Hydropower is a frequent target for criticism. Regardless of your views on global warming, turning a serene stretch of river into an artificial lake humming with electrical equipment can make you unpopular, and the announcement of any new hydropower project is often swiftly followed by outcries over habitat disruption and community displacement, among other concerns.

But hydropower’s clean energy bona fides are rarely questioned.

In fact, hydropower reservoirs do generate carbon emissions, and some scientists think these emissions could be substantial — maybe enough to cancel out the system’s green benefits.

Steven Bouillon, a carbon cycles researcher at the University of Leuven in Belgium, said the magnitude of emissions depends on the design of the reservoir. Bouillon is leading research in Africa to quantify emissions from inland water systems, including reservoirs.

“We’re quite convinced that for certain reservoirs, the effects on greenhouse gas emissions locally will offset the benefits of clean energy production,” he said.

When reservoirs are built — for hydropower or other purposes — the grass, vegetation and trees submerged underwater begin to slowly decompose, releasing the carbon dioxide they had been storing through photosynthesis for centuries.

How much of this carbon dioxide actually gets “outgassed” into the atmosphere is a product of several factors. The gas can bubble to the surface of the reservoir and escape; it can be released as the water passes through a dam’s hydroelectric turbines; and it can be released farther downstream. Some gas could also never make it out at all, buried in sediment in the reservoir or farther downstream, or carried all the way out to the ocean.

The size of the reservoir can make a difference. Shallow reservoirs with a wide surface area can emit more, because they’ve flooded more carbon-rich land, which can easily escape as gas out of the shallow water. Conversely, deep dams with a small surface area have much lower emissions. Read more

Courtesy: E & E Publishing, LLC