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How many Watts are in the Wind?

The power you can get out of the wind is determined by three factors.
  1. Wind Speed
  2. Wind Density (Determined by altitude and temperature)
  3. Turbulence
Let's get a few factors out of the way. The less turbulence the better and turbulence is very difficult to measure. For that reason, it is better to place your wind turbine away from obstacles and as high as you can get it and assume that you are then turbulence free (as you can get) then ignore this factor.

So how much power is available in a square meter 'window' of wind?
1/2 * density * ( windspeed * windspeed * windspeed )

Wind speed is an easy one to understand and measure. This is the speed in which the air is moving, measured in meters per second. You can buy meters to measure this, but your not much interested in what the wind speed is at the moment but what is the normal or average. If you approach or write to your local Bureau of Meteorology (Aus) they will have wind maps of your local area.

Density is tricker. At sea level and room temperature, density is about 1.3kg per cubic meter.

Wind Speed Power Description
2 Small than 1 Watt Can just feel the wind
5 5 Watts Enough to move but not lift leaves
10 60 Watts Enough to lift leaves
15 175 Watts Branches sway.
20 475 Watts Trees sway.


How much is useful though?

Much like solar panels don't get all the energy out of the sun, windmills can't get all the energy out of the wind. The Betz limits (from Albert Betz) postulates that you can get at most 59.25% of the available power of the wind. And no generator is 100% efficient either. Most are around the 35% mark from mechanical energy, the best are around the 80-90% mark. There are also friction losses etc that slow things down so in the end you can only expect about 25% efficiency. So for a wind speed of 10 mps with 60 Watts available we can only hope to get 15 Watts.

Wind Speed Power in the Wind Power from generator
2 Small than 1 Watt None
5 5 Watts ~1 Watt
10 60 Watts 15 Watts
15 175 Watts 44 Watts
20 475 Watts 119 Watts

So to get any power out of the wind you need to be exposed to a large surface and be in a position the gets an average wind speed of above 10 mps. Most commercial wind farms want an average of at least 15 mps and a blade span of 5+ meters.

Don't be discouraged though. Most areas are OK for home or hobby systems.
 

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