Hydropower as a CO2 slinger?

Some dam projects on the Mekong release as much CO2 as fossil fuels

The Ubol Ratana Dam in Thailand is gaining power from the damming of the Mekong River. But what does his greenhouse gas balance look like? © TrapperFrank / CC-by-sa 3.0
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Clean energy: Not every hydroelectric plant is automatically climate-friendly, as a study from the Mekong region in Asia points out. Especially large reservoirs, which were not freed from vegetation before being filled, can release as many greenhouse gases as fossil fuel power plants. The climate impact of such hydroelectric power plants must therefore always be assessed on a case-by-case basis, the researchers emphasize.

Hydropower is generally considered "clean", renewable energy. Because the turbines of such power plants are driven by the flow or the gradient between two reservoirs, fossil fuels are not necessary for this. But it has long been clear that larger dams and dams are a major intervention in nature. In the Amazon, even irreversible ecological damage threatens, as researchers recently warned.

But one aspect of hydropower has so far been little studied: the release of greenhouse gases from the reservoirs of hydroelectric power plants. "Although hydropower is often considered a climate-friendly energy option, such reservoirs also produce greenhouse gases such as methane, carbon dioxide and nitrous oxide (N 2 O), " explain Timo Räsänen of Aalto University and his colleagues.

Dams in the Mekong area as an example

How high these emissions from reservoirs are, the researchers have now examined the example of the Mekong region. Because of the rapidly growing economy in Southeast Asia, numerous hydropower plants have been built there in recent decades. For their study, the scientists analyzed the CO2 and methane emissions of the reservoirs for 64 existing and 77 planned plants in the Mekong region. This was based on models of typical emissions from comparable reservoirs and physical models of the gas exchange between water surface and atmosphere.

Greenhouse gas emissions and energy density of 141 existing and planned dam projects on the Mekong. © Räsänen et al / Aalto University

The result: "A large part of the reservoirs actually has relatively low emissions per unit of energy, " report R s nen and his colleagues. With emission rates of 26 kilograms of CO2 equivalent per megawatt hour, these hydropower plants were also part of other renewable energies. According to the researchers belonged to this category mainly smaller, already existing reservoirs. display

As much CO2 as a gas or coal power plant

But this positive balance was not for everyone: "But there are also a number of reservoirs on the Mekong with high emissions, " the scientists emphasize. After all, 32 of the 141 reservoirs emit significantly more CO2 and methane than are typical for renewable energies. "15 of these reservoirs produced between 278 and 9, 271 kilograms of CO2 equivalent per megawatt hour of electricity, " said R s nen and his colleagues. This is similar to electricity from fossil fuels.

But what is behind it? The researchers found that most of the CO2 and methane were released into the reservoirs, which covered a particularly large area, were located in very warm areas of the Mekong area and had a high erosion rate on the banks and surrounding plains s. Another factor is the original vegetation in the areas flooded by the reservoir: if this is not previously removed, it releases large amounts of greenhouse gases when rotting in the water.

Especially CO2 and methane are released when old vegetation in the reservoirs slowly rots. Aalto University

"Not per se climate-friendly"

"Our findings suggest that hydropower can not generally be considered as a low-emission form of energy, " said R s nen and his colleagues. "Because their emissions can reach the level of fossil fuel power plants." It was therefore important to carefully analyze each newly planned hydropower project with regard to its greenhouse gas emissions Facilities in warm climates.

"Our study could help to identify in advance dam projects that will potentially have high greenhouse gas emissions, " say the researchers. Then you can take preventive action against it. This could be, for example, the reduction of the open water surface of the reservoirs and, above all, the complete removal of the vegetation in the later flooded area. (Environmental Research Letters, 2018; doi: 10.1088 / 1748-9326 / aaa817)

(Aalto University, 06.03.2018 - NPO)