Comment by mapt | Hacker News Reader

Comment by mapt | Hacker News Reader

* I don't understand his explanation of starting torque. You can jump-start that problem by other means in the alternator. Torque and power should be decoupled anyway by blade pitch.

* There is such a huge difference between a 12m blade and a 60m blade that I don't see how the comparison is at all relevant. We played with smaller turbines for decades before we reached this sort of price efficiency.

* Betz' law explicitly disavows picking a number of blades: "Assumptions: 1. The rotor does not possess a hub and is ideal, with an infinite number of blades which have no drag. Any resulting drag would only lower this idealized value. ..."

* Betz' law is a three-dimensional consequence of convervation laws, not an observation about turbulent blade interactions. "Consider that if all of the energy coming from wind movement through a turbine was extracted as useful energy the wind speed afterwards would drop to zero. If the wind stopped moving at the exit of the turbine, then no more fresh wind could get in - it would be blocked. In order to keep the wind moving through the turbine there has to be some wind movement, however small, on the other side with a wind speed greater than zero. Betz' law shows that as air flows through a certain area, and when it slows from losing energy to extraction from a turbine, it must spread out to a wider area. As a result geometry limits any turbine efficiency to 59.3%."

* We already have a good idea what a moderate redesign of the fundamentals of a wind turbine looks like; It points downwind, it's huge like the existing turbines, and its two blades bend. They just need to solve the tower strike problem. https://www.technologyreview.com/s/401583/wind-power-for-pen... https://www.technologyreview.com/s/528581/two-bladed-wind-tu...

Furthermore, this idea that nobody's studying the number-of-blades problem is bogus too. When I worked abroad as a research assistant in the Wind Energy Project Group at the TU Delft (biggest tech university in the Netherlands), one of the researchers showed me a very interesting wind turbine that had a massive, elegantly curved blade on the one side and a big blob on the other. He explained, "We've been doing some simulations on these one-blade designs, even though they look goofy. This blob is just a counterweight, it is very cheap to produce because you don't need any advanced materials to use it. So the question is, if you had three times the budget to spend on just one blade, could you make it so good that it would outclass the three-blade model completely?"

In fact I asked why he wasn't going with 5 or 7 or 9 blades (he explained that even numbers tend to be Bad News) and he explained that the big problem was, as you increase the number of blades, the proportion of the rotor disk that consists of blade gets higher, which means the force on the tower component gets larger -- it goes something like the third power of wind-speed. Every few years or so, hurricane-force winds batter parts of Europe, and therefore there is a massively important design requirement that the wind turbine not completely be obliterated during these violent storms. An airplane with a funky propeller can of course be stored indoors when this happens; a turbine would require maintenance technicians to disassemble each one the day that the high-wind forecast was received.

Googling some of these related issues brings me to the page:

http://mstudioblackboard.tudelft.nl/duwind/Wind%20energy%20o...

However I cannot say with great certainty whether this is the same person that I heard these things from, or just a peer in the same field with the same general conclusions.

The power is also proportional to the swept area of the rotor disk, which increases by the square of the radius. This means the number of smaller windmills will have to go up by the square of the blade length.

12m blades have an area of 452m^2

60m blades have an area of 11,300m^2

At the same efficiency, you would need 25 times more windmills to generate the same power. Even if Cringely's theory is correct that smaller windmills can be made to be more efficient (doubtful), requiring an order of magnitude more physical hardware (towers, generators, etc.) is going to be a killer cost increase.

Did you factor in the increase in running time? If you can run at almost 90% of the time then you get a 4 fold increase in output as compared to the bigger turbines. So instead of 25 times as many, it's 5 times as many which isn't that bad.

That is going off the assumption that he'll get his optimal running time.

His basic assumption is one I've often seen... that a lot of very smart engineers working in a fast-moving and big money industry all make the exact same design mistake, because they're lazy and stupid and follow conventional wisdom rather than clever new thinking.

Which is sometimes true, but far more often, it's because those engineers know something you don't, because working on this problem is their job.

> They just need to solve the tower strike problem.

http://c2.com/cgi/wiki?JustIsaDangerousWord

The fact that this piece spent a lot of time vaguely implying that the Betz limit can be beaten is a testament to its insincerity. The entire point of that equation is that it applies before any real-world concerns are involved.

There are a few good points here (e.g. per-field profit is a different goal than per-rotor efficiency), but it feels a lot like an out-of-field expert assuming that no one in the field has ever been creative.

> I don't understand his explanation of starting torque

That's because Cringley doesn't, either. The starting torque is entirely provided by the wind, not the grid.

I think what he's talking about (not bothering to re-read) is the power required to magnetize the alternator components in the first place, so the alternator works at all. They use electromagnets rather than permanent magnets, because electromagnets are much cheaper, especially at that size. This is a fixed power requirement, so if the alternator can't generate more than that fixed power need to operate, there's no point in running.

Magnetizing power (and other constant-term losses) are a very small portion of the rated turbine power in a doubly-fed induction generator (DFIG).

Edit (one more thing): At this scale, permanent magnet (PM) machines aren't much more efficient than DFIG machines. It turns out that eddy current losses in the stator back iron remain as a significant term in both machines. But for a DFIG machine, the eddy current losses at light load are reduced by quite a bit relative to the losses produced by a PM machine. Since real-world wind turbines end up operating at partial load for much of their life, the effects basically wash out, or are even a net negative for the PM machine.

As an aside, the use of electromagnets occasionally produces hilarious failures. There have been some rare signaling errors that push a huge amount of power through the alternator and turn windmills into giant fans (at least until they shred themselves).

Tower strike, and less well known but probably equally important to longevity: tower thump.

Thanks for the clear explanation of Betz's law.

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