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2010/12/28-Emissions from Photovoltaic Life Cycles

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Emissions from Photovoltaic Life Cycles - Environmental Science & Technology (ACS Publications) http://pubs.acs.org/doi/full/10.1021/es071763q[12/27/2010 4:39:26 PM]

AbstractFull Text HTMLHi-Res PDF[1220 KB]PDF w/ Links[318 KB]Supporting Info Figures ReferencesCiting Articles Abstract1. IntroductionThe production of energy by burning fossil fuels releases many pollutants and carbon dioxide tothe environment. Indeed, all anthropogenic means of generating energy, including solar electric, create pollutants when their entire life cycle is taken into account. Life-cycleemissions result from using fossil-fuel-based energy to produce the materials for solar cells,modules, and systems, as well as directly from smelting, production, and manufacturing facilities. These emissions differ in different countries, depending on that country's mixture inEmissions from Photovoltaic Life CyclesVasilis M. Fthenakis

• f, Hyung Chul Kim f and Erik Alsema§PV Environmental Research Center, Brookhaven National Laboratory, Upton, New York, Center for Life Cycle Analysis, Columbia University, New York, NewYork, and Copernicus Institute of SustainableDevelopment, Utrecht University, Heidelberglaan 2, 3584 CS Utrecht, The NetherlandsEnviron. Sci. Technol.

, 2008 , 42 (6), pp 2168-2174DOI: 10.1021/es071763qPublication Date (Web): February 6, 2008 Copyright © 2008 American Chemical Society* Corresponding author tel: 631-344-2830 ; fax: 631-344-7650; e-mail:

vmf@bnl.gov., f Brookhaven NationalLaboratory., Columbia University., § Utrecht University.

ToolsAdd to FavoritesDownload CitationEmail a Colleague PermalinkOrder ReprintsRights & PermissionsCitation AlertsSciFinder LinksGet Reference DetailGet Substances Get Cited Get CitingExplore by:Author of this ArticleAny AuthorResearch TopicFthenakis, Vasilis M.

HistoryPublished In IssueMarch 15,2008Article ASAPFebruary 06,2008Received: July 17,2007Revised: December 19,2007Accepted: January 4,2008Recommend & Share CiteULike DeliciousDigg This Facebook NewsvineTweet ThisRelated ContentOther ACS content by these authors:Vasilis M. FthenakisHyung Chul Kim Erik AlsemaPhotovoltaic (PV) technologies have shown remarkable progress recently in terms of annualproduction capacity and life cycle environmental performances, which necessitate timely updates of environmental indicators. Based on PV production data of 2004-2006, this studypresents the life-cycle greenhouse gas emissions, criteria pollutant emissions, and heavymetal emissions from four types of major commercial PV systems: multicrystalline silicon, monocrystalline silicon, ribbon silicon, and thin-film cadmium telluride. Life-cycle emissionswere determined by employing average electricity mixtures in Europe and the United Statesduring the materials and module production for each PV system. Among the current vintageof PV technologies, thin-film cadmium telluride (CdTe) PV emits the least amount of harmfulair emissions as it requires the least amount of energy during the module production.

However, the differences in the emissions between different PV technologies are very smallin comparison to the emissions from conventional energy technologies that PV coulddisplace. As a part of prospective analysis, the effect of PV breeder was investigated.

Overall, all PV technologies generate far less life-cycle air emissions per GWh thanconventional fossil-fuel-based electricity generation technologies. At least 89% of airemissions associated with electricity generation could be prevented if electricity from photovoltaics displaces electricity from the grid.

ArticleDynamic Hybrid Life CycleAssessment of Energy and Carbon of Multicrystalline Silicon Photovoltaic Systems Environmental Science &

TechnologyMaterials Availability Expandsthe Opportunity for Large-Scale Photovoltaics Deployment Environmental Science &

TechnologyOptions for Near-TermPhaseout of CO 2 Emissionsfrom Coal Use in the United StatesEnvironmental Science &

TechnologyPublications A-Z HomelAuthors & Reviewers lLibrarians lACS Members lMobilelHelpLog In Register Cart Website Demos AnywhereEnviron. Sci. Technol.All Publications/Website ACS JournalsC&ENCASFthenakis,VasilisM.

