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Keywords:

  • offshore wind turbines;
  • axial induction factor;
  • power coefficient;
  • structural material;
  • rotor optimization;
  • BEM modelling

ABSTRACT

Wind turbine rotors are normally designed such that rotor power coefficient is maximized. Much of this methodology has been inherited from the aviation industry. This paper points out that designing machines for maximum rotor aerodynamic efficiency does not necessarily lead to a lower levelized cost of energy. The argument sits on the premise that levelized cost of energy is strongly influenced by machine capital expenditure (CAPEX) and annual energy production (AEP). We therefore assume that the true design objective is to minimize the CAPEX/AEP ratio. The basis of an alternative design path is presented, which centres on the minimization of total volume of structural material in the wind turbine. This is done whilst maintaining a given rated power. This alternative methodology requires the removal of conventional pre-set design variables and assumptions which relate to the maximization of rotor power coefficient. We examine how changing chord length, axial induction factor and aerofoil lift coefficient affect material volume in the blade. Following this, we use a custom-made blade element momentum programme to explore the relative CAPEX of machines with varying design axial induction factor and varying lift coefficient. This relative cost is calibrated to the 5 MW National Renewable Energy Laboratory offshore reference turbine. The effects on the rotor, drivetrain and tower are considered. For a 5 MW offshore machine, it is shown that an overall CAPEX/AEP reduction of over 2% can be achieved by using a low-induction rotor with blades possessing aerofoils operating at non-peak lift to drag ratios. This economy is delivered notwithstanding a 2.3% drop in design rotor power coefficient. Copyright © 2014 John Wiley & Sons, Ltd.