Low Speed Wind Turbines (VAWT)
By Lee Consavage
Vertical Axis Wind Turbine
I’m periodically asked about the performance of those funny looking wind turbines that are advertised as “Low Speed Wind Turbines.” These are the wind turbines with long vertical shafts. The blades spin around these long vertical shafts and for that reason are referred to as a Vertical Axis Wind Turbine (VAWT). This link shows an example of a VAWT:
They are cute looking, but before you get too excited about the VAWT though, I should tell you they don’t work very well. For one thing, “Low Speed Wind Turbine” is sort of an oxymoron since there is very little power available in low speed winds.
The amount of power available in the wind is related to the cube of the wind speed:
10 MPH Winds: So let’s say you install a wind turbine in an area with average wind speeds of 10 mph. The amount of power available to be captured is 10 x 10 x 10 = 1,000 units.
20 MPH Winds: Now let’s say you install a wind turbine in an area with average wind speeds of 20 mph. The amount of power available to be captured is 20 x 20 x 20 = 8,000 units. An 8 fold increase by doubling the wind speed.
5 MPH Winds: And finally, let’s say you install a wind turbine in an area with average wind speeds of 5 mph. The amount of power available to be captured is 5 x 5 x 5 = 125 units. An 8 fold decrease by cutting the wind speed in half.
So why waste the time and money to install a VAWT that produces very little energy.
I became very interested in the VAWT in the early 1980’s, when Sandia National Labs was designing and testing very large versions of them. This was a time when energy prices were soaring (sound familiar) and there was a tremendous interest in renewable energy. A wind research group was started at Sandia National Labs in Albuquerque, NM, to design and test both the VAWTs and Horizontal Axis Wind Turbines (HAWT), which is the more familiar wind turbine you see everywhere. In fact I do not know of any wind farms, domestically or internationally, that use VAWTs, even though they have been around for decades.
Manufacturer’s of these small VAWTs tend to exaggerate their performance since there are no standards in the wind industry to compare the performance of wind turbines. The solar industry does have standard tests to measure performance. The wind industry is currently working on standards for wind turbines. Until then, wind energy experts have coalesced around the idea of swept area to compare the performance of various wind turbines. So instead of referring to a 10 kW Bergey, they refer to the 21 foot diameter Bergey. Swept area is another important component in wind energy equations and the VAWTs are lacking in this dimension also.
Having said all that, if you do happen to know of anyone who has a VAWT that is performing well for them, let me know. I would like to know of any practical applications for VAWTs.
