A Numerical Study on Improving Airfoil Performance at Low Reynolds Numbers for Small-Scale Wind Turbines using Intentional Roughness

摘要

With an increasing demand for energy combined with the current dependence on non-renewable resources, the need for sustainable energy sources has become evident. Because no single renewable energy source can supply the bulk of the demand, a diversified energy portfolio is required. One promising renewable resource is wind energy, typically extracted with horizontal-axis wind turbines. Large-scale turbines, often installed on wind farms, are promising because of the large quantities of energy they can extract but must be installed in areas of high average wind speed, requiring an expensive and extensive power grid to be built throughout the United States. Small-scale units, on the other hand, can be optimized for much lower wind speeds. This allows them to be built near the areas of high energy consumption, which requires little grid development. In order to optimize small-scale wind turbines for low average wind speeds, blades designed for low Reynolds numbers are used. Another method of optimization, as shown in previous studies at Baylor, is to add certain types of roughness over a turbine blade. While roughness typically hinders turbine performance for Reynolds numbers 100,000 and higher, this study estimates an increase in torque produced of up to 20% by adding roughness on the surface of a 2 m diameter system that experiences Reynolds numbers below 100,000. The torque produced is a direct indicator of the power produced by the turbine. Turbines smaller than this would have more surface area experiencing Reynolds numbers below 100,000 and the improvement would be even greater. This paper analyzes the effects of roughness by examining the lift and drag produced by 2-D flow over the surface of an S823 airfoil using XFOIL, a numerical code. Performance will be analyzed with and without added roughness over a range of low Reynolds numbers: 25,000, 50,000, 75,000, 100,000, and 200,000. The results of this study will be used to develop a methodology for applying intentional roughness at locations of low Reynolds number (below 100,000) for increased power performance in low wind conditions.