Emissions from Photovoltaic LifeCyclesV A S I L I S M . F T H E N A K I S , *,†,‡H Y U N G C H U L K I M ,†A N D E R I K A L S E M A§PV Environmental Research Center, Brookhaven NationalLaboratory, Upton, New York, Center for Life Cycle Analysis,Columbia University, New York, New York, and CopernicusInstitute of Sustainable Development, Utrecht University,Heidelberglaan 2, 3584 CS Utrecht, The NetherlandsReceived July 17, 2007. Revised manuscript receivedDecember 19, 2007. Accepted January 4, 2008.Photovoltaic (PV) technologies have shown remarkableprogress recently in terms of annual production capacity andlife cycle environmental performances, which necessitatetimelyupdatesofenvironmentalindicators.BasedonPVproductiondata of 2004–2006, this study presents the life-cycle greenhousegas emissions, criteria pollutant emissions, and heavy metalemissions from four types of major commercial PV systems:multicrystalline silicon, monocrystalline silicon, ribbon silicon,and thin-film cadmium telluride. Life-cycle emissions weredetermined by employing average electricity mixtures in Europeand the United States during the materials and moduleproduction for each PV system. Among the current vintage ofPV technologies, thin-film cadmium telluride (CdTe) PVemits the least amount of harmful air emissions as it requiresthe least amount of energy during the module production.However, the differences in the emissions between differentPV technologies are very small in comparison to the emissionsfrom conventional energy technologies that PV could displace.As a part of prospective analysis, the effect of PV breederwas investigated. Overall, all PV technologies generate far lesslife-cycle air emissions per GWh than conventional fossil-fuel-based electricity generation technologies. At least 89% ofair emissions associated with electricity generation could beprevented if electricity from photovoltaics displaces electricityfrom the grid.1. IntroductionThe production of energy by burning fossil fuels releasesmany pollutants and carbon dioxide to the environment.Indeed, all anthropogenic means of generating energy,including solar electric, create pollutants when their entirelife cycle is taken into account. Life-cycle emissions resultfrom using fossil-fuel-based energy to produce the materialsfor solar cells, modules, and systems, as well as directly fromsmelting, production, and manufacturing facilities. Theseemissions differ in different countries, depending on thatcountry’s mixture in the electricity grid, and the variousmethods of material/fuel processing.Previous life-cycle studies reported a wide range ofprimary energy consumption for PV modules. Alsemareviewed studies of crystalline silicon photovoltaics from the1990s and found considerable variance among investigatorsin their estimates of primary energy consumption. In thosedays, manufacturing of solar cells was for the most part usingoff-spec products of electronic-grade silicon and variousallocation rules were applied to the energy andmaterial inputsfor each grade of silicon; also solar cells were much thickerthan the current ones (1). Meijer et al. evaluated 270-µm-thick Si PV with 14.5% cell efficiency fabricated fromelectronic-grade high-purity silicon (2). They estimatedenergy payback time (EPBT, the time it takes for a photo-voltaic (PV) system to generate an amount of energy equalto that used in its production) for the module only of 3.5years for the low level of insolation in The Netherlands (1000kWh/m2/yr). Jungbluth reported the life-cycle metrics ofvarious PV systems (2000 vintage) under average insolationin Switzerland (1100 kWh/m2/yr) (3). He estimated green-house gas (GHG) emissions in the range of 39–110 g CO2-equiv/kWh and EPBT of 3–6 years.There are a few life-cycle studies of thin-film PV tech-nologies; these include those of CdTe PV by Palz and Zibetta,Hynes et al.,and Kato et al., and theamorphous silicon studiesby Keoleian and Lewis (4–7). The CdTe studies were basedon R&D data and hypothetical production lines sincecommercial productionofsuch modulesstarted in 2004 (4–6).The study of Keoleian and Lewis was based on data from theearly operations of UniSolar, Alburn Hills, MI (7). Their studypresented that the EPBT of the frameless module of double-junction amorphous silicon with 5% efficiency is 4.6 yearsin Detroit, MI and 2.2 years in Phoenix, AZ. This study is notapplicable to the current production from the same companywhich comprises triple-junction modules of 8% efficiencies.Fthenakis and Alsema (8) reported the 2004-early 2005 statusof the EPBTs and of greenhouse gas (GHG) emissions in fourdifferent photovoltaic rooftop installations, namely ribbon-Si, multicrystalline Si (multi- or mc-Si), monocrystalline Si,and thin-film CdTe systems. Their corresponding EPBTs,under the average Southern European insolation of 1700kWh/m2/yr, were 1.7, 2.2, 2.7, and 1.1 years. The EPBT ofCdTe PV was much lower than that of the other systemsalthough its electrical-conversion efficiency was the lowestin the group (i.e., 9% for CdTe vs 11.5% for ribbon, 13.2% formulticrystalline Si, and 14% for monocrystalline Si).Reporting and comparing life-cycleemissions from energytechnologies that draw public health concern is an importantfacet in assuring the acceptability of any particular one. Inthis paper, we update the greenhouse gas emissions, andpresent the first comprehensive assessment of emissions ofcriteria pollutants and heavy metals, from cradle to gate, offour commercial PV systems based on the most recent data(i.e., 2004–2006): ribbon-silicon, multicrystalline silicon,monocrystalline silicon, and thin-film cadmium telluride.The heavy metal, toxic gas, and GHG emissions are the mainemissions from the considered commercial PV technologies.These are, for the most part, indirect emissions associatedwith the use of fossil fuelsin the generation of energy requiredin the life cycle of photovoltaics. Direct emissions of heavymetalsfrom mining and smeltingincluding particulatematterare also included, whereas liquid and solid waste are for themost part being recycled, and were not considered in thisstudy. The choice of electricity and fuel sources plays animportant role in determining the total emissions. In thiscontext, we investigated a scenario of PV breeding where PVsupplies a fraction of the electricity required in manufacturing.* Corresponding author tel: 631-344-2830; fax: 631-344-7650;e-mail: