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Wind power is far more site specific than solar. Good sites should be exposed and away from trees, large buildings and other obstructions which will cause damaging turbulence. Hill tops and saddles are ideal. Coastal sites are often suitably windy.
Wind power can be used to drive mechanical pumps – multivane windpumps – or converted to electricity using wind generators. Wind generators suitable for DIY installation range from about 50W to 300W. With some knowledge of rigging, machines up to 1.5 kW can be installed in the field without too much difficulty. These figures are the rated outputs at a specified speed (typically 12 m/s, ~30mph) – the generator will spend most of the time producing far less.
Wind turbines should be mounted on dedicated towers – guyed towers are generally the simplest to erect. Very small machines (up to 100W) can be mounted on poles attached to strong buildings, but this is best avoided as it will often cause noise and vibration.
Is there enough wind? It is common for people to think a site is a lot windier than measured data reveals. In general, the average windspeed for a good windpower site needs to be above 4m/s (9 mph) which means it generally be windy most of the time. Vegetation can provide some clues – if trees and bushes are leaning in one direction (‘flagging’) away from the prevailing wind this suggests strong regular wind.
You can get a much better idea of the resource with some wind data. Ideally, you can monitor on site using an anemometer at the proposed wind turbine height. Simple loggers can be found online for £200-300, or you can even improvise one using a bicycle speedometer. There may be long term wind data available from local airfields or meteorological stations. Although these won’t tell you the average at your site, they will reveal how wind varies through the day and the seasons. Treat any data gathered at 2m height with some scepticism, though, as it will be highly compromised by surrounding obstacles.
Wind speeds vary a lot according to local topology. Generally it’s better to be at the top of a hill, although a valley or saddle aligned with prevailing winds can funnel the wind and increase the speed. A clear view towards the prevailing wind (‘fetch’) is also desirable. Sharp edges – cliffs and crags – create lots of turbulence downwind and should be avoided.
As well as the terrain, the ground surface is important. Wind speeds increase with height above ground and this is affected by the ground cover. Grass and water make for smooth airflow with minimal turbulence and faster speeds near the ground. Rough surfaces – trees and buildings - create lots of turbulence and gusts and need high towers to get clean, fast air flow. As a rough guide, the turbine should be twice the height of any nearby obstacles – trees and buildings - and 5 times the height downwind to escape the turbulence. Turbulence exerts high loads on the blades and tail and can cause premature failure.
Why all the concern about windspeed? Theoretically, power in the wind is proportional to the cube of the windspeed and the swept area of the rotor (the square of the blade length). Doubling the rotor diameter gives you 4 times the energy, doubling the windspeed gives you 8 times the energy. In reality, this is modified by equipment characteristics, but in general a few hours of strong breeze will yield more energy than a sustained light breeze. The general rule is to use a big rotor and get the fastest, smoothest air flow for best results.
Setting up a wind power system is a bit more complex than solar photovoltaics (PV). You need a tall, well secured tower – at least 12m though 20m is far better. The equipment needs regular maintenance and needs to be robust and well-engineered. In areas with extreme winds – typhoons, for example – the tower should be arranged to allow easy lowering before high winds hit.
Compared to PV, windpower is more complex and often more costly, so why consider it? • Where security is an issue, wind turbines are less likely to be stolen (although guyed towers are vulnerable to sabotage and vandalism) • In areas with high cloud cover or short day length, wind can provide a good alternative or supplement to PV. • Wind turbines can be manufactured locally with fairly basic facilities, unlike PV modules. There are open source designs for small-scale wind turbines, the classic being the series by Hugh Piggott whose designs, originally from NW Scotland, have been built all over the world. His website (http://scoraigwind.co.uk) is a source of inspiration and real-life, hands-on advice.