This was once demonstrated by means of Eawag scientists at Lake Wohlen, close to Bern — a discovering which moderately tarnishes the popularity of hydropower as a climate-neutral method of producing electrical energy.
When they first noticed the knowledge, environmental chemist Tonya Del Sontro and her PhD manager Professor Bernhard Wehrli had been skeptical. But the rapidly top values stood as much as cautious research: on moderate, day-to-day emissions of methane (CH4) from Lake Wohlen quantity to greater than 150 mg in keeping with m2 floor house. This is by means of some distance the best possible emission charge recorded thus far for a temperate reservoir. At a water temperature of 17°C the speed is two times as top, which makes it similar to the emission charges noticed for tropical reservoirs.
Overall, the reservoir at the Aare produces 150 tonnes of methane a 12 months. This is set an identical quantity as is emitted yearly by means of round 2000 cows; in phrases of international warming possible, it’s an identical to the carbon dioxide emissions from 25 million automotive kilometres, since methane is set 25 occasions as potent a greenhouse fuel as CO2. “So hydropower isn’t quite as climate-neutral as people have assumed in the past,” says Del Sontro. At the similar time, she does no longer need to dramatize the findings of her analysis: despite the fact that the Aare hydropower plant is taken to be answerable for the entire methane emissions from the reservoir, and those are expressed in CO2-equivalents, a coal-fired energy station with the similar output produces 40 occasions as a lot CO2. However, the emissions from Lake Wohlen do point out that run-of-the-river reservoirs may also be important resources of methane emissions in temperate areas. As Wehrli notes, “That’s something which has previously been overlooked in greenhouse gas budgets.”
Methane manufacturing in Lake Wohlen is due to natural subject transported by means of the Aare, e.g. from Lake Thun. In the reservoir, the natural subject settles all of a sudden, present process microbial fermentation within the sediments. “In the summer,” says Del Sontro, “the water in Lake Wohlen sometimes looks like champagne, with masses of gas bubbles rising to the surface.” To find out about those bubbles, the Eawag scientists used customized fuel traps within the shape of inverted funnels. The analyses published that they had been principally composed of methane.