A special bellows design seals the pivoting blade mechanism of a new two-blade downwind wind turbine. The design protects the mechanism against damage and helps ensure reliable long-term operation.
Wind energy is the fastest growing segment of the renewable energy industry. It is also the most economic form of grid-connected renewable electricity at as little as 5 cents per kilowatt-hour. However, it is still more expensive than power generated by existing large thermal grid-connected power stations. But that may soon change.
New wind turbine technology developed by the Wind Turbine Co. (WTC) promises to slash 30 percent or more from the cost of today's wind-generated power. The technology can be found in a 250 kW proof-of-concept turbine, which was developed as a result of a contract between WTC and the U.S. Department of Energy's National Renewable Energy Laboratory. The company is also developing a 500 kW unit with help from the state of California. The company's innovations stem from the ground-up system level design of a two-blade downwind turbine, in which the turbine rotor blades operate downwind of the tower. Its principal advantage is its ability to shed excessive wind loads.
By contrast, conventional upwind machines, whose blades are upwind of the tower, must be built sufficiently strong to absorb all predictable wind loads and prevent blades from flexing so much that they strike the tower. While all blades flex to some degree, the potential for catastrophic failure with an upwind design mandates considerably heavier and therefore more expensive construction.
The costs of building a downwind design are considerably lower, although the electrical output of upwind and downwind oriented blades is the same. As a result, once WTC reaches full production, it envisions wind farms utilizing its turbines that will produce power for an unsubsidized price of 3.5 cents per kilowatt-hour or less, inclusive of all capital recovery, operations, maintenance, and other expenses. This will make the company's turbines competitive with today's low-cost gas turbine power plants.
Accommodating Blade Movement
Unlike an upwind design, which needs rigidity to prevent blade damage, WTC's downwind turbine is built to accommodate a wider range of blade movement. It includes blades with individual pitch control, a variable coning rotor, a highly integrated structure and drive train, load mitigating control strategies, and a guyed tubular tower.
With a wider range of blade movement, the downwind turbine's blades have enough flexibility to prevent damage. Lawrence W. Miles, president of WTC explains, "When you allow the blades to operate downwind of the tower, the wind blows past the tower and pushes the blades away from it, so the blades can bend quite a bit more."
The blades of the 250 kW machine measure 33 meters (approximately 110 feet) tip to tip, while the 500 kW unit now under construction has a gas rotor with a diameter of 44 meters. In addition to having a rotating pitch adjustment, the blades are individually hinged so that they can move back and forth depending on wind loads. To prevent excessive motion, they are collectively controlled by a hydraulic damping system.
Increasing Range Of Motion
Since wind farm turbines generally are serviced once every six months, turbine reliability is critical. They must operate dependably with minimal maintenance because repairs can be costly and downtime hurts profitability. Because of their range of blade motion, it was necessary to develop a means of sealing the moveable spinner and blade components to protect the mechanism that accommodates blade movement against damaging windblown particles and moisture.
Working in partnership with WTC engineers, A&A Manufacturing Co. Inc., a manufacturer of bellows, way covers, and other protective devices, produced a unique bellows design to withstand a range of constant motion while sealing out contaminants that could shorten the life of the blade pivot mechanism.
Four bellows are used per machine. The semi-circular bellows design measures more than two meters across its inside diameter. Construction is similar to that of heavy-duty bellows used on articulated transit buses. Its range of motion, according to Miles, is approximately 18 inches, with continuous frequency during operation. The basic material used for the bellows is a hypalon-impregnated fabric engineered to resist UV exposure and other outdoor conditions, and convolutions are reinforced with stainless steel edge channels.
With the bellows in place, WTC can keep its wind turbine churning out power and keep consumers out of the dark.
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