Cp = 43.1 %, a milestone in small windmills ?

[fritzblitz]

Hello all of you,

There is a german discussion board for small windmills. Link: http://www.kleinwindanlagen.de

There I found an interesting link of a small manufactured windmill designed for low wind, which has a diameter of 1,22 meters . They say it is supposed to generate 300 watts at 9 /m/sec. wind. Link: http://www.prevent-germany.com/windkraftanlagen/windgenerator-black-300-12v-niedrigwindgenerator.htm



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I took my pocket calculator and the well known formula: 0,35 x 0,5x 1.23 x dxdx0.785x windspeed xwindspeedxwindspeed. The result was an output of 183,3 watts. Can you tell me what is wrong here? my calculation, the Cp, or anything else? Or is the TSR  much better than I can imagine? Is that really possible, to achieve 300 Watts at 9 m/sec? Another interesting detail in their prospect: in case of stormy wind, it would be better to fix the propeller with a pice of string.

Fritz

[Airstream]

Turbine Site's Google German/English Translation

[hamitduk]

Nope, crap from China, translated to German.

[SparWeb]

Can't be right.

The raw wind energy is about 500 watts.  Accounting for the Betz limit, you can only capture 300 watts.  Nothing is made that "perfectly", so I don't believe the claim.

[Ungrounded Lightning Rod]

Yeah.  Using their own formula (which gives the energy in the wind) and multiplying by the (approximate) Betz limit of 59% I get about 308 watts, losing only about 2.5% of the theoretically recoverable power.

A HWAT is going to throw more than that downwind as "spinning the exhaust" even at very high TSR and I've never heard of a magneto that doesn't lose several percent.  (Even with superconducting wire you'd have the diode drop for a DC output.)  So something's bogus.

Best honest scenario I can come up with is poor wind speed measurement (or measuring the near-ground wind with the genny mounted high.)

Dishonest scenarios would include somebody calculating the Betz-limited power and claiming something a squidge below it or calculating the wind energy (not knowing about Betz) and throwing in a guess at losses that happens to be close to Betz' number.

[dnix71]

When I got the poorly translated Google version, it said something like it put out 3 phase a/c. That may explain the apparent high efficiency and low cut-in.

[dnix71]

9.6 meteres per second was the speed reported, not 9.

[dnix71]

9.36, my bad

Still hard to believe