Self-twisting blades for passive regulation of a small wind-turbine
It is widely recognisedt hat for small stand-aloneb attery chargingw ind-turbines, below lOkW in rated power, the issue of regulating the rotor speed is one that has not been resolved definitively, especially with regard to the runaway condition. Current methods of regulation work, but are prone to problems. It is desirable to find methods of regulation with reduced mechanical complexity. Building on earlier work at Reading University, a method of blade construction is examined which involves asymmetric lay-up of fibre-composite materials. This may be done in a manner which causes the blade to twist in response to centrifugal loads and thus the rotor speed of the turbine. There is a need to optimise the blade design to exploit this effect to the full. The equations describing the blade twist are developed and expresseds imply and explicitly in terms of blade shaped esignp arameters. Along with an established aerodynamic model, a system is developed for examining the expected steady state behaviour of a wide range of rotors of this type in order to find the most appropriate configuration. A range of existing aerofoil profiles is also examined in order to select the most appropriate choice and a shortlist of these is modelled numerically in 2D negative incident flow using an established model. In recognition of the inevitable flexibility of self-twisting designs of blade, a start is made on dynamic modelling of the rotor. A Rayleigh-Ritz approach is used in order to find both static blade deflections under loading and the blade's natural modal frequencies and shapes. A prototype design is developed, commissioned and tested under forced-rotation conditions to validate the twisting and bending models. The predicted modal frequencies are also compared with the results of vibration tests on the blades. Recommendations are made for further improvements.