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No, the area you should take into account is the area swept by the blades ( imagine a two-blade turbine turning fast or a ten-blade turning ive times slower, both are effectivly the same).

Also, there is no such thing as capturing 100% of wind energy. To do that, you would have to slow the wind down to 0 speed after your turbine, but then there would be no wind going through your turbine. If I remember correctly the theoretical max is something like 60%, and real-life turbines have a maximum of 25-30%, because they lose energy by rotating the flow. If I am not mistaken, vertical-axis machines perform even less good, 20% max. Remember, this is without friction losses or anything, just the energy you can extract from the wind while keeping it flowing.

Furthermore, it is absolutely pointless to design a turbine for huricane speeds. It would mean that you install an electric generator that will hardly ever perform at more than 15% of its capaciy, while still costing you 100% of its price.

Most wind turbines have a generator that will reach maximum capacity at 15m/s or so. At higher wind speeds, the blades are rotated to be less efficient, so that captured power stays constant (otherwise the generator fails). Somewhere between 20m/s and 25 m/s, the blades are switched to capture no wind at all, to diminish the loads on the structure.

These economics still hold for vertical axis machines, so it is save to say that a gigawatt turbine has to reach this power at no more than 20m/s, otherwise its gigawatt generator is a waste of money.

So: power at 20m/s is roughly 5000W/m2. Times 20% efficiency(ignoring friction losses etc.) is 1000 W/m2.
For a gigawatt, this means a capturing area (side view) of a million m2. Let's say the machine is 300 m high. Then it should  have a diameter of 3 kilometers. Good luck with your maglev bearings to lift that.

I googled a bit around, and a think there are two stories mixed-up. The Chinese have indeed made a maglev turbine, for small scale operations. This is not as futuristic as it sounds, frictionless magnetic bearings are already quite common in high-performance electric engines. This is definetly an interesting idea, but far from proven in real life.

The gigawatt story however is put out by a very unreliable company from Arizona that is using the real Chinese invention to make their own pie-in-the-sky idea sound believable; I suspect these are simply conmen trying to lure investors.

by GreatZamfir on Wed Nov 28th, 2007 at 08:44:25 AM EST
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