Small Turbine VAWTs and Rotor Loading

By Paul Gipe

February 25, 2008

Below is summary of principle characteristics of mini- and household-size Vertical Axis Wind Turbines on the international market. It is not intended to include every VAWT marketed worldwide.

The table has been sorted by the rotor loading at the rated power in W/m². It is important to note that the majority of the turbines have high rotor loadings relative to conventional Horizontal Axis Wind Turbines.

The characteristic loading of HAWTs is about 250 W/m². For example, the Bergey Excel has a rotor loading of 260 W/m² and that for the Skystream is 166 W/m². The Air X had a rotor loading of 373 W/m², a very high value for micro turbines. The Air Breeze, the newest model in the Air series, has a rotor loading of only 186 W/m². These turbines are all conventional HAWTs.

What does a high rotor loading mean? First, it doesn’t mean that the wind turbine is capable of generating more electricity than it’s HAWT competitors. It simply means that the wind turbine uses a much larger generator relative to the area swept by the wind turbine. It is the swept area that is the prime determinant of how much energy a wind turbine will capture. However, a high rotor loading does suggest that a VAWT manufacturer may be overstating what the turbine is capable of. Rotor loadings of greater than than 300 W/m² should be viewed with skepticism.

Another way to look at rotor loading is to consider a Standard Rated Power of 200 W/m². Many, though certainly not all VAWTs in the list, have rated their turbines at twice the Standard Power Rating.