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 â€” up from 1000 and 1400mm), and much lower rated power (500W and 1000W at 27mph â€” 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.