The water pumping windmills to the left look very different from modern, large wind turbines. But they are quite sensibly designed for the purpose they serve: The very solid rotor with many blades means that they will be running even at very low wind speeds, and thus pumping a fair amount of water all year round.

Clearly, they will be very inefficient at high wind speeds, and they will have to shut themselves down, and yaw out of the wind in order to avoid damage to the turbine, due to the very solid rotor. But that does not really matter: We do not want them to empty the wells and flood the water tank during a gale.

The ideal wind turbine design is not dictated by technology alone, but by a combination of technology and economics: Wind turbine manufacturers wish to optimise their machines, so that they deliver electricity at the lowest possible cost per kilowatt hour (kWh) of energy.

But manufacturers are not very concerned about how efficiently they use the wind resource: The fuel is free, after all.

It is not necessarily a good idea to maximise annual energy production, if that means that one has to build a very expensive wind turbine. In the next sections we shall look at some of the choices manufacturers have to make.

Relative Generator and Rotor Size

A small generator, (i.e. a generator with low rated power output in kW) requires less force to turn than a large one. If you fit a large wind turbine rotor with a small generator it will be producing electricity during many hours of the year, but it will capture only a small part of the energy content of the wind at high wind speeds.

A large generator, on the other hand, will be very efficient at high wind speeds, but unable to turn at low wind speeds.

Clearly, manufacturers will look at the distribution of wind speeds and the energy content of the wind at different wind speeds to determine the ideal combination of the size of the rotor and the size of the generator at different wind turbine sites.

Fitting a wind turbine with two (or more) generators can sometimes be an advantage, but whether it really pays to do it depends on the electricity price.

Tower Heights

In the section on wind shear , you have learned that taller towers generally increase a wind turbine's energy production.
Once again, whether a taller tower is worth the extra cost depends both on the roughness class, and the cost of electricity.