Texts for additional reading. Other sources of energy.

People do not usually think of hydropower, or energy from mo­ving water as coming from the sun. Yet the sun does provide this energy by evaporating water, which then falls to Earth as rain and runs from high ground to low.

Modern power plants harness water power for electricity. Usually the builders of a hydroelectric power plant need to dam a stream or river. By holding water in a reservoir, the dam raises the wa­ter level behind it much higher than the level of the power plant at the bottom of the dam. This increases the gravitational po­tential energy of the water. As electricity is needed, the water is released to rush down with terrific force. The falling water strikes and turns the blades of turbines. In turn, the turbines spin the generators to make electricity.

The sun is also the source of wind energy. Wind is caused by the flow of air from areas of high pressure to areas of low pre­ssure. These high- and low-pressure areas are a result of un­even heating of Earth’s surface by the sun.

Wind was one of the first forms of energy harnessed by humans. Sailors first used it to move ships in ancient times. Later, pe­ople throughout Europe and the Middle East used windmills for pumping water and grinding grain. Some old windmills still ope­rate in places such as Netherlands and the island of Crete.

After 1973, as the price of fossil fuels rose, people became more concerned about supplies of petroleum. Interest in wind power was rekindled. Thousands of wind machines are now in use. Some power machinery directly. For instance, they may use water on a farm.

When many wind machines are grouped together in wind farms they can generate as much electricity as power plant.

Energy.

Energy in great quantities is necessary to power civilization and meet human needs. Fossil fuels, the primary energy source, exist in limited supplies. However, energy is available from a number of natural sources. Humans are developing the technolo­gy to generate and harness energy for their work using alter­nate sources,

As the 1990s opened, fossil fuels provided most of the energy consumed world-wide. Of the alternative sources available in 1991, only nuclear power had the potential to replace fossil fuels on a large scale. Yet utility companies are discouraged by the costs and the public is varied of the hazards. Supplies for hydropower are limited and solar, wind and geothermal sources need more rese­arch to make them more efficient and practical for wide-scale use. During the 1980s, governments and industry devoted less re­search and fewer resources than in the 1970s to developing al­ternative sources of energy. The reason for this lack of inte­rest goes beyond change in administration: petroleum was inex­pensive relative to most alternative sources. During an era of relatively low oil prices, insufficient industry and consumer interest existed in the use of these fuels. In a crisis situa­tion, petroleum cannot be replaced immediately.

Vehicle manufacturers must plan three to five years in advance planning and construction of a coal-fired electrical plant takes seven years; a nuclear facility twelve years; a solar plant one to five years; and a geothermal facility three, wind farms can be constructed in six months, provided that the sites, supp­liers, constructors, and funds are in place.