Optimization method for wind turbine rotors

This paper presents a recently developed numerical multi-disciplinary optimization method for design of horizontal axis wind turbines. The method allows multiple constraints. The objective was minimum cost of energy, determined by the design giving fatigue and extreme loads and the annual production of energy. Time domain aeroelastic calculations and Rainflow counting provided the life time equivalent fatigue loads. A semi-empirical approach was developed for their sensitivities. This resulted in substantial savings in computing time. An optimization of a 1.5 MW stall regulated rotor demonstrated the design method, and the results showed that constraints on loads are important for the applicability of the optimization results. Shape optimization of the rotor resulted in maximum strain on more than 80% of the blade span and hence more efficient use of material. The cost of energy was reduced compared to a traditional design with the same swept area. The optimum specific power was found to 460 W/m(2), which is lower than that of modern Danish wind turbines. Studies for optimum airfoil characteristics showed that the airfoil sections should have a relative high maximum lift at the entire span including the tip region. An increase in the swept area should therefore involve a complete redesign of the rotor blades, and avoid the use of low maximum lift airfoils at the tip, which so far has been widely used to control peak power. (C) 1999 Elsevier Science Ltd. All rights reserved.