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Small-Scale Wind
Small wind turbines generally have a lower energy output than large commercial wind turbines, but their size can differ significantly: So called Micro wind turbines may be as small as a fifty watt generator and generate only about 300 kWh per year. They are used for boats, caravans, miniature refrigeration unit, but also for fence-charging and other low-power uses. In comparison to that household-size turbines reach diameters of 9-meter, can have a rated power of 20kW and produce about 20000 kWh per year for homes, farms, ranches and small businesses.[1]
Small units often have direct drive generators, Direct current output, aeroelastic blades, lifetime bearings and use a vane to point into the wind. Larger, more costly turbines generally have geared power trains, alternating current output, flaps and are actively pointed into the wind. Direct drive generators and aeroelastic blades for large wind turbines are being researched.[2]
Small wind turbine technology
Nearly all small wind turbines today are upwind, horizontal-axis turbines, which means the rotor is spinning in front of the tower. There are some turbines using two blades, while the majority of the recent turbines comes supplied with a three-blade rotor, which in general terms makes the turbine run more smoothly and last a longer time. The prevalent blade materials are composite materials as fiberglass, while only a few products in this class still use wood. Instead of the yaw motors of the big wind turbines, small wind turbines often use tail vanes to point the rotor to the wind.
Because of the sometimes demanding environmental conditions, the robustness of a turbine is a very important parameter, which can be estimated only very roughly: Experience has shown, that the weight of the turbine in relation to the area swept by the rotor can be used as a criterium. For example a turbine with a relative mass of 10 kg/m2 should be more robust than a turbine with 5 kg/m2.[3]
