Tuesday, October 5, 2010

How Does Hydroelectricity Create Greenhouse Gases?

By Roger Vanderlely

Hydroelectricity is often presented as a green energy source, but how true is this? Hydro has several environmental and aesthetic drawbacks that make it less than ideal compared to the clean energy provided by sources such as wind and sun.

The most visible environmental aspect of Hydroelectricity is its impact on the environment, as it requires the damming of rivers. This causes the flooding of low lying land behind the dam and while this is often not desirable, the reality is that it is just an alteration to the environment. If the dam was to remain filled with water all the time, this would have no environmental effect outside the dam area.

Why is it then that Hydroelectric installations are associated with high levels of methane production? When organic matter from plants and animals breaks down without oxygen present, methane is formed. This anaerobic process is very similar to the ones that resulted in the formation of the fossil fuels we use today.

The following describes a section of land that is submerged by the construction of a hydroelectric facility.

The valley behind the dam fills with water and all the plants drown and start t rot. Since they are under water they are not exposed to much oxygen gas, so they decay anaerobically. This produces a variety of products including significant amounts of methane. This methane is absorbed into the water.

So far all is normal, just the same as any other permanent flooding. But since this is a power station and in most cases also an urban water supply, the levels tend to fluctuate annually. The water levels drop in dry times. This exposes sections of land at the bottom of the dam.

As soon as these areas are exposed they blossom with new plant life. This happens at the edge of the water in the dam as the water level drops. Since most Hydroelectric dams are quite shallow, a great deal of land is exposed annually in this way.

Eventually the rains come and the dam refills, filling the new plants with water. Then it is the turn of these plants to rot anaerobically, releasing even more methane into the water.

This continues year after year, resulting in a slow but steady increase in the amount of methane absorbed in the water of the dam. This is a problem because methane is not very soluble in water. When the water passes through the dam's turbines it escapes the water and enters the atmosphere.

Methane is a dangerous greenhouse gas. It is approximately 21 times more effective at trapping heat in the atmosphere than Carbon Dioxide. This means that electricity from a Hydroelectric plant can be up to three times more polluting per energy unit than the same energy from a coal or oil fired plant. This figure depends on the climate and geography the Hydro plant is located in, as these factors determining the amount of vegetation added to the dam each year. The Intergovernmental Panel on Climate Change (IPCC) has recognized this issue and now includes methane from Hydroelectricity in national emissions totals.

While Hydroelectricity is a renewable energy source it is not as environmentally friendly as it looks. If there is discussion about whether to build a new Hydro plant, methane emissions have to be considered. Far cleaner are the options of solar and wind power that once built have no emissions whatsoever.

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