The IEC standard 61400-2 Design Requirements for Small Wind Turbines is applicable to wind turbines with a rotor swept area smaller than 200 m2, or around 17 m in diameter. Small wind turbines could be 100W or so in rated power, to tens of kW in rated power. However, the descriptions in the section will focus on machines up to a few kW in power rating, which mostly provide DC output (usually by rectifying the output of the permanent magnet alternator)instead of AC output. Their main application is for charging batteries to function as a stand-alone power supply system, probably in conjunction with PV panels. There are also small wind turbines for rooftop applications. However, such practice is relatively new and more operating experience on rooftop applications needs to be gathered.
As explained in Large Wind Turbine - Technology Outline section, the power of the wind flowing through a certain area is given by
Materials under the following topics in the Large Wind Turbine - Technology Outline section are generally applicable to small wind turbines, except for a number of aspects which will be described below.
A small wind turbine mainly comprises of three major parts - a rotor, a nacelle with a tail, and a tower or a mounting strurcture. Most small wind turbines are of the the horizontal axis type. Small wind turbines of vertical axis type are also available but they are less common.
For most machines, the rotor directly drives the generator without the need for a gearbox. With DC output, the rotor speed can vary greatly depending on wind strength. In light wind conditions, the rotor rotates gently and as wind speed increases, the rotor rotates faster and faster.
There is no active yawing mechanism as in the case of a large wind turbine. The "tail" of the small wind turbine automatically aligns the wind turbine towards the wind.
When the wind speed gets too high, the overspeed control mechanism is activated to protect the small wind turbine from damage. One common type of such mechansim is the "furling" mechanism by which the whole rotor is turned away from the wind to reduce the projected swept area.
In Hong Kong, the Hong Kong Observatory have been using small wind turbines to provide electrical power to the measurement and telecommunication equipment of automatic weather stations in remote locations. There are also a number of small wind turbines installed in urban environment, mostly as research and demonstration projects. The 1 kW wind turbine on the roof of EMSD Headquarters is grid-connected.
Due to the small size, small wind turbines produce power at a higher unit cost than that electricity from large wind turbines. Besides, they turn faster and hence produce more noise than large turbines.
This flash illustrates small wind turbines. The paragraph above describes small wind turbines.