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Cycle Gas Emissions Table 1. Harmonization Parameters Parameter Value from Solar Solar Irradiation (kWh/m2/yr) 1,700 | 2,400 Lifetime 30 years Over the last thirty years, hundreds of life cycle assessments Crystalline Module Efficiency (LCAs) have been conducted and published for a variety of residential and utility-scale solar photovoltaic (PV) . Mono-crystalline 14.0% These LCAs have yielded wide-ranging results. Variation could be Multi-crystalline 13.2% attributed to differences in evaluated (i.e., differing Module Efficiency system designs, commercial versus conceptual systems, system operating assumptions, improvements over ) and (a-Si) 6.3 % LCA methods and assumptions. The National Renewable (CdTe) 10.9% Laboratory (NREL) recently led the Life Cycle Assessment (LCA) Copper diselenide (CIGS) 11.5% Harmonization Project, a study that helps to clarify inconsistent Performance Ratio and conflicting life cycle GHG emission estimates in the published literature and provide more precise estimates of life cycle GHG Ground-Mounted 0.80 emissions from PV systems. Analysts developed and applied a Rooftop 0.75 systematic approach to review LCA literature, identify primary sources of variability and, where possible, reduce variability in and were included in this analysis. Harmonization was performed by life cycle GHG emissions estimates through a process called adjusting published GHG emission estimates to achieve consistent “harmonization.” values (Table 1) for these key technical parameters: 1. Solar irradiation, the average energy from the , in Published results from 400 studies of PV systems including kilowatt-hours per square meter per year (kWh/m2/yr). (c-Si) (mono-crystalline and multi-crystalline) and 2. Operating lifetime of the PV system and components (years). thin film (TF) (amorphous silicon [a-Si], cadmium telluride [CdTe], 3. Module efficiency, the percentage of the converted and copper indium gallium diselenide [CIGS]) were reviewed and to by the module. screened. Seventeen studies passed the screening (providing 46 4. Performance ratio, the ratio of electricity estimates of life cycle GHG emissions for these PV technologies) actually produced by the system, after accounting for losses, to

Life Cycle Stages Upstream Processes Operational Processes Downstream Processes LCA of Energy Systems

Photovoltaics (PV) LCA can help determine environmental burdens from “cradle • Raw Materials Extraction to grave” and facilitate comparisons • Materials Production of energy technologies. Comparing • Module Manufacture • Generation • System/ life cycle stages and proportions • System/Plant Component • System/Plant Operation Decommissioning of GHG emissions from each Manufacture and Maintenance • Disposal stage for PV and shows that, • Installation/Plant for coal-fired power , Construction during operation ~40 g CO2eq/kWh emits the vast majority of GHGs. ~60% - 70% ~21% - 26% ~5% - 20% Coal For PV power plants, the majority of GHG emissions are upstream of • Coal operation in materials and module • Power Plant • Raw Materials Extraction • Coal Preparation manufacturing. Decommissioning • Construction Materials • Coal Transport • Waste Disposal Manufacture • Coal Combustion • Coal Mine • Power Plant Construction • Power Plant Operation Rehabilitation and Maintenance ~1,000 g CO2eq/kWh <1% >98% <1% Source: Burkhardt et al. (2012) and Whitaker et al. (2012) Photos from iStock/19291390 and iStock/1627655, Top (left to right): Photo from iStock/13737597, NREL/PIX 18553, iStock/12123595, NREL/PIX 16933, NREL/PIX 18968, NREL/PIX 19163 NREL is a national laboratory of the U.S. Department of Energy, Office of Energy Efficiency and , operated by the Alliance for , LLC. 1,800 differences in GHG emissions from ground- 250 mounted and roof-mounted systems were Maximum Maximum 1,600 th observed for c-Si or TF PV technologies. 75th Percentile 75 Percentile 200 Median Median 25th Percentile th Given the large number of previously 1,400 25 Percentile Minimum Minimum e/kWh) 150 Indiv. Estimates published life cycle GHG emission 2 1,200 estimates for c-Si and TF PV systems and their narrow distribution after 100 1,000 harmonization, the results of this research provide an initial estimate potentially 50 800 useful for decision makers and investors. Additional studies on TF systems are needed 600 0 to understand the key sources of variability Published Harmonized Published Harmonized Crystalline Silicon Thin Film in life cycle GHG estimates. LCAs of both (41 estimates/13 references) (5 estimates/5 references) 400 c-Si and TF PV should continue as module Life Cycle Emissions (g CO and utilization efficiencies improve and as 200 PV manufacturing locations shift. Future assessments should also consider the 0 systems-level effects of integrating variable Published Harmonized Published Harmonized Published Harmonized Published Harmonized Published Harmonized Photovoltaics Concentrating Nuclear Coal generation sources onto the existing grid (C-Si and Thin Film) (O shore and Onshore) ( ) (Sub- and Supercritical, to better understand the impacts on GHG (Trough and Tower) IGCC, Fluidized Bed) emissions from conventional generation Estimates 46 36 126 99 164 sources.

