The question of which energy technologies generate the most greenhouse-gas emissions - cradle to grave - now has a more precise answer, thanks to a meta-analysis of life cycle assessment (LCA) studies done by the U.S. Department of Energy's (DOE) National Renewable Energy Laboratory (NREL).

The new, robust analysis weighed the emissions estimates per kilowatt-hour from raw materials, manufacturing, transportation, operation, and decommissioning to get the best apples-to-apples comparisons.

NREL's LCA Harmonization Project gives decision-makers and investors more exact estimates of greenhouse-gas emissions for renewable and conventional generation, clarifying inconsistent and conflicting estimates in the published literature and reducing uncertainty.

The analysis found that from cradle to grave, coal-fired energy releases about 20 times as much greenhouse gas into the atmosphere per kilowatt-hour as solar energy.

Wind and nuclear energy are on relative par with solar energy. Natural gas generation wasn't included in the final analysis but is generally assumed to emit about half as much greenhouse gas per kilowatt-hour as coal.

"Study of studies" narrows estimates

Until recently, emissions estimates ranged wildly, sometimes because vested interests had a stake in demonstrating that a certain technology's emissions were high or low. For instance, if decommissioning costs aren't included in a total-emissions estimate for nuclear energy or natural gas, those studies give artificially low figures.

NREL looked at more than 2,000 studies across many generation technologies, applied quality controls, and greatly narrowed the range of estimates to reach reliable medians for greenhouse-gas emissions.

This "study of studies" narrowed the huge ranges of estimates sometimes as much as 90 percent, presenting a more reliable look at the likely greenhouse-gas emissions from different technologies.

"This methodology allows you to arrive at a better precision, so you can say with more certainty that this is the benefit you get from using this technology rather than that technology," said NREL Senior Scientist Garvin Heath, who led the project. "Anyone who wants a true comparison of the greenhouse-gas costs should benefit from this."

Investors "need to be very forward looking," Heath said. A power plant is long lived, and its attributes and shortcomings are locked in for decades. That's why investors push for estimates of greenhouse-gas emissions before they invest.

"As a society, we need to better understand what the effects of our energy choices are," Heath said. "Greenhouse gases and climate change are a part of the discussion. As we try to envision what our future energy system will look like, we need an accurate picture of what that transition will mean."

"The application of meta-analysis to life cycle assessment is an important advance. Decision-makers seeking to make greener choices need a way to make sense of the information that is coming at them so quickly now. This can help," said Peter Crane, Yale University's dean of Forestry and Environmental Studies.

Learn more about NREL's LCA Harmonization Project and download and visualize the numerical results and bibliographies on OpenEI.org.