The Strathprints institutional repository (https://strathprints.strath.ac.uk) is a digital archive of University of Strathclyde research outputs. It has been developed to disseminate open access research outputs, expose data about those outputs, and enable the management and persistent access to Strathclyde's intellectual output.1 Abstract-The objective of this paper is to optimize direct drive permanent magnet synchronous generators for offshore direct drive wind turbines in order to reduce the cost of energy. A 6MW wind turbine design is assumed and parametric electromagnetic and structural generator models are introduced for a surface-mounted magnet generator topology (using magnets with high BHmax) and a flux-concentrating variant (using magnets with lower BHmax). These are optimized using a hybrid Genetic Algorithm and Pattern Search process and the results show that the surface-mounted permanent magnet generator produces the lower cost of energy. The choice of objective function is addressed and it is found that a simplified metric incorporating generator cost and losses proxy produces similar designs to a full cost of energy calculation. Further steps to improve the quality of the model include the effect of generator mass on the design and cost of the turbine tower and foundation, which can add €0.4m to the turbine cost. Further optimizations are carried out to show the impacts of magnetic material costs (doubling this leads to a €1.1/MWh increase in cost of energy) and generator diameter limits (increasing the upper limit from 6m to 8m leads to a 0.9% drop in cost of energy) have on the choice of optimum independent variables.Index Terms-Cost of energy, direct drive wind turbine, optimization, permanent magnet generator, structural model, tower and foundation.
I. INTRODUCTIONgrowing proportion of offshore wind turbine designs are now based on directly driven permanent magnet synchronous generators. Direct drive machines can offer higher reliability and reduced maintenance cost because of the omission of the gearbox from the drive train [1]. Some of the downsides of these generators include their large size (due to the high torque rating), requirements for large quantities of rare earth permanent magnets and the significant generator structures required to maintain the small air-gap clearance against the large attraction forces between the rotor and the stator [2]. The generator designer needs to deliver a number of performance characteristics including high efficiency, low power losses at part load, high availability, low machine mass, reduced volume and lower material and manufacturing costs. Normally the designers employ some element of optimization to achieve the best balance of these aspects [3].The main purpose of this paper is to examine the process of optimizing large, low speed generators for offshore direct drive wind turbines, exploring the different objective functions that a A. McDonald and N. A. Bhuiyan are with the Institute of Energy and Environment, Department of Electronic and Electri...