Makani's presentation in AWEC2017

[PierreB]

I think there are a lot of mistakes in my now old calculation:

@PierreB

Verification: 33 x 1.2/2 x 70.666666 x70.666666 x 70.666666 x 0.0857575 = roughly 600 kW.

Not, the result would be roughly 6 MW, although it is 600 kW but with other numbers.

@PierreB

It looks that the turbines are (10 times too) large due to E-VTOL requirement, then the thrust coefficient is lower during generation.

No, it was what I thought, but the turbines are well sized.

So I would prefer take another basis of hypothetical calculation on:

@PierreB

Let us take an example (with some approximations) from the onboard turbines: the diameter of a rotor of Makani M600 is 2.3 m. So 8 rotors lead to a rotor area of roughly 33 m². The nominal wind speed for Makani is 9 m/s. If the L/D ratio is 12, then 6 with the onboard turbines and the losses you mention, that lead to a reasonable kite (apparent wind ) speed of 54 m/s. Air density is assumed to be 1.2, power coefficient is assumed to be 0.2 (losses of efficiency due to a desired lower induction factor leading to larger rotor area; other losses because their trajectory is not rectilinear; other losses…) :

1/2 x 33 x 1.2 x 54³ x 0.2 = a little more than 600 kW.

The usual formula of wind energy by swept area is applied for this time.

Now from the supposed 40 m² of the wing, assuming a lift coefficient of 2, a L/D ratio of 12 (with tether drag), an efficiency of 0.65 after cosine cube loss: 40 x 1.2 x 2/27 x 2 x 10³ x 12² x 0.65 = about 660 kW. Derived Loyd’s formula is used.

So the value of the wing roughly matches the value of the onboard turbines.

These are only assumptions and I expect that some other possible mistakes can be corrected.