水力發電仍會釋出溫室效應氣體

The reservoirs of many large dams built to generate hydroelectric power do produce greenhouse gases, the World Commission on Dams says in a comprehensive report issued earlier this month. This finding is in contrast to the widespread assumption that such emissions are zero or negligible.

"Greenhouse gases are emitted for decades from all 30 dam reservoirs in the boreal and tropic regions for which measurements have been made," the commission says.

The World Commission on Dams, based in Cape Town, South Africa is made up of an international team of 12 experts. It is an independent think tank set up and financed by aid agencies, industry, governments, and non-governmental organizations to look at what the commission calls "the good, the bad and the ugly impacts of dams around the world." Established in 1998, the Commission's mandate is to research, peer review and write the most independent, authoritative and comprehensive cross examination of dams and water and energy resource development ever undertaken.

Chairman of the World Commission on Dams Kader Asmal signs the final report at the London launch November 16. (Photo courtesy World Commission on Dams)

In its final report, "Dams and Development: A New Framework for Decision-making," presented to the United Nations on November 17, the commission writes, "All large dams and natural lakes in the boreal and tropical regions that have been measured emit greenhouse gases."

The commission's findings were informed by the most recent research on this issue in Canada, Brazil, French Guyana and Finland.

In northern boreal climates like Canada and Scandinavia available studies so far suggest that emissions from hydropower reservoirs are low, the commission says.

For Brazil, of ten dams studied, emissions vary from dam to dam with a 500-fold difference between lowest and highest. The lowest emissions are on similar levels to Canadian lakes and reservoirs, the highest annual gross emissions reach the ranges of thermal energy plants, although life cycle assessment, and determination of net emissions, is needed before definitive comparisons can be made.

World's largest storage dam, Uganda's Owen Falls Dam. The hydroelectric station at the dam supplies most of the electricity requirements of Uganda, and parts of Kenya. (Photo courtesy Faculty of Engineering, Kasetsart University, Thailand)

The flooded biomass alone does not explain the observed gas emissions, the commission says. "Carbon is flowing into the reservoir from the entire basin upstream, and other development and resource management activities in the basin can increase or decrease future carbon inputs to the reservoir," according to the commission's report.

The commission advises that hydropower cannot be automatically assumed to emit less greenhouse gas than the thermal alternatives which use fossil fuels to generate power. Net emissions should be established on a case by case basis.

Hydroelectricity provides for 80 percent of all the renewable power generated in the United States, and provides about one-fifth of the world's electricity. Of the 75,000 dams in the U.S., less than three percent have hydroelectric generating stations, according to the National Hydropower Association (NHA), an industry oranization representing 61 percent of non-federal hydroelectric capacity in the United States.

Ideas for generating electricity with water but without building dams to hold back rivers in reservoirs are in development, according to the NHA. Because no reservoirs are created, a free flow method for generating power may not create greenhouse gas emissions.

The NHA says an idea that is ready for field testing would use tidal or river currents to generate electricity without impounding water behind a dam. Requiring a current of three knots or better, these submerged units would be clustered together to generate from 300 kW to as much as 300 MW. They could be used either on or off the grid, providing power that is "cost competitive with other intermittent renewable energy sources," the association says.

Clairville Dam and Reservoir, Humber River, Toronto, Ontario, Canada (Photo courtesy Environment Canada)'

In Canada, hydropower represents 62 percent of the electricity production. The Canadian Hydropower Association joined with the International Hydropower Association and the International Energy Agency in writing a report issued on November 21, "Hydropower and the World Energy Future." The report declared hydropower to be, "the main supply of clean, renewable energy, and as an energy that will play a key role in the future world energy mix."

"One way to reduce greenhouse gas emissions while continuing to supply the growing energy demand is to increase the development of clean, renewable energy sources such as hydropower," says Canadian Hydropower Association executive director Pierre Fortin.

The World Commission on Dams does not condemn all dam reservoirs as greenhouse gas emitters. The commission calls for "explicit assessment of future net greenhouse gas emissions of a project" through full life cycle assessments to compare available options.

For the hydropower option, the commission proposes guidelines to assist in these assessments. These include measuring carbon flows in the natural pre-impoundment watershed and assessing how these will change following dam construction.

The commission calls for further research to improve current understanding of how greenhouse gases cycle within existing basin and reservoirs and to improve capacity to predict future net emissions of new reservoir projects.

The 12 commissioners who make up the World Commission on Dams represent a wide range of interests including commercial engineers, academics, activists and administrators of government agencies. To learn more about the commissioners, visit: http://www.dams.org/about/commissioners.htm