First the basics… underpinning the physics of wind power is a ‘cubic law’ – the power output of a wind turbine is approximately proportional to the cube of wind speed. So double the wind speed and the power goes up by eight times (2x2x2) – halve it, and it falls to one eighth, roughly.
So the value for money of a domestic wind turbine and its cost of reducing carbon depends very sensitively on the average wind speed at the site. And this, it turns out is why domestic wind turbines a sucha rotten green-buy…
The DTI has a national wind speed database which is best accessed, with useful supporting information, from the British Wind Energy Association web site. I used this to calculated the wind speed for my own area, and it comes to 4.8 metres per second at 10m above ground.
However, the rated power of the typical wind turbine (I picked the Futurenergy 1kW system at random) is based on wind speed of 12.5 m/sec… in other words, a force 6 “strong breeze”. In fact, there are very few locations that have an average wind speed this high… see wind speed map of the UK – most of the country is below 6 m/sec – and this map is for wind speed at 25 metres above ground (a domestic turbine would be 5-10).
So for typical wind speeds expect much less power than it says on the tin. The main chart is derived from the better generation (an excellent site about practical green energy) and shows the approximate cubic relationship between wind speed and power output, but based on real measurements for real micro wind turbines – I’ve added the manufacturer’s rated power and wind speed and power for my location to it. You can see a 90% fall in performance – I get about 100 watts for every rated kilowatt. But the cost doesn’t change. So if I slip on the energy-numbers anorak, I calculate that a 1kW system costing just under £2,000 fully installed, would produce electricity at 22p/kWh and save carbon emissions at a cost of €450 per tonne of CO2 – more than 40 times the cost of the EU Emissions Trading System. The economics are very poor (though not as bad as for solar PV). See all data on this spreadsheet [XLS version].
Other issues arise from attaching a moving structure to your house: the advice is ‘don’t’. These things need to be mounted on a pole or the noise and vibration will drive you mad, and a stiff wind might damage your house.
Aren’t people just trying to make a difference?
Some will object that people just want to make a green gesture – but I think people should be told when they are doing things that are terrible value for money and encouraged to spend it well, which usually means energy saving. Otherwise, there is a risk of a breach of trust and descent into cynicism about personal action and investment in the environment.
What about large wind turbines?
Large wind turbines are much better value for money, though of course with impacts of their own. They are placed in high wind-speed sites, high above the ground, and their blades have large swept area – the power output is proportional to the square of the rotor diameter as well as cube of wind speed. This is why large wind turbines have important economies of scale and why they are leading the rise of renewables.