Power from the wind
Wind speed distribution is derived by taking readings over a year. The distribution pattern we get will plot like the Weibull distribution curve above. Average wind speed above is 6.5 meters per second but will vary for different locations.
Should we pick a wind turbine that optimize efficiency at the average wind speed?
Let us look a how much power is in the wind.
| Wind Speed | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 |
| Power /SqM | 0.6 | 4.9 | 16.5 | 39.2 | 76.2 | 132.3 | 210.1 | 313.6 |
| Wind Speed | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 |
| Power /SqM | 446.5 | 612.5 | 815.2 | 1058.4 | 1345.7 | 1680.7 | 2067.2 | 2508.8 |
At the low wind speeds there is very little energy to harness. Doubling of the wind speed increases the power by a factor of eight.
Pick a wind turbine that optimize efficiency at the average power available. For a 2.0 type distribution above the average power is at 8.7 meters per second.
Wind Turbine Efficiency : The Efficiency of a wind generator is calculated by taking the output at each wind speed divided by the power available.
| Wind Speed | 4 M/S | 5 M/S | 6 M/S | 7 M/S | 8 M/S | 9 M/S | 10 M/S |
| Efficiency | 0.01 | 0.15 | 0.25 | 0.35 | 0.44 | 0.40 | 0.38 |
Number of Blades : A wind turbine output voltage increases as the RPM increases. The ideal is to get a turbine that generates higher output voltage at the low wind speeds. This will enable it to start charging batteries at low wind speeds.
Higher number of blades optimises performance at lower wind speeds.
Generator Size : The higher the rated generator output the more power is needed to turn it. Conversely the smaller the generator the easier it is to spin in the wind. The faster you spin the generator the higher the output voltage
Rotor Size : The larger the rotor the larger it's swept area and greater the amount of power it can harness from the wind. The smaller the rotors the faster it can spin in low winds enabling it to start generating in low speeds.
Voltages : In order to charge the battery the output voltage of the wind turbine have to be higher than the battery's voltage. For a given generator because of back emf from the battery, a 12 volt battery bank will charge up at lower wind speeds but will require higher wind speeds to charge at 24 or 48 volts. This is because the generator's output voltage increase as the RPM increase. This is why small wind generators have 12 volt outputs only.
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