Читать книгу Wind Energy Handbook - Michael Barton Graham - Страница 101

The power output of a turbine running at constant speed is strongly governed by the chosen, operational rotational speed. If a low rotation speed is used, the power reaches a maximum at a low wind speed, and consequently it is very low. To extract energy at wind speeds higher than the stall peak, the turbine must operate in a stalled condition and so is very inefficient. Conversely, a turbine operating efficiently at a high speed will extract a great deal of power at high wind speeds, but at moderate wind speeds it will be operating inefficiently because of the high drag losses. Figure 3.56 demonstrates the sensitivity to rotation speed of the power output – a 33% increase in rpm from 45 to 60 results in a 150% increase in peak power, reflecting the increased wind speed at which peak power occurs at 60 rpm.

At low wind speeds, however, there is a marked fall in power with increasing rotational speed, as shown in Figure 3.57. In fact, the higher power available at low wind speeds if a lower rotational speed is adopted has led to two speed turbines being built. Operating at one fixed speed that maximises energy capture at wind speeds at or above the average level will result in a rather high cut‐in wind speed, the lowest wind speed at which generation is possible. Employing a lower rotational speed at low wind speeds reduces the cut‐in wind speed and increases energy capture. The increased energy capture is, of course, offset by the cost of the extra machinery.