The Scoop on Vertical Axis Wind Turbines

http://www.motherearthnews.com/renewable-energy/vertical-axis-wind-turbines-comparison.aspx

One type of wind turbine that’s been getting a lot of attention these days is the vertical axis wind turbine. Even famous Hollywood stars like Ed Begley, Jr., are touting their supposed advantages over conventional wind turbines.

In a vertical axis wind turbine (VAWT), the blades are attached to a central vertical shaft. The shaft is attached to an alternator located at the bottom of the shaft, sometimes even at ground level. When the blades rotate, they spin the rotor of the generator, producing electricity.

Vertical axis wind machines have been around for a long time, about 3,000 years. The reason you see so few of them is that they’ve failed miserably.

That hasn’t stopped inventors (who are apparently unaware of their disappointing history) from rolling out new models and singing their praises. One refrain in their song of praise is that VAWTs can capture wind from any direction, which is true of horizontal wind turbines, too. The difference is that VAWTs are always oriented into the wind whereas conventional wind turbines turn into the wind as wind direction changes, thanks to the tail vane. No big deal there.

Proponents also like to claim that VAWTs are immune to turbulence that wrecks havoc with horizontal axis wind turbines (HAWTs). Turbulence is crazy airflow that occurs downwind from buildings and other obstacles on the ground, known as ground clutter. In turbulent areas, winds can flow in a number of different directions. This, in turn, is pretty hard on conventional wind turbines and one reason they are mounted on tall towers. Tall towers place them in the smoother, stronger, more energy-rich higher-level winds.

The VAWT does have a slight advantage in dealing with wind direction shifts, but turbulence also wreaks havoc on their blades, increasing fatigue. Fatigue leads to failure. Blades can crack and break.

Another supposed advantage is that VAWTs can be mounted close to the ground — even on top of buildings — where they capture ground-level winds. This eliminates the need for tall and costly towers and the need to obtain the zoning variances sometimes required to install horizontal axis wind turbines on tall towers.

Although VAWTs can capture ground-level winds, just like any turbine installed on a short tower, ground-level winds are subject to friction. This slows wind down as it sweeps across the land. Both friction drag and turbulence in lower-level winds around buildings and trees decrease the power available to a turbine mounted at ground level — so much so that there is very little extractable energy in them. The lower the wind speed, the less electricity a turbine will produce.

In addition, dead air spaces form behind buildings and other ground clutter. Placing a VAWT in such a location renders it useless. Even though you can mount a VAWT at ground level, there’s not much energy for them. You could mount a solar electric module inside your garage, too, but it’s not going to do much.

Unfortunately, years of experience with VAWTs has been rather discouraging, to say the least. Hundreds of commercial VAWTs were installed in California in the late 1980s and early 1990s. They all failed and were removed from service. These were not experimental units, but production units.

VAWTs are also less efficient than horizontal axis wind machines. For a given swept area, they just don’t extract as much wind energy as a well-designed HAWT. In addition, the blades of VAWTs are prone to fatigue created by centrifugal forces as the blades spin around the central axis. The vertically oriented blades twist and bend as they rotate in the wind. This causes the blades to flex and crack. Over time, this causes the blades to break apart, leading to catastrophic failure.

VAWTs are less reliable and efficient than HAWTs. All in all, they just don’t stack up against horizontal axis wind turbines. Don’t be fooled by the chatter of the ill informed — our friend and ally Ed Begley, Jr., included.

To learn more about VAWTs, check out Wind Power: Are Vertical Axis Turbines Better? You also may want to sign up for some of our courses at The Evergreen Institute or read my newest book, Power from the Wind. This book should be available by mid April in major bookstores and online at The Evergreen Institute and New Society Publishers.


Contributing editor Dan Chiras is a renewable energy and green homes expert who has spent a lifetime learning life’s lessons, which he shares in his popular blog, Dan Chiras on Loving Life. He’s the founder and director of The Evergreen Institute and president of Sustainable Systems Design. Contact him by visiting his website or finding him on .