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KWH Confusion
By CoolBreeze, Section Wind Posted on Wed Oct 28, 2009 at 05:17:31 PM MST
Need to understand kwh numbers in the Homebrew Windpower book

On page 307 of the Homebrew Windpower book (appendix F,Useful wind data, Power output per swept area for a good turbine with cP of 0.35)the table shows 1.75 watts per square ft. of swept area at 10mph. By my calculation that would give a monthly kwh total of 1.75w x 24hours/day x 30days/month = 1.26kwh/month.
However on page 308 (Energy output per swept area per month for a good turbine with cP of 0.35)the table shows an energy output per square foot of swept area at 10mph of 2.078kwh.
Why the difference? I did that same math at 14mph and the numbers don't match at that wind speed as well.

KWH Confusion | 4 comments (4 topical)

Re: KWH Confusion (3.00 / 0) (#1)
by Flux on Thu Oct 29, 2009 at 12:57:50 AM MST

I don't have the book so I can't check what you are questioning, but are you comparing like things.

In the second case I suspect it is the power out for an AVERAGE 10 mph windspeed. If this so it would include the Rayleigh distribution where the effects of higher and lower winds above the nominal are included.

The second figure is something like the value I would expect if this is what is being considered.

Flux

Re: KWH Confusion (3.00 / 0) (#3)
by Ungrounded Lightning Rod on Thu Oct 29, 2009 at 08:27:31 PM MST

If so you're talking about an arithmetic average of the wind speed but the power going up with the cube of the wind speed (and power collected by simple mills typically with something near the square).

So the amount collected when the wind is above average is far more than the amount short when the wind is below average. Thus both numbers might be right.

[ Parent ]

Re: KWH Confusion (3.00 / 0) (#2)
by ADMIN on Thu Oct 29, 2009 at 09:53:19 AM MST

I'll have to look into that. There is probably something missing in our description.
All that data was from Mike Klemen --- it's on his website here:
http://www.ndsu.edu/ndsu/klemen/Perfect_Turbine.htm
DAN FINK

Re: KWH Confusion (3.00 / 0) (#4)
by scoraigwind on Fri Oct 30, 2009 at 11:08:12 PM MST

The average of the power (which depends on the cube of the windspeed) is generally about twice the power you would expect from the average wind. This doubling takes place when the wind varies in a typical way called the Rayleigh distribution. It's because stronger winds have way more power than weaker winds. The more widely the windspeed varies, the more the mean of the cube becomes more than the cube of the mean windspeed. The factor is known as the 'energy pattern factor'
Hugh Piggott http://www.scoraigwind.co.uk

KWH Confusion | 4 comments (4 topical)

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