a life-cycle analysis on thin-film cds/cdte pv modules

Solar Energy Materials & Solar Cells 67 (2001) 279}287

A life-cycle analysis on thin-"lm CdS/CdTe PVmodules

Kazuhiko Kato!,*, Takeshi Hibino", Keiichi Komoto#,Seijiro Ihara$, Shuji Yamamoto%, Hideaki Fujihara&

!Electrotechnical Laboratory, 1-1-4 Umezono, Tsukuba, Ibaraki 305-8568, Japan"Matsushita Battery Industrial Co., Ltd., 1-1 Matsushita-cho, Moriguchi, Osaka 570, Japan#Fuji Research Institute Corporation, 2-3 Kandanishiki-cho, Chiyoda-ku, Tokyo 101, Japan

$Nippon Institute of Technology, Miyashiro, MInami-Saitama, Saitama 345-8501, Japan%PVTEC, 1-22-12 Otowa, Bunkyo, Tokyo 112-0013, Japan

&NEDO, Sunshine 60, 3-1-1 Higashi-Ikebukuro, Toshima, Tokyo 170-6027, Japan

Abstract

Authors have evaluated the life cycle of a thin-"lm CdS/CdTe PV module to estimate theenergy payback time (EPT) and the life-cycle CO

2emissions of a residential rooftop PV system

using the CdS/CdTe PV modules. The primary energy requirement for producing 1m2 of theCdS/CdTe PV module was similar to a-Si PV module at annual production scale of 100MW.EPT was calculated at 1.7}1.1 yr, which was much shorter than the lifetime of the PV systemand similar to that of a-Si PV modules. The life-cycle CO

2emissions were also estimated at

14}9 g-C/kWh, which was less than that of electricity generated by utility companies. ( 2001Published by Elsevier Science B.V. All rights reserved.

Keywords: Life-cycle analysis; CdS/CdTe solar cell; Energy payback time; CO2

emissions

1. Introduction

CdS/CdTe solar cell, a type of thin-"lm solar cells, have the potential to bemass-produced at low cost, even though current shipment of CdS/CdTe PV modulesare limited (1.2MW in 1998) [1]. In Japan, the CdS/CdTe solar cell, as well as the a-Sisolar cell, is now being developed under the `New Sunshine Programa initiatedby MITI. This program has a target of 140Yen/W-module at annual production

*Corresponding author.

0927-0248/01/$ - see front matter ( 2001 Published by Elsevier Science B.V. All rights reserved.PII: S 0 9 2 7 - 0 2 4 8 ( 0 0 ) 0 0 2 9 3 - 2

Fig. 1. A schematic of a thin-"lm CdS/CdTe solar cell.

capacity of 100MW. Since toxic substances are employed in the manufacture of thesecells, however, they will not be introduced on a large scale before their environmentalaspect has been studied.

Recently, life-cycle analysis is being popular to evaluate environmental aspects ofvarious products including solar cells. Though many studies on life-cycle of Si-basedsolar cells have been reported [2], life-cycle analyses for CdS/CdTe solar cell havebeen reported only by Hynes et al. [3] and Alsema et al. [4,5].

We are now researching on life cycle of the thin-"lm CdS/CdTe PV modules underdevelopment in the `New Sunshine Programa, and estimated energy payback timeand life-cycle CO

2emissions as the "rst part of our study.

2. Major assumptions

2.1. General description of CdS/CdTe PV module production process underevaluation [6]

Fig. 1 depicts a schematic of a thin-"lm CdS/CdTe solar cell; its production processis described in Fig. 2. A TCO-layer is deposited on a previously cleaned substrateglass. CdS-layer is deposited on the TCO-layer with organic cadmium compound byMOCVD. After grooves have been formed on the CdS-layer by a laser, CdTe-layer isformed using the atmospheric pressure close spaced sublimation technique. This isfollowed by a thermal treatment carried out with CdCl

2, and mechanical patterning is

done. Finally, CdS/CdTe solar cell is completed by screen printing with both carbonand silver contact. Fig. 3 shows the PV module structure that we employed.

Various technological advances in the module production, which are expectedaccording to expansion of production scale in the near future, have been assumed inthis study. Major technological parameters for the production of CdS/CdTe PVmodule is given in Table 1.

