Hydroelectric produces carbon dioxide

How clean is hydropower?

In the past it was just small water mills that knew how to extract energy from the flow of rivers. Today it is often large dams and reservoirs that already cover 80 percent of the total energy demand in countries like Brazil.

"The reservoirs for hydropower plants can be found all over the world, but especially in Canada, Brazil and Sweden. In Brazil, this branch of energy is growing very quickly. If you look at a map, all of the reservoir projects look like a disease. But It is well known that China and India are now building very large dams. "

The ecologist Fabio Roland of the Brazilian University of Juiz de Fora is observing this development with some concern. Because the reservoirs not only often impair the ecosystem of the rivers or make forced relocation necessary. Although they are mostly advertised as being emission-free, they produce greenhouse gases such as carbon dioxide and methane. In extreme cases, the emissions caused per megawatt can even be as high as with a coal-fired power plant. Because when rural areas are flooded by reservoirs, a variety of chemical processes take place on the ground, explains Fabio Roland.

"The soil is sometimes very rich in carbon. And this carbon is processed into carbon dioxide by bacteria in the water. At the same time, these bacteria consume oxygen. And when an oxygen-poor layer has formed on the ground, methane is also produced there."

Fabio Roland took a closer look at these emissions with colleagues from Sweden, Canada and the USA and evaluated the data from 85 different reservoirs. In a current study in the magazine "Nature Geoscience" they estimate that greenhouse gases equivalent to 288 million tons of carbon dioxide are released annually by the hydropower plants. That is about a third of Germany's total carbon dioxide emissions. However, according to Roland, there are major differences in the emission rate between the individual hydropower plants.

"We found two important factors: On the one hand, the emissions decrease sharply as the reservoirs get older. The highest emissions are therefore immediately after the flooding. On the other hand, the emissions rise more and more the closer the reservoir is to the equator We have the highest emissions in the lower latitudes, that is, in the tropics. In Sweden, for example, greenhouse gas emissions are very low because the water is not as rich in nutrients and the forest is not as dense as in the tropical regions. "

A reservoir in the tropics releases around ten times as much greenhouse gases on average as its equivalent in the temperate zone. How quickly the emissions then halve depends entirely on the landscape that was flooded. It takes an average of ten years, says Fabio Roland. But there are also areas in Brazil that achieve low emission rates much more quickly. The Cerrado savannah, for example, is not so densely overgrown, so that reservoirs there have often halved their greenhouse gas emissions after just five years. Fabio Roland therefore advocates that before dams are built, not only the effects on the ecosystem but also the resulting greenhouse gas emissions should be taken into account.

"We have to examine the projects very carefully before they are built. And that requires knowledge of a wide variety of sciences so that we can develop hydropower plants that damage the environment as little as possible."

Knowledge that is not only important for hydropower plants, as Fabio Roland emphasizes. Because they are only responsible for eleven percent of the emissions caused by artificial freshwater lakes. The remaining 89 percent go to the account of reservoirs that were created for other purposes such as the supply of drinking and industrial water or flood protection.

To the topic portal "Future of Energy"