Dual Purpose Design of Small Wind Turbine Blades

Experimental studies of the starting behaviour of small wind turbines have shown that the initial “idling period”, characterised by small rotor acceleration, is usually much longer than the subsequent period of rapid accleration to reach operational rotor speed. Idling obviously reduces the power generation potential of any turbine. The experimental results also imply that most starting torque is generated near the hub, whereas power-producing torque is concentrated near the tip. Therefore this paper considers the computational design of blades for small wind turbines for the dual purposes of (i) efficient power production at rated wind speeds and (ii) rapid starting at smaller wind speeds. Standard blade element theory is used to determine the power coefficient, which is the first objective function to be maximised. A modified blade element method gives the start, whose inverse is the second objective function. During the idling period, the blade angles of attack are relatively large, allowing the lift and drag to be modelled using “generic” flat-plate equations with a small adjustment for the particular aerofoil. No power is extracted from the wind while the blades are accelerated by the aerodynamic torque. Starting is deemed to be complete at a tip speed ratio representing the end of the idling period, which considerably less than the period for power generation. The design is by “differential evolution”. An initial population of blade types is generated randomly and allowed to evolve through random modification and selection of the fittest blades to comprise the next generation. The blade “genes” are the chord and pitch of each blade element; the aerofoil section is the same for all elements and all blades. In multi-objective optimisation, most interest focusses on the members of the population which have at least one objective function greater than every other member, as the final design is usually chosen from these blades. The geometry of the best power extracting blade obtained from the evolutionary optimisation was compared with that arising from standard blade element theory. Good agreement was found for the case of an aerofoil having a pronounced peak in its lift to drag ratio but only a small Reynolds number dependence in the lift coefficient at which the ratio is a maximum. Using an aerofoil with a much flatter lift to drag characteristic, the evolutionary method easily adapted to the significant Reynolds number dependence of the aerofoil characteristics. The effects of blade inertia and the resistive torque of the generator and drive train are also considered. In all cases studied, the best power-producing blade always had relatively poor starting performance, but it was usually possible to choose an alterntive blade with only a small reduction in power output but with a starting time decreased by a factor of around two.