Diffuser augmentation of wind turbines

Abstract

One of the more promising advanced concepts for overcoming the economic deterrents to widespread use of windpower is the Diffuser-Augmented Wind Turbine (DAWT). The diffuser controls the expansion of turbine exhaust flow, producing a highly subatmospheric pressure at the turbine exit. The low static pressure induces greater mass flow through the turbine in contrast to a conventional turbine design of the same diameter. Thus, the output power of the DAWT is much larger than for an unshrouded turbine. Our wind tunnel investigation of models of two diffuser design concepts is directed toward unconventional, very short, cost-effective configurations. One approach uses the energetic external wind to prevent separation of the diffuser's internal boundary layer. Another method uses high lift airfoil contours for the diffuser wall shape. Diffuser model tests have indicated almost a doubling of wind power extraction capability for DAWTs compared to conventional turbines. Economic studies of DAWTs have used these test data and recent (1975) cost projections of wind turbines with diameter. The specific power costs ($/kW) for a realistic DAWT configuration are found to be lower than conventional wind turbines for very large size rotors, above 50 m diameter, and for rotor diameters less than about 20 m. The cost-to-benefit assessment for intermediate size rotors is affected by the uncertainty band of cost for these rotor sizes.