Moon Power

From the energy of wind-driven waves, we can now turn to the wind itself. Windmills are another ancient method of obtaining power; those used by the Persians in AD 644 to drive pumps for irrigation had cloth sails on a vertical axle. The traditional "picturesque" windmill with a tall tower and a horizontal axle appeared in Europe at the beginning of the 14th century, and its numbers grew until, as we saw, steam made its takeover late in the 18th. In some countries, though, they reached a standard of efficiency, especially when applied to the generation of electricity, as pioneered by Professor P. La Cour in Denmark. Wind-generated electricity was used up to and through World War II in Denmark, with over 480 MWh being obtained from 88 windmills in January 1944. The world's largest windmill was built in America. At Grandpa's Knob, Vermont in 1941 a 33.5,-tall windmill produced 1.25 MW with the first synchronous electric generator.

Wind is notably capricious element. The UK's electrical needs could be met by tapping only a fraction of it's available wind energy, but doing so economically is another matter. To compete with normal power stations, windmills or aerogenerators would have to be sited on hill-tops or around the coastline, where average wind speeds approach 32 kph. Even then a 50m rotor would produce an average output of only some 350 kW. Modern windmills would not be as attractive as their predecessors; resmbling more closely a large electricity pylon, a fast windmill could well also be noisy. They do ahve the advantage that the wind blows most when energy is most needed - in the winter.

The power of a windmill depends on the area swept by its blades or rotor, its efficiency, and wind speed. The power available from the wind varies as the cube of the wind speed - which means that the power in a 100 kph wind is 1,000 times greater than one of 10 kph. Some sortof governor or method of shutting down in high wind speeds is needed. Rotors are classified by their "tip speed ratio", which is blade tip speed divided by wind speed. The theoretical possible efficiency - 59% - is unlikely to exceed 455 in practice. Most works best at 30-50 kph.

One solution to the visual and noise problem could be to site windmills offshore. A cluster of 400 windmills in an area 10 x 10 km could, according to some studies, give a total of 1 GW, with an average of 400 MW - comparable with a convetional power station, or 1.7% of our annual needs. As to storage during calm periods, among novel suggestions are to use excess electricity to electrolyse water, piping the hydrogen ashore to be stored for as fuel (perhaps in fuel cells); and the use of a flywheel to even out fluctuations. A light fibre composite material for this purpose stores twenty times more energy than a steed flywheel, according to US work develloped be Exeter University.