windgen.org vawt discussion

crazyguy wrote:

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http://www.youtube.com/watch?v=5CcgmpBGSCI

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 This is the downwind run with the wheels driving the propeller. At first the drag on the prop starts blowing the car downwind, as the wheels turn they drive the prop to blow against the wind making the car go faster than the wind.

The car was later modified by replacing the prop with a turbine to drive the wheels to run into the wind.

 

sjh7132 wrote:

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PASVAWT wrote:

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Here's an interesting bit of information to think about: The amazing Blackbird windmill-driven land yacht achieved a speed directly into the wind of 2 times the wind speed. It fed its own power into the wheels to drive it forward. That is partially analogous to a VAWT driving an AT forward. What is especially interesting is that to do that, the Blackbird's windmill has to operate at reduced power so as to achieve the best lift to drag ratio for the blades, and thereby minimize the rotor drag.

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 That thought had crossed my mind..... Is there an operating point where the AT is not close to Betz and it has much less drag so that the power to drag ratio is lower and it's more efficient?  It sounds like that might be the case.  If I get bored late tonight I'll see if I can calculate thinking that way.

 

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 I just checked to make sure. The Blackbird is a downwind vehicle that uses wheel power to drive a propeller.

Oops. I just found this for the modified blackbird.

http://www.nalsa.org/BlackBirdDDWSR/UpwindSubmisionReportNALSA.pdf

sjh7132 wrote:

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PASVAWT wrote:

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My understanding of what that 80% maximum means is that the combination would be equal to 59.3 times .80, or 47.44 maximum efficiency. If a practical efficiency of 50% for the AT is used, then the actually efficiency would be roughly 40% times 50%, or 20% of the energy passing through the VAWT.

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 Ah then there is a difference in how we even define efficiency.  My definiation is what percent of the power put into moving the aerodynamic transmission comes out as electricity.  So it's a percentage of total power in, not of the Betz limit.   So really we are comparing my 30% against 47% which is a lot closer.

 

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 Hi Steve,

I think that we are saying the same thing but assuming different efficiencies for the AT.

My understanding is that an AT can capture and transmit only 80% of the power being produced by the VAWT, and that is the theoretical limit for the AT, not the practical limit, which would be much lower.

However, I don't think that the Betz limit applies to the AT. That is because the AT can bring the air passing through it to a complete stop, and air will still flow through the AT -- because the AT is moving away from the air behind it. Where an AT would become inefficient is if it starts pulling air along behind it. But as we've discussed, the true wind will be acting on the air behind the AT most of the time and sweeping it away. So that should help a lot. But how to calculate all those effects is far beyond me.

A very efficient computer controlled cycloturbine operating at, say, 55% efficiency combined with a very efficient AT operating at, say, 60% efficiency, would give a combined efficiency of 33%. That should be possible some day. But probably not soon. I'm going to stick with a likely combined practical efficiency of about 20%, and focus on cuttng costs so as to maximize the swept area.That assumes that the AT will achieve a 50% efficiency, and the VAWt will achieve a 40% efficiency. That seems reasonably conservative.

And if the VAWT achieves only 20% efficiency, then the combined efficiency will be only 10%. That might be the case for a single-blade Bird Windmill equipped with an AT mounted on the blade (on the rocking axis so as to not affect blade pitching).

Here's an interesting bit of information to think about: The amazing Blackbird windmill-driven land yacht achieved a speed directly into the wind of 2 times the wind speed. It fed its own power into the wheels to drive it forward. That is partially analogous to a VAWT driving an AT forward. What is especially interesting is that to do that, the Blackbird's windmill has to operate at reduced power so as to achieve the best lift to drag ratio for the blades, and thereby minimize the rotor drag.

An ordinary efficient windmill has a hard time going faster than about half the speed of the wind when heading directly upwind.

The 6 part video lecture by Mac Gaunaa, et. al. on the subject of using a rotor to sail directly upwind and directly down wind faster than the wind may be relevant to how to optimize an AT. It may need to minimize its drag in order to maximize its power. But I'm just guessing.

(Incidentally, the basic concepts in that lecture are ones that I published 6 years before he did. But he did the math, which I could not. On the other hand, I showed that, by moving to a higher level of abstraction, one equation could be used to describe both situations -- whereas he used separate equations.)

My understanding of what that 80% maximum means is that the combination would be equal to 59.3 times .80, or 47.44 maximum efficiency. If a practical efficiency of 50% for the AT is used, then the actually efficiency would be roughly 40% times 50%, or 20% of the energy passing through the VAWT.

If this is true, then to be practical, the swept area would need to be especially large. Power generating kites do achieve an extremely large swept area relative to the size of the blade. The Bird Windmill does too, although not nearly as large as power generating kites, because the TSR is only 2, whereas the kites probably operate at a TSR of around 6 or even more.

Kites have a further advantage in that they are higher where the winds blow stronger and more often. They have a high capacity factor.

However, the Bird Windmill is potentially much cheaper than the kites because it requires no complex controls. And the AT should be much smaller and cheaper than a conventional generator. 

The VAWT would be over-sized with a low solidity ratio so as to provide clean air to the AT for as much of the time as possible -- down wind as well as up wind.

Under those circumstances, a VAWT equipped with an AT might work.

The technique can be reversed, conceptually. Think of the small HAWT as the wind turbine. Think of the large VAWT as a wind concentrator. The concentration ratio is much higher than can be achieved by shrouds or other concentrating devices. But the VAWT must be very cheap and very large in order to be practical. The Bird Windmill satisfies that requirement. But it's too soon to be sure the combination would work technically. I think it can, but I still need to test it.