Thin-Film Photovoltaic Power Generation Offers Decreasing

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Thin-Film Photovoltaic Power Generation Offers Decreasing Greenhouse Gas Emissions and Increasing Environmental Cobenefits in the Long Term Joseph D. Bergesen,† Garvin A. Heath,‡ Thomas Gibon,§ and Sangwon Suh*,† †

Bren School of Environmental Science & Management, University of California, 2400 Bren Hall, Santa Barbara, California 93106-5131, United States ‡ National Renewable Energy Laboratory, Golden, Colorado 80401, United States § Industrial Ecology Programme, Department of Energy Process and Engineering, Norwegian University of Science and Technology (NTNU), Høgskoleringen 5, NO-7491 Trondheim, Norway S Supporting Information *

ABSTRACT: Thin-film photovoltaic (PV) technologies have improved significantly recently, and similar improvements are projected into the future, warranting reevaluation of the environmental implications of PV to update and inform policy decisions. By conducting a hybrid life cycle assessment using the most recent manufacturing data and technology roadmaps, we compare present and projected environmental, human health, and natural resource implications of electricity generated from two common thin-film PV technologiescopper indium gallium selenide (CIGS) and cadmium telluride (CdTe)in the United States (U.S.) to those of the current U.S. electricity mix. We evaluate how the impacts of thin films can be reduced by likely costreducing technological changes: (1) module efficiency increases, (2) module dematerialization, (3) changes in upstream energy and materials production, and (4) end-of-life recycling of balance of system (BOS). Results show comparable environmental and resource impacts for CdTe and CIGS. Compared to the U.S. electricity mix in 2010, both perform at least 90% better in 7 of 12 and at least 50% better in 3 of 12 impact categories, with comparable land use, and increased metal depletion unless BOS recycling is ensured. Technological changes, particularly efficiency increases, contribute to 35−80% reductions in all impacts by 2030.

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INTRODUCTION Given the abundance of solar insolation on the earth’s surface and rapidly falling costs, photovoltaic (PV) technologies could play an important role in transitioning the world to a lowcarbon economy.1,2 Recent reports from the Intergovernmental Panel on Climate Change3 (IPCC) and the National Renewable Energy Laboratory4 (NREL) have reiterated that PV power generation can help mitigate greenhouse gas (GHG) emissions from the electricity sector by replacing fossil-fuelbased electricity generation.5,6 From 2009 until the end of 2012, PV installations more than quadrupled to a global total of over 100 GW.7 If this growth rate continues, most of the PV capacity that will be in operation in just a few years has not yet been manufactured and deployed. These new systems will be the product of near-term technological advances, with capabilities and environmental impacts that differ from PV systems manufactured today. Thin-film PV technologies including cadmium telluride (CdTe), and copper indium gallium selenide (CIGS) comprise a significant fraction of the global PV market. In the United States (U.S.), thin-film production has exceeded crystalline silicon production since 2006.8 The Americas produced 12% of global thin-film modules in 2012.7,9 Growth is fueled in part by © 2014 American Chemical Society

the maturation of thin-film technology, evident in rising efficiencies for both CdTe and CIGS modules.10 In 2012, one CIGS manufacturer verified a record 15.7% module efficiency, while the CdTe record stands at 16.1%.11,12 Both CIGS and CdTe module efficiencies are expected to improve substantially over the coming decadeto more than 20% and 19%, respectively.13−15 These efficiency improvements and changes in material composition are needed to reduce module costs, but they also have the potential to lead to different environmental and resource impacts.13,14,16 Previous assessment of long-term changes in environmental impacts of PV technologies has been limited.16 Such assessments have not fully addressed the benefits and trade-offs that could result from changes to cell and module design or the environmental profile of sources of energy and materials “upstream” of PV manufacturing. Rapid changes in thin-film technologies warrant the reevaluation of their impacts as the technologies mature and Received: Revised: Accepted: Published: 9834

