Power Coefficient

The Power Coefficient

Power coefficient graph The power coefficient tells you how efficiently a turbine converts the energy in the wind to electricity. Very simply, we just divide the electrical power output by the wind energy input to measure how technically efficient a wind turbine is.
In other words, we take the power curve, and divide it by the area of the rotor to get the power output per square metre of rotor area. For each wind speed, we then divide the result by the amount of power in the wind per square metre.
The graph shows a power coefficient curve for a typical Danish wind turbine. Although the average efficiency for these turbines is somewhat above 20 per cent, the efficiency varies very much with the wind speed. (If there are small kinks in the curve, they are usually due to measurement errors).
As you can see, the mechanical efficiency of the turbine is largest (in this case 44 per cent) at a wind speed around some 9 m/s. This is a deliberate choice by the engineers who designed the turbine. At low wind speeds efficiency is not so important, because there is not much energy to harvest. At high wind speeds the turbine must waste any excess energy above what the generator was designed for. Efficiency therefore matters most in the region of wind speeds where most of the energy is to be found.

Higher Technical Efficiency is not Necessarily the Way Forward
It is not an aim in itself to have a high technical efficiency of a wind turbine. What matters, really, is the cost of pulling kilowatt hours out of the winds during the next 20 years. Since the fuel is free, there is no need to save it. The optimal turbine is therefore not necessarily the turbine with the highest energy output per year.
On the other hand, each square metre of rotor area costs money, so it is of course necessary to harvest whatever energy you can - as long as you can keep costs per kilowatt hour down. We return to that subject later on the page about optimising wind turbines.