Our local community turbine rotates at a constant speed of 39 revolutions per minute. What is the mechanism that allows greater power to be generated in strong winds, in terms I can explain to my grandchildren?
鈥� For good reasons, our electricity supply works not by providing a single, steady flow, but by flitting backwards and forwards. This is called alternating current, or AC for short. The electricity generated by wind turbines must keep in time with this, so all of them rotate at the same speed. When the wind blows harder, adjustments inside the turbine make it harder to turn the rotors, generating more electricity with each rotation.
It is exactly like trying to pedal your bike at a continuous speed. When you leave the flat and start going uphill, it becomes much harder to maintain the same pedalling rate.
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Tim Stevenson, Great Missenden, Buckinghamshire, UK
鈥� Older designs for wind turbines aim to keep the blades rotating at a constant speed because this enables optimal conversion of the movement into electrical energy for use in the national grid. Too slow and the turbine doesn鈥檛 catch enough wind energy; too fast and the blades and other components suffer excessive stress. If left to itself, the turbine would spin faster as the wind increases, just like the plastic windmills we buy at the seaside.
We convert the wind into electricity by allowing the turbine to spin freely and then 鈥渂raking鈥� it to capture as much energy as possible. But instead of a mechanical brake such as those on a car or bicycle, which remove the energy from a spinning wheel, we use an electrical 鈥渂rake鈥� that allows us to capture energy and convert it into electricity rather than waste it as heat.
We don鈥檛 want to stop the turbine completely, but allow it to turn at a constant speed regardless of how hard the wind is blowing.
If we want to freewheel a bicycle down a gentle hill at a steady speed, we apply the brakes gently and they warm up by the time we reach the bottom. If the hill becomes steeper, but we want to keep the same speed, we squeeze the brakes harder and they get even hotter because they convert more energy into heat (don鈥檛 test this by touching the brakes).
鈥淥lder wind turbines rotate at a constant speed because it makes for easier conversion into electricity鈥�
Similarly, when we want our turbine to turn at 39 revolutions per minute in a steady breeze, we only apply a small amount of electrical braking, and generate a small amount of power as a result. When it鈥檚 blowing a gale, we can really apply the 鈥渂rakes鈥� hard to get a lot more power out, even though the blades are spinning at the same rate.
Steve Agar, Hexham, Northumberland, UK
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