HYDROPOWER AND WIND ENERGY

Electricity can be produced from flowing water by

· Large-scale hydropower, in which a high dam is built across a large river to create a reservoir. Some of the water stored in the reservoir is allowed to flow through huge pipes at controlled rates, spinning turbines and producing electricity.

· Small-scale hydropower, in which a low dam with no reservoir (or only a small one) is built across a small stream and the stream’s flow of water is used to spin turbines and produce electricity.

Hydropower supplies about 99% of the electricity in Norway, 75% in New Zealand, 50% in developing countries, and 25% in China. According to the United Nations, only about 13% of the world’s technically exploitable potential for hydropower has been developed, with much of this untapped potential in South Asia (especially China), South America, and parts of the former Soviet Union.

Because of increasing concern about the environmental and social consequences of large dams, there has been growing pressure on the World Bank and other development agencies to stop funding new large-scale hydropower projects. In 2000, the World Commission on Dams published a study indicating that hydropower is a major emitter of greenhouse gases. This is because reservoirs that power the dams can trap rotting vegetation, which can emit greenhouse gases such as carbon dioxide and methane.

Small-scale hydropower projects eliminate most of the harmful environmental effects of large-scale projects, but they can
1) threaten recreational activities and aquatic life, 2) disrupt the flow of wild and scenic rivers, 3) destroy wetlands. In addition, their electrical output can vary with seasonal changes in stream flow.

Is producing electricity from waves, tides and heat stored in water a useful option? Twice a day in high and low tides, water that flows into and out of coastal bays can spin turbines to produce electricity. However, most analysts expect tidal power to make only a tiny contribution to world electricity supply. There are few suitable sites, and construction costs are high.

The kinetic energy in ocean waves, created primarily by wind, is another potential source of electricity. Most analysts expect wave power to make little contribution to world electricity production, except in a few coastal areas with the right conditions (such as western England). Construction costs are moderate to high and the net energy yield is moderate, but equipment can be damaged or destroyed by saltwater corrosion and severe storms.

Japan and the United States have been evaluating the use of the large temperature differences (between the cold, deep waters and the sun-warmed surface waters) of tropical oceans for producing electricity. If economically feasible, this would be done in ocean thermal energy conversion plants anchored to the bottom of tropical oceans in suitable sites.

Saline solar ponds, usually located near inland saline seas or lakes in areas with ample sunlight, can be used to produce electricity. Heat accumulated during the day in the denser bottom layer can be used to produce steam that spins turbines, generating electricity. A small experimental saline solar pond power plant on the shore of the Israel side of the Dead Sea operated for several years but was closed in 1989 because of high operating costs.

Freshwater solar ponds can be used to heat water and space. A shallow hole is dug and lined with concrete. A number of large black plastic bags, each filled with several centimeters of water, are placed in the hole and then covered with fiberglass insulation panels. The panels let the sunlight in but keep most of the heat stored in the water during the daytime from being lost in the atmosphere. When the water in the bags has reached its peak temperature in the afternoon, a computer turns on pumps to transfer hot water from the bags to large insulated tanks for distribution.

Saline and freshwater solar ponds use no energy storage and backup systems, emit no air pollution, and have a moderate net energy yield. Freshwater solar ponds can be built in almost any sunny area and have moderate construction and operating costs.

Wind power is the world’s fastest-growing energy resource with an average growth of 30% per year during 1990s and more that $4 billion in sales of wind turbines in 2000. In 2000, individual wind turbines and wind farms (clusters of 20 ¾ 100 wind turbines) worldwide produced almost 15.000 megawatts of electricity, enough to meet the needs of 5.2 million homes. This was more than three times the capacity in 1995 and 1.300 times the capacity in 1980.

Despite its rapid growth, wind power produced only about 1% of the energy used in the United States in 2000 because it is still in its infancy. There has been launched a program designed to have 5% of the country’s energy produced by the wind by 2020. In 2000, the price of electricity produced by wind in the United States was about the same as that produced by new gas- and coal-fired power plants. Within a few years, wind power could be the country’s cheapest way to produce electricity. The Midwestern United States are called the «Saudi Arabia of wind», the Dakotas and Texas alone have enough wind resources to meet all the nation’s electricity needs.

Denmark (with wind generating about 13% of its electricity) is the world’s largest user of wind and producer of wind turbines. Wind power also is being developed rapidly in Germany (the world third largest user of wind power), Spain, and India (the world’s number-two market for wind energy).

In the long run, electricity from large wind farms in remote areas might be used to make hydrogen gas from water during off-peak periods. The hydrogen could then be fed into a pipeline and storage system.

Date: 2016-01-14; view: 579