Wind Conditions

Before choosing a turbine, the site conditions need to be understood, so that the choice of turbine suits the physical and other constraints.

 Before choosing a turbine, the site conditions need to be understood, so that the choice of turbine suits the physical and other constraints. Cut-in speed is a key issue.

Many turbines only start to work (or ‘cut in’) at a minimum wind speed. If the wind speed is lower than the cut in speed, then the turbine won’t turn at all. Some turbine designs work even at very low wind speeds, and there are designs of VAWT with a small Savonius rotor attached to a larger Darrieus rotor – the small one helps get the big one turning.

Air turbulence is another crucial issue. Some turbines work poorly in turbulent conditions, while others work well, even taking advantage of the funnelling effect round buildings in order to increase the energy output. Similarly, frequently changing wind direction is more of a problem for some turbine designs than for others.

Box: Wind Speed and Mass Flow Rate

The most important factor affecting a free-standing wind turbine is the wind speed. Energy output increases with the cube of the wind speed, so if the wind speed is doubled, the output is increased eight times (2x2x2=8).

With funnelling devices, other factors come into play resulting in a less dramatic,  but still useful, increase in energy output. Here, it is not simply the wind speed that matters, but rather the mass flow rate of the wind. The mass flow rate is a measure of how fast the air flows and how much ‘push’ or energy it has in a given volume. Funnelled wind goes faster (i.e. more litres per second flow past) but it has less energy per litre.

In practice, this means that a funnelling device that doubles the speed of the wind will result in about double the energy output. This has led several designers to make use of the funnelling effect of wind around buildings. From shaped ‘diffusers’ to carefully designed building forms, there are many variations on the basic theme: by increasing the speed of the wind through the turbine, the energy output can be increased.