Are domestic wind turbines an eco-con?
- 30 Nov 06, 02:40 PM
It is time to talk turbines. Very uncharacteristically I’ve been biting my tongue on the subject as I waited until I got my own windmill. Then last week, just three days before the thing was due to be installed, a representative of Windsave rang to say the company had decided that my property isn’t suitable and that their installers would not be coming round.
Windsave, as regular viewers will know, offered me a turbine back in March when the Ethical Man experiment first began. It is also the company which, with much fanfare, sells its turbines through B&Q. Windsave has promised to come round to my house next Thursday to explain its decision.
But if Windsave isn’t going to put a turbine up then I should put my cards on the table: I am now persuaded – unless someone can convince me otherwise – that domestic wind turbines are little more than an eco-con.
Why do I believe this? Well, it is a question of physics.
I know this is rightfully a subject for my colleague Steve Smith but let me do my best to explain (and I would be very grateful if you could write in if you can improve upon or add to my explanation).
As I understand it (with a little help from the web) you can calculate how much power the wind can deliver with this equation:
Power in watts = (collection area in square feet) x (wind speed)3 x (0.0054)
The problem with domestic turbines lies in that little cube function on the wind speed. Consider what it means: because power is related to the cube of the wind speed, when the wind is blowing hard you get a lot of power but when it drops the output falls disproportionately. Indeed, it rapidly becomes almost non-existent.
The cartoonist, artist and engineer Tim Hunkin explains it very well on his website. Conveniently for me he takes the example of Windsave, the company that had promised me a turbine.
Windsave boasts that its 1.75 metre turbine will generate 1kW of power at speeds of 12.5 metres per second, a pretty strong breeze. Not bad. But what happens when the wind falls? By the cube law, halve the wind speed to six metres per second (a moderate breeze) and you now get 120 Watts – that’s one non-energy saving bulb. Hum, not so good.
My house is on the flanks of the highest hill in London and is relatively exposed but I’m told that average wind speeds are likely to be between 4 and 5 metres per second.
What would that do to the output from my Windsave turbine? At those speeds I’d be lucky to get 25 Watts. That is barely enough for two energy saving light bulbs and nowhere near enough to live up to the company’s promise of reducing my electricity bills by “up to 30% a year”.
So does that mean that wind power is a dead end? Once again that is answered by that equation.
The problem with Windsave’s wind turbines (and those of its rivals) is that they are too small. Just as power falls disproportionately when wind speed drops the collection area increases disproportionately as you increase the length of the turbine blades. So a big turbine can generate reasonable amounts of power even in a relatively low wind.
So what is the upshot of all of this? Well, unless these calculations are very wrong my advice to you if you are considering buying a domestic wind turbine is don’t do it. It will probably take more carbon to make the thing than it will ever generate in usable power.
But there is a wider implication too. Micro wind power is one of the ways the government suggests that this country will achieve the sixty per cent cut in emissions that is due to be enshrined in the new climate change bill. The Energy Saving Trust estimates that domestic turbines could supply up 4% of all the UK’s electricity needs and could cut carbon dioxide emissions by 6%.
If my argument is right then those figures are nonsense and the government will need to look elsewhere for the carbon cuts it needs.
This is a pessimistic conclusion. Please prove me wrong if you can.