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of 1 kWh of generated electricity, not including losses from transmission and distribution. The system boundary for the PV life cycle includes production of raw materials, water, and energy needed for thin-film module manufacture, transportation, construction of PV generation facilities, operation and maintenance (O&M), and facility decommissioning. We include all of the system components necessary for PVs to supply grid-voltage electricity: arrays of modules and balance of system (BOS) components such as module supports, wiring, inverters, and high-voltage transformers (for utility-scale systems). The study considers small-scale (distributed-scale) thin-film PV systems, i.e., rooftop systems, and utility-scale systems. Energy storage is not considered in this study because it is not yet widely utilized for PVs.40,41 The impact of integration of variable PVs on the grid is not included because its effects have been shown to be relatively small for near-term penetration projections.42 Life Cycle Impact Assessment. According to the ISO 14040 series of LCA standards, life cycle impact assessment (LCIA) is needed to better understand the “magnitude and significance” of emissions of a multitude of individual pollutants.43 LCIA is recommended by the International Energy Agency (IEA) photovoltaic power systems programme (PVPS) task 12 guidelines and has proved valuable for comparing energy technologies with differing emissions profiles (e.g., PV- and fossil-fuel-based electricity), locating environmental hot spots and setting priorities for impact mitigation.37,44,45 We employ two prominent LCIA methods to characterize the potential impacts from a comprehensive set of environmental stressors (emissions and natural resources) on human health, the environment, and natural resources.46,47 We use the U.S. Environmental Protection Agency’s TRACI 1.0 method to characterize the potential environmental impacts of emissions on the following categories: climate change (kg of CO2equivalent (equiv)), human health carcinogens (kg of benzeneequiv), human health noncarcinogens (kg of toluene-equiv), ecotoxicity (kg of 2,4-dichlorophenoxyacetic acid (2,4-D)equiv), acidification (mol of H+-equiv), eutrophication (kg of nitrogen-equiv), photochemical oxidant formation (kg of nitrogen oxides (NOx)-equiv), and respiratory effects (PM2.5equiv).48 We use the ReCiPe 2008 LCIA method to characterize natural resource consumption for its potential impacts on metal depletion (kg of iron (Fe)-equiv), fossil depletion (kg of oil-equiv), water depletion (m3), and land used (m2·year), since these categories are not included in TRACI.49 Lastly, we express the characterized results of U.S. thin-film PVgenerated electricity as a percentage of the impacts of the 2010 U.S. electricity grid mix, enabling an exploratory discussion of the effectiveness of using thin-film PVs to mitigate GHG emissions from the electricity sector and of the potential environmental benefits and trade-offs that may occur. Life Cycle Inventory Models for Thin-Film PV Technologies. We developed life cycle inventories (LCIs) for ground-mounted utility-scale (1 MW capacity) and roofmounted distributed-scale (3 kW) CIGS and CdTe PV systems. The following section summarizes the production of CIGS and CdTe modules and complete systems and discusses the technological changes likely to occur from 2010 to 2030. Most materials usage, energy inputs, and cost data on thinfilm PV manufacturing were obtained from NREL manufacturing cost models.13,14,50 Compiled through collaborations with CIGS and CdTe industry partners, the cost models are

as the upstream environmental impacts of industry and the economy change. Life cycle assessment (LCA) enables consistent comparisons between renewable and fossil-based energy technologies by accounting for environmental burdens accrued both upstream (materials production, component manufacturing, and facility construction) and downstream (dismantling and disposal) of power plant operation. LCAs of PV technologies published within the past decade, summarized in two LCA harmonization studies, have shown that electricity generated from PVs offers substantially lower emission of GHGs compared to fossil-fuelbased electricity generation technologies and that thin-film CdTe and CIGS modules have slightly lower median GHG emissions than crystalline silicon technologies.17,18 The LCA literature evaluating thin films, however, is dominated by amorphous silicon19−23 and CdTe24−27 with no CIGS studies based on recent, commercial-scale production data.20,21 While technological change has been previously incorporated into LCAs of silicon and CdTe PV technologies in the European Union’s New Energy Externalities Development for Sustainability (NEEDS) project,16,28 the environmental impacts of thin-film technologies in impact categories other than climate change (e.g., human health, ecotoxicity, acidification, water use, and natural resources consumption) have not been reported comprehensively in the literature.20,25,29,30

