Air Quality

Published on May 9th, 2012 | by Silvio Marcacci


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

May 9th, 2012 by  


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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About the Author

Silvio is Principal at Marcacci Communications, a full-service clean energy and climate policy public relations company based in Oakland, CA.

  • RHytonen

    (apologies for typos. I’m used to a post edit function and have a BAD kb.)

    • Bob_Wallace

      You can edit your comment after posting. Look at the bottom of your comments.

  • RHytonen

    What about immediate and cumulative infrastructure, public health, and employee risk of the lcalized processes throughout the lifecycle? Particularly the risks f buclear nd gas/oil recommend a greatly extended lifecycecle for analysis. The biggest costs are not quatifiable or monetizable. Live near fracking/compression/pipeline/export
    operations, or have them impinge near you, and you will see. Work at one, and you will see much more clearly – even though possibly within as much as a decade later.

  • Pingback: Marcacci Communications()

  • RockSahay

    can anyone share what is the emission impact on the investment into energy efficiency.

  • lftrsuk

    Atmospheric pressure plant – there’ll be a fraction of the material used and energy consumed in the manufacture. Do you will make them fare better in the analysis?

  • lftrsuk

    LCA of PWRs as good as the best renewables. But breeder reactors use about one thirtieth of the fuel of a PWR, for a given power output, so how good will their LCA look?

    If you care about minimising environmental impact, then support the global deployment of breeder reactors. Your only decision is: do you prefer LMFBRs or MSBRs:

    • Bob_Wallace

      It ain’t the fuel cost.

      It’s the construction and financing costs.

      That’s what prices all nuclear off the table.

      • RHytonen

        That and the ongoing risk of operation. Human operators are fallible, and designers too subject to their employers’ notorious greed, corruption of “regulation,” and cost cutting.

  • silviomarcacci

    Hi Susan,

    The final harmonized median coal LCA emissions were 979 g CO2eq/kWh and the maximum as-published emissions were roughly 1,400 grams.

  • Silvio, I see you plowed through all the assessments at the journal… So what did you get for the final harmonized coal figure ranges? Can you include those for a reference case – to put these figures in perspective? God forbid someone thinks this means “everything pollutes”.

    It’s relative.

    ( I botched a story on this earlier and was well advised by an astute reader (Bill Woods) who caught my error.)

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