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example vawt
The best brains are working on VAWT

Recent howling winds have been like sweet music to one US company, which says its new vertical wind turbine (VAWT) is substantially more efficient than traditional propeller designs at wind power generation. VAWT is not a new idea and there are already many designs on the market but they usually turn out to be spurious. And new entrants are up against the might of companies like GE Power, which has recently announced it is transforming the company towards green power.

Officials at low-profile Terra Moya Aqua Inc. unveiled their new turbine power generator Friday, saying the design already had attracted interest from both domestic and foreign buyers.

“We have people nationally and internationally who want to buy this turbine now,” said Ron Taylor, TMA’s founder and chief executive officer.

Company officials said its just a question of wind power energy and green power: traditional propeller-driven turbines are able to convert 25 percent to 40 percent of wind power and green power into transmittable energy. But TMA’s design is 43 percent to 45 percent efficient, creating up to 80 percent more energy from the same wind.

That energy is generated even though the vanes are moving slower than on traditional propeller models, meaning the turbines are less noisy and less dangerous to birds, the company said. And since they stand no taller than 96 feet, the turbines can be used in industrial areas where taller propeller-driven models are not allowed.

Former Gov. Jim Geringer, who serves on TMA’s board of directors, said the design improvements could help persuade doubters of wind’s potential.

“To some people, wind is a four-letter word,” Geringer said. “With what we’re talking about here, it’s anything but a four-letter word.”

Minnows like Terra stand no chance against GE Energy, a $17 billion unit of the industrial, finance and media conglomerate, which forecasts its revenue from wind energy to grow by 50 percent to $3 billion in 2006.

The wind business has already grown ten-fold since GE bought the business from Enron in 2002.

“Right now we’re pretty shocked by demand for wind,” John Krenicki, president and CEO of GE Energy, told Reuters in an interview. In 2007, GE targets sales to grow to $4 billion and increase at a rate of more than 10 percent annually through the end of the decade, he said.


VAWTs come in two flavors: lift- and drag- based designs. Drag-based designs work like a paddle used to propel a canoe through the h2o. 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 wind measurement 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 energy, more efficiently.


Stacked Savonius

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 for power supply. 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 energy output, you might try getting your machine to lift something heavy (safely). One horsepower (HP) equals 550 ft-pounds/sec. If it lifts 100 pounds 5.5 feet in one second, it is one horsepower(HP). Another way to measure output would be to sample the torque and RPM:

Horsepower(HP) = torque x rpm / 63000

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


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 energy) sinusoidal output — just like cranking on a cycle — 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.


McDonnell Aircraft
Vertical Axis Giromill

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.

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