Optimisation of design and operation of generators for offshore vertical axis wind turbines

A process for optimising both the design and operation of the generator for a large offshore vertical axis wind turbine (VAWT) is developed. The objectives of the optimisation process are to minimise additional costs and losses in the generator to allow for a fair evaluation of the impact of the VAWT environment on the powertrain. A spectrum of torque control strategies was tested based on the ratio, q, of the allowed electrical torque variation to the inherent mechanical torque variation. Equations relating q to the generator losses were established. The effect of q on the energy extracted by the rotor was also investigated and incorporated into the optimisation process. This work shows that a variable q strategy with respect to wind speed can improve turbine performance across the range of operational wind speeds depending on the torque loading from the rotor blades. In turn, this also allows for the torque rating of the generator to be reduced from the peak torque rating that would otherwise be expected, creating an opportunity to downscale the generator size, reducing costs. The optimisation of powertrain design and operation should be carried out at as high level as is possible, ideally using the fully factored Cost of Energy (COE) to guard against unexpected losses due to excessive focus in one COE factor (for example reducing upfront cost but in turn reducing availability).