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MATERIALS AND METHODS This study employs hybrid life cycle assessment31,32 (HLCA) to compare the present and future environmental and natural resource impacts of a kilowatt-hour of electricity from thin-film PV to a reference case of U.S. grid electricity in 2010. HLCA combines process-based LCA with environmentally extended, economic input−output (EIO) analysis to capture a more complete picture of a product’s environmental impacts than either method alone. In particular, we use the tiered hybrid approach,31,32 a method that has been frequently applied to PVs and other energy technologies.33−39 In the tiered hybrid model, the life cycle impacts of each material or energy input to PV systems is computed using the most appropriate of either process LCA or EIO methods. In this study, process LCA is used when physical data on materials and energy used in component manufacturing are available. EIO is employed to determine the impacts of proprietary inputs and capital goods based on costs. Goal and Scope Definition. The goal of this study is to explore potential benefits and trade-offs from using U.S.manufactured and -deployed thin-film PVs to mitigate GHG emissions from the electricity sector in the long term. To address gaps in previous literature, we examine a broad set of environmental and resource impacts of U.S.-manufactured CdTe and CIGS of current and future design, using the most recent manufacturing data and technology roadmaps. By comparing the impacts of thin-film PV-generated electricity to a consistent baseline of the current U.S. electricity mix, we investigate how environmental impacts may change as PV provides a greater portion of U.S. electricity and as thin-film technologies change over time. We highlight areas of environmental concern in the life cycles of these technologies (“hot spots”) and prioritize the technological improvements that may be most effective at mitigating and reducing environmental impacts. To compare with grid electricity, the resources and emissions required to produce electricity from thin-film PV and grid electricity are scaled to the functional unit 9835

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Table 1. Selected CIGS and CdTe Technological Improvements from 2010 to 2030 CIGS module efficiency semiconductor layer emitter layer buffer layer, per m2 module annealed soda lime glass substrate capital costs, per m2 module

technological improvement category

CdTe

ref

12% → 20.8% 2 μm → 1 μm CIGS; increased Ga:In ratio 70 nm → 20 nm CdS $2 → $1.5 TCO

11.6% →19% 2.5 μm → 1 μm CdTe

efficiency increase dematerialization

NREL roadmaps13−15 NREL roadmaps13−15

100 nm → 20 nm CdS $2 → $1.5 ZnO

dematerialization dematerialization

NREL roadmaps13−15 NREL roadmaps13−15

3.2 mm → 2.2 mm, add antireflex coating $26 → $8

3.2 mm → 2.2 mm, add antireflex coating $26 → $8

dematerialization

NREL roadmaps13−15

dematerialization

author calculation based on NREL cost models13−15,50

CdS, cadmium sulfide; TCO, transparent conducting oxide; ZnO, zinc oxide.

making strides toward these goals (evident in announcements of record module and cell efficiencies11,12), CdTe maintains a larger market share and production scale, implying that CdTe technology might more quickly be able reach its full potential. PV Plant Construction, BOS, and Land Use. Data on the BOS and construction for a ground-mounted, fixed-tilt, utilityscale PV plant, was adapted from an accounting by Mason et al. that includes a bill of materials for construction and O&M for a 1 MW PV plant in Arizona, USA.57 The BOS, construction, and electrical installation for 3 kW rooftop PV systems were adapted from the ecoinvent database.51 For both ground- and roof-mounted systems, construction, O&M, module supports, and wiring are scaled on a per m2 panel basis, while transformers, inverters, and electrical installation are scaled to the power output of the PV system. End-of-life (EOL) recycling scenarios consider the recycling of steel, aluminum, and copper used in both the ground- and roof-mounted BOS. To account for the burdens and benefits accrued by the recycling of those materials at end-of-life, we use ecoinvent’s processes and recovery rates for the production of secondary steel (90%), aluminum (79%), and copper (76%).58 To estimate land use by ground-mounted PV systems, an average fixed-tilt packing factor of 47% was assumed, meaning that for every m2 module, ∼2 m2 of land is used.59,60 This area is equivalent to what Ong et al. called “direct land use.”59 In addition, we also consider life cycle land use upstream of the power plant, albeit small. By contrast, total land use by PV facilities accounts for any additional access roads, fences, and construction areas surrounding the PV arrays. While not accounted for in our LCA, on average it can add as much as 30−40% more land to our estimates based on the empirical analysis by Ong et al.59 System Assumptions. We generally follow the system assumptions recommended by the IEA-PVPS guidelines.61 We use average U.S. solar irradiation of 1800 kWh/m2/year (latitude tilt) to calculate the electricity generated by a PV system over its lifetime. We assume a 30 year module lifetime for CIGS and CdTe.17,18 We assume typical PV system performance ratios of 0.8 for ground and 0.75 for roof-mounted systems, representing energy conversion losses due to voltage transformation, AC/DC inversion, wiring resistance, soiling, degradation, and misalignment and shading (for residential systems).61 Modeling Technological Change in the Background. To assess the changes in impacts from thin-film PVs in the future, it is also essential to consider changes in the mix of technologies supplying grid electricity for PV manufacturing (a key contributor to the manufacturing impact of PV modules62),

