CFD modelling of the performance of various wind turbine rotors with experimental verification

Abstract

The Blade Element Momentum Method (BEMM) is often used in the initial design of horizontal axis wind turbine (HAWT) rotors. The BEMM has many simplifying assumptions and limitations, therefore simulation and testing are essential for a successful design. Climate change concerns and the depletion of fossil fuels have created a global imperative for increased use of renewable energy. An important source of renewable energy is wind which is mostly harvested by means of large HAWTs. This research was focussed on performance comparison of two, almost identical, small HAWT rotors. The first rotor was designed using the conventional BEMM. The second was designed using an adapted BEMM which is currently under research at Cape Peninsula University of Technology (CPUT). A particular area of interest was the performance of HAWT rotors at low (off-design) wind speeds. Performance comparison of these two rotors by simulation and physical testing at half the design wind speed was the central objective of this research. Research included the creation of solid models for Computational Fluid Dynamics (CFD) simulation and manufacturing of the conventional and Adapted (ADP) rotors for physical testing. The vehicle-top mounted test rig and instrumentation were built for physical testing to capture the power output of the rotors. Flow through the rotors and rotor power output were analysed using ANSYS Fluent software. CFD simulation results and physical test result were interpreted and compared. Procedures for the solid modelling and the CFD analysis formed part of the output of this research. Simulation predicted a 1.06 % increase and physical test results revealed a 6.06 % increase of peak performance for the ADP rotor. Physical results had lower power output than the simulation results due to losses present during field testing. The power output and power coefficient curves of the simulation and field test were compared for each rotor. An unexpected outcome from physical test results was that the ADP rotor power peak occurred at a significantly higher rotational speed than the power peak of the standard (STD) rotor due to the ADP rotor’s design change. The field tests produced lower measured power output compared to the CFD predicted power output which was likely due to the oversized generator. Although we were not able to accurately measure the absolute power, we were still able to make use of the relative electrical power output to make the comparison between the ADP and Standard (STD) rotor characteristics. Simulation and physical results confirmed that the ADP rotor outperformed the STD rotor at peak performance rotational speed. It is recommended that the ADP design approach be considered for rotors that have a hub ratio in the region of 20 %. Further recommendations were also made for solid modelling, simulation, the vehicle-top mounted test rig and aspects of the methodology.