Passive flow control devices for a multi megawatt horizontal axis wind turbine

Abstract

Renewable energy is an environmentally friendly alternative to the use of fossil fuels. In response to reducing the dependency on fossil fuels, the European Wind Energy Association (EWEA) has made a policy to increase the overall renewable energy consumption by three times the current amount by the year 2020. Critical to achieving this target will be the use of wind turbines. There is a scope to increase the performance of wind turbines by utilising the existing techniques from the aeronautical industry. One of these techniques is the use of passive flow control which involves no moving parts that has a reduced complexity compared to active flow control techniques. Initially eleven flow control devices applicable for a wind turbine were identified. Out of the eleven devices four possible flow control devices were selected for this research project (vortex generator, vortex trapping, passive ventilation and sinusoidal leading edge wing). These four concepts have been evaluated for their applicability for a wind turbine. A novel CFD work was conducted for a wedge type vortex generator for wind turbine blade. A number of different configurations as well as its performance over different operating conditions were assessed. The wedge type vortex generator (VG) showed the most benefit for a wind turbine blade and it is recommended for wind turbine application, specifically for the root part of the blade. It was found that up to 0.2% increase in AEP is possible with integration of this VG at the blade root, which corresponds to about €30,000 worth of additional energy production of a 5MWturbine per annum. A series of theoretical studies using Blade Element Momentum (BEM) theory was conducted to establish the requirements for an optimum wind turbine blade. Based on this investigation it was found that the current root geometry is unable to attain the optimum lift force and in fact it produces negative torque. One of the interests of this project was to identify ways to reduce the blade loads. This simple BEM based investigation was conducted to quantify potential the chord reduction available with the use of conventional vane and passive air-jet vortex generators (PAJVG). Findings from this exercise showed that large chord reductions are possible with the use of these devices. Only the PAJVGs were able to attain chord reduction up to 10% while allowing the blade to operate within a safe stall margin. Extensive number of 2D CFD simulations was conducted to validate the current 2D CFD methods. Baseline 2D CFD methods have been successfully validated for the pre-stall angle of attack. The effect of modelling transition for a 2D and 3D wind turbine simulation has been established using the Menter’s SST γ-θ transition model.