Structural modelling of blades for small wind turbines
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Small wind turbines are defined as having a power output and rotor diameter less than 50 kW and 16 m respectively. Turbines of this class are typically used to provide off-grid generation and can be sited in locations which experience comparatively high wind turbulence due to obstacles such as dwellings, vegetation, and unfavourable topography. These wind regimes can be detrimental to both power output and component fatigue life. Small wind turbines have several significant design and operational differences compared to large commercial scale wind turbines. In order to maintain an ideal tip speed ratio, small wind turbines usually operate at higher rotor speeds (~200-600 rpm) compared to commercial scale wind turbines (~10-30 rpm), therefore increasing both the centrifugal loading and number of fatigue cycles experienced during a blades design life. The rotor is usually aligned passively to the inlet wind direction via the means of a tail fin. This method of yaw control can result in complex load cases acting on the blades due to both gyroscopic forces and rotor yaw error. Key differences in blade design of small wind turbines include; relatively small masses (where mass has been shown to approximate a cubic relationship to blade radius), the lack of an internal spar member, relatively high stiffness (particularly in the lead-lag direction), and the lack of a circular blade root transition region and pitch control mechanism. When considering blade development for small wind turbines, design tools such as computational fluid dynamics (CFD), and aeroelastic modelling software packages are not frequently used due to limited budgets and time constraints. In this study we compare several different methods used to model the structural behaviour of a 5 kW Aerogenesis small wind turbine blade (2.5 m in length). These include a finite element (FE) model incorporating the layup properties of the fibreglass composite, a simple isotropic plate FE model, and a simple beam model. Aerodynamic loads will be determined via the unsteady blade element momentum method (BEM) where algorithms to account for effects such as dynamic inflow, dynamic stall, yaw error, and wind shear are included. Metrics including blade deflection, blade stress, and computation time will be compared for a range of aerodynamic load cases appropriate for small wind turbine operation (i.e. high rotor speeds, large yaw errors, etc.). The structural models will then be compared according to solution accuracy and computational time. The results of this study are expected to aid the design of small wind turbine blades in an accurate and time effective manner. Particular applications may include the prediction of lifetime fatigue loading, and blade design via iterative genetic algorithms whereby computational time is paramount.