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Energy from the Wind - Practical Answers

This means that the power density in the wind will range from 10W/m² at 2.5m/s (a light breeze) to 41,000W/m² at 40m/s (a hurricane).

so assuming 100% eficiency from this design, and hurricane speed winds you're talking 24,400 square metres of blade area.

(take into account that this is ultimate bodge physics and I really have no idea what I'm talking about, these are numbers I pulled off a random page on the internet)

Any idiot can face a crisis - it's day to day living that wears you out.

by ceebs (ceebs (at) eurotrib (dot) com) on Mon Nov 26th, 2007 at 07:47:33 PM EST
[ Parent ]
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
[ Parent ]
I knew someone who actually knew what they were talking about would comment sooner or later.

Any idiot can face a crisis - it's day to day living that wears you out.
by ceebs (ceebs (at) eurotrib (dot) com) on Wed Nov 28th, 2007 at 09:06:55 AM EST
[ Parent ]
I am not an expert, but I followed some courses on wind energy, and I guess I know some people who are experts. Accidentally, I almost did an internship at a Chinese wind turbine manuacturer. This was their largest company, making normal turbines, and they only had a small share of even the Chinese market.

They were expanding very fast, with clearly a lot of govenment money behind them. However, their main goal was to get closer to the level of western manufacturers, not making new concepts (how hiring unexperienced western students was going to help that goal, I do not know, but I suspect they just wanted name recognition in western academia).

From this I deduce that even the Chinese are not thinking their maglev turbines will be succesful in the short term, or they would put their money elsewhere.

By the way I found the site of the company developing the maglev turbine:

http://www.zkenergy.com/en/gsjj.asp

At the moment they are making small (300kW) horizontal (regular) turbines using magnetic bearings that would be especially good for low-wind conditions. Apparently they are used to power street lights.

Their R&D plans include a 200kW and later 2000kW vertical turbine, using maglev, but currently they only have a 20kW turbine. If they can build the 2000kW, and the technology is sound, they could become serious players.

by GreatZamfir on Wed Nov 28th, 2007 at 09:38:20 AM EST
[ Parent ]

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