5.2.4 ELECTRICITY GENERATION

5.2.4.1 Generator rating

Figure 5.10 Typical nacelle equipment

 

If the wind turbine is to produce electricity, it will drive a generator via a speed increasing gearbox, all contained within a nacelle (fig 5.10). It may appear from 5.2.3 above that the highest possible wind speed should be chosen for the rated power of a generator. But this in turn requires large heavy machinery for high maximum power, which will be very inefficient at lower wind speeds. For the maximum electrical output per year it has been shown that the rated wind speed should only be just over twice the average annual wind speed for the site.

 

The financial return required on an investment may however restrict the size of machine even more, and many wind farms use turbines for which the rated wind speed is only about 1.6 times the average wind. The rated power is still 4 times that at the average wind speed (because of the cubic relationship), and the turbine is idle for over a quarter of the hours in the year when the wind speed is below the 'cut-in' value and no net power can be produced (see fig 5.8).

 

5.2.4.2 Typical performance

Source: Vestas Fig 5.11 Electric power output for a large machine

 

Because the energy in the wind increases with the cube of the wind speed, the power output of a wind turbine must also follow this trend, at least for the lower wind speeds above the cut-in value. As the rated wind speed is approached however, the way in which the power is contolled becomes important. A pitch regulated rotor may be able to deliver steadily increasing power right up to the rated value; but if the wind speed then increases further, the blade must be rotated in the hub to reduce the aerodynamic effect and prevent any further power increase.

 

The resulting discontinuity in the power characteristic may be undesirable because a gusty wind will result in repeated oscillation of the blade angle and ineffective control. The alternative method, stall regulation, produces a smoother and more gradual transition from the steep rise onto the rated power plateau without any need to rotate the blade (for example see fig 5.11), although there may be some small performance penalty at lower wind speeds. The wind generator for the example shown actually uses pitch regulation at the higher wind speeds only, since otherwise the effect of stall regulation could be to reduce power output under these conditions. The red line also shows the effect of adding a smaller low speed generator in order to maximise the output at low wind speeds, and the consequent additional losses at higher winds.