What does “cp.max“ actually mean?

06/28/2019 15:00

Often friends and partners, even within the wind industry, are telling us: “We know what you are doing, but what does cp.max actually mean?” That is not surprising since these five letters and the dot hardly give a clue about its meaning. To understand what cp.max stands for you have to immerse a bit in the basics of wind energy.

Wind energy is available to us on an unlimited scale. Due to the uneven warming of our earth by the sun there is always wind and it cannot be exhausted. This is one of the most important benefits of using wind energy.

But there are physical limits to the efficiency of energy conversion. Scientifically, this theme was first described in the 1920s by the German physicist Albert Betz. Through a simple thought experiment, the basics of his research can be easily understood: If a windmill would withdraw 100 % of the energy from the wind, the air behind the rotor would stand still. Hence no fresh wind could blow through the rotor, because the air behind the rotor is not transported away. The wind turbine would be aerodynamically blocked. This means: if a wind turbine tries to extract too much energy from the wind, then the system sabotages itself.


If a wind turbine takes too much energy from the wind, the air behind the rotor cannot be removed. The windmill has thus blocked itself and the wind does not blow through the rotor anymore, but flows around the "obstacle".


The "power coefficient" describes which part of the energy of the wind is converted by the rotor. In physics this value is called "cp". Albert Betz for the first time calculated the maximum amount of energy that can be taken from the wind with the wind turbines we know today. It is about 59 %.

That means from an aerodynamic point of view, an ideal wind turbine can convert a maximum of 59 % of the wind energy. And that is exactly the maximum efficiency of the rotor, which is called ”cp.max”. Behind these incomprehensible letters hides the maxim of our company: the optimal setting of the rotor blades to maximize the energy yield.

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