How ‘Drafting’ can help wind turbine efficiency

VAWTs closely spaced - Mariah Power
VAWTs closely spaced - Mariah Power
If you don’t know what drafting is, you see it all the time in car races and bicycle races – its when the cars or bicycles follow close behind the leader and enjoy the benefits of decreased wind resistance.   Now researchers are finding that the same benefit occurs with Vertical Axis Wind Turbines (VAWT).

One of the benefits of VAWT’s is that they can be spaced close together, much closer than traditional wind turbines (or Horizontal Axis Wind Turbines,  HAWTs).   Horizontal axis wind turbines, if placed too close together, actually lose efficiency because the turbine in the front disrupts the air flow so much that the trailing turbines don’t get the full benefit from the wind.   As a result, the really large HAWTs need to be spread out across acres and acres of land in order to be efficient.

But two fluid dynamicist researchers at the California Institute of Technology have found that the drafting effect works to VAWT’s benefit, and the closer together the vertical axis wind turbines are, the better. Robert Whittlesey and John Dabiri measured airflows at a prototype VAWT setup in Glendora, California, and compared the data with studies of how water flows through schools of swimming fish.   Using that information, they then changed the geometric arrangement of the turbines to measure the impact on performance.

What they found showed how drafting paid off.  Arranging the turbines just like schools of fish increased performance the most.   Tightly spacing the turbines can produce as much electricity as horizontal turbines, while using about 1% of the land.   Experiments are continuing – in fact they recently found out that by alternating the rotational direction of the turbines (for example, the lead turbine being clockwise, then the next row of two being counterclockwise, then the turbines in the next row of three being clockwise, etc), even more efficiencies are gained.

One further benefit, VAWTs are considered to be much less harmful to birds, and tightly packing the VAWT arrays could make them even less harmful – the faster the turbines spin, the more solid they appear to birds.


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