Wind yields in the UK and Germany

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I read your article Price and Credibility on the Bottom Line" (Windpower Monthly, April 1997) with considerable interest. I have two points.

According to Paul Gipe's Wind Energy Comes of Age, the power in the wind in watts is P=1/2pAV3, where p is the air density, A the area intercepting the wind and V the wind's velocity. Assuming the same air density and rotor diamter, the 7.6 m/s given as median for wind speed in the UK would generate 84% more watts than the 6.2 m/s in Germany. In my not too well wind educated mind, this would alone suffice to explain the big difference in wind costs in the UK and Germany. Speculations -- although reasonably well based -- regarding differences in capital costs would thus not be necessary.

Your article says the yield "of a German plan is 808 kWh per square metre of rotor swept area. From a sample of similar size in Britain it is 1154 kWh/m2. In other words, the productivity of British wind farms -- on average -- is about 43% higher." Again, given the difference in average wind speeds, shouldn't the British plants generate 84% more kWh per square metre? If this is true the British plants would seem toave a productivity inferior to that of the German ones.

The energy content of the wind does increase according to the cube of the wind speed, but the energy delivered by typical wind turbines does not increase in the same way. The main reason is that energy is lost above the rated wind speed, where power starts to be limited. The higher the mean wind speed, the more energy is lost. All energy data in the article were based on outputs of actual machines. David Milborrow

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