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Forum Index : Windmills : Windmill specifications
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GWatPE Senior Member  Joined: 01/09/2006 Location: AustraliaPosts: 2127 |
The power figures quoted from a manufacturer not correlating with established physics, seems to come up too often. Readers should be aware of some fundamental and empirically determined "laws" related to windmills. The first is the energy in the wind, and secondly is Betts Limit. the energy in the wind follows a cubic relationship to the wind velocity, and for a particular cross section of the wind, approx a maximum 60% can be harvested by a machine. The Kinetic energy available is proportional to the mass of air passing a point, and its velocity. There are many online calculators, including this one. For a standard 10m^2 cross section of wind, with a velocity of 10m/s, then close to 6.2kW of energy will pass in that second. This is the maximum energy. If the velocity is reduced to 5m/s, the total energy for this same cross section will be 1/8 of this, or 775W. Now the empicical limit is for only 60% that could be extracted. There are also factors for aerodynamic blade efficiency, and then transmission losses, and then alternator electrical efficiency, that continue to reduce the power that can be harvested. Whenever we see published windmill data, we should relate back to the maximums that are possible. Windmill comparisons will continue to a problem until a verification system of published data is in place. There may be online comparisons already, but readers seem to need reminding of the basics. Gordon. become more energy aware |
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KarlJ Guru  Joined: 19/05/2008 Location: AustraliaPosts: 1178 |
makes sense to me my 3m diameter for 7m area i'm lucky if i get 250W in 5m/s which in fact doesnt look that bad given only 60%max of 540W in the wind or 321W at betz limit. all goes pear shaped after 8-10m/s where the available vs the output are a loooong way apart  Luck favours the well prepared |
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| GWatPE Senior Member Joined: 01/09/2006 Location: AustraliaPosts: 2127 |
I have just been reading a design abstract of an axial flux alternator. This was 6.5MW unit, intended for a direct drive windmill application at 13rpm. Here is a link to the article. It seems that the designers are OK with only 80% efficiency at maximum power. Imagine over a MW of heat in the windings, that has to be radiated. Worth a read to keep DIY in perspective. I read another article where small scale windmills above the few kW should only be installed by qualified persons. Here is a link to another well rated windmill. At least the outputs agree with physics. Gordon. become more energy aware |
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Gizmo Admin Group  Joined: 05/06/2004 Location: AustraliaPosts: 5124 |
Interesting read Gordon. It seams like the biggest engineering challenges are how to assemble the thing. Glenn The best time to plant a tree was twenty years ago, the second best time is right now. JAQ |
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| JimBo911 Senior Member Joined: 26/03/2009 Location: United StatesPosts: 262 |
Multi layered-axial generator (Fascinating) Great read, thanks Gordon Jim |
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| KarlJ Guru Joined: 19/05/2008 Location: AustraliaPosts: 1178 |
So Gordon I can see you building the 1:20 scale version.... just because you can. 2.1KW at 180rpm sounds good. Much better than the 3.~T of magnets for the fullsize version! obviously stacking stators works fine if you can use that magnetic material in the middle without doubling up the magnets.. 48 poles is going to be fun lots of counting coils there remind you of the solar car one perhaps! Luck favours the well prepared |
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| GWatPE Senior Member Joined: 01/09/2006 Location: AustraliaPosts: 2127 |
Hi Karl, Not today or tomorrow or next year. Is not like the solar car motor either. Gordon. become more energy aware |
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| domwild Guru Joined: 16/12/2005 Location: AustraliaPosts: 873 |
Great read, Gordon. Taxation as a means of achieving prosperity is like a man standing inside a bucket trying to lift himself up. Winston Churchill |
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| GWatPE Senior Member Joined: 01/09/2006 Location: AustraliaPosts: 2127 |
Hi Karl, This plateau of the output power is a good thing, unless you are a salesman. Gordon. become more energy aware |
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| KarlJ Guru Joined: 19/05/2008 Location: AustraliaPosts: 1178 |
I guess that prevents burnout but OZ may disagree, he seems to dislike the F&P... I should have mentioned thanks for the link, i enjoyed that read, especially as plenty of it has been covered here on this forum by you and OZ. imagine 3T of neos! 
yeah baby..... Cheers Karl Luck favours the well prepared |
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