Wind turbines for low wind speeds defy Betz limit efficiency


Wind turbine designers often pooh-pooh the idea of wind turbines able to efficiently work at low wind speeds. Archimedes gets around most such arguments by using a design that is closer to that of a water wheel than to traditional wind turbine designs.

Many designers of utility scale wind turbines dismiss the idea of using turbines to capture energy from light breezes and in areas where there isn't a lot of wind. Their argument comes from Betz's law which calculates the maximum power available from the wind. The law uses principles of conservation of mass and momentum of the air stream flowing through an idealized cylinder that extracts energy from the wind stream. Importantly, it assumes the use of aerodynamic propeller blades in the turbine. Betz's law says no turbine can capture more than 59.3% of the kinetic energy in wind. Practical utility-scale wind turbines can usually get to 75% to 80% of the Betz limit. So at low wind speed, there just isn't much energy left to harvest.

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But one way to get around the problem at low speeds is to avoid using aerodynamic blades and instead use something that looks more like a water wheel than a traditional wind turbine. That is the approach used by a Dutch firm called Archimedes for its Liam F1 Urban Wind Turbine. The turbine blades are in the shape of an Archimedes screw, notable in that hydro systems often use Archimedes screw designs in pumping systems characterized by low head pressures. They also typically have an efficiency curve that is flat across a wide range of flow rates.

archimedes turbineMarinus Mieremet, cofounder of Archimedes, puts it this way: "Generally speaking, there is a difference in pressure in front and behind of the rotor blades of a windmill. However, this is not the case with the Liam F1. The difference in pressure is created by the spatial figure in the spiral blade. This results in a much better performance. Even when the wind is blowing at an angle of 60 degrees into the rotor, it will start to spin. We do not require expensive software: Because of its conical shape, the wind turbine yaws itself automatically into the optimal wind direction. Just like a wind vane. And because the wind turbine encounters minimal resistance, it is virtually silent. “ Mieremet also says the efficiency is about 80%.

The wind turbine cuts in at about 4.5 mph. Its maximum output is 1.5 kW which it reaches when winds hit a little over 11 mph. The firm says it is targeting home owners with the Liam. 

Also, Archimedes isn't the first wind tubine firm to go with a design principle based on that of water wheels. Wind Sail Receptor in Nevada has a pinwheel-like design that functions more like a water wheel than like an aerodynamic turbine. 

Discuss this Blog Entry 14

Marcus Burt (not verified)
on Jun 4, 2014

Looks promising!!
Where can one aquire one?

How much?
Marcus Burt

Russell McMahon (not verified)
on Jun 13, 2014

The Archimedes turbine is NOT claimed by it's makers to exceed the Betz limit. A number of people have made this claim but it appears to be based on slightly unusual wording in descriptions of the turbine. The claim is - efficiency of about 90% OF the Betz limit. ie 90% x 59.3% =~ 53%. Their graphs show ~ 52% peak and a creditable > 40% across a wide range of wind speeds and TSrs.

David Wilshusen (not verified)
on Jul 2, 2014

I know this will sound far fetched but here goes. I know nothing on the mathmatical side of this so maybe you could help me.What kind of power could this produce with a constant wind speed of say 60 mph? I see in my mind a archimedes wind turbine that would be designed to fit in a wind shaft and under the hood of an electric car generating at least enough power to recharge the batteries for long distance travel with out stopping for recharging, A much smaller version of this wind mill of course that is able to withstand 60 mph driving speeds. Please reply as my brain is locked on this idea and I'm not sure how this might power an electric cars motors. I wish I could talk to some one that could see my vision. Thanks in advance.

W3XWT (not verified)
on Jun 23, 2014

Given the quoted "water wheel" approach, has anyone tried a modified Pelton Wheel design for a wind turbine? (Pelton Wheels have seen muse for ages in the Eastern Sierra Nevada by the City of Los Angeles, SDG&E, and PGE for hydropower generation...)

AJ2X (not verified)
on Jun 24, 2014

I remember some vertical-axis wind machines from the late '70s and early '80s that somewhat resembled Pelton wheels. If I recall correctly, they were claimed to be effective at low wind speeds, and the vertical axis meant that they accepted wind from any direction. Don't recall any efficiency claims.

on Jun 23, 2014

Of engineering and design interest: "... Their argument comes from Betz's law which calculates the maximum power available from the wind. ..." and is a ratio of 16/27 (in the wikipedia definition) ...

Which just happens to be the inverse of the Golden Ratio (27/16 = 1.68...)

AnonymousGB (not verified)
on Jun 23, 2014

Golden Ratio is (1 + sqr(5)) / 2 or ~ 1.618...

on Jun 23, 2014

Can anyone see which way the windstream moves relative to the rotor? L->R, or R->L or front-to-back ?

Anonymouse (not verified)
on Jun 25, 2014

Looks like the tapered end faces the wind - left-to-right w.r.t. the photo. So it will require some structure to direct it?

on Jul 26, 2016

It says in the article: "Because of its conical shape, the wind turbine yaws itself automatically into the optimal wind direction." I assume it sits on a rotating mount and just spins to face the win on its own.

Curt (not verified)
on Jun 23, 2014

For a wind turbine of any design to capture 100% of the kinetic energy of the wind in its swept area, the air would have to leave the turbine at zero velocity. This would present SLIGHT flow continuity problems.

ChrisP (not verified)
on Jun 23, 2014

Lee - this is yet another "drag" design. These people are not experts in the field. Every one of these "try a different approach" turbines is not as efficient as a regular 2 or 3 blade turbine. If they were as efficient as they say then they would be used to power aircraft. This turbine has a huge area of blades which creates friction drag. I think you have been taken in by hype.

Eric M. Jones (not verified)
on Jun 23, 2014

Wishful thinking! There is still that imaginary air column to consider. If the Archimedes turbine extracts 100% of the available energy from a three-meter diameter column of air; how does that compare to 50% from a one-hundred meter column of air Bubela?
I certainly wouldn't put money into this pretty but wistful design.

Shane (not verified)
on Jun 28, 2014

I looked at this device's spec sheet and it appears one need about a 45 MPH wind to get the promised 1.5 KW output. The system needs something like 21 MPH for 500 Watt generation. I have looked at wind data for my home in Mesa, Arizona and my folk's home in Payson Arizona, each with roughly a 5 MPH peak sipped during daylight hours, which is the cut in speed for this generator.

Without knowing the cost of the generator, and the peak winds in the windy part of the year here (February through approximately May), it is difficult to conclude this system would make a reasonable return on investment.

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