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8. Here Comes the Sun
- Johns Hopkins University Press
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G ∫ HERE COMES THE SUN The main renewable energy sources are wind power and solar power—the subjects with which we begin this final chapter. We start o√ with a survey of wind power technology and capabilities. You might reasonably wonder why wind power should fall under this chapter title. In fact, winds are driven by solar power.1 We then consider again the possibilities of solar power, which we first glimpsed in the introduction. In the second part of the chapter we look at the mix of power generating technologies which, it seems to me, o√er the best prospects for the future. The Answer, My Friend, Is Blowin’ in the Wind Wind energy is all around us and is free, in the sense that nature delivers it right to our doorstep, whether we want it or not. Harnessing that energy is not free—quite the reverse—and yet power from the wind is being increasingly realized. The sails of wind power research and development are filling, so to speak. Wind is a clean, renewable energy source; exploiting it does not generate greenhouse gases or cause chemical pollution. Fromthewindmillsofold,wehaveoverthelastseveraldecadesdeveloped wind turbines of various shapes and sizes. The vertical-axis wind turbine (VAWT) has been around since the 1920s and has found several applications, mostly small scale. The most widespread and familiar of these is the cup anemometer, used to measure wind speed. There are also larger eggbeaterlike designs that generate power. These have one advantage over the much more common horizontal-axis wind turbine (HAWT) design: they do not need to be turned into the wind. This is a distinct advantage in areas where the wind direction is highly variable and unreliable—for example, on the tops of urban buildings. Generally though, VAWT designs are uncommon because they rotate slowly and therefore generate only a little power.2 ∞π∫ Lights On! HAWT designs usually have three blades; they vary in size from 200-W micro-turbines to large megawatt turbines. The micro-turbines, with blades 0.5–1 m long, are used for generating power for sailboats in harbor, RVs, electric fences, and water pumps (one of the original windmill applications), and are used by hobbyists. Stepping up in size we find mini-turbines that generate a few kilowatts; these have blades up to 3.5 m long and are used to power remote cabins or households. I will concentrate here on commercial turbines, designed to generate power for industries or, more usually, for an electricity grid. The blades for these turbines range from large—20 ft— to gigantic, as long as 165 ft (fig. 8.1). The giants can generate serious amounts of electrical power. They stand atop 300-ft towers and are usually grouped to form a wind-power plant, or wind farm. As with many technologies, scale a√ects economics and e≈ciency. Microand mini-turbines are 20%–25% e≈cient, whereas large turbines can be up to 40% e≈cient. The power of a turbine depends on the swept area of its blades, so it increases as the square of blade length. Power also depends sensitively on the winds, increasing as the cube of wind speed. (You may FIG. ∫.∞. Transportation of a large wind turbine blade in Texas. Photo by Alexi Kostibas. [44.215.110.142] Project MUSE (2024-03-28 14:37 GMT) Here Comes the Sun ∞πΩ recall from chapter 2 that we showed this dependence on blade length and wind speed to be the case for windmills; the calculation is in the appendix.) This is why scaling up a wind turbine yields a disproportionate increase in power. Another reason is the wind profile—the increase in wind speed with height above the ground. Friction with the ground slows the wind; at increasing heights there is a consequent increase in its speed. Atmospheric scientists assume this speed to increase as the one-seventh power of height.3 So, doubling the height of a turbine tower from, say, 60 feet to 120 feet increases mean wind speed by 10%. Assuming that turbine blade length increases proportional to tower height, we find that the wind power impinging upon the turbine blades increases by about 540%. All of this means that turbines for wind farms have to be large to be economical. Installation costs for wind turbines are very high. For a small turbine, reckon upon spending $230 per square foot of swept area, and perhaps $93 per square foot for a large turbine. Recall that...