Search Anywhere Search Emissions from Photovoltaic Life Cycles - Environmental Science & Technology (ACS Publications) http://pubs.acs.org/doi/full/10.1021/es071763q[12/27/2010 4:39:26 PM]the electricity grid, and the various methods of material/fuel processing.Previous life-cycle studies reported a wide range of primary energy consumption for PVmodules. Alsema reviewed studies of crystalline silicon photovoltaics from the 1990s and found considerable variance among investigators in their estimates of primary energy consumption. In those days, manufacturing of solar cells was for the most part using off-spec products of electronic-grade silicon and various allocation rules were applied to the energy and material inputs for each grade of silicon; also solar cells were much thicker than 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 estimated energy payback time (EPBT, the time ittakes for a photovoltaic (PV) system to generate an amount of energy equal to that used in its production) for the module only of 3.5 years for the low level of insolation in The Netherlands(1000 kWh/m 2/yr). Jungbluth reported the life-cycle metrics of various PV systems (2000vintage) under average insolation in Switzerland (1100 kWh/m 2/yr) (3). He estimatedgreenhouse gas (GHG) emissions in the range of 39-110 g CO 2-equiv/kWh and EPBT of 3-6 years.There are a few life-cycle studies of thin-film PV technologies; these include those of CdTe PVby Palz and Zibetta, Hynes et al., and Kato et al., and the amorphous silicon studies byKeoleian and Lewis (4-7). The CdTe studies were based on R&D data and hypotheticalproduction lines since commercial production of such modules started in 2004 (4-6). The studyof Keoleian and Lewis was based on data from the early operations of UniSolar, Alburn Hills, MI (7). Their study presented that the EPBT of the frameless module of double-junctionamorphous silicon with 5% efficiency is 4.6 years in Detroit, MI and 2.2 years in Phoenix, AZ.This study is not applicable to the current production from the same company which comprisestriple-junction modules of 8% efficiencies. Fthenakis and Alsema (8)2005 status of the EPBTs and of greenhouse gas (GHG) emissions in four different photovoltaicrooftop 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 1700 kWh/m 2/yr, were 1.7, 2.2, 2.7, and 1.1 years. The EPBT of CdTe PV wasmuch lower than that of the other systems although its electrical-conversion efficiency was thelowest in the group (i.e., 9% for CdTe vs 11.5% for ribbon, 13.2% for multicrystalline Si, and 14%for monocrystalline Si).Reporting and comparing life-cycle emissions from energy technologies that draw public healthconcern is an important facet in assuring the acceptability of any particular one. In this paper,we update the greenhouse gas emissions, and present the first comprehensive assessment of emissions of criteria pollutants and heavy metals, from cradle to gate, of four commercial PVsystems based on the most recent data (i.e., 2004-2006): ribbon-silicon, multicrystallinesilicon, monocrystalline silicon, and thin-film cadmium telluride. The heavy metal, toxic gas, and GHG emissions are the main emissions from the considered commercial PV technologies.These are, for the most part, indirect emissions associated with the use of fossil fuels in thegeneration of energy required in the life cycle of photovoltaics. Direct emissions of heavy metals from mining and smelting including particulate matter are also included, whereas liquidand solid waste are for the most part being recycled, and were not considered in this study.The choice of electricity and fuel sources plays an important role in determining the total emissions. In this context, we investigated a scenario of PV breeding where PV supplies afraction of the electricity required in manufacturing.2 Life Cycle of Silicon and Thin-Film CdTe PhotovoltaicsThe life cycle of PV systems starts with the mining of quartz sand (silicon PV) or metal ore(CdTe PV). The silica in the quartz sand is reduced in an arc furnace to obtain metallurgical grade silicon, which has to be purified further into electronic grade or solar grade silicon(Figure 1). The silicon purification step involves either the Siemens process in which areactor chamber with trichlorosilane (SiHCl

3) and hydrogen (H

2) gases is heated to 1100-1200°C for growing silicon rods, or the modified Siemens process in which silane (SiH) and Emissions from Photovoltaic Life Cycles - Environmental Science & Technology (ACS Publications) http://pubs.acs.org/doi/full/10.1021/es071763q[12/27/2010 4:39:26 PM]Figure 1. Simplified process-flow diagrams from mining to systemmanufacturing stages, namely cradle-to-gate for (a) mono-, ribbon-, and multi-Si PVs, and (b) thinfilm CdTe PVs. Detailed descriptions of the life cycles are available elsewhere (10, 12, 13)

