Types of windmill
The type of windmill we see in our landscape today is still limited to the horizontal axis wind turbine (HAT). The axis is situated in a virtually horizontal position in the wind direction and the two or three blades turn perpendicular to the wind direction in the so-called rotor plane. The axis of the blades ends up in the ‘nacelle’ at the top of the mast, where the power generator is situated. The nacelle is also used to keep the rotor surface perpendicular to the wind. This is usually achieved using a ‘yaw motor’, which is driven electrically, by a small wind vane.
Standard generators have been built for a rotational speed that is far higher than that of the main axis of the wind turbine. So, a gearbox, which is also situated in the nacelle, must ensure that the higher rotational speed is achieved. Energy losses in the gearbox and generator are small, generally less than 5%. The nacelle also contains a mechanical brake (to stop the windmill in the event of excessive wind speeds) and a control unit.
VAT turbines are used too, particularly in Canada and the US: these ‘whiskers’ are vertical axis wind turbines that do not rely on the wind direction. They also have the advantage that no yaw system is needed and the generator can be positioned directly under the turbine. Disadvantages are a slightly lower yield and periodic variations in the wind load on the blades. However, VAT turbines would seem to be an attractive option in relation to the development of wind turbines in built-up areas, precisely because of their non-reliance on wind direction.
Lift principle and resistance machines
Both systems (HAT and VAT) usually work on the basis of the lift principle. The aerodynamic characteristics of the rotor blades (blades) ensure that they develop a speed that is higher than the wind speed itself. The effective force (lift) is virtually perpendicular to the forward direction of the rotor blade (in the direction of the wind). Lift machines achieve a higher yield and require less material than the so-called resistance machines. The principle on which these machines are based is that a body moves in the same direction as the wind direction. An example of this is the cup-anemometer, which is well known from wind speed meters. One side of the cup (the hollow side) has a greater air resistance than the other side (the rounded side). The hollow side moves with the wind (but always moves more slowly than the wind itself), while the rounded side moves against the wind, in order to return the hollow side to the position necessary for energy generation.
How many blades?
Driving past them in our cars, we are surprised that windmills usually have three and sometimes two blades. What’s the difference? Technically there are differences, which we will return to later, but the most important reason for the preference for three blades is aesthetic. Research has shown that people generally appreciate three-bladed wind turbines with a closed conical tower the most. The preference for three blades is probably connected to the optical perception of the turning rotor, which presents a far more peaceful scene for a three-bladed turbine than for a two-bladed one.
Technically speaking, a turbine with more (broad) blades has a lower rotational speed and greater force than a turbine with a small number of (narrow) blades, when the same amount of energy can be produced from wind. In the case of small turbines, a free choice is possible in principle, depending, for example, on the properties of the power generator.
The new, big windmills always have three blades and, as such, a low speed of rotation. This is because of noise pollution, which, to date, has played an important role in the installation of windmills on land. Sound production is largely determined by the tip speed of the blades. At lower wind speeds, the speed must generally be kept below 60 to 65 m/s. For a small windmill with a rotor diameter of 10 metres, this limits the rotational speed to 125 rotations per minute (rpm). For a large windmill, with a diameter of 80 metres, this will be just 16 rpm. So, to be able to achieve sufficient energy from the approaching air a third blade is needed.
In the case of wind at sea, noise pollution plays a less prominent role and fast, easy installation is vital. Therefore, the discussion of whether a windmill should have two or three blades is very much the subject of discussion again with the big windmill builders.