For alternative methods of energy generation to make sense they need to do more than simply appear to be ‘ecological’: they need to make sound ecological and economic sense. And nothing could appear more ecological than sticking a wind turbine on one’s roof! But does this make either economic or ecological sense?
My colleague Neil Campbell was kind enough to send me a link to a site which conducted extensive tests on small wind turbines. I have analysed the results below, but the results are simple to understand: unless you have no connection to the electrical mains, wind power makes no sense at all on a small scale. A basic physics analysis indicates that the extractable power is proportional to the diameter of the rotor squared. So a 5 metre diameter can extract as much as 5 x 5 = 25 times more power than a 1 metre diameter turbine. Wind turbines are not cheap, and wasting money and resources on a wind turbine is as wasteful as wasting it on anything else.
The Table below shows in turn, the name of a wind turbine, its cost in euros, its actual output in a favourable wind environment (kWh per year), what this output equates to as an average power (W), the cost per unit of electricity if the wind turbine last for 10 years (compare with around £0.20 for a maximum unit cost in the UK); a rough estimate of the maximum average output if wind speeds are a consistent 5 metres per second (11 m.p.h.) for 30% of the time, and a record of how much of that plausible maximum the wind turbines actually achieved. I evaluated the plausible maximum using the formula
Power ~ 0.6 x 0.5 x (air density) x π x rotor radius^2 x wind speed ^3
|Output (kWh/Year)||Average Output
||Cost per kWh depreciated over a 10 year Life||Plausible maximum average output||Fraction of Plausible Maximum achieved|
|Energy Ball||1||4304||73||8.3||5.90 €||11||78%|
|WRE 030||2.5||29512||404||46||7.30 €||66||69%|
|WRE 060||3.3||37187||485||55.4||7.67 €||115||48%|
The data above teach three lessons
- The last column tells me that wind turbines are roughly as efficient as they can be. In this context I take any answer between 50% and 150% to be roughly equal to 100%.
- To make more economic sense the only way forward is (a) to use large diameter blades or (b) to reduce the cost of each turbine.
- 4 metre and 5 metre diameter turbines are on the borderline of making economic sense if they are mounted on a good tower in a windy place. But a 5 metre diameter turbine is a serious engineering undertaking and would probably need a 25 metre tower to make sense. Would your neighbours object? You know they would!
So in summary, small scale wind power doesn’t make sense. And its the alignment of ecology and economics that makes that clear.
And physics of course:-)