.4hydrogen (H

2) gases are heated to 800 °C for the same, resulting in lower energy consumption (9).A detailed life-cycle inventory (LCI) of crystalline silicon modules for polycrystalline siliconfeedstock purification, crystallization, wafering, cell processing, and module assembly with the CrystalClear European Commission project (10). The sources of LCI data for this projectinclude 11 commercial European and U.S. photovoltaic module manufacturing companies supplemented by numbers from the literature. Depending on the type of cell material,crystallizing or growing the silicon wafer follows, along with tailoring the wafers. Cellmanufacturing and subsequent module assembly, which is virtually equal for each module, concludes their life cycle (Figure 1). Each module assembly typically consists of seventy-two0.125 m x 0.125 m solar cells with silver contacts in front and back sides. Ethylene-vinylacetate and glass sheets encapsulate the PV module to provide protection from the physical elements during operation. Crystalline silicon modules typically have aluminum frames foradditional strength and easy mounting.Fthenakis (11) described the material flows of cadmium (Cd) and emissions from the entire life-cycle stages of cadmium telluride (CdTe) PV. The life cycle starts with the production of Cd andTe which are byproducts of Zn and Cu smelting. Cd is obtained from a waste stream of Znsmelting, namely particulates collected in electrostatic precipitation and bag house and slimes collected from Zn electrolyte purification stages. Te is recovered from the slimes producedduring electrolytic copper refining, which also contain Cu, Se, and other metals. The slimes aretreated with dilute sulfuric acid to extract Te. After cementation with copper, CuTe is leached with caustic soda to produce a sodium-telluride solution that is used as the feed for Te and TeO 2. Cadmium is further processed and purified either through leaching and vacuum-distillation, or through electrolytic purification followed by melting and atomization or vacuum-distillation, to produce the 99.999% purity required for the synthesis of CdTe. Tellurium is alsofurther purified by the above-mentioned methods. CdTe is produced from Cd and Te viaproprietary processes.We analyzed in detail the life-cycle inventory of CdTe PV modules in commercial scaleproduction (12). The data were obtained at a CdTe PV manufacturing plant in Perrysburg, OHwith a 25-MW production capacity (www.firstsolar.com). The dimensions of this plant's typicalframeless CdTe module are 1.2 m x 0.6 m with an electricity conversion efficiency of 9% (The efficiency of the modules produced by this plant has increased to 10% as of September 2007;this improvement is not reflected in the current article). The cadmium telluride (CdTe)absorber layer and cadmium sulfide (CdS) window layer in First Solar's production scheme are laid down by vapor transport deposition (VTD), based on subliming the powders and condensingthe vapors on glass substrates. A stream of inert carrier gas guides the sublimed dense vaporcloud to deposit the films on glass substrates at 500-600 °C with a growth rate over 1 µm/s

