The shape of things to come

In America, The National Wind Technology Centre, operated by the National Renewable Energy Laboratory, has been indulging in informed speculation on the shape of wind turbines in 2000, perhaps looking beyond the three design projects recently funded: Kenetech KVS-50, a WTC-1000 from the Wind Turbine Company and a Z-56 from Zond (Windpower Monthly, May 1996). The NWTC anticipates three likely configurations for 2000: two for horizontal axis designs and one for a vertical axis unit. The first configuration represents a low-risk design path incorporating the more conventional three bladed rotor. Innovations include a larger rotor diameter using advanced airfoils and trailing edge flaps for overspeed control. At 800 kW, this design is twice the size of current machines in the US.

The second machine incorporates higher-risk design options, including a 50 m diameter, two bladed, teetered downwind rotor. The stall controlled rotor also has actively controlled ailerons for power clipping in response to gusts. The use of a variable speed generator is intended to increase energy capture over a broad range of wind speeds. Although high risk because of its innovations, the philosophy offers high potential for reduced weight and therefore cost.

Although no vertical axis designs were included in the latest release of funds, NWTC sees their simplicity as the key. Cost-effective manufacturing techniques (primarily for blade production) will produce significantly less expensive blades, based on cost per unit length. VAWT blades are not geometrically complex (no twist or taper), thus making manufacturing processes such as extrusion and pultrusion viable candidates to reduce costs. In addition, the inherent advantage of the VAWT configuration, having all drive train and generator parts at ground level, creates opportunities for using components with high weight or large physical size. Of recent interest in this regard is the direct, or linear drive generator, which eliminates the need for a gearbox and provides the advantages of variable speed operation at a very competitive price.