representative of current (∼2012) thin-film manufacturing and allow us to model LCIs for typical CIGS and CdTe manufacturing in the U.S. Per module capital costs (for factory construction and semiconductor manufacturing machinery), amortized over the useful life and expected output of the facility, were also provided by the cost models. Long-term conversion efficiency potentials (21% for CIGS and 19% for CdTe) and projected future technological changes in manufacturing and module material composition are obtained from NREL technology roadmaps.13,14 Life cycle emissions resulting from most direct inputs to PV manufacturing were calculated using the ecoinvent 2.2 processbased LCI database,51 including direct manufacturing emissions, glass and metals production, electricity generation, and transportation. When possible, ecoinvent’s U.S. processes were used, and all ecoinvent processes requiring electricity were modified to demand the U.S. electricity mix. Emissions resulting from manufacture of capital goods and proprietary materials such as transparent conducting oxides (TCOs) were calculated based on their cost using the Comprehensive Environmental Data Archive52,53 (CEDA), an environmentally extended input−output database of the U.S. economy. Production of CIGS and CdTe from 2010 to 2030. As described in NREL’s cost analysis, to produce a CIGS module in 2010, a 2.0 μm semiconductor layer consisting of varying proportions of copper (Cu), indium (In), gallium (Ga), and selenium (Se) is co-evaporated or sputtered onto a 0.65 μm molybdenum (Mo) back contact layer.13,54 To determine the quantities of CIGS metals and Mo required for manufacturing, we account for typical material utilization rates from coevaporation and the recovery of any unutilized metals, which are then sold back to supplier at ∼30% of cost.13,55 Quantities of other material and energy inputs were provided by the NREL cost models and the ecoinvent database. The majority of CdTe modules are produced by a sublimation transport of the semiconductor layers.14 We use ecoinvent data on the proportions of Cd to Te within the 2.5 μm thick semiconductor layer for 2010,56 and we account for typical utilization and recovery rates of these metals during manufacture.55 NREL cost models provided most material, energy, and cost inputs, and we take values for manufacturing electricity consumption from Fthenakis et al.24 By 2030, NREL roadmaps project that CIGS and CdTe module efficiencies will likely increase through more efficient use of materials in manufacturing. Also, economies of scale and increased production can reduce capital costs on a per module basis. Table 1 summarizes expected changes in CIGS and CdTe module manufacturing by 2030. While both technologies are 9836

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Figure 1. Environmental and resource impacts of ground-mounted thin-film PVs from 2010 and 2030 normalized to those of the 2010 U.S. electricity grid mix. Assumes average U.S. irradiation of 1800 kWh/m2/year. 2010 results include no end-of-life recycling, but 2030 results include recycling of aluminum, copper, and steel used in the balance of the system. The logarithmic scale is necessary to display large variations in impacts relative to the U.S. grid, but it can obscure differences between CdTe and CIGS. Numerical results are reported in SI Table S11.

