Ten years ago shock waves resounded through the energy sector. They were generated by a publication based on measurements taken at an HEP plant in Brazil.

It reported that CO₂ and methane from rotting organisms in HEP reservoirs could, in the worst case scenario, be responsible for well over a quarter of the planet's total climate emissions from anthropological sources.

But the latest from this field is nothing if not good news: In sub-tropical Laos, SINTEF has taken measurements from a 30 year-old HEP reservoir. They reveal that the organisms in the water and on the reservoir floor take up more CO₂ from the atmosphere than the reservoir itself releases in the form of climate gases, converted to CO₂ equivalents.

Reassuring for the industry

The figures from Brazil were published by the respected World Commission on Dams. The report concluded that the world's HEP reservoirs are responsible for between 1 and 28 per cent of the planet's climate gas emissions.

"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.

The ecosystem decides

The climate gases released by an HEP reservoir originate from carbon derived from the decomposition of buried organisms and organic material brought in by streams and from human sources. However, at the same time, algae, phytoplankton, zooplankton and fish living in the reservoir will take up CO₂ from the atmosphere.

"Emissions levels are greatest at the start, but decline when the buried organisms are fully decomposed. We also investigated a ten year-old reservoir in Laos, where there was a balance between gas uptake and release," says Harby.

He explains that the vitality of the reservoir's ecosystem and the volume of unconsolidated sediment that buries the dead organisms together determine whether or not the climate gas balance is in the red or the black over the lifetime of the reservoir in question.

"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," says Atle Harby.

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