2.2. Life-cycle analysis

Major primary energy requirement (PER) for the production of thin-"lmCdS/CdTe PV modules as shown in Fig. 2 was taken into account, i.e. energy required

280 K. Kato et al. / Solar Energy Materials & Solar Cells 67 (2001) 279}287

Fig. 2. Evaluated process steps for the production of thin-"lm CdS/CdTe solar cells.

Fig. 3. Evaluated structure of CdS/CdTe PV module.

for manufacturing input materials, capital equipment and buildings, direct processenergy, and indirect process energy such as air conditioning and lighting. Althoughfacilities for waste management and decommissioning/recycling are important forCdS/CdTe solar cells, they were not considered in this study. Electrical energyrequired for both production and annual electrical output of the PV system wereconverted into equivalent primary energy with recent average conversion e$ciency ofthe utilities in Japan (0.35).

2.3. Energy payback time and life-cycle CO2 emissions

`Energy payback time (EPT)a } the number of years required to recover thelife-cycle energy input by annual energy output } can be calculated by dividing thetotal PER for the PV system by the equivalent annual primary energy output of thePV system.

The amount of CO2

emitted during the manufacture of the PV system, on the otherhand, was estimated by using both the PER and CO

2emission rates of di!erent

energy forms. Life-cycle CO2

emissions (g-C/kWh) can be estimated by dividing the

K. Kato et al. / Solar Energy Materials & Solar Cells 67 (2001) 279}287 281

Table 1Major assumptions for CdS/CdTe PV module production

Annual production 10MW 30MW 100MW

CellE$ciency (%) 11 12 13Dimension (width]length) (mm) 455]910 910]910Overall yield (%) 88Substrate thickness (mm) 3.0TCO-layer thickness (m) 0.5CdS-layer thickness (m) 0.07 0.06 0.05CdTe-layer thickness (m) 3Carbon-paste requirement (g/m2) 18.5 17.0 15.0Ag-paste requirement (g/m2) 31 28 25

ModuleDimension (width]length) (mm) 910]910Fraction of e!ective cell area (%) 94 96E$ciency 10.3 11.2 12.4Front cover "lm thickness (m) 50Back cover sheet thickness (mm) 0.50 0.45 0.40Al frame requirement (g/m) 280Butyl rubber requirement (g/m) 57 52 36Silicone rubber requirement (g/m) 23 22

total CO2

emissions throughout the life cycle of the PV system by the life-cycleelectrical output. The lifetime of the PV system was assumed to be 20 yr in this study.

3. Results and discussion

3.1. Primary energy requirement (PER) and CO2 emissions during CdS/CdTe PV moduleproduction

Table 2 shows the estimated total PER and the total CO2

emissions of the thin-"lmCdS/CdTe PV module.

Total PER for the CdS/CdTe PV module production was estimated at 1803MJ/m2

(17.5MJ/W) at 10MW/yr, 1514MJ/m2 (13.5MJ/W) at 30MW/yr and 1272MJ/m2

(10.2MJ/W) at 100 MW/yr. Total CO2

emissions were also calculated at 26.5 kg-C/m2 (258 g}C/W) at 10MW/yr, 22.8 kg-C/m2 (204 g-C/W) at 30 MW/yr and 18.6 kg-C/m2 (149 g-C/W) at 100MW/yr.

PER for cell production accounted for more than 50% of the total PER for moduleproduction. In the cell production process, a lot of primary energy was required forTCO-layer deposition, CdS-layer deposition and CdTe-layer deposition, in whichlarge part of PER was the direct process energy. It should be noted that substrate glassalso contributed to the increase of PER for cell production.

A fraction of the PER for module fabrication, which was one third of the total PERat the production scale of 10 MW/yr, increased to approximately 40% at 100 MW/yr.


Table 2Estimation results of primary energy requirement and CO

2emissions for production of thin-"lm

CdS/CdTe PV module by production process

Annual production scale 10MW 30MW 100MW

Energy CO2

Energy CO2

Energy CO2

(MJ/m2) (kg-C/m2) (MJ/m2) (kg-C/m2) (MJ/m2) (kg-C/m2)

Cell productionSubstrate glass 159 2.6 159 2.6 159 2.6Substrate cleaning 4 0.0 3 0.0 3 0.0TCO-layer deposition 164 1.8 115 1.3 100 1.1CdS-layer deposition 192 2.1 141 1.6 122 1.4Laser patterning 12 0.1 11 0.1 7 0.1CdTe-layer deposition 265 2.9 192 2.1 159 1.8Thermal treatment by CdCl