(13). Depositing layers of common metals followed by series of scribing and heat treatmentforms interconnections and back contacts. No rare metals/elements are used in the backcontact layers.Table S1 in the Supporting Information of this paper presents the material compositions ofsilicon- and CdTe-modules. Glass is the heaviest part of PV module components, particularly ofthe frameless CdTe module where two panes of glass ensure structural toughness. The double-glass design eliminates the need for an aluminum frame which accounts for a significant Emissions from Photovoltaic Life Cycles - Environmental Science & Technology (ACS Publications) http://pubs.acs.org/doi/full/10.1021/es071763q[12/27/2010 4:39:26 PM]Figure 2. Life-cycle emissions from silicon and CdTe PV modules. BOS is theBalance of System (i.e., module supports, cabling, and power conditioning).Conditions: ground-mounted systems, Southern European insolation, 1700 kWh/m 2/yr, performance ratio of 0.8, and lifetime of 30 years. Case 1: currentdatabase. Case 2: Union of the Co-ordination of Transmission of Electricity(UCTE) grid mixture and Ecoinvent database. Case 3: U.S. grid mixture andFranklin database.fraction of emissions for the silicon modules. The use of CdTe powder per m 2 of thin film CdTemodule is minimal compared with silicon modules since the thickness of cell materials of the former is 3 µm compared with 270-300 µm for silicon modules. The CdTe module also requiressmaller amounts of gases, liquids, and other consumables than does a silicon module.3 GHG and Criteria Pollutant EmissionsWe estimate the emissions GHG, SO 2, and NO x during the PV life cycles. Together with theheavy metal emissions assessed later in this paper, these emissions comprise the main hazardsto the environment and human health from energy use and materials extraction during the PVlife cycle. These emissions are normalized by the electricity generated during the life cycle ofPV. The major parameters for the life cycle, i.e., lifetime electricity generation of a PVsystem, include conversion efficiency (E), solar insolation (I), performance ratio (PR), andlifetime (L). The total lifetime electricity generation (G) per m 2 of PV module is calculated asfollows: G = E x I x PR x L. We consistently use, for our own analysis, the Southern Europeanaverage insolation of 1700 kWh/m 2/yr, a performance ratio of 0.8, and a lifetime of 30 years.Alsema and de Wild report that the GHG emissions of Si modules for the year 2004 are withinthe 30-45 g CO 2-equiv/kWh range, with an EPBT of 1.7-2.7 years for a rooftop applicationunder Southern European insolation of 1700 kWh/m 2/yr and a performance ratio (PR) of 0.75(8, 10). Their estimates are based on the electricity mixture for the current geographicallyspecific production of Si (Figure 2, Case 1).Fthenakis and Kim (12) recently investigated the GHG emissions and EPBT of CdTe PV modules,based on U.S. production and insolation conditions (insolation = 1800 kWh/m 2/yr; performanceratio 0.8; lifetime 30 years). Their estimates were 24 g CO 2-equiv/kWh of GHG emissions, and1.1 yrs of EPBT for ground-mounted installations. In the following we updated the previousestimates and normalized them for constant solar irradiation, performance ratio, and electricity mixture. Figure 2, Case 2 shows emissions corresponding to upstream electricity forthe average grid mixture for continental Europe (Union of the Co-ordination of Transmission ofElectricity, UCTE), and Figure 2, Case 3 shows the same for the average U.S. grid mixture. Themost commonly used LCA databases, Ecoinvent for the European grid and Franklin for the U.S.grid mix, are employed for the energy and emission factors (14, 15).The production of polycrystalline silicon is the most energy-consuming stage of the siliconmodule's life cycle; it accounts for 45% of the total primary energy usage in the multi-Si module life cycle (10). Electricity demand during CdTe film deposition accounts for the greatestuse (i.e., 54%) of primary energy in the CdTe module life (12). The estimated emissions fromCase 1 which is based on the electricity mix of CrystalClear project, are lower than those from Cases 2 and 3, mainly because of the higher portion of hydropower and natural-gas-firedcombined-cycle power plants currently used by the producers of polycrystalline solar gradesilicon (Table S2 in the Supporting Information). For the same reason, the emission estimatesbased on the UCTE grid mixture (Case 2) typically are lower than those based on the U.S. grid(Case 3) (i.e., the former is a cleaner fuel mix). The life-cycle emissions from mono-Si PV are greater than those from other Si PVs mainly because the mono-Si requires substantial energy during the ingot growing process (by Czochralski crystal pulling).

