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In this paper, the main objective is to optimize permanent magnet synchronous generators for offshore direct drive wind turbine, examining the best choice of magnet grades, BH max, and working temperature. A surface-mounted Nd–Fe–B generator is designed electromagnetically and structurally and optimized for different rated powers of 6, 8, and 10 MW. The results show that the cost of energy decreases as the wind turbine's rated power increases. Further optimizations were carried out using different neodymium magnet grades, and it was found that the higher magnet grades produce a lower cost of energy. In addition, steps were taken to estimate the effect of magnet temperature. A detailed thermal model is used to calculate the cooling airflow requirements to bring the magnet operating temperature from 120 to 80 °C. Allowing the use of cheaper temperature grades of magnets, the additional cooling reduces winding losses and improves the effective BH max of the magnets.

A large proportion of offshore wind turbine designs are now based on directly driven permanent magnet synchronous generators using rare earth materials. During last few years, the price of Nd-Fe-B has increased and fluctuated significantly. The large price fluctuations encourage us to look at flux-concentrating Nd-Fe-B generator (where flux-concentrating characteristics helps to increase flux density in the air gap) and optimize that for offshore wind turbine generator. In this paper, a 6 MW generator using Nd-Fe-B magnet is designed analytically in MATLAB, where magnets are placed between magnetically conducting pole shoes to reinforce the air gap flux. The generator design is optimized for the best performance machine and lowest cost of energy. Further optimization is performed to compare the results with a 6 MW surface-mounted Nd-Fe-B generator. It is found that, the flux-concentrating Nd-Fe-B generator gives better cost of energy compare to surface mounted Nd-Fe-B generator. The effect of variable power factor and sensitivity to wind conditions are also estimated in this study.

In the past few years interest in the use of low speed permanent magnet generators for direct-drive wind turbine generator applications has increased significantly. The significant fluctuations in NdFeB magnet prices has encouraged designers to optimise magnet utilisation and to look at alternative magnet materials for wind turbine electrical generators. In this paper an analytical design model is developed for 6 MW offshore direct-drive wind turbine generators using different magnet materials (one with surface mounted NdFeB and another with flux concentrating ferrite magnet). Finite element method models are used to check key dependent variables calculated by the analytical models. The generator designs are optimised using a hybrid optimisation method incorporating a Genetic Algorithm and Pattern Search approaches. This is applied for four different objective functions, the first two which concentrate on maximising rated torque per unit magnet mass or unit of generator active material cost. They are simple and quick to execute but prioritise cost reduction and ignore lower efficiencies leading to lower turbine energy yields and hence poor cost of energy. A third objective function which seeks to minimise the sum of the generator active material cost and the costs of lost revenue over a finite number of operational years. This gives similar results to a fourth objective function which is an explicit turbine cost of energy calculation. The cost of NdFeB magnets affect the cost of energy of the surface mounted generator which tested with different cost €40/kg, €60/kg and €80/kg. The ferrite magnet generator being better when the NdFeB magnet price rises to €80/kg.

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 6 MW 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 ${\rm{BH}}_{{\rm{max}}}$ ) and a flux-concentrating variant (using magnets with lower ${\rm{BH}}_{{\rm{max}}}$ ). 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 €0.4 m 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 6 to 8 m leads to a 0.9% drop in cost of energy) have on the choice of optimum independent variables.