Friday, April 07, 2006

 

Energy - Hydropower

As of 2003, the sources of US energy included the following

* Oil 39%
* Natural gas 24%
* Coal 23%
* Nuclear 8%
* Hydropower 3%
* Other 3%

The only "renewable" resources on this list are Hydropower and the mysterious "Other". Hydropower is used to create electricity, and therefore contributes very little to the energy required to move cars, trucks, and machinery around.

Sadly, Hydropower isn't exactly renewable, unless you are capturing energy without a dam at natural waterfalls. When is the last time you saw a working water wheel producing electricity?

The reason dams are often considered non-renewable is that all resevoirs behind dams fill with sediment over time. This happens in every resevoir, although the rate that this happens varies by quite a bit, depending primarily on the size of the resevoir compared to the amount of sediment flowing into it. A small resevoir on a muddy river, for example, compared to a large resevoir fed by a clear river will have very different rates of sedimentation. However, the clear river resevoir will still lose capacity over time.

The build-out phase of dam projects in the US is drawing to a close. We've captured pretty much all the energy we can capture from rivers.
Hydropower dam projects will continue to provide a fraction of power for the US, but since almost all available sites are already constructed, and some major sites will not have the lifespan originally predicted, this proportion cannot truly grow and may actually contract.

It sounds, therefore, somewhat hypocritical to come out against hydropower projects in other countries since we've managed to "tame" almost every river at home.

The largest and most high-profile dam project would probably be the Three Gorges Dam in China.

"The Three Gorges Dam is designed to operate under conditions practically untested in the world and never before tested in such a large structure. Projections of controlling sedimentation within the reservoir are subject to significant uncertainties. China has about 83,000 reservoirs built for various purposes, of which 330 are major in size. Sediment deposition in 230 of them has become a significant problem, resulting in a combined loss of 14 percent of the total storage capacity. In some, more than 50 percent of the storage capacity has been lost." Hu Chunhong, 1995, Controlling Reservoir Sedimentation in China, Hydropower and Dams, March issue, pp. 50-52.

Flood control is a tremendous advantage of large dam projects. The number of people affected and killed by flooding in China below the site of the dam is tremendously high.

Both India and China also are planning very large river diversion projects that are intended to solve water shortage/surplus issues based on geography. In some analyses, though, the cost of moving the people to the water is less than moving the water to the people.

Despite the flood control advantages, a dam site has a limited lifespan for energy production. The local geography changes quite a bit with sedimentation of the resevoir. There is no guarentee that the site can be used forever, again, or even as long as the dam structure lasts.

Can a dam be reconstructed? In theory, yes. Dam reconstruction means either raising the body of the dam or dredging out the resevoir behind the dam. Successful dam reconstruction projects usually include replacement or rehabilitation of smaller collapsed structures. Reconstruction of a large dam, like the Hoover Dam, is in a different category. I can't find an example of it ever having been successfully done.


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