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Comment by _p8hc | Hacker News Reader

Why, out of curiosity?

Not parent, but aside from the novelty factor:

For small scale energy production small VAWTs [0] are relatively cheap, they offer more W/m2, and they are much more easily serviceable and installable.

For large scale production it's orders of magnitude less efficient, plus having multiple kites on the same plane brings in the wake effect [1] in full force. That's why regular turbines are generally laid in lines rather than wide surfaces, or with significant separation between them.

[0] https://en.wikipedia.org/wiki/Vertical-axis_wind_turbine

[1] https://www.wind-energy-the-facts.org/wake-effect.html

Okay, but why is everything wind-related so impossibly big? What's the physics behind needing blades so big that they can't fit into a cargo container? Instead of several very large turbines, why not 1000 or 10,000 of them?

Power goes up exponentially with size. To get good RoI, you need to go huge.

Power is roughly proportional to the wind turbine’s cross sectional area. So power scales by the square of blade length but isn’t quite exponential.

Yes, it's not exponential at all. Larger blades also need to be thicker, and the tower stronger etc. But still, it's economies of scale like everywhere else.

It's more than square, because bigger also means taller which means more wind.

It's more than squared, it's cubed. If you consider height advantages, it's going to produce 22 times more energy for each doubling of the blade length. http://large.stanford.edu/courses/2011/ph240/parise1/

And not only is there more output, it's more valuable per unit.

Higher winds are much more stable. Both in having less completely still days, and having the median be much closer to the maximum. A wind turbine that is built big enough starts having a large part of it outbut be effectively baseload instead of intermittent, and as more renewables are built out, baseload capacity is increasingly more valuable than intermittent.

Not only does the area argument apply (see Betz limit), but also you get higher wind speeds at higher altitudes.

I think the Makani documentary does a good job explaining the problem in less than 2 min [1].

[1] https://youtu.be/qd_hEja6bzE?t=392

Regarding the size x quantity, I believe the reason lies in where the best wind is, which is usually high above the ground. The roughness of the terrain and obstacles generate shear and turbulence, which translates into more stress for the components. The higher wind has a more uniform distribution across the rotor and is higher in magnitude than in lower heights. So for small wind turbines to have access to the best wind, you would have to build expensive structures to reach there, making it infeasible. Hence kite approaches like the one posted and Makani (with different principles).

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