“Oh no, not again!” I’m saying to myself, and really hoping that — just once — I’ve done my sums wrong.

You’ve seen my rant about how the initial public specification of the WindSave rooftop wind turbine was an impossibility (and, in fairness to them, how the corrected specs are much more like the thing). And you may have seen that I’ve written about the Swift before. But the Renewable Devices Swift was all over the blogosphere (a hateful word, I must say) today; both Treehugger and sustainablog were on it.

So I download the very pretty PDF spec sheet. Here’s the technical table from the document, which is dated 19 November 2004:

So that’s a diameter of 2m, rated wind speed of 10.5ms^-1, and a rated power of 1.5kW. Plugging that into my simplified C_p equation

C_p = P / ( 0.48106 d² v³ )

which gives:

C_p = 1500 / ( 0.48106 × 2² × 10.5³ ) = 0.67

As this is higher than the Betz Limit of 0.59, the claimed power output of the Swift wind turbine is theoretically impossible.

I don’t know how to put this, but rooftops are sheltered places; if they get any wind at all, it tends to be turbulent and highly directional. You get huge updrafts, none of which help generate power. I know of some very open sites that struggle to get the 3.5ms^-1 cut-in speed of the Swift, and that speed is at 50m+, not on a rooftop.

Renewable energy, for me, is about using the appropriate technology for the right location. Devices like the Swift are a distraction from the whole conservation/renewables agenda.