Emissions from Photovoltaic Life Cycles - Environmental Science & Technology (ACS Publications) http://pubs.acs.org/doi/full/10.1021/es071763q[12/27/2010 4:39:26 PM]Figure 3. Life-cycle atmospheric Cd emissions for PV systems fromelectricity and fuel consumption, normalized for a Southern Europe averageinsolation of 1700 kWh/m 2/yr, performance ratio of 0.8, and lifetime of 30yrs. Ground-mounted BOS (18) is assumed for all PV systems;comparisons with other electricity generation options.4 Heavy Metal EmissionsWe followed the direct and indirect (due to energy use) emissions of heavy metals (arsenic,cadmium, chromium, lead, mercury, and nickel) during the life cycles of the four PV technologies we studied. The CdTe PV can emit Cd both directly and indirectly whereas the crystalline Si PV stages would emit such only indirectly.4.1 Direct Cd EmissionsFthenakis (11) compiled the direct, atmospheric Cd emissions from the life cycle of CdTe PVmodules based on 30 years of module lifetime, 9% efficiency, and the average U.S. insolation of 1800 kWh/m 2/yr. The total direct emissions of cadmium during the mining, smelting, andpurification of the element and the synthesis of CdTe are 0.015 g/GWh. The total directemissions of cadmium during module manufacturing are 0.004 g/GWh (11). Emissions duringaccidental releases (e.g., fires) are extremely small, if any. Such emissions could add to the total of 0.02 g/GWh. The latter have been investigated experimentally with the aid of high-energy synchrotron X-ray microprobes (16). Cd emissions from the life cycle of CdTe modules(Table S3 in the Supporting Information) are estimated to be 90-300 times lower than thosefrom coal power plants, which are 2-7 g Cd/GWh (17).4.2 Indirect Cd Emissions due to Electricity and Fuel UseWe hereby accounted for Cd emissions in the generation of electricity used in producing a PVsystem. Electricity generation by fossil fuels creates heavy metal emissions as those arecontained in coal and oil and a fraction is released in the atmosphere during combustion. Theelectricity demand for PV modules and BOS were investigated based on the life-cycle inventoryof each module and the electricity input data for production of BOS materials. Then, Cdemissions from the electricity demand for each module were assigned, assuming that the life-cycle electricity for the silicon-and CdTe-PV modules are supplied by the UCTE grid.Indirect Cd emissions include those from using fossil fuel, such as natural gas, heavy oil, andcoal for providing heat and mechanical energy during materials processing, for climate controlof the manufacturing plant, and for the transportation of materials and products throughout the life cycle of PV modules. The dominant sources of such indirect Cd emissions were found tobe the use of coal during steel-making processes and the use of natural gas during glass-makingprocesses. The cadmium emissions from natural gas use are indirect, from the boiler materials and from the electricity supply needed in the boiler, not from the burning of gas itself.The complete life-cycle atmospheric Cd emissions were estimated by adding the Cd emissionsfrom electricity and fuel demand associated with manufacturing and materials production for PV module and Balance of System (BOS). These are shown in Figure

3. The results show thatCdTe PV displacing other electricity technologies actually prevents a significant amount of Cdfrom being released to the air. Every GWh electricity generated by CdTe PV module can prevent around 4 g of Cd air emissions if used instead of or as a supplement to the UCTEelectricity grid. The direct emissions of Cd during the life cycle of CdTe PV are 10 times lowerthan the indirect emissions due to the electricity and fuel use in the same life cycle, and about 30 times lower than those indirect emissions in the life cycle of crystalline photovoltaics.Other heavy-metal emissions from the life-cycle electricity and fuel usage of PV systems areshown in Figure

4. The calculated emission factors are the products of electricity and fuelusage during the life cycle of PV modules and the emissions factors taken from the LCA databases. The heavy-metal emission factors in Cases 1-4 are based, respectively, on thefollowing grid mixtures and databases: Case 1, Ecoinvent database and the grid mixture of the Emissions from Photovoltaic Life Cycles - Environmental Science & Technology (ACS Publications) http://pubs.acs.org/doi/full/10.1021/es071763q[12/27/2010 4:39:26 PM]Figure 4. Life-cycle atmospheric heavy-metal emissions for PV systems(normalized for Southern European average insolation of 1700 kWh/m 2/yr,performance ratio of 0.8, and lifetime of 30 yrs). Each PV system isassumed to include a ground-mounted BOS as described by Mason et al.

