NREL Publishes Cradle-to-Grave Assessment of Greenhouse Gases from Energy Sources

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Renewables are widely assumed to generate far fewer greenhouse gas emissions than fossil fuels, but a precise accounting of the differences in energy generation technologies has never been completed — until now.

The National Renewable Energy Laboratory (NREL) has developed a new approach to determine the cradle-to-grave emissions profiles of various forms of energy generation. The results are not surprising, but will serve as an important input on long-term energy infrastructure decisions.

Two-Phase Process

Emissions were evaluated in two phases during the Life Cycle Assessment (LCA) Harmonization Project. First, analysts evaluated more than 2,100 published LCAs for electricity-generation technologies. Then, NREL developed a meta-analytical procedure called “harmonization” that applied common metrics to 25 percent of published references and accurately narrowed the large range of previous and distinct LCA estimates by up to 90 percent.

As expected, the LCA harmonization determined that renewables, even when considering component manufacturing and plant decommissioning, generate far fewer emissions than coal. Somewhat surprisingly, nuclear also placed within the lowest LCA emissions range of all renewables.

Harmonized LCA emissions profiles

Concentrated solar power (trough), wind energy (onshore and offshore), and nuclear (pressurized- and boiling-water) had the smallest emissions profiles, with median LCA emissions of 13 grams CO2 equivalent per kilowatt-hour (g CO2eq/kWh), 11-12 g CO2eq/kWh, and 12-13 g CO2eq/kWh, respectively. Concentrated solar power (tower) and photovoltaic solar’s median LCA were both slightly higher at 46 g CO2eq/kWh and 45 g CO2eq/kWh, respectively.

LCA emissions for coal were orders of magnitude higher than either renewables or nuclear. Median coal LCA emissions were 979 g CO2eq/kWh and maximum emissions were roughly 1,400 grams. The comparative numbers could not be more distinct — coal remains, by far, the greatest contributor of greenhouse gas emissions across the electricity-generation sector.

Wider Range of Non-Harmonized Results

While solar, wind, nuclear, and coal assessments were harmonized to produce much more accurate LCAs, the larger body of 2,100 as-published LCA estimates included all generation technologies (NREL does plan a harmonization of some of these technologies in the future). While the as-published estimates are less precise, the results are still quite informative.

As-published LCA estimates (non-harmonized)

The non-harmonized studies found geothermal power to be in the same LCA emissions range as concentrating solar power and wind energy, with 50-80 g CO2eq/kWh, while hydropower and ocean energy fell into the lowest range of all generation sources, with LCA emissions of 4-14 g CO2eq/kWh and 8-23 g CO2eq/kWh, respectively. One outlier to this set is reservoir hydropower, which has LCA emissions over 150 g CO2eq/kWh.

Biopower, generally defined, had the largest range of LCA emissions impact of all renewables at 16 to 360 g CO2eq/kWh, depending on the specific type of fuel source, with its maximum range overlapping with the minimum range for natural gas. Biopower’s higher emissions impact was attributed to land use changes and methane emissions from landfilling of biomass wastes.

Among fossil fuels, natural gas had the lowest median LCA emissions at around 500 g CO2eq/kWh and oil was second at around 850 g CO2eq/kWh. Both fuel sources remain worse polluters than renewables and nuclear, but still fall far below coal. However, when carbon capture and sequestration (CCS) technology is applied, natural gas LCA emissions fall within the range of all renewables and nuclear, while coal LCA emissions are reduced by roughly 75 percent.

Coal power emissions image via Shuttertock

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