photochemical oxidation, ecotoxicity, and eutrophication. In 2010, for instance, ground-mounted CdTe and CIGS are estimated to emit approximately 20 and 22 g of CO2-equiv/ kWh, respectively. These more recent estimates contrast somewhat with previous assessments showing CIGS to have life cycle GHG emissions nearly twice as high as those of CdTe.18 Noncarcinogen and water depletion-related impacts from U.S. thin films were at least 80% lower than the U.S. grid, while carcinogenic emissions were 50−60% lower. Land occupation was comparable to grid electricity for groundmounted PV systems in 2010, a result supported by previous assessments of land use by electricity generation technologies.64,65 Results indicate, however, that electricity from thinfilm PVs will likely place additional stress on metal resources, with metal depletion estimated at approximately 2.8 times (for CdTe) to 3.3 times (for CIGS) that of the average U.S. grid mix in 2010. Environmental and Resource Impacts in 2030. Assuming efficiency improvements and changes to module design and the background economy based on NREL roadmaps and IEA projections, we project reductions in all life cycle impact categories for U.S.-manufactured CdTe and CIGS manufactured in 2030 when compared to their 2010 impacts. For instance, life cycle GHG emissions from CdTe and CIGS were reduced by 69% compared to their 2010 estimates, to 6 and 7 g of CO2-equiv/kWh, respectively. Carcinogenic human health impacts from thin-film PVs in 2030 reduced by approximately 50% to be 75% lower than the 2010 grid mix without considering EOL recycling. While we expect that the stress on metal resources will decline as module efficiency increases and fewer materials are used in manufacture, we project that metal depletion for thin films in 2030 will remain twice as high as that of the 2010 grid mix unless recycling of transformers, inverters, wiring, and other BOS components is ensured, as the 2030 results show in Figure 1. Contribution and Decomposition Analysis of Technological Improvements. In this section, we use contribution analysis to identify the manufacturing inputs and environmental stressors that contribute most to life cycle climate change, metal depletion, and carcinogenic emissions. Also, we use decomposition analysis to analyze the effectiveness of different technological improvements at reducing the

as well as future changes in the energy efficiency and environmental profiles of key upstream industrial processes and economic sectors (henceforth, collectively referred to as the “background economy”). To examine a range of future possibilities, we bounded changes in the background economy in 2030 based on the IEA’s BLUE Map scenario (halving global energy-related GHG emissions by 2050) and Baseline scenario (business as usual).63 Electricity in the 2030 BLUE Map scenario includes increased generation by renewables and utilization of carbon capture and sequestration on fossil facilities. To model changes to the background economy in further detail than given by the IEA scenarios, we updated upstream energy requirements, material requirements, and emissions in ecoinvent processes representing the aluminum, copper, nickel, iron and steel, and glass manufacturing sectors in 2030. These changes reflect increased adoption of emissions controls and so-called best available technologies (BATs) as represented by the NEEDS project’s “realistic optimistic” scenario.28 We also updated the CEDA database to reflect the changes in energy efficiency and GHG emissions in the aluminum, steel, chemicals, cement, and paper sectors projected by each IEA scenario from 2010 to 2030.63 Additionally, we performed regression analysis on historical emissions of criteria pollutants to approximate possible changes in emissions intensity per dollar of sector output in the CEDA database.

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RESULTS Figure 1 summarizes the profiles of the potential impacts from a kilowatt-hour of electricity generated by ground-mounted CdTe and CIGS from 2010 to 2030 (BLUE Map scenario), normalized to the 2010 US electricity grid mix. Results for roofmounted PVs are similar to those of ground-mounted PVs in all categories except land occupation (since roof-mounted PVs use no land directly). For simplicity, the discussion of results henceforth will focus only on ground-mounted PVs, with all other results reported in the Supporting Information (SI). Environmental and Resource Impacts in 2010. For 2010, we estimate that the per kilowatt-hour life cycle impacts of U.S. thin-film PV technologies are at least 90% lower than those of the U.S. grid mix in 7 of 12 categories: acidification, greenhouse gas emissions, fossil depletion, respiratory effects, 9837

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Figure 2. Decomposition of impact reductions by technological improvement categories from 2010 to 2030 for climate change, carcinogens, and metal depletion. Each chart shows the decrease in impact as a result of each category of technological improvement: BOS recycling, efficiency, dematerialization, and background changes.

