I wonder if ANEW expressed their "efficiency" as the power harnessed from the wind ( typical 40%) as a relation to the Betz limit as follows...: 40/59.3 = 67.45 ??
The aerodynamic efficiency is a factor 1.5 higher than the Cp. So a Cp of 0.4 means an aerodynamic efficiency of 0.6. But mostly people don't know the difference in between the Cp and the aerodynamic efficiency and when they talk about the efficiency, they mean the Cp. So I think that this company really believes that they have measured a Cp of 0.7. But if they have measured the wind speed some metres below the rotor, they have measured a wind speed which is much lower than the undisturbed wind speed and this explaines the unrealistic high Cp.
But even if they really mean an aerodynamic efficiency of 0.7, this means a Cp of 0.467 and this is also much too high for a Darrieus rotor
Is Darrieus somehow inferior to HAWT ?
If so then why ?
Yes, a Darrieus rotor is inferior to a well designed HAWT and the discussion why, is conducted already many times on this forum. I give all the disadvantages of Darrieus rotors in my public report KD 215. Altough this question about Darrieus rotors is completely out of topic, I will give the main points again and a hope that this ends the ever lasting discussion with people who favour Darrieus rotors.
1) The angle of attack varies in between a large positive angle if the blade is at the front side and a large negative angle if the blade is at the back side. Therefore one has to use a symmetrical airfoil which has a lower Cl value for the minimal Cd/Cl ratio than an asymmetrical airfoil with the same thickness. The average Cd/Cl ratio during a whole revolution is a lot higher than for a well designed HAWT which can be designed such that the airfoil is used at the lift coefficient for which the Cd/Cl ratio is minimal. The maximum Cp decreases at increasing Cd/Cl ratio and the maximum Cp of a Darrieus rotor is therefore a lot lower. A maximum Cp of 0.35 is already very high for a big Darrieus rotor.
2) A symmetrical airfoil is very sensible for the Reynolds number (see KD 601 figure 1). So a low Cd/Cl ratio is only possible for a rather large chord and for a rather large wind speed. That is why small Darrieus rotors perform very badly.
3) The blades of a H-Darrieus rotor are connected to the hub by spokes and these spokes have aerodynamic resistance. The power needed for this resistance reduces the Cp.
4) The lift coefficient varies such that the thrust points inwards the rotor if the blade is at the front side and outwards the rotor if the blade is at the backside and this shakes the blade terribly.
5) If the rotor is running at a low tip speed ratio, the angle of attack is that large that airfoil stalls if it is at the front and at the back side. Then the airfoil has a very high resistance resulting in a negative Cp. So the rotor must be started by an electric motor to pass the lambda area for which the Cp is negative.
6) The power at high wind speeds can't be reduced by turning the rotor out of the wind. So a Darrieus rotor needs a big mechanical brake to reduce the maximum rotational speed and if this brake isn't working automatically, the rotor can be destroyed easily in a big storm
7) The optimum tip speed ratio of a Darrieus rotor is about 4.2 (see derivation given in KD 601). Big HAWT's have an optimum, tip speed ratio of about 8. The required chord decreases about with the square of the local tip speed ratio and the needed solidity for a big HAWT is therefore much lower. This results in a much lighter and so a much cheaper rotor for the same power. So Darrieus rotors will always loose the battle with big HAWT's if the economics are taken into account. That is the main reason why you only see big HAWT's and no big Darrieus rotors.