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Micro wind turbines: the options

The most important general aspects to consider when choosing whether or not to install a small wind turbine have already been reviewed under Micro wind turbines: the basics. Here we look at the various designs, how they are mounted, and the likely costs and benefits.

Building-mounted turbines

Various small turbines are designed to be mounted on a building, typically on a mast attached to the wall, gable end, or roof. The maximum rated output of such models generally lies in the range 400 W to 1.25 kW, with units costing typically from about £1500 upwards. Most can either connect directly to the grid through the ring mains circuit via an inverter, or be used to charge batteries as a means of storing energy on site. However, some, notably the Windsave turbines, are suitable only for grid connection.Swift micro turbine

Useful operating life expectancy is generally quoted at 10-20 years, and payback obviously depends on how well the turbine performs and the cost of the electricity saved. Reliability was mixed with earlier designs, and products and installers should now adhere to standards set under the Microgeneration Certification Scheme.

The Swift rooftop turbine, with a rated maximum power output of 1.5 kW, can be connected to the grid or used for off-grid applications (courtesy of Renewable Devices Swift Turbines Ltd)

There is controversy about the effectiveness of building-mounted turbines. At best, they can supply only a fraction of the electricity required by the typical UK household. A report published by the Buildings Research Establishment concluded that performance is very sensitive to local variations in wind speed and direction, and in most urban areas of the UK such turbines are unlikely to repay the initial financial investment, or even the carbon cost of their manufacture and installation.

A survey conducted by consultants Encraft in 2009 monitored 26 such installations at various sites across the UK. It found that on average they generated just over 4% of the claimed maximum output, even allowing for time when the turbines were switched off. For comparison, larger turbines on favourable sites can reach up to 30%.

Mast-mounted turbines

Where space permits, the installation of a larger turbine mounted on a custom-made mast may be a better option. These can range in output upwards from 600 W to 6 kW. Height of the turbine hub should be 10-20+ metres above the ground, to benefit from higher wind speeds and reduced turbulence. Some masts are free-standing steel poles, whereas others use guyropes to help stabilize the structure.

Cost of installation ranges typically from £5000 up to £20,000 or more. Preparation of a suitable foundation for the mast is often an additional cost.

Grants

For turbines installed after 15 July 2009, payments for electricity generated are made under the Feed-in Tariff (FIT) scheme. This started on April 2010; see the article on FITs for further details. To be eligible for the FITs scheme, both the turbine equipment and the installer must be certified under the Microgeneration Certification Scheme.

Planning consent

Installation of a wind turbine generally requires planning permission. Factors such as visual impact, noise, and architectural character of the surrounding buildings can all influence the outcome. Check with your local authority first.

Points to consider

Several factors can contribute to poor performance of domestic wind turbines:

  • Unsuitable location - the general wind speeds are too low, or the turbine is mounted too low, or is subject to turbulence from other buildings, etc. Sites are seldom ideal in towns and cities
  • Unreliability - including turbines, ancillary equipment, and failure of the mast or the structure to which it fastens
  • Excessive noise or vibration - causing disturbance or complaints from neighbours resulting in turbines being switched off
  • Unrealistic claims by manufacturers or inaccurate windspeed data