Hydroelectric Energy - The Methane Factor
While this is not desirable, it is merely an alteration to the environment. If the dam remained flooded this would not pose long term ecological threats to us from this modification. What does cause problems, however, is the significant amount of methane generated by hydroelectric energy installations. Methane is produced when organic matter breaks down anaerobically (without oxygen). This is similar to the processes that resulted in the formation of coal and oil and natural gas. The following diagrams represent a section of land that is submerged by the construction of a hydroelectric facility.
First the land is submerged and the vegetation with it.
The vegetation drowns and begins to rot. Since there is very little available oxygen, the plant material breaks down to form, among other things, methane that is absorbed by 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 and edges of the dam.
When exposed, these damp patches of earth that are rich in organic material blossom with new plant life. Shallow dams have more of this edge and bottom area exposed annually.
Eventually the rains come again and this new vegetation is covered in water.
This vegetation then also rots anaerobically, adding further methane to the water in the dam.
This continues on an annual basis, resulting in a steady supply of methane in the dam water from newly added plant matter. Methane is generally insoluble in water so when the water passes through the turbines of the power station it is released into the atmosphere. Methane is approximately 21 times more effective than Carbon Dioxide as a greenhouse agent. This makes hydroelectric energy anywhere up to three times more polluting per megawatt of electricity generated than the equivalent coal or oil fired power station. This figure is dependent on the climate the dam is located in and the geography of the region. The effect is significant nonetheless, with the Intergovernmental Panel on Climate Change (IPCC) now taking hydroelectric energy methane production into account when inventorying national greenhouse gas emissions. |
