Number of blades
Why not multiple blades?
Wind turbines are built to catch the wind's kinetic (motion) energy. The question arises why modern wind turbines are not built with a lot of rotor blades, like the old "American" windmills. Turbines with many blades or very wide blades, i.e. turbines with a very solid rotor, however, will be subject to very large forces, when the wind blows at a hurricane speed (remember that the energy content of the wind of the wind varies with the third power of the wind speed).
Wind turbine manufacturers have to certify that their turbines are built, so that they can withstand extreme winds which occur, say, during 10 min once every 50 years. To limit the influence of the extreme winds turbine manufacturers therefore generally prefer to build turbines with a few, long, narrow blades. In order to make up for the narrowness of the blades facing the wind, turbine manufacturers prefer to let the turbines rotate relatively quickly.
Why not an even number of blades?
Modern wind turbine engineers avoid building large machines with an even number of rotor blades. The most important reason is the stability of the turbine. A rotor with an odd number of rotor blades and at least three blades can be considered to be similar to a disc when calculating the dynamic properties of the machine.
A rotor with an even number of blades will give stability problems for a machine with a stiff structure. The reason is that at the very moment when the uppermost blade bends backwards, because it gets the maximum power from the wind, the lowermost blade passes into the wind shade in front of the tower.
Most modern wind turbines used for generating electricity have three blades, although some have two or even one. Three blades have the particular advantage that the polar moment of inertia with respect to yawing is constant, and is independent of the azimuth position of the rotor. This characteristic contributes to relatively smooth operation even while yawing. A two- blade rotor, however, has a lower moment of inertia when the blades are vertical than when they are horizontal. This 'imbalance' is one of the reasons that most two-bladed wind turbines use a teetering rotor. Using more than three blades could also result in a rotor with a moment of inertia independent of position, but more than three blades are seldom used. This is primarily because of the higher costs that would be associated with the additional blades.
The Danish 3-bladed concept
Most modern wind turbines are three-bladed designs with the rotor position maintained upwind (on the windy side of the tower) using electrical motors in their yaw mechanism. This design is usually called the classical Danish concept and tends to be a standard against which other concepts are evaluated. The vast majority of the turbines sold in world markets have this design.
The basic design was first introduced with the renowned Gedser wind turbine. Another characteristic is the use of an asynchronous generator.
Two-bladed (teetering) concept
Two-bladed wind turbine designs have the advantage of saving the cost of one rotor blade and its weight, of course. However, they tend to have difficulty in
penetrating the market, partly because they require higher rotational speed to yield the same energy output. This is a disadvantage both in regard to noise and visual intrusion. Lately, several traditional manufacturers of two-bladed machines have switched to three-bladed designs.
Two- and one-bladed machines require a more complex design with a hinged (teetering hub) rotor as shown in the second picture, i.e. the rotor has to be able to tilt in order to avoid too heavy shocks to the turbine when a rotor blades passes the tower. The rotor is therefore fitted onto a shaft which is perpendicular to the main shaft, and which rotates along with the main shaft. This arrangement may require additional shock absorbers to prevent the rotor blade from hitting the tower.
One-bladed wind turbines exist and they save the cost of another rotor blade. If anything can be built, engineers will do it. One-bladed wind turbines are not very widespread commercially, however, because the same problems that are mentioned under the two-bladed design apply to an even larger extent to one-bladed machines.
In addition to higher rotational speed, and the noise and visual intrusion problems, they require a counterweight to be placed on the other side of the hub from the rotor blade in order to balance the rotor (look second figure). This obviously negates the savings on weight compared to a two-bladed design. The aesthetic factor of the appearance of imbalance is another consideration. The presumed advantage is thus that the turbine can run at a relatively high tip speed ratio, and that the cost should be lower because of the need of only one blade.
- Two blades: general explanation
- Three blades: general explanation