


The design, from Selsam Innovations, Fullerton, Calif. (selsam.com), puts several rotors on a single carbonfiber driveshaft. A 7ftdiameter turbine typically produces a peak power of 800 W. "The recordsetting model is at a 5,000ft 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 selfpositioning 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 singlerotor 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."