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Dense Packing Increases Efficiency for Wind February 17, 2010

Posted by OldGuy in Alternatives, Wind Power.
Tags: , , ,
1 comment so far

While it’s generally understood that horizontal wind turbines are more efficient than vertical blade turbines, the effect of the blade catching the air tends to slow that air down, meaning each turbine needs a significant separation from one another to reach that efficiency.

Students at Caltech were looking for ways to improve wind turbine efficiencies, and have found a way to increase the power efficiency per foot of land area by studying the way fish swim. 

When in a school, fish swim in an offset pattern that creates what’s called a Kármán vortex street.  In fluid dynamics, a Kármán vortex street is a repeating pattern of swirling vorticies (whirlpools) that are formed when a fluid (which could include water or air) passes over objects.  Under the right conditions, the separation and recombining of the fluid is what causes the effect.

When air passes around an object – especially circular cylinders, like power plant cooling towers, it tends to move to one side, which creates a low pressure on the other side, pulling the air back in a wave pattern.  These eddies are shed continuously from each side of the body, forming rows of vortices in its wake.  The further it gets from the object, the smaller the oscillation, and eventually the regular pattern disappears.  But in the first few feet past the object, it can cause havoc.  It’s especially troubling when the object is moving through a relatively slower fluid (like an airplane flying through the air).

This effect also has positive effects. Their interaction helps keep schools of fish synchronised and reduces the total propulsive power needed per fish. A similar effect reduces the fuel consumption of vehicles travelling in a platoon.

What the students did was arrange the turbines in a way to catch the vortices.  that drove them to vertical blades, since the eddies would dissipate in a traditional horizontal blade wind farm.  But by installing vertical turbines in a Kármán vortex street, the turbulance from one helped power the next.  And because they were closer together, more of them could be mounted on smaller patches of real estate, and support structures could be combined for more efficient transmission.

According to the researchers, “these configurations significantly reduced the land use for vertical axis wind turbine wind farms, resulting in array power density increases of over one order of magnitude compared to operational horizontal axis wind turbine wind farms”

Patents have been filed.

sources: 

http://www.technologyreview.com/blog/arxiv/24813/

http://en.wikipedia.org/wiki/Von_Karman_vortex_street

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Note: the comment to the article mentioned  a Russian company, ‘SRC Vertical’, whose wind turbines are marketed in the USA by a company called ‘Wind-sail’, that was funded by the U.S. Department of Energy to build VAWT’s, has built VAWT’s with an efficiency of 38%, which is up there with the best HAWT’s, and they reckon they can increase the efficiency up to 45%.

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