Windswept_ The Story of Wind and Weather - Marq de Villiers [126]
By 1918 more than a hundred local utilities in Denmark had at least one wind turbine, usually putting out no more than 20 or 35 kilowatts. In total, wind already accounted for about 3 percent of Danish electricity consumption. Most of these turbines were locally owned and operated, often by cooperatives of farmers, and sited close to the devices they were designed to power. This wide distribution, with the concomitant wide acceptance, was a major reason why Denmark remained the world leader in wind turbines by the year 2005. At the turn of the millennium an astounding 5 percent of the Danish population owned at least one share in a wind turbine, and the Danish public came to see them as natural parts of the landscape, both urban and rural; and their relatively small scale, absent the grandiosities of the first American incarnations, meant that it was easy to make small, incremental, and cumulative design improvements.
By the 1940s, the largest wind turbine ever attempted was built by a Vermonter, Palmer Cosslet Putnam, on a hillside called Grandpa's Knob near his home. It had two seventy-foot stainless steel blades each weighing eight tons, and generated enough power, 1.25 megawatts in a 30 mile an hour wind, to power about two hundred homes, fed through the local utility. In 1945 a blade tore loose, demolishing a few trees along its arc, and the turbine was never repaired. Cheap electricity produced by cheap coal and cheap oil put most of the research efforts on hold.
Until the 1970s, when OPEC's first oil shock hit, setting off another wave of R&D in both America and Europe.
A wind turbine is the opposite of a fan. Instead of using electricity to make wind, wind turbines use wind to make electricity. The wind turns the blades, which spin a shaft, which connects to a generator and makes electricity. So far so simple. What makes it effective is the force that wind generates.
As described earlier, the force exerted by the wind on a structure varies by the square of its wind speed—that is, the force exerted by a 24-mile-per-hour wind is four times the force exerted by a 12-mile-per-hour wind. It's not just wind that does this—the ratio applies to all kinetic motion. If you double the speed of a traveling car, say, it will take four times the power to bring it down to a standstill, in accordance with Newton's second law of motion. But the wind's energy as it applies to windmills is greater still—it increases with the cube (the third power) of its velocity, not the square. That is, if the wind speed is twice as fast, it contains eight times as much energy (2X2X2). This apparently puzzling fact is explained by the fact that the wind passes through the turbine, so that if the speed of the wind doubles, twice as many slices of wind pass through the turbine each second, and each slice contains the usual four times as much energy, yielding up the eight-times result.
The speed of the winds, then, is critical, and has great practical implications. Take an average wind speed for a day of an arbitrary 11.2 miles an hour. Say the wind in London blows all day at that speed, 11.2 miles an hour, but the winds in Paris blow at 8.2 miles an hour half the day and 14.2 the other half. The averages are the same, but in practice Paris would get as much power from half a day as London did the whole day. The shorter but faster