of those metals is important when considering the scalability and cost volatility of specific technologies, they compose only a fraction of the total mass of metals used in the life cycle of PV electricity generation. The ReCiPe method characterizes a larger set of metals than prior analyses, and it estimates the possible societal economic impact resulting from heightened demand of a given metal by the PV technologies (based on estimated marginal price increases to consumers due to a decline in ore grades that result from increased demand), otherwise holding world demand constant.49 These results help identify the metals contributing most to this potential economic impact and prioritize innovations in PV system design that would reduce the use of those metals. For both CIGS and CdTe in 2010, copper is the largest contributor to metal depletion, followed by iron, manganese, and nickel (Figure 3). These metals are used for inverters, transformers, wiring, and BOS, thus explaining the similarity of results between CIGS and CdTe. To put these results in perspective, producing the 83 GW of capacity needed for PVs to generate 2.7% of U.S. electricity in 2030, as described by the IEA Blue Map scenario, would require 0.6 million metric tons of copper (assuming current material requirements), equal to more than 50% of the copper refined in the U.S. in 2013.57,71

potential environmental impacts of thin-film PVs in the long term. For this analysis, we assigned technological improvements to four categories: (1) recycling of copper, iron, steel, and aluminum in BOS, (2) module efficiency increase, (3) dematerialization, and (4) background changes discussed previously. Climate Change. While the life cycle GHG emissions per kilowatt-hour from the current generation of thin-film PVs are just 3% of those of the U.S. grid mix in 2010, changes in GHG emissions are important to assess because mitigation of climate change is a driving motivation for the increased use of renewable energy. Figure 2 shows that of the four categories of improvements, module efficiency increase reduces GHG emissions the most, closely followed by background change due to decarbonization of the U.S. grid, which is the main contributor to PV’s GHG impact. Module dematerialization, specifically using less glass and fewer metals in the semiconductor layer, and consequently less electricity for manufacture, has a smaller effect on reducing life cycle GHG emissions for thin-film PVs. Metal Depletion. Previous literature considering the metal requirements of PVs has focused on the availability and recovery of critical materials used in the module’s semiconductor, buffer, and emitter layers.9,14,55,66−70 While scarcity 9838

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comprise only a tiny fraction of the carcinogenic emissions over the thin-film life cycle. This result is supported by previous assessments showing emissions of cadmium over the CdTe life cycle are minimal in comparison with coal-fired generation (0.02 g of Cd/GWh vs 2−7 g of Cd/GWh, respectively).25,74 Because of the large contribution of copper production to carcinogenic emissions, BOS recycling provides the largest reduction in impact by 2030. In this case, under the avoided burden philosophy, recycling BOS copper is expected to displace carcinogenic emissions from future copper production, lowering the quantity of those emissions attributable to PVs. Similar to metal depletion, life cycle carcinogenic emissions are strongly linked to demand for copper, meaning both impact categories can be improved through recycling and efficiency increase (Figure 2). Unlike metal depletion, background changes result in a larger 14−15% decrease in carcinogenic emissions by 2030 due to increased pollution control in the copper industry as modeled by the NEEDS project.28 Sensitivity and Uncertainty. In this section we systematically assess the sensitivity of results to changes in quantities of manufacturing inputs, as well as discuss how results could change depending on different assumptions regarding key parameters and scenarios. The NREL cost models only provide single-point estimates for the quantities of manufacturing inputs to thin-film PV systems. Absent data on the uncertainty of input quantities, we assess the sensitivity of climate change, carcinogens, and metal depletion results to +10% perturbations in the quantities of all manufacturing inputs. Sensitivity results in 2010 are presented in SI Figures S7 and S8. For each technology, climate change, carcinogens, and metal depletion results show greater than 1% sensitivity to quantities of 6 of about 30 manufacturing inputs. Carcinogenic emissions and metal depletion are both most sensitive to the quantity of copper demanded by PV systems, while climate change is most sensitive to electricity and steel demand. As copper can be manufactured through different processes and in different locations, life cycle estimates of the carcinogenic emissions by U.S. thin-film PV could vary based on copper origin.58,75 However, the results reported in this work are robust for the mix of copper consumed in the U.S. Also, recent improvements in thin-film module efficiency since 2011, particularly for CdTe, could result in potentially lower per kilowatt-hour GHG emissions, assuming no significant changes in module manufacturing. First Solar reported 13.4% average production module efficiency at the end of 2013, potentially lowering CdTe estimates to 17 g of CO2-equiv/kWh.76 Efficiency improvements would have a smaller effect on metal depletion and carcinogens, as demand for transformers and inverters scales linearly with module efficiency. Life cycle impacts normalized per kilowatt-hour are inversely proportional to the lifetime electricity production of the PV system. In addition to efficiency, lifetime generation is affected by solar irradiation, system performance ratio, and system lifetime.62 Because impacts scale linearly and inversely with these parameters (a 10% change in a parameter causes a 10% change in impacts; see the framework provided by the NREL harmonization studies17,18), the results of this study can be easily adjusted to meet different assumptions. For example, in this study we assume a value for irradiation that is conservative for the U.S. (1800 kWh/m2/year), whereas many utility-scale PV installations are likely to be located in more optimized locations, such as the U.S. Southwest, where irradiation is

