Wind turbines are the piece of alternative technology hardware that seems to create the most enthusiasm in their designers and users. Part of their appeal is visual: they move in response to the wind, unlike solar collectors which lie quietly absorbing heat from the sun (if you are lucky) or insulation which you cannot even see. wind turbines have some of the charm of early aeroplanes with their spinning blades and light construction; and they offer ample opportunity for the amateur engineer to try out his or her theories, and perhaps invent some new or improved method for harnessing the energy of the wind. In East Anglia model wind turbines, ranging from mass-produced plastic ones to large-scale replicas of old corn mills, rival gnomes as garden ornaments in rural areas, suggesting that the sight of revolving blades brings pleasure to many.
Alternative technology wind turbines are most commonly used to make electricity; they are often given names which are somewhat more technological than ‘wind turbines’ (Don Quixote ias a lot to answer for) such as wind generators, aerogenerators or even ‘wind energy conversion systems’. Except in very windy areas, wind turbines used to make electricity are not really economic if you need batteries to store the electricity, because the cost of batteries and their replacement is so high, but if you live in an isolated house with no mains electricity and you want electric lights and say a radio, television or stereo set, a wind turbine may be the practical answer. If cost effectiveness is not your concern, Of if you can obtain scrap materials, go ahead and build a wind turbine anyway. They are fun, a splendid symbol of alternative technology, and a kinetic demonstration of the use of natural sources of energy.
Selecting a site – The formula
Before building or buying a wind turbine you need to know if the site where you intend to put it is windy enough to make it worthwhile. The energy available from a wind turbine can be calculated using a formula devised by the Building research Establishment, whereby E is approximately equal to 10 V503.
E is the energy available in the wind in kWh per year per square metre of swept area, the swept area being the circle or other shape described by the rotating blades of the wind turbine. The equation allows for the loss of energy from very low and very high speed winds which the mill will not be able to use.
V50 is the wind speed exceeded 50 per cent of the time. The map shows values of V50 for different parts of the USA, and these values should be multiplied by the following factors to take account of the proposed site:
- city 0.7
- suburban 0.8
- open 1.0
- coastal 1.1
- hilltop 1.2
The values for V50 on a decent wind speed map will give you a rough idea of what you are likely to obtain in the way of energy in the wind, which is not the same as the output from the wind turbine. However, an individual site may be 50 per cent more or less windy than it appears from the map depending on topography, the presence of trees, and other factors. If you are serious about windpower you must choose the proposed site carefully and then measure the wind there.
The general principle when choosing the site is to find the most exposed one possible. The best place would be a bare hilltop, preferably a smoothly rounded hill which tends to accelerate the wind over it; the worst possible location would be in a forest clearing. Try to choose a location where the wind turbine will be at least 150-200 metres from any trees or other obstructions, as these will block the wind and cause turbulence that may tend to damage the wind turbine. If you are going to use the wind turbine to generate electricity it will have to be fairly close to your house, or the cost of the cable linking wind turbine and house will be excessively high. Most sites are not large enough to allow much scope in the placing of the wind turbine and the chosen location usually involves some compromise.
Using an anemometer
To establish most precisely the amount of energy available at a site you will need an anemometer, preferably a ‘cup anemometer’. This is a device which is blown round by the wind: as it revolves it turns a counter like the mileometer of a car. This clocks up the number of kilometres or metres of wind that have been past the anemometer. For example, if you set the thing up and after an hour the counter reads 10km the mean wind speed during that period has been 10km per hour. If you read the anemometer each day you can then divide each result by 24 hours to give a daily mean wind speed in km per hour. The figure clocked up in a year, divided by 8760 (the number of hours in a year), will show the mean annual wind speed, which is fairly close to the V50 speed. If the speed is in km per hour it can be converted to metres per second for use in the equation by multiplying it by 0.28 — similarly figures in metres per second can be divided by 0.28 to convert them to km per hour. Using the mean annual wind speed and the daily mean wind speed, you will have some idea of the distribution of wind energy throughout the year.