Photovoltaics: Life-cycle analyses

1 Photovoltaics: Life-cycle Fthenakisa,b,*, KimaaCenter for life cycle Analysis, Columbia University, New York, NY, USAbPhotovoltaic Environmental Research Center, Brookhaven National Laboratory, Upton, NY, USAA vailable online 23 February 2010 Communicated by: Associate Editor Yogi GoswamiAbstractLife- cycle analysis is an invaluable tool for investigating the environmental profile of a product or technology from cradle to Life-cycle analyses of energy technologies are essential, especially as material and energy flows are often interwoven, and divergentemissions into the environment may occur at different Life-cycle -stages. This approach is well exemplified by our description of materialand energy flows in four commercial PV technologies, , mono-crystalline silicon, multi-crystalline silicon, ribbon-silicon, and cadmiumtelluride. The same Life-cycle approach is applied to the balance of system that supports flat, fixed PV modules during operation.

2 We alsodiscuss the Life-cycle environmental metrics for a concentration PV system with a tracker and lenses to capture more sunlight per cell areathan the flat, fixed system but requires large auxiliary components. Select Life-cycle risk indicators for PV, , fatalities, injures, and max-imum consequences are evaluated in a comparative context with other electricity-generation pathways. 2009 Elsevier Ltd. All rights :Photovoltaics; Life-cycle analysis; Life-cycle assessment; Environmental and health effects; Energy payback times1. IntroductionPhotovoltaics (PV), made from semiconducting materi-als, convert photons into electricity. When sunlight hitsthese materials, photons with a certain wavelength triggerelectrons to flow through the materials to produce directcurrent (DC) electricity. Commercial PV materials includemulti-crystalline silicon, mono-crystalline silicon, amor-phous silicon, and thin film technologies, such as cadmiumtelluride (CdTe), and copper indium diselenide (CIS).

3 Atypical PV system consists of the PV module and the bal-ance of system (BOS) structures for mounting the PV mod-ules and converting the generated electricity to alternatecurrent (AC) electricity of the proper magnitude for usagein the power analysis (LCA) is a framework for consideringthe environmental inputs and outputs of a product or pro-cess from cradle to grave. It is employed to evaluate theenvironmental impacts of energy technologies, and theresults are increasingly used in decisions about R&D fund-ing and in formulating energy policies. Informational pub-lications for decision-makers in the European Community(European Commission, 2003) and in Australia (AustralianCoal Industry Association Research Program (ACARP),2004) indicated that photovoltaics have relatively highenvironmental impacts compared with other technologies, , greenhouse-gas (GHG) emissions of 180 g in Germany, and 100 g CO2-eq/kWh in Australia,10 and times higher than the GHG emissions of thenuclear-fuel cycle for each country, and 45% and 23% ofthose of combined cycle (CC) natural-gas power generationin the same impacts reflect the fossil-fuel-based energy used inproducing the materials for solar cells, modules, and sys-tems; however, the data used in these studies were outdated0038-092X/$ - see front matter 2009 Elsevier Ltd.

4 All rights *Corresponding author. Address: photovoltaic Environmental Re-search Center, Brookhaven National Laboratory, Upton, NY, Fthenakis). online at Energy 85 (2011) 1609 1628and some assumptions made were invalid. In this paper wesummarize the results of PV Life-cycle analyses based oncurrent data for three silicon and one thin-film technolo-gies, emphasizing basic metrics including energy paybacktimes (EPBTs), GHG emissions, criteria pollutant emis-sions, toxic metal emissions, human injuries and Previous studiesPrevious Life-cycle studies reported a wide range of pri-mary energy consumption for Si-PV modules. Alsemareviewed such analyses from the 1990s and found consider-able variance between investigators in their estimates ofprimary energy consumption (Alsema, 2000). Normalizedper m2, the researchers reported 2400 7600 MJ of primaryenergy consumption for mc-Si, and 5300 16,500 MJ formono-Si modules. Besides uncertainties in the data, heattributed these differences mainly to the assumptionsand allocation rules that each author adopted for modelingthe purification and crystallization stages of silicon.

5 Inthose days, solar cells were mostly tailored from off-specproducts of electronic-grade silicon not directly fromsolar-grade silicon, so that multiple allocation rules mightwell be applied to the energy and material inputs for eachgrade of silicon; currently, only 5% of solar cells are fromoff-spec electronic-grade silicon (Alsema, 2000; Alsemaand de Wild-Scholten, 2005). Selecting only those processsteps needed to produce solar-grade silicon, Alsema sown estimates were 4200 and 5700 MJ/m2for mc-Si andmono-Si modules, respectively (Alsema, 2000). These val-ues correspond to an energy payback time (EPBT) of years, and Life-cycle greenhouse-gas emissions of46 and 63 g for mc-Si PV with 13% efficiencyand mono-Si with 14% efficiency, respectively, underSouthern European (Mediterranean) conditions: insolationof 1700 kWh/m2/year, and a performance ratio of balance of system (BOS) components, such as amounting support, a frame, and electrical componentsaccount for additional years of EPBT, and 15 of GHG et al.

