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"VAWTs come in
two flavors:
lift-based and
drag-based
designs."

 

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What Are Vertical-Axis Wind Turbines (VAWTs)?

by Eric Eggleston and AWEA Staff

It can be very difficult to find information on vertical axis wind turbines (VAWT). Here's a basic summary of VAWT technology.


A cup anemometer is
a drag-type vertical
axis wind turbine

VAWTs come in two flavors: lift- and drag- based designs. Drag-based designs work like a paddle used to propel a canoe through the water. If you assume that the paddle used to propel your canoe did not slip, then your maximum speed would be about the same speed you drag your paddle. The same holds true for the wind. The three-cup anemometers commonly used for measuring wind speed are drag-based vertical-axis wind turbines. If the velocity of the cups is exactly the same as the wind speed, we can say that the instrument is operating with a tip speed ratio (TSR) of 1. The ends of the cups can never go faster than the wind, so the TSR is always 1, or less.

A good way of determining whether a VAWT design is based on drag or lift is to see if the TSR can be better than 1. A TSR above 1 means some amount of lift, while TSR below 1 means mostly drag. Lift based designs can usually output much more power, more efficiently.


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Stacked Savonius
rotor

 

The Savonius: A Useful, Drag-Type VAWT

Yet drag-based VAWTs can be useful. They can be made many different ways with buckets, paddles, sails, and oil drums. The Savonius rotor is S-shaped (when viewed from above) and apparently originated in Finland. A good Savonius turbine might exceed a TSR of 1, but not by much. All of these designs turn relatively slowly, but yield a high torque. They can be useful for grinding grain, pumping water, and many other tasks; but are not good for generating electricity. RPMs above 1000 are generally best for producing electricity; however, drag-based VAWTs usually turn below 100 RPM. One might use a gearbox, but then efficiency suffers and the machine may not start at all easily.

Should you have already built a low-RPM VAWT and wish to calculate its power output, you might try getting your machine to lift something heavy (safely). One horsepower equals 550 ft-pounds/sec. If it lifts 100 pounds 5.5 feet in one second, it is one horsepower. Another way to measure output would be to sample the torque and RPM:

Horsepower = torque x rpm / 63000

Torque in. (inch x pounds) (1 hp = 746 watts)


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DOE's 500-kW variable
speed Darrieus machine

 

Darrieus Lift-Type Vertical-Axis Machines

There are also lift-based vertical-axis types like the "eggbeater" Darrieus from France (first patented in 1927.) Each blade sees maximum lift (torque) only twice per revolution, making for a huge torque (and power) sinusoidal output -- just like cranking on a bicycle -- that is not present in HAWTs. And the long VAWT blades have many natural frequencies of vibration which must be avoided during operation. For example, a 500-kW two-bladed vertical-axis turbine we have on site has two or three rotational speeds that must be gone through quickly to get up to operating speed and several modes within the operational band which the control must avoid. A well-designed HAWT has none of these problems.

VAWTs are very difficult to mount high on a tower to capture the higher level winds. Because of this, they are usually forced to accept the lower, more turbulent winds and produce less in possibly more damaging winds.

Guy cables are usually used to keep the turbine erect. They also impose a large thrust loading on the main turbine bearings and bearing selection is critical. Like all types of turbines, replacing main bearings requires that the turbine be taken down.


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McDonnell Aircraft
Vertical Axis Giromill

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ASI/Pinson
Cycloturbine

Other Lift-Type Vertical Axis Configurations

Darrieus' 1927 patent also covered machines with straight vertical axis blades called Giromills (photo at left).

A variant of the Giromill called the cycloturbine (below left) uses a wind vane to mechanically orient a blade pitch change mechanism.

There are not many easy-to-find references devoted to vertical-axis turbines. The wind energy group of Sandia National Labs in Albuquerque, New Mexico, has done a lot of research on Darrieus vertical-axis technology. Straight-bladed VAWTs were explored by the National Wind Technology Center at NREL.

VAWTs have not performed well in the commercial wind turbine market. The cylcoturbine was marketed commercially for several years. The Giromill never progressed beyond the research stage. In the summer of 1997, the last U.S. Darrieus VAWT company went bankrupt.


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1998 by the American Wind Energy Association.
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