Figure 3. Contribution of metals and PV system components to life cycle metal depletion results for ground-mounted CIGS PV systems in 2010.

The greatest reduction in metal depletion from 2010 to 2030 comes from end-of-life BOS recycling of copper, iron, steel, and aluminum (Figure 2). We assume that such recycling will displace the future production of those metals and thus lower the impact of metal depletion attributable to PV, following the common “avoided burden” approach for recycling, after accounting for material losses and the impacts of recycling processes.72,73 Efficiency increase also produces a sizable reduction in metal depletion. As module efficiency increases, less wiring and frames are used per kilowatt-hour generated. However, inverters and transformers scale with the power rating of the PV system, so increasing module efficiency does not reduce demand for metals by inverters and transformers. Finally, module dematerialization produces a surprisingly small reduction because metal depletion is dominated by the BOS. Carcinogens. We estimate the carcinogenic emissions of thin-film PV electricity in 2010 to be only 39−43% of those of the 2010 U.S. grid (Figure 1). Figure 4 reveals that arsenic contributes a majority of carcinogenic emissions for CIGS in 2010, with emissions originating predominantly from the production of copper that is used for transformers, inverters, and wiring. Direct emissions from module manufacture (release of semiconductor-, buffer-, and emitter-layer metals, e.g., Cd)

Figure 4. Contribution of emissions and PV system components to life cycle carcinogenic emissions for ground-mounted CIGS PV systems in 2010. HxCDF, 1,2,3,4,7,8-hexachlorodibenzofuran. 9839

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efficiency of power electronics could contribute to PV costs approaching grid parity while also possibly mitigating key environmental and natural resource impacts of PV. Given the reduction in impacts seen from technological improvements by 2030, we find that technology changes can influence the assessment of impacts of U.S. thin-film PVs, implying that technological change could be important for other rapidly changing technologies. Results of this study can provide the basis for more informed policy and investment decisions that could help to prioritize research and development into technological improvements that further lessen the impacts of thin-film PVs. Limitations and Future Research. Three areas for future research are recommended in this section. First, the technology roadmaps and background economy scenarios considered in this work do not account for all potential technological improvements to thin-film PVs. Further work should consider possible changes in the use of materials for transformers and inverters, which contribution and decomposition analyses suggest might be effective at further reducing key impacts (metal depletion and carcinogenic emissions). Second, while metal depletion results are useful for comparing the impacts on metal resources by disparate technologies and identifying priorities for reducing the overall impact of PV on metal resources, we recommend further examination of the scarcity and availability of specific metals. Such examination has begun but can be expanded to include a more comprehensive list of metals used in PV modules and systems.9,14,55,66,68 Furthermore, metal depletion results do not consider the changing external costs of the environmental impacts of metal production over time, among other issues. Lastly, due to a lack of technology-specific information, our analysis does not consider the potential benefits of module recycling and disposal. Previous assessments have shown that the recycling of modules and recovery of the constituent metals will be feasible,78,79 also evident in the fact that U.S. thin-film manufacturers have already developed recycling programs to manage modules at EOL.80 Future research could examine technology-specific recycling processes and further investigate the recovery and possible releases of semiconductor metals during the use and end-of-life stages of thin-film modules, which a recent study suggests could be a concern for flexible, building-integrated panels (not considered in this work).81

