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Roof mounted wind turbines
Part of a special report from Building for a Future Winter 2005 Volume.15 No.3

If you are one of the many people who have read or heard that the next generation of roof mounted wind turbines will provide all the electricity needs of the average semi in the UK, this article is for you. Nick Martin is sceptical and believes that the predicted rush for domestic roof-mounted turbines will further damage the precarious reputation that wind power already has in the UK ...

Don't believe the hype. I can tell you now that for the average house in the UK a roof mounted wind turbine will not provide anywhere near your elctrical power requirements estimated to be an average of 4200kWhrs. Nor will they save you 400 per year in electricity bills. They might, however, on a typical roof top installation generate 600-1000kWhrs a year if you are lucky with the site of your house. Less output is the more likely scenario. As such you might expect to get savings a long way short of the £400 advertised by some.

Predicted paybacks of 3 years are also fantasy unless you get a free one. Even a payback time of 20 yrs is unlikely unless you get a substantial grant for the total package and the product remains maintenance free for that period (will the manufacturers supply a 20 year warranty? I don’t think so.)

Am I opposed to wind power? On the contrary, I am an ardent supporter. That is why I raise issue with products that are likely to do a disservice to the wind industry. The products in question may be well engineered and good value (there appears to be no independent assessment on this issue) but to over-rate their energy production will leave a lot of disappointed owners and dissatisfaction with wind power generally.
I am not the only sceptic on this issue - many share my concerns for very valid reasons.

To start I am going to shamelessly lift some information from Steve Wade of Wind and Sun contained in his excellent catalogue and design guide. Now in its 19th edition, it is a must-have for anyone interested in small scale renewable energy.

Wind energy
The wind is a by product of uneven heating of the Earth's atmosphere by the sun, and has a very complex distribution pattern. Though widely spread, winds are most prevalent along coasts, at higher elevations and at higher altitudes.

The power in the wind is proportional to the cube of wind speed. Twice the wind speed gives eight times the power, small differences in average wind speed cause large differences in available wind energy. So for optimum performance it is important to find a site which offers the highest overall wind speeds.
Avoid locations with excessive gustiness or turbulence, since these will reduce the output from a wind turbine and lead to undue wear and strain on component parts.
Siting should take account of exposure to prevailing winds. Factors such as surface roughness and obstructions are important, eg woodland or built up areas will create higher turbulence than open grassland. Cliff tops are to be avoided and wind generators must be sited clear of obstructions to the wind. A useful rule is to place the turbine at a distance from any obstacle (building etc) of at least ten times the height of the obstacle, or on a tower that is at least twice that height ...

To read the rest of Nick Martin's six page assessment of the potential for energy from roof mounted wind turbines order your own copy of this edition or subscribe to Building for a Future now!

Nick Martin

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