Design Considerations, Performance Enhancing Techniques, and Wind Tunnel Testing for Small-Scale, Low Reynolds Number Wind Turbines

Wind turbines have become a significant part of the world’s energy equation and are expected to become even more important in the years to come. A much-neglected area within wind turbine research is small-scale, fixed-pitch wind turbines with typical power outputs in the 1–10 kW range. This size wind system would be ideal for residential and small commercial applications. The adoption of these systems could reduce dependence on the aging U.S. power grid. It is possible to optimize a small-scale system to operate more efficiently at lower wind speeds, which will make wind generation possible in areas where current wind technology is not feasible. This investigation examines the use of the S818 airfoil, a typical blade root airfoil designed by the National Renewable Energy Laboratory (NREL), as a basis for the design of low Reynolds number (less than 200,000) systems. The literature shows that many of the airfoils proposed for wind turbine applications, including the S818, only have lift and drag data generated by numerical simulations. In previous research at Baylor, 2-D simulations published by NREL have been shown to predict an optimal design angle of attack (which is the angle at which L/D is maximized) up to 2.25° different from actual wind tunnel data. In this study, the lift and drag generated by the S818 airfoil has been measured experimentally at a Reynolds number of approximately 150,000 and compared with NREL simulation data, showing a discrepancy of 1.0°. Using the S818 airfoil, a set of wind turbine blades has been designed to collect wind turbine power data in wind tunnel testing. Design parameters investigated include the effect of design tip speed ratios (TSR) (1, 3, and 7) and the influence of the number of blades (2, 3 and 4) on power generated. At the low Reynolds numbers tested (ranging from 14,000–43,200 along the blade for a design TSR of 3 and a wind speed of 10 mph), the effect of roughness was explored as a performance enhancing technique and was seen to increase power output by delaying separation. Under these low Reynolds number conditions, separation typically occurs on smooth blades. However, the roughness acted as a passive flow control, keeping the flow attached and increasing power output. Preliminary data suggest that as much as a 50% improvement can be realized with the addition of roughness elements for a TSR of 3. Additionally, the increase in power output due to roughness is comparable with the increase in power due to adding another smooth blade.