Chapter 16 WATER, WIND, BIOMASS AND GEOTHERMAL ENERGY

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296 CHAPTER 16 production of 500 MW of electricity on 142 acres of the Wao Kele O Puna rain forest. Molten underground rock abounds in this volcano-studded area, but the concern of environmental groups over the fate of some of the rain forest habitat may delay this project. (See Time, August 13, 1990, p. 68.) Wind Energy The energy of the wind can be viewed as ‘secondary’ or indirect solar energy. The winds are caused by the uneven heating of earth's atmosphere, with consequent differences in the atmospheric pressure at different locations. The familiar land-sea (onshore-offshore) breeze cycle is a good illustration of this phenomenon. During the day, the land heats to a greater extent than the nearby sea; as the air over the land is heated, it rises and is replaced by the breeze that brings air from the cooler sea. After sunset, the land cools faster than the sea; therefore, the now warmer air is over the sea and as it rises it is replaced by the breeze that brings air from the cooler land. As we know from everyday experience, the daily wind patterns can be quite variable. But the monthly average speeds and directions of winds are surprisingly predictable and can be relied upon for conversion of their energy into more useful forms such as electricity. India Denmark Germany U.S.A. 0 200 400 600 800 1000 1200 1400 1600 1800 World capacity, megawatts 8000 6000 4000 2000 0 1980 1985 1990 1995 2000 Figure 16-5. Trends and current status of wind-based electricity generation, in megawatts. [Source: Vital Signs 1996, Worldwatch Institute.] The energy of the winds has been used for centuries to pump water for irrigation, to carry sailing ships across the oceans and to turn millstones to grind flour from grain. For
WATER, WIND, BIOMASS AND GEOTHERMAL ENERGY 297 centuries, windmills were a trademark of many European countries. In The Netherlands water needed to be pumped from the below-sea-level land; in Spain's Castille windmills were immortalized by Cervantes in Don Quixote. In the U.S., by the early 1900s, several million windmills were in use to pump water. In today's age of cheap and efficient electric motors, the revived interest in wind energy is focused exclusively on electricity generation. Figure 16-5 summarizes the current capacity in the world and the leading nations. It is seen to be of the same order of magnitude as world's geothermal energy, though somewhat smaller. Europe remains the region of fastest growth of wind turbines, especially in the 1990s. After a period of fast growth in the 1980s (spurred by tax credits, which expired in 1985), the U.S. capacity hasn't changed much in the 1990s. California leads the way, as it does in most other energy-related initiatives. For example, 80 square miles of hills east of San Francisco (Altamont Pass) are already ‘decorated’ with 7000 wind turbines; another 4000 are spinning outside Palm Springs. The period of stagnation in the U.S. is expected to cease by the turn of the century; for example, a 400-MW plant was scheduled to be added at Buffalo Ridge, Minnesota by 2002. But it is very unlikely that optimistic predictions, such as 10% of the nation's electricity by the year 2000, will be realized. Wind industry's complaints about the lack of significant government support, in contrast to the situation in Europe, may be a problem. INTERNET INFO For the most recent developments in the wind energy industry, see the site of the American Wind Energy Association, at www.igc.apc.org/awea. In addition to the obvious flexibility and reliability that are needed to respond to winds of variable speed, the key issue in the use of wind turbines is their efficiency. Conversion of the kinetic energy of the moving mass of air into the rotation of the blades – placed perpendicular to the direction of the wind – and subsequently into electricity can never be 100% efficient. (If it were, the wind would stop right there, at the blades.) The classic Dutch windmills had (and still have) an efficiency in the range of 5-10%. Modern wind turbines have maximum efficiencies in the range of 35-40%, and typical efficiencies in the range of 20-25%. The theoretical maximum is 60%. But, as in the case of geothermal energy, the ‘fuel’ for the power plant is free and relatively low efficiencies do not necessarily mean that the economics are unfavorable (see Chapters 17 and 18). From Chapter 3, we know that the kinetic energy (of wind, for example) can be expressed as follows: Ek = mv2 2 , where m is the mass of the air and v is its speed. Now, power (P) is energy divided by time (t), so the power of the wind will be: P = mv2 2t .
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Chapter 16 WATER, WIND, BIOMASS AND GEOTHERMAL ENERGY

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