[thedoc (OP)]

When wind turbines create electricity they reduce energy from the air. How much does the wind slow down after passing through the turbine? Can you put an efficiency number on a turbine like the binders on solar panels
Asked by Mark Harnett


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[nilak]

I'm wondering, how much power is left over the surface area behind the turbine ? This shows the magnitude of interaction between the turbine and the environment. If I had to guess I would say something like 90%.

[alancalverd]

Wikipedia has clear description of Betz's Law

if all of the energy coming from wind movement through a turbine was extracted as useful energy the wind speed afterwards would drop to zero. If the wind stopped moving at the exit of the turbine, then no more fresh wind could get in - it would be blocked. In order to keep the wind moving through the turbine there has to be some wind movement, however small, on the other side with a wind speed greater than zero. Betz' law shows that as air flows through a certain area, and when it slows from losing energy to extraction from a turbine, it must spread out to a wider area. As a result geometry limits any turbine efficiency to 59.3%.

[Colin2B]

.......on the other side with a wind speed greater than zero. Betz' law shows that as air flows through a certain area, and when it slows from losing energy to extraction from a turbine, it must spread out to a wider area.

As shown here:
https://www.google.co.uk/search?q=wind+farm+fog+image&ie=UTF-8&oe=UTF-8&hl=en-gb&client=safari#imgrc=csQhW9ZItKMwdM%3A

[nilak]

Yes, they are much more efficient. Typically the modern turbines operate at around 40% wind power converted to mechanical power.

If the wind blows at 30 mph, behind the turbine it will average 25mph, for 60% power left over the surface area.
http://people.bu.edu/dew11/turbineperformance.html