Should Large Hydroelectric Plants Be Considered Renewable Energy?

Should large hydroelectric plants be considered renewable  energy? 

Large hydro is often included in state Renewable Energy Standards (RES), but it’s long been controversial, because of the associated greenhouse gases (GHG), habitat destruction and other concerns.

An international team of scientists has amassed the largest data set to date on GHG emissions from hydro plants, and find they actually emit a relatively small amount of both carbon dioxide and methane. 

Their analysis, published in Nature Geoscience, looked at 85 hydro plants around the world and found they emitted 48 million metric tons of carbon a year. That’s much lower than previous estimates of 64 million metric tons based on analysis of fewer plants.

Hydro reservoirs are responsible for less than 16% of the total carbon dioxide and methane emissions from all types of human-made reservoirs combined, the researchers say.

"Our analysis indicates that hydroelectric reservoirs are not major contributors to the greenhouse gas problem," comments Dr. Jonathan Cole, a limnologist at the Cary Institute of Ecosystem Studies and one of the authors. "But there are some caveats. To date, only 17% of potential hydroelectric reservoir sites have been exploited, and impacts vary based on reservoir age, size, and location."

Carbon dioxide and methane are two of the main GHG created by human activities. Carbon dioxide is produced during the combustion of nearly any organic material; methane has a variety of industrial sources. Both gases are also produced naturally through decomposition, particularly in wetlands and lakes.

When rivers are dammed to make the reservoirs needed for hydroelectricity, flooding creates lake-like conditions that generate carbon dioxide and methane. Emissions are the highest following reservoir construction, due to decomposing vegetation and soil organic matter. As reservoirs age, emissions decline, with cold-water systems stabilizing more rapidly than their warm-water counterparts.

Lead author Nathan Barros, of the Federal University of Juiz de Fora further explains, "The bottom line is that per unit of energy, hydroelectric generation produces much less carbon dioxide and methane emissions than previously thought, but impacts are not equal across all landscapes."

The amount of GHG vary depending on where they are built, at what latitude and the amount of biomass in the watershed. Reservoirs in tropical locations, such as the Amazon, emit more methane and carbon throughout their lifecycles.

Hydro supplies an estimated 20% of the world’s electricity and accounts for over 85% of electricity from renewable sources. Future development is expected globally.

The paper’s authors urge careful consideration of site location and design. "During the environmental impact phase, it should be a goal to minimize the amount of carbon dioxide and methane emitted per unit of energy generated," Cole notes.

To truly tease apart the emissions generated by hydroelectricity, the authors call for a study that assesses a site’s carbon budget before and after reservoir construction. Pre- and post flooding analysis would clarify the net carbon impact of hydroelectric reservoirs.

Editor’s note: It should also be noted that emissions are not the only environmental concern surrounding man-made reservoirs. The destruction of natural habitats and the release of toxic methylmercury into the aquatic environment are two serious concerns surrounding old and new reservoir construction.