267 0.7 53 0.6 50 0.6

Mechanical patterning 19 0.2 18 0.2 8 0.1Carbon-contact formation 86 1.0 71 0.8 63 0.7Ag-contact formation 19 0.2 18 0.2 13 0.2Passivation 14 0.2 13 0.2 8 0.1Performance test 2 0.0 1 0.0 0 0.0Subtotal 1002 12.1 796 9.7 693 8.6

Module fabricationAl frame 280 3.5 280 3.5 280 3.5Back cover sheet 169 2.8 152 2.5 118 1.9Other materials 164 5.8 152 5.4 96 3.5Other energy input 6 0.1 6 0.1 4 0.0Subtotal 619 12.2 590 11.5 498 9.0

Overhead 182 2.2 128 1.6 82 1.0Module total 1803 26.5 1514 22.8 1272 18.6

(MJ/W) (g-C/W) (MJ/W) (g-C/W) (MJ/W) (g-C/W)17.5 258 13.5 204 10.2 149

Di!erent from the cell production, almost all the PER for module fabrication resultedfrom material input. PER for manufacturing Al frame particularly contributed toroughly half of the PER for module fabrication.

As shown in Table 2, the total PER per 1W of the CdS/CdTe PV module decreasedby 7.3MJ from 10MW/yr to 100 MW/yr. E$cient use of direct process energy due tomass-production resulted in the maximum contribution to the reduction in the totalPER, 50%, and the contribution of improvement in cell e$ciency accounted for 30%.

Fig. 4 shows a comparison of total PER per 1 m2 between the CdS/CdTe PVmodule and a-Si PV module that we estimated previously [7]. The CdS/CdTe PVmodule needed more direct process energy and less energy for manufacturing materialinput than the a-Si PV module in the cell production process. CdS/CdTe solar cellsand a-Si solar cells seem to be direct-energy intensive and material intensive, respec-tively. With respect to CdS/CdTe PV module fabrication, energy content of both backcover sheet and sealant increased PER more than the reduction in PER by using noEVA. Subsequently, the total PER per 1m2 of the CdS/CdTe PV module was


Fig. 4. A comparison of total PER per 1m2 between the CdS/CdTe PV module and a-Si PV module.

Fig. 5. Energy payback time of a rooftop residential PV system using the CdS/CdTe PV modules.

approximately on the same level as the a-Si PV module except for the case of30MW/yr. But it must be noted that the CdS/CdTe PV module had less PER per 1 Wthan the a-Si PV module because of the di!erence in cell e$ciency.

3.2. Energy payback time (EPT) and life-cycle CO2 emissions of a rooftop residentialPV system

Fig. 5 depicts the EPT of a rooftop residential PV system using the CdS/CdTe PVmodules. EPTs of the PV system using poly-Si and a-Si PV modules, which the


Fig. 6. Life-cycle CO2

emissions of a rooftop residential PV system using the CdS/CdTe PV modules.

authors previously estimated [7], are also shown. For production of the poly-Si solarcells, two near-future technologies, the `Direct Reduction Processa for Solar-grade Si(SOG-Si) production and `Electromagnetic Castinga for producing poly-Si ingots,were assumed. It was supposed that the PV system that had 0.81 of performance ratiowas connected to a utility grid. Capacity of the PV system was determined based onavailable rooftop space for PV module installation (approximately 30m2) and modulee$ciencies shown in Table 1. Concerning balance of system (BOS), PER and CO

2emissions for manufacturing array support (steel), cabling (copper) and power condi-tioner were taken into consideration. The annual irradiation in Japan ranges from1200 to 1630, and 1430kWh/m2/yr, which is almost the same as average annualirradiation in Tokyo, was employed in this study.

EPT of the PV system with poly-Si PV modules was 2.4 yr at 10MW/yr, 2.2 yr at30MW/yr and 1.6 yr at 100MW/yr, all of which were longer than when using eithera-Si PV modules or CdS/CdTe PV modules due to the high PER for SOG-Siproduction and wafer production. It seems to be di$cult for the poly-Si PV modulesto reduce PER by mass-production because the production of poly-Si solar cell ismaterial-intensive if SOG-Si is regarded as material input. Shorter EPT of the PVsystem with CdS/CdTe PV modules at 10MW/yr and 30MW/yr than when usinga-Si PV modules resulted from low e$ciency of the a-Si PV module (8% at 10 MW/yr,10% at 30MW/yr and 12% at 100MW/yr). However, both the EPTs for CdS/CdTePV modules and a-Si PV modules were the same at 100MW/yr, 1.1 yr.