(18). The four types of PV modules and corresponding efficiencies areribbon-Si 11.5%, multi-crystalline Si 13.2%, monocrystalline 14%, and CdTe 9%.Figure 5. Breakdown of heavy-metal emissions for PV modules. UCTE gridmix and Ecoinvent database are used for heavy-metal emission factors ofelectricity, fuel, and materials.CrystalClear project in which electricity mix of gas-fired combined cycle and hydropower wasused for production and purification of polycrystalline silicon (10, 14); Case 2, Ecoinventdatabase for medium-voltage electricity of the UCTE grid (14); Case 3, Franklin database forthe U.S. average grid mixture (15); and Case 4, emission factors of a recent study by Kim andDale for the U.S. grid mixture (19). The last one adopts the DEAM LCA database and the eGRIDmodel from the U.S. Environmental Protection Agency. Emissions across different data sources vary greatly, with the factors quoted by Kim and Dale (19) being the highest. In particular, theCr emissions for this source are 5 times higher than the Ecoinvent estimate. The CdTe PV module performs the best, and replacing the regular grid mix with it affords significantpotential to reduce those atmospheric heavy-metal emissions.4.3 Direct Emissions during Material ProcessingOther than the Cd emissions depicted above, direct, heavy-metal emissions from materialsprocessing have not been determined by the present study for several reasons. First, emissionsduring processing highly depend on the selection of the system's boundary, and therefore, the allocation method that an LCA study adopts. For example, a significant amount of Cd emissionsare allocated to recycled aluminum because unwanted copper scraps containing Cd as an alloyadditive are mixed and melted with aluminum scrap during recycling. In this case, allocation can be avoided (according to ISO guideline) if the Cd emissions are assigned to the primarycopper alloy production. Moreover, the amount of unabated emissions may significantlydecrease with technological progress and stricter regulatory standards. For instance, Cr emissions from steel production using an electric arc furnace based on one database is 15 timeshigher than that cited from another database (1.5 mg/kg of steel from one database vs 0.1mg/kg of steel from another). Finally, the composition of the metal, in other words, the amount of impurities mixed with matrix metal, often decides heavy-metal emission factors.According to the Ecoinvent database, low-alloyed converter steel generates six times morearsenic, and 150 times more chromium than unalloyed steel (14). Therefore, estimating heavy-metal emissions directly from materials processing, i.e., from mining, smelting, andpurification, entails large inherent uncertainties.For demonstration purposes, we first compared the heavy-metal emissions from electricity andfuel, with the direct emissions from material processing based on heavy-metal emission factorsfrom the Ecoinvent database (Figure 5). Direct heavy-metal emissions from copper, lead, andsteel alloying processes together with aluminum recycling that is unrelated to electricity or fuels have been estimated for the multi-Si PV module. It is shown that the electricityconsumption is the most important source of heavy-metal emissions for As, Cd, Hg, and Niemissions. The high fraction of direct Pb emission from material processing is related to solar glass manufacturing, which accounts for about 80% of such Pb emission. However, this resultmay be an overestimation since Ecoinvent database assumes a construction-grade glass for thesolar glass, although the glass used in PV modules typically contains less Pb because lead is not added to solar glass as an additive (20). In fact, the lead emission factor of glass manufacturingin the ETH-ESU database (the predecessor of Ecoinvent) is lower than that in Ecoinvent by afactor of 250 (14, 21). For the above reasons estimates of direct heavy-metal emissions carryhigher uncertainties than the energy-related emissions. Further work is required to improvereliability of estimating such emissions.

Emissions from Photovoltaic Life Cycles - Environmental Science & Technology (ACS Publications) http://pubs.acs.org/doi/full/10.1021/es071763q[12/27/2010 4:39:26 PM]Figure 6. Greenhouse gas emissions profile for PV modules when using aPV breeder that supplies electricity for PV production. Insolation of 1700 kWh/m 2/yr, performance ratio of 0.8, and lifetime of 30 yrs are assumed.BOS is not included. In 2005, the total (100%) electricity corresponded to250 kWh/m 2 for mc-Si and 59 kWh/m 2 for CdTe PV.5 PV BreederAs part of prospective analysis, one could evaluate the effect of increased PV penetration inthe quality of the energy mixture used in PV production. At the limit, all electricity used inPV manufacturing can be generated by onsite or nearby PV. In this section, we explorepotential benefits of returning electricity generated by PV to the PV fuel cycle. As the electric power generated by PV is variable to insolation, an electrical storage system will be needed tofully meet the electricity demand for the PV production; with the reality of today's electricitygrid (22), around 30% of the electricity required for PV production can be supplied without astorage system. In Figure 6, we illustrate the effect of incrementing electricity supply from aPV breeder scheme (i.e., PVs that supply electricity to the PV life cycles) of multicrystalline(mc)-Si and CdTe PVs. For mc-Si, around 250 kWh per m 2 of electricity is required throughoutthe consecutive process of polycrystalline silicon, wafer, cell, and module production, whileproducing the same area of CdTe requires 59 kWh of electricity (10, 12). If the considered PVbreeder system supplies 30% of the electricity required in each Si PV production process, i.e.,

