The title of this article, while recalling one of Bob Dylan’s famous antiwar and civil-rights anthems of the ‘60s, addresses another potential solution to our energy crisis. The energy crisis is not only a matter of energy shortage, but also of the pollution and global warming that the present sources of energy are causing.
In my last column, “Let the Sunshine In,” I addressed the issues of generating electric energy from the sun’s rays. Actually, the wind is a form of solar power that has been converted to mechanical energy. Moreover, just as the sunshine does not shine all the time, the wind does not blow all the time. The more the wind blows, the more power that can be produced by the wind turbines.
Most studies and predictions about world wind energy indicate that it can support the world’s energy needs several times over. The map below, compiled by researchers at Stanford University, shows wind speeds at many sites around the world. More than 13% of those sites experience winds fast enough to power modern wind turbines. Altogether, these energy-producing sites could generate 72 terawatts of electricity, or more than five times the roughly 14 terawatts used worldwide in 2002.
How fast the wind blows, how often and when — all these issues play a significant role in determining the cost of generating power from wind. A common term used to rate a wind-turbine output is the ”capacity factor,” or the ratio of power produced over a period to the amount of power a turbine could produce if it ran at full capacity over the same time. The power output from a wind turbine also rises as a cube of wind speed. In other words, if wind speed doubles, the power output increases eight times. Therefore, higher-speed winds are easier and less expensive to capture.
Just as the dc output of solar cells is unregulated, the ac coming from wind turbines is not regulated, since it depends on the wind, which is not a constant "fuel." The raw ac from the turbines is essentially unusable, since it varies both in frequency and in voltage. This is particularly true for small wind turbines (up to 10 kW). Large wind turbines (500 kW and up) are very similar even though some have a mechanical gear that orients the pitch of the blades so that the propeller always spins at the same speed. In that case, electronic inverters and regulators are unnecessary, but due to the change in pitch and mechanical gearing, the efficiency of the turbine is lowered.
A more-efficient approach is to take the turbine output, which varies between 0 V and 600 V of unregulated and frequency-varying ac, and rectify it and feed it to a dc-ac inverter. In many instances, the output of the inverter will need to be converted again for charging batteries or powering electronic equipment. Given the need for such multiple conversions, the use of the inverter to generate well-regulated ac sounds like it would make for a very inefficient system. But if you consider that today’s inverters that produce a high-voltage fixed-frequency output can be 97% efficient, then the multiple conversions can be justified.
Electricity generated by the wind can be sold to the utility or credited to the user against future consumption the same way that power from solar or other sources is sold. Besides the motivation to reduce global pollution, one can be motivated by the multiyear payoff for the initial investment of installation and equipment costs.
At present, mechanically regulated wind generators are expensive and not very efficient. As the industry matures, the trend will be to replace the mechanical gearing with designs that take the raw ac produced by the wind generators, rectify it and electronically convert it to ac that is usable by the electrical grid. Since the sun shines and the wind blows somewhere all the time, interconnecting all the generating locations will enable the constant supply of electricity to all locations.
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