Machine Design

Novel windmill captures more energy

A novel 7-ft.-diameter wind turbine has set a record by producing 5.3 kW in a 32.5-mph wind.

Doug Selsam holds a windmill design with 21 blades each 20-in. diameter, and a 0.75-hp generator. The design has only one moving part. Shafts on all designs are positioned at a slight angle from horizontal, to take rotors out of the wake of each other.

Selsam's record-setting design uses seven 7-ft-diameter rotors on a 60-ft tower. Propellers are from a Whisper H-40 windmill from Southwest Windpower. At sea level, the unit is capable of 5.4 kW at 30 mph, 4.0 kW at 25 mph, over 2.0 kW at 20 mph, and over 1 kW at 16 mph. Four rotors downwind and three upwind provide for passive control on smaller units. Larger versions, however, will have active yaw control.

An 8-MW version of the multirotor generator will have a service hatch that will give technicians access to the hardware.

The design, from Selsam Innovations, Fullerton, Calif. (, puts several rotors on a single carbon-fiber driveshaft. A 7-ft-diameter turbine typically produces a peak power of 800 W. "The recordsetting model is at a 5,000-ft elevation so 5.3 kW corrects to 6.0 kW at sea level," says inventor Doug Selsam. The design has been tested for six months as part of the California Energy Commission research program.

In light winds, the shaft slightly tilts forward so each rotor stays away from the draft of the preceding one, thereby exposing each to the breeze. In stronger winds, the self-positioning driveshaft is parallel to the wind, placing downwind rotors in the wake of those upwind. "The rotors 'draft' each other like race cars. This provides protection from overspeed," says Selsam.

"The design's breakthrough is that it's the only way to increase the swept area of a wind turbine without increasing the diameter. The design also maintains the light blade weight and high rpm of a smaller turbine, while taking advantage of the increased swept area and higher power output of a larger turbine, essentially the best of both worlds," says Selsam.

"Even uncorrected for sea level, we get a solid average of 4.0 kW at 27 mph," he adds. " Lowwind speed performance is superb. Because we use more rotors, we get the same power at half the wind speed of a traditional turbine of the same diameter. And in the same breeze, we get six times the power of a convention single-rotor windmill," he says.

Selsam adds that other wind engineers criticized the design as unworkable because they suspected the driveshaft would fly out of control. "As it turns out, each rotor is a node of stability," he says. "Other pluses are lighter total rotor weight, and higher rpm, for a given power rating. Blade weight grows as a function of the diameter cubed, while swept area is a function of diameter squared. That means smaller blades sweep more area per unit mass and turn at higher rpms, which can eliminate a gearbox usually needed to drive a generator."



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