Deutsche Welle Radio presents recent research from scientists in the Brazilian Amazon that is showing that many hydroelectric dams cause the release of more greenhouse gas for the
equivalent amounts of electricity created, that would be released if that electricity was created by burning fossil-fuels. (Audio link to DW Radio's program, Living Planet - the piece on the methane dams is a little under half way through the 30 min program)
The culprit is dam design and methane, the latter is created by the floating plants that feed off the rich nutrients that the dam traps, and is concentrated in dissolved form by the pressurised water on the dam side of the dam wall.
When the dam water is released to drive the turbines, water pressure radically decreases, and the water releases its methane as gas just as carbonated water releases co2 when the bottle is opened.
There are things that can be done to mitigate these problems; purposely harvesting the methane for clean electricity, and to move the turbine water intakes away from the pressurised methane rich water. And to design future dams to take these issues into account.
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