Life-cycle CO2

emissions of the rooftop residential PV system with CdS/CdTe PVmodules is shown in Fig. 6 together with our previous estimate of poly-Si PV moduleand a-Si PV module. The life-cycle CO

2emissions of the PV system with poly-Si PV

modules were the highest. Regarding life-cycle CO2

emissions, the CdS/CdTe PVmodule and a-Si PV module produce approximately the same amount. These esti-mates suggest that all PV modules shown in this "gure can be useful in reducing CO

2


emissions compared to the recent average CO2

emission rate of utility companies inJapan (approximately 100 g-C/kWh).

4. Conclusion

In this study, the life cycle of a thin-"lm CdS/CdTe PV modules, which are beingdeveloped under the `New Sunshine Programa, was analysed and both total primaryenergy requirement (PER) and CO

2emissions during the CdS/CdTe PV module

production were estimated assuming various technological progresses and mass-production.

Total PER for producing 1 m2 of the CdS/CdTe PV module resulted in 1803MJ/m2

at 10 MW/yr, 1514MJ/m2 at 30 MW/yr and 1272MJ/m2 at 100 MW/yr. E$cient useof direct process energy due to mass-production contributed to 50% of the reductionin PER for CdS/CdTe PV module production at 100MW/yr compared with at10MW/yr. Compared with a-Si solar cells, the production process of CdS/CdTe solarcells was direct-process energy intensive. Thickness of CdS-layer and CdTe-layer hadno e!ect on the primary energy requirement for the cell production. The modulestructure that employs no EVA did not result in the reduction in the PER for modulefabrication due to high PER for back cover sheet and sealant. To reduce PER for theCdS/CdTe PV module, less energy-intensive materials have to be used. Consequently,the PER for producing 1m2 of the CdS/CdTe PV module was similar to a-Si PVmodule at 100MW/yr of annual production scale.

Furthermore, both EPT and life-cycle CO2

emissions of a rooftop residential PVsystem using the CdS/CdTe PV modules were estimated. The EPT resulted in 1.7 yr(10MW/yr) to 1.1 yr (100MW/yr), which was much shorter than the expected lifetimeof the PV system (20 yr) and similar to the EPT for a-Si PV modules at 100 MW/yr.Life-cycle CO

2emissions for CdS/CdTe PV modules were also on the same level of

that for a-Si PV modules: 14 g-C/kWh (10MW/yr) to 9 g-C/kWh (100MW/yr). Theywere quantitatively much less than the recent average CO

2emission rate of utility

companies in Japan. A series of our studies on life-cycle of PV modules has made clearthat residential rooftop PV systems are useful as alternative energy sources and canreduce a great deal of CO

2emission in Japan. But we have to take into account for

energy requirement for BOS components when these thin-"lm solar cells will be usedfor large-scale PV systems.

Environmental aspects must be evaluated for CdS/CdTe PV modules, however,since the CdS/CdTe PV modules uses toxic materials. Additional energy might berequired for processing toxic waste. It is also important to discuss decommissioningand recycling end-of-life CdS/CdTe PV modules.

Acknowledgements

This work has been supported by the Technical Committee on Evaluation ofPhotovoltaic Power Generation System that is organized in the PVTEC


(Photovoltaic Power Generation Technology Research Association) under the R&Dcontract with NEDO.

References

[1] P. Maycock, PV News, February 1999.[2] E. Niewlaar, E.A. Alsema, Report on th IEA PVPS Task I Workshop, Utrecht, the Netherlands, 1997.[3] K.M. Hynes, A.E. Baumann, R. Hill, First World Conference on PV Energy Conversion, Hawaii, 1994,

pp. 958-961.[4] E.A. Alsema, B.C.W. van Engelenburg, 11th E.C. Photovoltaic Solar Energy Conference, Montreux,

1992, pp. 995}998.[5] E.A. Alsema, Summary Report 96074, Dept. of Science, Technology and Society, Utrecht University,

1995.[6] M. Murozono, J. JSES, 24 (1) (1998) 25}28.[7] PVTEC: Survey study on photovoltaic power generation evaluation, JFY1998, NEDO Contract

Report, 1999.


a life-cycle analysis on thin-film cds/cdte pv modules

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