Comment by JackC | Hacker News Reader

Comment by JackC | Hacker News Reader

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OK, so: Betz's Law says that the maximum energy that any wind turbine can extract is 16/27 of the kinetic energy contained in the wind that it blocks. Modern three-blade wind turbines reach 75-80% of the theoretical maximum, across the entire area their rotors sweep out.[1]

That's why the classic windmill design is so classic: its efficiency scales with the square of the length of the rotors. The energy extracted by a windmill is (very roughly):

    wind_speed * 16/27 * 0.8 * π * rotor_length^2

But then the wind speed also increases with height.[2] So the game to design an efficient windmill is to sweep out as much cross-sectional area as possible, as high up in the air as possible, and that's how we get the iconic windmills we have.

I don't know anything about anything -- I just read this stuff on Wikipedia. But until a third-party engineer says otherwise, I'm super skeptical that a device like this with minimal high-altitude cross-section is anywhere in the game.

[1] http://en.wikipedia.org/wiki/Betz%27s_law [2] http://en.wikipedia.org/wiki/Wind_profile_power_law

The article did say it's less efficient, but you can put twice as many in the same space which supposedly more than makes up for the power deficit. Plus no noise pollution or threat to birds and lower maintenance costs. Obviously a lot of technologies look good at this point, before they are widely deployed and tested, but I wouldn't count it out yet.

My first thought was "even if this is less efficient, the silence and lack-of-bird-killing means it would be viable to install places where NIMBYism etc. mean that traditional turbines can't be".

i.e. it may be sufficiently in the game to compete effectively against "no turbines" even if it can't compete against turbines directly.

It also seems like you'd be able to fit more of them into the same space. The company also claims that they're (allegedly) cheaper to manufacture and are (allegedly) cheaper to maintain. Even if the efficiency is less than those of a traditional windmill, it would still make sense to use them from a cost savings perspective alone.

That's if everything the company claims is true. Too many other alternative energy companies have made similar claims and failed to follow through for me to take this with anything other than a grain of salt.

edited for clarity.

> I just read this stuff on Wikipedia.

Same here. I've also read up on a lot of different designs, and have been paying attention for a while now.

There are a couple of points that have occurred to me about wind power. 1) Most supposedly exciting, new designs are minor variations on older designs with minor improvements in efficiency. 2) Some esoteric design are actually good for something, but not for massive wind farms. Think wind turbine as art in a public park, etc. 3) Media love esoteric designs and want to hype it as the next thing, even though they are nowhere near the efficiency of a big 3-blade on a tall pole design.

The efficiency loss can be overcome with better operational aspects: requiring less maintenance, being able to install anywhere, etc.

Betz's Law applies to turbines, but it this thing really a turbine? "No moving parts" and the fact that it operates via oscillation rather than rotation implies that this is not in fact a turbine. If it is not a turbine then there is no reason to assume Betz's Law must apply to it, perhaps there is something else that governs efficiency in this case?

Betz's law arises from the fact that you can't very well stop air dead after you've extracted energy from it - it has to keep on moving to make space for new air. (Otherwise pressure would build up indefinitely, a clearly non-physical situation). So it applies to all such devices, rotors or no. (It doesn't apply to fluids because fluids don't compress, but that is a completely different story).

I am no means an expert, but from my understanding you are only partially correct. Betz's law essentially says that because the velocity of the fluid exiting the cross section of the turbine is lower than the velocity or the fluid entering, the exiting fluid must consume more volume, and that their is a calculable limit to that difference. If these devices are in fact using vortices/turbulent airflow to induce resonance then perhaps different rules apply to the amount of energy that can be extracted?

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