Windsave, again

Anent my previous rant about Windsave claiming impossible efficiencies, they’ve made some changes to their website. The machines now have larger diameters (1250 and 1750 mm — up from 1000 and 1400mm), and much lower rated power (500W and 1000W at 27mph — down from 750 and 1200).

Plugging in those numbers to Cp = P / ( 0.48106 d2 v3 ), we get more realistic efficiencies of 0.378 and 0.386 (for the small and large machines, respectively).

The Lakota turbine we installed last week has a nominal rated power of 900W at 28.8 mph for a 2.09m diameter rotor. It has a very conservative Cp = 0.20, although David Cooke says that typically they see 1,000 Watts at around 25mph (a Cp of around 0.34).

At the other end of the scale, the Lagerwey LW52 is a 51.5m diameter machine rated at 750kW at 12ms-1. This advanced utility scale, variable pitch machine has a Cp = 0.34.

Windsave’s revised figures are much more credible, but until we have real figures backed by a few years of installations, there’s little more we can say about them. I’m a little concerned that, although there are claims that 1000s of these machines have been sold, there’s not a single real photo of one on the web.

I’m going to enjoy putting up an anemometer and logging system alongside the urbine downtown. We’ll see how it runs.

WindSave: All Betz Are Off!

Update, May 2005: While it’s true that WindSave appear to have made their device obey the laws of physics (at least according to their published spec), I’m keeping this posting intact.

If the data on their website are to be believed, WindSave cannot generate the figures they claim. There’s much geeky theory involved, but basically, they’re claiming efficiencies that cannot be attained.

In short, they are claiming coefficients of performance of 0.724 and 0.887 for their turbines. Unfortunately, the theoretical maximum efficiency for a wind turbine — the Betz limit — is 0.593. So something, somewhere, is screwy. I’m pretty sure it’s not my sums, as they’ve been verified by an external source.

I worry that the UK energy minister, Brian Wilson, has been taken in by this. Five minutes with a calculator and a wind energy primer shows that these things are too good to be true.

Continue reading “WindSave: All Betz Are Off!”