 24 V Load: one 60 W television; three 20 W fluorescent lamps Power condit ioning:one 24 - 110 V d.c.-to-d.c, converter for televison;

three 24 - 150 V d.c.-to-a.c, inverters for tube lights Battery: two 12 V, 120 A h (20 h rate) lead acid batteries Wire used for connections: 3/20 in and 7/20 in gauge copper wire Array structure: painted mild steel flat plate 1.5 in × 0.25 in

A. 2. Micro irriga tio n

Array output: 300 W Number of modules: 20 Array area: 100 ft 2 Voltage: 48 V Water head: 5 m Cultivated area: 1 ha Load: one 48 V, 300 W d.c. motor pump Power conditioning: pulse-width modulated switching regulator Battery: 30 A h, 48 V (20 h rate) Wire used for connections: 3/20 in and 7/20 in gauge copper wire Array structure: painted mild steel flat plate 1.5 in X 0.25 in

Documents

Impact of balance-of-system costs on photovoltaic electric power systems