Ten years ago, a report based on measurements taken at a hydroelectric plant in Brasil said that the gas (CO2 and methane) emissions of hydroelectric power (HEP) reservoirs account for 1 to 28% of the total greenhouse gas emissions caused by man. The report said that the CO2 and the methane came from the rotting organisms contained in the HEP reservoirs.
Now, SINTEF, the largest research group in Scadinavia, did some measurements in a 30-year-old HEP reservoir from Laos. Their calculations showed that the previous study was wrong and that the living organisms in the water and on the reservoir floor actually consume more CO2 from the atmosphere than the rotting ones get to release greenhouse gases, converted to CO2 equivalents.
“Our findings in Laos indicate that the true figure is much closer to 1 than 28 per cent. The measurements in Brazil failed to take account of the uptake of CO2 by the reservoir’s ecosystem. However, the results attracted much attention and were used to draw erroneous conclusions at global level,” says Atle Harby, Senior Research Scientist at SINTEF Energy Research.
What the previous study failed to mention is that time after the reservoir is built when dead organisms actually emit greenhouse gases is limited. Harby also mentioned that in a ten year-old reservoir they also investigated in Laos, there was a close balance between the gas uptake and release, and the uptake grows as the dam gets older.
“The bedrock, soil and water quality in the 30 year-old Laotian reservoir have combined to promote prolific organic production, and are thus also responsible for the high uptake of CO2 from the atmosphere. It is also likely that large volumes of sediment are deposited on the bottom,” he says, also mentioning that the vitality of the ecosystem in the HEP reservoirs also plays an important role in its climate gas balance.
Hydroelectric power has been considered one of the greenest sources of renewable energy and accounts for some 20 percent of the world’s energy needs, providing huge greenhouse gas reductions worldwide. Only in 2006, hydroelectric plants had a 2998 TWh energy production capacity.