What do wind farms and fish schools have in common? Not enough, according to an analysis by Bob Whittlesey and buddies at the California Institute of Technology in Pasadena. And here’s why.
Most wind farms use horizontal axis turbines because they generate significantly more power from the air flowing through them than vertical axis turbines. That’s why you seen plenty of wind farms using horizontal axis turbines and very few using vertical axes.
But there’s a problem with horizontal axis turbines. The maximum power a rotor can extract is proportional to the cube
of the windspeed. But sinc, they slow the air passing through them, neighbouring turbines need to be around ten turbine diameters apart. And that places a significant limit on the power that can be generated from a given area of land.
The anecdotal evidence is that vertical axis turbines do better–by some accounts closely packing these turbines can actually increase their power output. Today Whittlesey and co explain why.
The secret appears to be a well known aerodynamic effect called a Karman vortex street, a term you may not know but will surely be familiar with from the example below, wich shows the cloud patterns downwind of the Juan Fernandez Islands off the coast of Chile.
These repeating patterns of vortices in an air stream around a body are relatively common. In fact, fluid dynamicists have well developed models for simulating the way 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.
So for the Caltech kids, it was but a small matter to redesign such a model to simulate the vortices in a 16 x 16 farm of vertical axis wind turbines and to study the effect of close packing the turbines.
It turns out that the additional acceleration that the vortices add to the wind can increase the power of vertical axis turbines. And although they still do not have the power efficiency of horizontal axis turbines, this means they can be packed much more closely together.
That’s important because it increases the power that can be genetated from a given area of land and not by a small amount. “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,” say the Caltech boys.
Yep, an order of magnitude increase in power density.
Of course, the model is only two dimensional and does not account for complex flow behaviour such as vortex shedding. But is it worth looking into in more detail? You betcha. Caltech has already filed patents.