Crystalline Silicon and Thin Film Photovoltaic Results – Life Cycle Assessment Harmonization
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Over the last 30 years, hundreds of life cycle assessments (LCAs) have been conducted and published for a variety of residential and utility-scale solar photovoltaic (PV) systems with wide-ranging results. The inconsistencies in these results can be attributed to the technologies evaluated—such as differing system designs, real-world versus conceptual systems, or technology improvements over time—and life cycle assessment methods and assumptions.
To better understand greenhouse gas (GHG) emissions from commercial crystalline silicon (C-Si) (mono- and multi-crystalline) and thin-film (amorphous silicon (a-Si), cadmium telluride (CdTe), and copper indium gallium diselenide (CIGS)) PV power systems, the NREL LCA Harmonization Project developed and applied a systematic approach to review LCA literature, identify primary sources of variability and, where possible, reduce variability in GHG emissions estimates through a meta-analytical process called "harmonization."
The published results from 400 studies of PV systems were reviewed and screened. Seventeen studies passed the screening (providing 46 estimates of life cycle GHG emissions for PV technologies) and were included in this analysis.
All published PV data that passed the screening were harmonized by adjusting published GHG estimates to consistent values of four key PV system performance parameters: solar irradiation (1,700 kWh/m2/yr is presented here; 2,400 was also investigated), module efficiency, performance ratio, and operating lifetime. Slightly different approaches were taken for the other harmonization steps based on differences in the published data.
The individual published life cycle GHG emissions estimates for thin film PV technologies were also harmonized to a consistent system boundary, through addition of the balance-of-system components to the upstream stage, and consistent global warming potentials (GWPs) of GHG emissions. No system boundary adjustments were made in the C-Si PV LCA harmonization process and only one GWP adjustment were made to published data owing to limitations in reporting by the underlying studies. The published and harmonized life cycle GHG emissions for PV technologies are summarized in the figure below. Because the pool of estimates was small for thin-film technologies, statistical analysis to assess variability before and after harmonization was not conducted (and single points are shown in the figure).
Overall the collective life cycle assessment literature shows that PV power production is similar to other renewables and much lower than fossil fuel in total life cycle GHG emissions. The distribution of published GHG emissions estimates for PV systems becomes narrower and shifts down after harmonization as follows:
- Overall, harmonization reduced the variability (based on the Interquartile Range) across all evaluated PV technologies from 33 g CO2e/kWh to 12 g CO2e/kWh, a decrease of 65%.
- Of the parameters harmonized, adjusting reported data to a consistent solar irradiation assumption had the greatest impact on reducing the variability in estimated GHG emissions from C-Si PV technologies.
- Harmonization reduced the median of the published estimates across all evaluated technologies from 54 g CO2e/kWh to 44 g CO2e/kWh, a reduction of approximately 19%.
- The factor most responsible for the decrease in median is adjustment to a consistent solar irradiation value of 1,700 kWh/m2/yr and operating lifetime.
Comparison of Harmonization Impacts on Specific Photovoltaic Technologies
- OpenEI: Data, Visualization, and Bibliographies
The median as-published life cycle GHG emissions estimate for c-Si PV is 57 g CO2eqkWh; the harmonized median is 45 g CO2eq/kWh. Harmonization reduced the IQR from 44-73 g CO2eq/kWh to 39-49 g CO2eq/kWh, a reduction of 62%. Additional analysis comparing mono-Si and multi-Si technologies, and ground-mounted with roof-mounted systems suggest that these system differences are not key factors in life cycle GHG emissions from c-Si PV.
Harmonization reduced the published life cycle GHG emissions for thin-film PV systems by 35%-55%. The published ranges for thin-film PV systems are reduced from 19-95 to 18-52 g CO2e/kWh after harmonization for the total thin-film PV systems. No significant differences in GHG emissions from ground-mounted and roof-mounted systems were observed.
The harmonization process, through systematically adjusting estimates to reflect a consistent set of several important parameters, increased the precision of life cycle GHG emission estimates in the literature for c-SI and thin film PV. Given the large number of previously published life cycle GHG emission estimates of c-Si and thin film PV systems and their narrow distribution post-harmonization, the harmonized results represent an initial estimate potentially useful for policymakers. Additional studies on thin film systems are needed to understand the key sources of variability in life cycle GHG estimates. Life cycle assessments of both c-SI and TF PV should continue as module and utilization efficiencies improve and as PV manufacturing is shifted to Asia and impacts of introducing variable generation sources onto the grid are better characterized.