How much power can you expect to produce with a homemade wind turbine?
Wind speed |
5mph |
10mph |
20mph |
35mph |
4 FT (1.21m)Blade Dia |
.9 Watts |
6.8 |
54.5 |
292.2 |
6 FT (1.82)Blade Dia |
1.9 |
15.3 |
122.7 |
657.4 |
8 FT (2.43) Blade Dia |
3.5 |
27.7 |
221.7 |
1188.3 |
10 FT (3.04) Blade Dia |
5.5 |
43.7 |
349.9 |
1875.2 |
This table can give you a general idea what you might produce with a wind turbine. It assumes a conservative power coefficient (CP, efficiency of the system) of 15%, small commercial built turbines could be as much as double CP. |
This table can help you to predict how much power you will be able to generate with a wind turbine. You need to taken into account the size of you blades and the speed of the wind. The table assumes that your wind turbine will catch 15% of the raw power in the wind. This percentage is known as the power coefficient (CP).
The raw power in the wind depends on the density of air, the speed of the wind, and the diameter of your blades. Wind speed is the critical component though, the energy in wind velocity increases by the cube! That means the power in the wind will increase dramatically with wind speed.
The swept area of a wind turbine is the area in which the blades (propeller, wing, catch spoons, or sails) spin and catch the wind. So the bigger the blades the bigger the swept area, area is always square, so increasing your blade size will also dramatically increase your energy.
As you can see the power in the wind varies enormously. There are only a few watts available in light wind and huge amounts under high wind. It is not easy or sometimes even practical to design a machine which can convert the wide range of power effectively, so you should design it to handle medium or your average wind.
The wind is always changing, and the power fluctuations can be extreme. We need to harvest the energy when it is here and then store it for the calm periods. It is also a good idea to have some power source as a backup. Generally small wind electric systems use lead acid battery, which stores our surplus power during windy days and during the calm periods we can draw from that power.
A quick guide to predicting energy capture
Energy capture in a given time is the average power multiplies by the hours.
Here are just general averages of different sites, the higher you got up the fast the wind is –
Site | Average Wind Speed |
Trees and Buildings | 6 MPH |
Large Open Field | 10 MPH |
Hilltops or Coast lines | 13 MPH |
Average power output from a wind turbine is not the same as it instantaneous power output when wind speed is average. I recommended calculating it yourself using this equation:
Wind Turbine Power Equation:
P = 0.5 x rho x A x Cp x V3 x Ng x Nb
where:
P = power in watts (746 watts = 1 hp) (1,000 watts = 1 kilowatt)
rho = air density (about 1.225 kg/m3 at sea level, .9 kg/m3 at 4000 ft less higher up)
A = rotor swept area, exposed to the wind (m2)
Cp = Coefficient of performance (.59 {Betz limit} is the maximum theoretically possible, .35 for a good design)
V = wind speed in meters/sec (20 mph = 9 m/s)
Ng = generator efficiency (50% for car alternator, possibly 70% more for a permanent magnet generator, home made generator 50%)
Nb = gearbox/bearings efficiency (depends, could be as high as 95% )