Size of turbines
Power output increases with the swept rotor area
When a farmer tells how much land he is farming, he will usually state an area in terms of hectares or acres. With a wind turbine it is much the same story, though in wind farming a vertical area is farmed instead of a horizontal one.The area of the disc covered by the rotor determines how much energy can be harvested in a year.
The picture gives an idea of the normal rotor sizes of wind turbines: A typical turbine with a 600 kW electrical generator will typically have a rotor diameter of some 44 m. If the rotor diameter is doubled, then the rotor area gets four times bigger. That also means that the power output from the rotor gets four times bigger.
Rotor diameters may vary somewhat from the figures given here, because many manufacturers optimise their machines to local wind conditions: a larger generator, of course, requires more power, i.e. strong winds, to turn at all. So if a wind turbine is installed in a low wind area, then the annual output will be maximized by using a fairly small generator for a given rotor size (or a larger rotor size for a given generator). For a 600 kW machine rotor diameters may vary from 39 to 48 m. The reason why more output cam be taken from a relatively smaller generator in a low wind area is that the turbine will be running more hours during the year.
Reasons for choosing large wind turbines
There are the following reasons:
1. There are economies of scale in wind turbines, i.e. larger machines are usually able to deliver electricity at a lower cost than smaller machines. This comes because the cost of foundations, road building, electrical grid connection, plus a number of components in the turbine, like the electronic control system, are somewhat independent of the size of the machine.
2. Larger machines are particularly well suited for offshore wind power. The cost of foundations does not rise in proportion to the size of the machine, and maintenance costs are largely independent of the size of the machine.
3. In areas where it is difficult to find sites for more than a single turbine, a large turbine with a tall tower uses the existing wind resource more efficiently.
Reasons for choosing smaller wind turbines
There are the following reasons:
1. The local electrical grid may be too weak to handle the electricity output from a large machine. This may be the case in remote parts of the electrical grid with low population density and little electricity consumption in the area.
2. There is less fluctuation in the electricity output from a wind park consisting of a number of smaller machines, since wind fluctuations occur randomly and therefore tend to cancel out. Again, smaller machines may be an advantage in a weak electrical grid.
3. The cost of using large cranes and building a road strong enough to carry the turbine components may make smaller machines more economic in some areas.
4. Several smaller machines spread the risk in case of temporary machine failure, e.g. due to lightning strikes.
5. Aesthetical landscape considerations may sometimes dictate the use of smaller machines. Large machines, however, will usually have a much lower rotational speed, which means that one large machine really does not attract as much attention as many small, fast moving rotors.