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Micro wind turbines: the basics

Wind is one of the most abundant forms of renewable energy, especially in the UK, but we still generate only about 0.5% of our electricity from wind. Wind turbines harness the energy in wind and convert it into electricity. A micro wind turbine is generally regarded as one whose power output lies in the range 100 watts (100 W) to 6 kilowatts (6 kW).

These machines are most suitable for domestic installations or remote sites off the grid where small amounts of electricity are required. They can be mounted directly on buildings, or on a suitable mast.

Although wind energy is a clean and sustainable form of energy, the efficiency of wind turbines depends very much on their design and where they are sited. Even the best designs produce some noise and vibration, and your neighbours might not share your appreciation of their striking visual appearance! So, think carefully before going ahead, and view manufacturer's claims with scepticism.

How do they work?

grid-connected turbine

Scheme showing components of a grid-connected wind turbine.

The blades of the turbine are rotated by the wind and, via a rotor shaft, drive a generator. Most small turbines produce electricity in the form of direct current (DC). This has to rectified by a DC/AC inverter to produce alternating current (AC) at a voltage (230 V) suitable for domestic appliances. Any surplus electricity can be exported to the grid. In larger models a gearing mechanism enables the generator to produce some electricity even in very light winds.

Alternatively, the DC generated by the turbine can be stored in batteries, and then used to power appliances as required. Again, a DC/AC inverter is needed, but such a system can operate entirely separately from the grid.

The power generated by a turbine is proportional to wind speed cubed. So, if the wind speed doubles, the power output increases eightfold. This emphasizes the importance of wind speed in relation to performance.

 

Can wind turbines work for me?

It is said that the performance of any wind turbine depends on three factors: location... location... location! Manufacturer's promises are often highly optimistic, and assume virtually ideal operating conditions. Principal factors affecting operating efficiency are:

  • Wind speed - according to the Energy Saving Trust (EST), the average wind speed should be more than 5 metres per second (6 m/s). The EST's Wind Speed Prediction Tool will give some indication of the suitability of your site for a wind turbine. Beware, though, that windspeed can be affected by local geography, buildings or other structures nearby. Also, studies have shown that the database predictions often overestimate average speeds. If possible, conduct your own wind speed survey first.
  • Height above ground - wind speed increases in proportion to height above the ground, so the higher the better. However, accessibility is vital for servicing and maintaining the unit.
  • Turbulence - turbines perform best with smooth steady air currents, and should ideally be positioned well away from buildings, trees, or other structures that will cause turbulence. Frequent changes of wind direction affect performance, even in strong winds.

Other crucial factors

  • Noise - turbines can be noisy, especially in high winds, due not only to mechanical noises but whistling of air past the mast or guy ropes.
  • Vibration - installations on buildings have sometimes caused the building to vibrate, on occasion resulting in structural failure; also, vibration of the turbine itself increases wear
  • Safety - manufacturers and installers should now adhere to a performance and safety standards set by the Microgeneration Certification Scheme. Check that your turbine and the installer comply with these standards.
  • Maintenance - service checks are needed periodically to ensure efficient operation, and lifespans can be up to 20 years; batteries will need replacing every few years.
  • Planning permission - this is nearly always required, so consult your local planning department well in advance of installation