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On the face of it, wind-generated power is an attractive proposition in the UK, given that it is one of the windiest countries in Europe. What’s more, the windiest months are between December and February, when the demand for electricity is highest; during this period a wind turbine can generate a third of its annual output.

Wind turbines vary enormously. At one extreme are the large-scale commercial wind farms, in which turbines generate electricity by the megawatt. On the other, there are the micro turbines, which are suitable for individual dwellings or small community projects with an output of 500-20,000W.

Wind speed and power

Wind speeds increase with height above ground level. The higher the wind speed, the more power generated. While average speeds in the UK are suitable for generating power, wind speed data is generally only available for 10-50m above the ground – which is higher than many micro turbines.

This means that turbines do not operate at 100% efficiency. The average capacity factor (that is, actual energy produced compared with the theoretical maximum it could produce) for the UK is 25-35%. Maintenance time must also be included.

For micro turbines the capacity factor may be much lower, as they tend to be at low levels and surrounded by objects that disrupt wind flow. In fact, wind speeds may less than the 3-5m/s required to turn the turbine’s blades.

Specifiers should check at what wind speed the turbine produces its rated power output. This may be higher than average speeds, which will mean lower than expected outputs and longer payback periods. A specialist appraisal should be carried out to determine whether or not a micro wind turbine is economically worthwhile.

Specification options

There are two main specification options: horizontal-axis wind turbines, which are the more common, and vertical-axis turbines, which are quieter but less efficient.

Typically, electricity generating wind turbines have two to six blades and a tail fin or servo motor. These tend to be made of fibre-reinforced polymers and have a life expectancy of 20-30 years. Output is related to the blade’s swept area.

The electricity is best used directly by the home. It is less efficient for local battery storage and less still it exported to the grid.

Mounting

Micro wind turbines are generally mounted on steel tubular masts, lattice towers or directly on the building. They should be designed by a structural engineer to account for foundations, loads and vibrations.

Standards

BS EN 61400 is the standard for small scale wind power generation. Evidence for the reliability and durability of wind turbines is not readily available. Is is expected that some components will require replacement during their life. Experience from the marine sector suggests that turbines have a service life of 20-25 years, but in the US, some claim longer lives. Manufacturer's guarantees tend to be limited to between two and five years.

The BRE has produced a micro-generation installation standard for wind turbines (MIS 3003:2007) that enables contractors to obtain third-party certification. This focuses on installation and energy performance rather than the durability.

Provision for lightning protection may be required to BS EN 62305 and electrical installations should be to BS 7671.

Whole life cost issues

There are many cost benefits to bear in mind. Grants may be available from the low-carbon buildings programme in England and Wales and reduced VAT may be applicable. Exporting surplus electricity to the grid could provide an income. It is also worth noting that at the end of its life, much of the wind turbine may be recycled.

However, additional costs may also arise, for instance from the need to commission environmental impact reports, particularly assessment of noise. There also may be protracted planning periods.