Farmingdale State College hosted an event May 11 at which Rep. Steve Israel (D-Huntington) introduced Arun Majumdar, Director of the Defense Advanced Research Projects Agency - Energy (DARPA-E). Long Island entrepreneurs were present and some had interesting displays around the room. One was Flynn+Stott Architects, Southampton, representing Urban Green Energy of China. They make several vertical axis wind turbines which can be mounted on buildings and other structures. Richard Stott says they work well in turbulent wind that would be present around buildings. The 1 kW model has three blades, spaced 120 degrees apart around a (non-existent) cylinder. The blades have a helical pitch such that the top of each blade is directly above the bottom of an adjacent blade. The concave surface of one blade airfoil faces the wind when that blade is moving in the direction of the wind, so there is always some spot to act as a cup to catch the wind from any direction to get the rotor started. Once started, I am completely baffled by how the lift and drag forces work.

The rotor height is 2.7 m and the diameter is 1.8 m. The product of these two numbers is 4.86 m2. Their specifications say the swept area is 3.24 m2. The ratio is 2/3. I thought it should be the larger number. Can somebody explain this 2/3 factor? Anyway, at the rated wind speed of 12 m/s, the kinetic energy transport rate through the 3.24 m2 area is 3.33 kW. The rated output is 1 kW, so the power coefficient (the ratio of power out to power going through the rotor's swept area) is 0.30, if we use their specification for the swept area. Using my 4.86 m2 area, the power coefficient is 0.20. Horizontal axis wind turbine makers have claimed 0.50, and 0.45 is commonly claimed for water turbines.

The blades are carbon fiber and fiberglass. The vertical shaft can pass through the roof and extend to the alternator and lower bearing some distance below. There is electronic overspeed protection. The cut out speed is 30 m/s and the cut in speed is 3 m/s. Power output varies as the cube of the wind speed, so at cut in, the output power is 1/64 of the 1 kW output at 12 m/s (27 mph, a pretty strong wind).