Active control of flow separation using compressed air egress and ingress for low-speed wind turbine blades

Active control of flow separation using compressed air egress and ingress for low-speed wind turbine blades

Wind energy is one of the most promising alternative energy sources for the continuously increasing demand for worlds’ energy consumption. In recent decades, there has been a steady and growing demand for energy, which has led to a shift to renewable resources. During many decades the wind turbine design, in particular, wind turbine blade design has been intensively researched to extract maximum possible energy from the wider range of wind flows. However, there have been many unresolved problems in the performance of small wind turbine blades due to insufficient energy generation at low-velocity wind regimes. Therefore, this project will investigate methods to improve the performance of small horizontal axis wind turbines (sHAWT) operating in low wind speed sites.

The primary cause for the reduction in energy generation at low wind velocity regimes was identified as the flow separation. In this project, suction and blowing techniques were used as active flow controls (AFC) to reduce flow separation in order to overcome the effect of an adverse pressure gradient (APG) and thus improve aerofoil performance. The data for a sHAWT with a 10 m diameter and a capacity of 25 kW were adopted. Two types of aerofoils were used, S823 and S822 aerofoils. Lift and drag coefficients were obtained with/without AFC technology. The range of tested angles of attack was between 0 and 21° and therefore considered suitable for predicting turbine performance. Suction speeds were changed to improve aerofoil performance, with -5 and -35 m/s suction speeds determined for low (less than 9°) and high (at least 9°) angles of attack respectively.

The research concluded that the deterioration of aerodynamic properties occurs at an AoA of 18°. It was also found that applying suction at 18% of the chord length on the upper surface gave good results at all angles of attack. The use of a boundary layer suction method delays the flow separation from the upper surface of the blade's aerofoil and even prevents separation for all but the highest angles of attack. The purpose of this method is to reduce the momentum flow in the high-pressure areas of the suction surface of the aerofoil. Although the use of such technologies may improve the performance of wind turbines, many challenges (such as design, maintenance and construction costs) must be met to reduce or mitigate the occurrence of separation in the boundary layer.

Moreover, the research found that the enhanced sHAWT operated very efficiently at a rotational speed of 60 and 110 rpm with lower/ higher wind speeds than 9 m/s respectively. The rates of improvement were variable with each wind speed, so the average rate of improvement was 15% as a result of using the early suction (S1) technique. The research also demonstrated that the annual energy production for a site with an average annual wind speed of 6 m/s is 22% higher compared to the selected standard wind turbines.