silicon, wafer, cell, and module, as well as in CdTe PV production process, 6 and 2 g/kWh ofGHG emissions will be curbed from the case that uses UCTE grid mix (Figure 6). A recent studydemonstrates that large-scale PV plants can utilize compressed air to store electricity (23),which could enable a 100% electricity supply for PV manufacturing. This would reduce around50% of life-cycle GHG emissions for both Si and CdTe PVs (Figure 6). A similar exercise by Paccaet al. resulted in greater GHG reductions, i.e., 68% and 82% for multi-Si and amorphous-Si PV, respectively (24). The greater reductions are related to the higher CO 2 emissions from thebackground electricity being replaced in their study, i.e., the U.S. average grid mix, which is 45% more carbon-intensive than the UCTE grid (14, 15). A PV breeder system could directlysupply a large part of the electric energy used in manufacturing. This scenario indicates thepotential of further reducing GHG emissions in the future by employing more carbon-free electricity generation. Other electricity generation and production related-parameters, forinstance, photon-to-electricity conversion efficiency and cell/film thickness are also advancingin parallel and would also result in reduced emissions.6 DiscussionUsing data compiled from the original records of twelve PV manufacturers, we quantified the emissions from the life cycle of four major commercial photovoltaic technologies and showed that they are insignificant in comparison to the emissions that they replace when introduced inaverage European and U.S. grids. According to our analysis, replacing grid electricity withcentral PV systems presents significant environmental benefits, which for CdTe PV amounts to 89-98% reductions of GHG emissions, criteria pollutants, heavy metals, and radioactive species.For roof-top dispersed installations, such pollution reductions are expected to be even greateras the loads on the transmission and distribution networks are reduced, and part of the emissions related to the life cycle of these networks are avoided. It is interesting thatemissions of heavy metals are greatly reduced even for the types of PV technologies that makedirect use of related compounds. For example the emissions of Cd from the life cycle of CdTe Emissions from Photovoltaic Life Cycles - Environmental Science & Technology (ACS Publications) http://pubs.acs.org/doi/full/10.1021/es071763q[12/27/2010 4:39:26 PM]

1.2.3.4.5.6.

7.8.9.particulate control devices. In fact, life-cycle Cd emissions are even lower in CdTe PV than incrystalline Si PV, because the former use less energy in their life cycle than the later. In general, thin-film photovoltaics require less energy in their manufacturing than crystalline Si photovoltaics, and this translates to lower emissions of heavy metals, SO x, NO x, PM, and CO 2.In any case, emissions from any type of PV system are expected to be lower than those from conventional energy systems because PV does not require fuel to operate. PV technologies provide the benefits of significantly curbing air emissions harmful to human and ecological health. It is noted that the environmental profiles of photovoltaics are further improving as efficiencies and material utilization rates increase and this kind of analysis needs to be updated periodically. Also, future very large penetrations of PV would alter the grid composition and this has to be accounted for in future analyses.

AcknowledgmentThe crystalline silicon research was conducted within the Integrated Project CrystalClear and funded by the European Commission under contract SES6-CT_2003-502583. The BOS and cadmium telluride research was conducted within the PV Environmental Research Center, BNL, under contract DE-AC02-76CH000016 with the US Department of Energy. We thank M. de Wild-Scholten (ECN), A. Meader (First Solar), T. Hansen (Tucson Power Electric), J. Mason (Solar Energy Campaign, NY), Terry Jester (Shell Solar), and a number of industry experts who contributed to the collection of background data. Also we thank the following companies for their help: Deutsche Cell, Deutsche Solar, Evergreen Solar, First Solar, HCT Shaping Systems, Isofoton, Photowatt and Shell Solar (currently Solar Word). Links to the industry-provided Material Inventory Data are listed below as citations 25 and 26; updates of mc-Si inventoriesindicating significant improvements are listed in citation 27.Supporting InformationMaterial and energy inventories and emissions of individual contaminants. This material isavailable free of charge via the Internet at http://pubs.acs.org

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