6 (2003)more recently assessed a slightlyhigher energy expenditure of 4900 MJ/m2to produce anmc-Si module. They assumed that the 270-lm thick SiPV with cell efficiency was fabricated from elec-tronic-grade high-purity silicon, which entails greaterenergy consumption. Their corresponding EPBT estimatefor the module was years excluding BOS components; , higher than Alsema s earlier determination of The increase stems mainly from the low level of inso-lation in the Netherlands (1000 kWh/m2/year) comparedwith the average for Southern Europe (1700 kWh/m2/year), and, to a less degree, from the higher energy estima-tion for silicon (Alsema, 2000; Meijer et al., 2003). Jungb-luth reported the Life-cycle metrics of various PV systemsunder environmental conditions in Switzerland in 2000(Jungbluth, 2005). He considered the environmentalimpacts for 300-lm thick multi- and mono-Si-PV modulewith and conversion efficiency, on which of the two materials he evaluated,and their applications ( , fac ade, slanted roof, and flatNomenclaturea-Si amorphous siliconACalternate currentBOS balance of systemc-Sicrystalline siliconCdS cadmium sulfideCdTe cadmium tellurideCIGS copper indium gallium selenideCIS copper indium diselenideDCdirect currentDNI direct normal irradianceEPBT energy payback timeESP electrostatic precipitatorsFBR fluidized bed reactorGaAs gallium arsenideGHG greenhouse gasGWP global warming potentialHCl hydrogen chlorideHCPV high-concentration PVHF hydrogen fluorideLCA Life-cycle analysis (or assessment))

7 LCI Life-cycle inventoryLPG liquefied petroleum gasmc-Si multi-crystalline siliconmono-Simono-crystalline siliconNAICS North American Industry Classification SystemNG natural gasNOxnitrogen oxidePMparticulate matterPRperformance ratioPSA probabilistic safety assessmentPSIPaul Scherrer InstitutePVphotovoltaicsRMP Risk Management ProgramSiH4silaneSiHCl3trichlorosilaneSO xsulfur oxideTeO2tellurium dioxideTPE thermoplastic elastomerUCTE Union for the Coordination of Transmission ofElectricityVTD vapor transport Fthenakis, Kim / Solar Energy 85 (2011) 1609 1628roof), he arrived at figures of 39 110 g ofGHG emissions and 3 6 years of EPBT for the averageinsolation of 1100 kWh/m2/year in that country. Heassumed that the source of silicon materials was 50% fromoff-grade silicon and 50% from electronic-grade silicon,which is distant from the composition of the current(2005) PV supply (Alsema and de Wild-Scholten, 2005;Jungbluth, 2005).

8 There are fewer Life-cycle studies of thin-film PV tech-nologies; evaluations of the Life-cycle primary energy con-sumption of amorphous silicon ranged between 710 and1980 MJ/m2(Alsema, 2000). The differences are largelyattributed to the choice of substrate and encapsulationmaterials. The lowest estimate, made byPalz and Zibetta(1991), considered a single glass structure, while the highestone byHagedorn (1992)was based on a double-glass con-figuration to protect the active layer (Alsema, 2000; Palzand Zibetta, 1991). For CdTe PV,Hynes et al. (1994)basedtheir energy analysis on two alternative technologiescurrent at that time. The first employed non-vacuumelectro-deposition of a absorber layer (CdTe), inconjunction with chemical-bath deposition of the layer (CdS); the second method deposited bothof these layers , 5-lm thick absorber layer and thick window layer by thermal evaporation primary energy estimate for the first technologywas 993 MJ/m2, and that for the second was 1188 et al.

9 S (2001)energy estimates were pertinentto the scale of annual production; they suggested thatenergy consumption will decline as the scale of productionrises; they cited values of 1523, 1234, and 992 MJ/m2forframeless modules with annual capacities of 10, 30, and100 MWp,1respectively. However, these earlier estimatesfall far short of describing present-day commercial-scaleCdTe PV production that, unlike previously, now encom-passes many large-scale production life cycle of photovoltaicsThe Life-cycle stages of photovoltaics involve (1) the pro-duction of raw materials, (2) their processing and purifica-tion, (3) the manufacture of modules and balance of system(BOS) components, (4) the installation and use of the sys-tems, and (5) their decommissioning and disposal or recy-cling (Fig. 1).Production starts with mining the raw materials ( ,quartz sand for silicon PV; Zn- and Cu-ores for CdTePV), and continues with their processing and purification(Fig.)

10 2)(Fthenakis et al., 2008). The silica in the quartzsand is reduced in an arc furnace to metallurgical-grade sil-icon, which must be purified further into electronic-grade or solar-grade silicon, typically through a Siemens process. Crystalline silicon modules typically are framedfor additional strength and easy mounting. The recentLCAs for crystalline silicon are based on Life-cycle inven-tory (LCI) data provided, collectively, by eleven Europeanand US photovoltaic companies participating in the Euro-pean Commission s Crystal Clear project. The data setswere published in separate papers byAlsema and deWild-Scholten (2005)and byFthenakis and Alsema (2006).The Life-cycle inventories of the minor metals used inthin-film PVs such as Cd, In, Mo, and Se, are closelyrelated to the production cycle of base metals (Zn, Cu).The allocations of emissions and energy use between theformer and the latter during mining, smelting and refiningstages are described elsewhere (Fthenakis et al.