What are Vertical-Axis Wind Turbines(VAWTs)?

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.

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

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 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. (See Links.) 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.

Basic Principles of Wind Resource Evaluation

Wind resource evaluation is a critical element in projecting turbine performance at a given site. The energy available in a wind stream is proportional to the cube of its speed, which means that doubling the wind speed increases the available energy by a factor of eight. Furthermore, the wind resource itself is seldom a steady, consistent flow. It varies with the time of day, season, height above ground, and type of terrain. Proper siting in windy locations, away from large obstructions, enhances a wind turbine's performance.

In general, annual average wind speeds of 5 meters per second (11 miles per hour) are required for grid-connected applications. Annual average wind speeds of 3 to 4 m/s (7-9 mph) may be adequate for non-connected electrical and mechanical applications such as battery charging and water pumping. Wind resources exceeding this speed are available in many parts of the world.

Wind Power Density is a useful way to evaluate the wind resource available at a potential site. The wind power density, measured in watts per square meter, indicates how much energy is available at the site for conversion by a wind turbine. Classes of wind power density for two standard wind measurement heights are listed in the table below. Wind speed generally increases with height above ground.

Classes of Wind Power Density at 10 m and 50 m(a)
	10 m (33 ft)		50 m (164 ft)
Wind Power Class	Wind Power Density (W/m2)	Speed(b) m/s (mph)	Wind Power Density (W/m2)	Speed(b) m/s (mph)
1	<100	<4.4>	<200	<5.6>
2	100 - 150	4.4 (9.8)/5.1 (11.5)	200 - 300	5.6 (12.5)/6.4 (14.3)
3	150 - 200	5.1 (11.5)/5.6 (12.5)	300 - 400	6.4 (14.3)/7.0 (15.7)
4	200 - 250	5.6 (12.5)/6.0 (13.4)	400 - 500	7.0 (15.7)/7.5 (16.8)
5	250 - 300	6.0 (13.4)/6.4 (14.3)	500 - 600	7.5 (16.8)/8.0 (17.9)
6	300 - 400	6.4 (14.3)/7.0 (15.7)	600 - 800	8.0 (17.9)/8.8 (19.7)
7	>400	>7.0 (15.7)	>800	>8.8 (19.7)
(a) Vertical extrapolation of wind speed based on the 1/7 power law
(b) Mean wind speed is based on the Rayleigh speed distribution of equivalent wind power density. Wind speed is for standard sea-level conditions. To maintain the same power density, speed increases 3%/1000 m (5%/5000 ft) of elevation. (from the Battelle Wind Energy Resource Atlas)

In general, sites with a Wind Power Class rating of 4 or higher are now preferred for large scale wind plants. Research conducted by industry and the U.S. government is expanding the applications of grid- connected wind technology to areas with more moderate wind speeds.