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Siting a small wind turbine

Many potential users of small wind turbines think how nice it would be to put up a wind turbine next to their house and use the free power of the wind. Unfortunately, whilst the wind is free, the means to extract the power from it is not. Buying and installing a turbine a turbine costs money, and there are also operation and maintenance costs. Consequently serious consideration much be given to siting the turbine to get the best performance and reliability from it.

The output from a wind turbine is highly sensitive to wind speed. It is essential that turbines should be sited away from obstructions, with a clear exposure or fetch for the prevailing wind. Wind speed also increases with height so it is best to have the turbine high up, and most small turbines have towers much higher relative to their diameter than large ones.

It is generally agreed that the ideal position for a wind turbine generator is a smooth hill top, with a flat clear fetch, at least in the prevailing wind direction. The wind speeds up significantly near the top of the hill and the air flow should be reasonably smooth and free from excessive turbulence. Excessive turbulence causes fatigue damage and shortens a turbine's working life.

In practice, especially for very small machines which need to be located near to the user, ideal siting will not be easy. As far as possible though, keep away from local obstructions such as large trees and houses, or use a taller tower to ensure that the turbine is well above the obstructions.

Before considering the installation of a wind turbine the potential site should be assessed. Initial indications of wind strength and direction can be obtained by observing the deformation of vegetation and trees, and in many cases the user may already have a good feel for the winds in the locality. However impressions can be deceptive.

A more reliable way to evaluate the wind resource, which is strongly recommended when there is doubt over whether the wind is strong enough, is to take regular measurements over a period of several months, preferably a year. It is not straightforward to use data even from nearby sites, and probably the nearest meteorological station or airfield where records have been kept is many miles away. However, measurements taken at a proposed site can be compared with measurements taken elsewhere at the same time, and used as a guide to the probable correlation over longer periods. In addition computer models re available for professionals to use, although such predictions should be applied with care.

Clearly the expenditure on site assessment should be in relation to the proposed size and cost of the installation. Very little or no cost is justified in evaluating a site for a 50W battery charger, whereas a 60kW wind turbine, for example, would in general merit site measurements.

Basic requirements

  1. Get a reliable estimate of the winds to be expected at the proposed site. There is no substitute for actual measurements. The turbine manufacturers should be prepared to help. You can get an estimate of the annual mean wind speed for a given location by entering the OS grid reference into the UK wind speed database
  2. Mount the turbine on as high a tower as possible and well clear of obstructions, but do not go to extremes. Easy access will be required for erection and foundations for the tower may be needed depending on the size and tower type. It is also important to ensure that the wind turbine can be easily lowered for inspection and maintenance.
  3. Try to have a clear, smooth fetch to the prevailing wind, e.g. over open water or smooth ground. If possible site the turbine on a smooth hill.
  4. Use cable of adequate current carrying capacity (check with the turbine supplier. This is particularly important for low voltage machines). Cable costs can be substantial.
  5. Consult your local council as to whether you need planning permission. You should try to minimise the environmental impact of the turbine, and it will be helpful to inform your neighbours of your plans at early stage.
  6. For larger machines you may have to pay rates. This can make a big difference to the economics of the installation, again you should find this out by consulting your local council.

If the machine is intended for connection to the grid you must discuss it with your local electricity company. For details, see Generating electricity for the UK Electricity system

Further information can be obtained from Using Wind Power, A Guide to Remote Power and Domestic Applications, which provides a good general explanation of how wind energy works and how it can be used domestically, and includes tables for calculating the costs.
(35 pages. Send large SAE plus £1 in stamps to BWEA office).

The publication Off the Grid, priced at £5.50 is also recommended. Available from the Centre for Alternative Technology (CAT) Tel: 01654 702 400, Fax: 01654 702 782