around 2400 kWh/m2/year. In that case, the results reported here would overestimate the impacts of PV installations in the U.S. Southwest by a factor of 2400/1800 = 1.33. Lastly, we can compare the 2030 impacts of U.S. thin-film PV under the Baseline and BLUE Map scenarios to explore the effect of different assumptions regarding changes to the background economy, particularly future electricity mixes. For climate change, an impact linked closely to electricity sources, thin-film PV impacts in 2030 under the Baseline scenario are 20% higher than under the BLUE Map scenario (albeit resulting in a difference of less than 2 g of CO2-equiv/kWh). 2030 results for carcinogens and metal depletion are essentially unaffected by the choice of scenario, partially because both scenarios assume the same future improvements in pollution control technologies for manufacturing sectors. Additionally, we considered the possible migration of the same thin-film manufacturing process as considered in the main analysis to lower-cost locales such as China. While some air-pollutionrelated impacts increased as much as 60% owing to the more coal-reliant grid mix in China, the comparison of PV relative to the U.S. electricity mix was essentially unaffected. Based on these similarities among results in each scenario, we find our 2010−2030 estimates of thin-film PV impacts to be robust with regard to uncertainty in future manufacturing location and primary energy sources for electricity and that impacts are affected much more by direct manufacturing inputs and the key parameters discussed above.

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DISCUSSION Strategies and Recommendations. Results of this study suggest that U.S.-manufactured CdTe and CIGS both offer environmental co-benefits in comparison with fossil-based electricity in 10 of 12 impact categories in 2010, comparable land occupation, and greater depletion of metals if BOS recycling is not implemented. Furthermore, analysis of advances in PV technology and structural changes in the economy by 2030 show that the impacts of CIGS and CdTe are likely to decline into the future for all impact categories considered. For both technologies, carcinogenic emissions and metal depletion are closely linked to the production of metals, especially copper, used in the transformers, inverters, and wiring that are needed to deliver PV power to the grid. To mitigate the impact of thin-film PV electricity on metal resources and further reduce its potential impacts on human health, reuse and recycling of electrical system components are potentially effective for sustainable development of high grid penetration of PV. Beyond BOS recycling and innovations in power electronics, increasing module energy conversion efficiency has the broadest effect on reducing all impacts, while module dematerialization and background changes provide smaller improvements. The U.S. Department of Energy (DOE) reports that the average cost of utility-scale PVs has dropped from $0.21 to $0.11/kWh since 2011, making strides toward the SunShot Initiative goal of $0.06/kWh in 2020 (competitive with grid electricity).77 To reach this goal, total system costs must decline to $1/W for utility-scale installations.4 Costs of modules alone were benchmarked at $1/W (CdTe) and $1.6/W (CIGS) in 2011.14,15,50 The efficiency and material improvements discussed in this work could reduce modules costs to $0.5/ W, but BOS and power electronics costs must also be reduced 75% to meet SunShot targets.4,14,15,50 Innovations improving the performance, durability, economies of scale, and material

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ASSOCIATED CONTENT

S Supporting Information *

Methods for hybrid analysis, impact assessment, changes to the background economy, calculation of the benefits of recycling, and structural decomposition, unit process inventories for CIGS, CdTe and BOS, impacts of electricity mixes, roofmounted PV, baseline scenario, and sensitivity analysis results. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: (805) 893-7185; fax: (805) 893-7612; e-mail: suh@ bren.ucsb.edu. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. 9840

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge Michael Woodhouse and Alan Goodrich of NREL for supplying data and providing review and Anastasiya Lazareva (UCSB) and Patrick O’Donoughue (NREL) for their valuable contributions. Support for J.D.B. was provided by the ConvEne IGERT Program (Grant NSF-DGE 0801627). Support for G.A.H was provided by the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy.

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