Comparison of direct-drive PM generators for tidal turbines

The aim of the paper is to compare the design of direct-drive permanent-magnet (PM) generators associated with horizontal axis tidal turbines. The turbine/generator couplings are examined. These turbine/generator couplings consist of a POD and Rim-driven assembly. In a Rim-Driven association the electrical generator active parts are inserted in a duct surrounding the blades. For POD generator insertion, the electrical machine is placed in a nacelle located on the turbine axis. To achieve the generators sizing, a design rated point related to an industrial MCT is defined. The used design models include an electromagnetic model which is linked to a thermal model in an optimization procedure that goals to minimize the active parts cost. Firstly, a single rotor/single stator PM axial flux generator and a radial flux PM generator are designed for a rim-driven MCT specification. For these generators sizing, a comparison of the machines active parts and the machines geometrical dimensions are carried out. Secondly, radial flux PM generators are designed for Rim-Driven and POD assembly and a comparison is performed for this study case. Finally, the influence of the POD diameter on the generator electromagnetic design is studied. It shows that the active parts costs are minimized, when the generator diameter is around 1/3 of the turbine diameter for the considered specifications. These performed comparisons between the considered study cases aims to help designers in their technologies choices.