References 17 10 49 27 53 References Hsu, D.; O’Donoughue, P.; Fthenakis, V.; Heath, G. ; Summary of results of the systematic review and harmonization of estimates of life cycle GHG Kim, H.; Sawyer, P.; Choi, J.; Turney, D. (2012). “Life emissions for selected technologies, with a focus on PV technologies. See Cycle Greenhouse Gas Emissions of Crystalline Silicon www.nrel.gov/harmonization for more . Photovoltaic Electricity Generation: Systematic Review and Harmonization.” Journal of Industrial (16:S1); pp. S122-S135. the electricity calculated based on the • Life cycle GHG emissions from c-Si direct current-module efficiency and and TF PV technologies appear broadly Kim, H.; Fthenakis, V.; Choi, J.; Turney, D. (2012). “Life Cycle Greenhouse Gas Emissions of Thin-film irradiation. similar; the small number of estimates Photovoltaic Electricity Generation: Systematic Review and for TF technologies limits robust Harmonization.” Journal of Industrial Ecology (16:S1); pp. GHG emission estimates were also comparisons. S110-S121. harmonized to a consistent system boundary, Whitaker, M.; Heath, G.; O’Donoughue, P.; Vorum, M. as well as global warming potentials Of the harmonization parameters investigated, (2012.) “Life Cycle Greenhouse Gas Emissions of Coal- for methane and nitrous oxide. Other adjusting reported results to a consistent Fired Electricity Generation: Systematic Review and Harmonization.” Journal of Industrial Ecology (16:S1); pp. potential sources of variability that were solar irradiation assumption had the greatest S53-S72. not considered in this study include silicon impact on reducing the variability in estimated thickness and silicon type for the c-Si GHG emissions from c-Si PV technologies. PV system, and the upstream electricity mix Solar irradiation directly influences the used in manufacturing processes for both power generated from a PV system and the c-Si and TF PV systems. varies by location and season, time of day, and . In the LCA literature on PV technologies, the assumed solar irradiation The life cycle GHG emissions for c-Si and 2 TF PV power systems are compared with ranged from 900 to 2,200 kWh/m /yr. When these values were adjusted to 1,700 National Renewable Energy Laboratory other electricity generation technologies in 2 the figure on this page. These results show kWh/m /yr (typical for southern Europe), 15013 Denver West Parkway, that: the variability in the interquartile range Golden, CO 80401 • Total life cycle GHG emissions from of life cycle GHG emissions for c-Si PV 303-275-3000 • www.nrel.gov solar PV systems are similar to other technologies was reduced by 48%. Using a higher irradiation estimate than 1,700 kWh/ renewables and nuclear energy, and 2 2 much lower than coal. m /yr (i.e., 2,400 kWh/m /yr which is typical NREL is a national laboratory of the • Harmonization increases the precision for the Southwestern U.S.) would result in U.S. Department of Energy, Office of life cycle GHG emission estimates proportionally lower GHG emissions. of Energy Efficiency and Renewable for c-Si and TF PV, reducing variability Energy, operated by the Alliance for in the interquartile range (75th minus Adjustment to a consistent operating Sustainable Energy, LLC. 25th percentile value) by 65%. lifetime is also a driving factor in decreasing NREL/FS-6A20-56487 • November 2012 • Harmonization has a small effect on the the variability of the harmonized data. central estimate for each technology, Additional analysis comparing mono-Si reducing the median by approximately and multi-Si technologies suggest that 20%. Median values for both PV these do not significantly differ in life cycle GHG emissions. In addition, no significant technologies are below 50 g CO2e/kWh. Printed with a renewable-source ink on paper containing at least 50% wastepaper, including For more information about the NREL LCA Harmonization Project and other technologies: www.nrel.gov/harmonization. 10% post consumer waste.