Small vertical-axis wind turbines could finally start dotting the landscape – even cities – if a team from Stanford University has its way.
Provided they are properly arrayed, they could increase the energy yield of a single vertical axis turbine tenfold – and thus finally compete with traditional, horizontal axis turbines, which individually generate more energy, but also take up much more space in the landscape.
And being smaller and more compact, they would also present less threat to birds.
Vertical-axis turbines are not totally new – researchers have been looking into this design for over a decade, and several of them have already been installed around the world. There are even two on the second level of the Eiffel Tower, and Icelandic company Ice-Wind has been testing these turbines for years, as have the Canadian province of Quebec, California, and several other places.
Their Achilles heel until now has been poor efficiency: one vertical axis turbine produces a mere 0.1% of the output of an equivalent horizontal turbine.
The drawback of conventional turbines is that they take up a lot of space. Their arrays are huge: to be truly efficient, they are spaced far apart, to avoid energy-sapping turbulence and sharp drops in wind speed.
Vertical-axis turbines, on the other hand, are much more compact and can be packed much denser, says Anna Craig, a Stanford mechanical engineering PhD candidate who led the study. Typically they are about 10 times shorter than the white pylons we’re so used to seeing, and of a more compact, cylindrical shape, with blades parallel to the pole and rotating around it like a top.
Learning from fish
So to boost the yield, Craig’s team suggests placing the vertical axis turbines in clusters - either by themselves, or peppering them among conventional turbine arrays. Placed in empty spaces between big turbines, the small ones would capture the ‘wasted’ wind and also reduce turbulence for the traditional ones around them, increasing their output, say the scientists.
Individual small turbines would also take advantage of each other's turbulence, and could outperform traditional wind farms.
“The presence of each vertical-axis turbine creates regions of faster wind on its sides,” says John Dabiri, a Stanford mechanical engineer and co-author of the research. And, he adds, the turbines “can increase the area in contact with the wind without increasing the footprint of the system, since the system can be tall and slender.”
The idea mimics the way schooling fish arrange themselves, so that they are propelled forwards by the turbulence of the group, optimising their movement in interaction with their neighbours.
This method has already been applied to traditional wind farms – for instance, by General Electric and Berkshire Hathaway Energy. But Dabiri and his colleagues say that arrays of small vertical-axis turbines can outperform them because they respond to turbulence much better.
To get the results, the researchers simulated the wind using water flow in a large water tank. They set up a 10-foot-long ‘array’ by mounting 300 rotating cylinders that represented vertical-axis turbines, and tested 10 arrays with different configurations.
If it works, the idea would make vertical axis turbines “a suitable choice for applications in an urban setting, where space is scarce, wind speeds are low, and the wind can be reliable in both direction and speed,” says Abel-John Buchner, a mechanical engineer at Monash University, Australia, who was not involved in the research.
Stability on water
And the arrays of these smaller turbines could be scaled to much larger sizes offshore, where wind speeds are greater, he adds – making them ideal floating wind turbines. Their centre of gravity is low because the generator is located quite low on the tower, which increases stability on water.
But not everyone is convinced. Hans Bernhoff, an engineering professor at Uppsala University in Sweden, says that comparing a widely-spaced array of traditional turbines and a closely-packed group of the vertical axis ones may not be fair. “This does not rule out the possibility of designing the conventional turbines for closer spacing if this is the main limiting factor,” he says. “Claiming a 10 times increase is misleading, as the traditional turbines are not optimised for this case.”
Compact arrays of small vertical-axis turbines can also be more efficient only if the wind direction is relatively constant, adds Bernhoff. And before they are being grouped together, their design has to change, he thinks – those made today cause the material to experience fatigue because of ever-changing aerodynamic loads.
Craig agrees that despite the new concept, the small turbines need much more research before they can compete with the conventional wind farms. So don’t expect the traditional huge white towers with three blades to disappear from our fields and seas any time soon.
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