Offshore wind energy has received a lot of interest as one important renewable energy source. One promising way to reduce the Levelized Cost of Electricity (LCOE) of offshore wind energy is by developing large wind farms and turbines with large ratings. The average wind turbine size has reached 4.2 MW in 2015 and turbine sizes of 6-8 MW have already been seen in the wind market. Even larger turbine sizes are managing to pave their way from studies to market. The trend towards larger ratings and more offshore installations asks for innovations in power generation, which requires lower weight and cost, smaller size, higher efficiency and reliability. Due to the high current-carrying capability and no DC losses of the superconductors, superior power to weight/volume ratio with high efficiency of a superconducting generator can be achived. Moreover, direct current (DC) transmission has been put forward for the offshore wind farms mainly due to the overall economic benefit, as they are located far away from the land. Hence, this thesis introduces a DC generation and transmission scheme which consists of superconducting DC wind generators and superconducting DC cables as a possible technical solution. This enables a highly efficient and compact generator and in addition a new and also very efficient generator connection scheme at DC.
The work presented in the thesis focuses on the feasibility study and design of a superconducting DC wind generator. In part, an optimisation method will be developed by taking superconducting tape length (cost), mass, volume, and efficiency into a simplified objective function. All necessary analytical equations will be derived to connect the electromagnetic design and mechanical design with properties of the superconducting tapes and iron materials. To increase the design accuracy, analytical equations to calculate flux density distribution in the superconducting DC generator will be verified by finite element analysis. Not only the active parts but also inactive structural materials will be included in the mass calculation. Based on the design method, the design of a 10 kW superconducting DC generator demonstrator will be described. The losses of the demonstrator and its commutation, torque and efficiency at different wind speeds will be addressed. As first steps towards the demonstrator, properties of key components, superconducting tapes, iron materials and a superconducting coil, will be tested and characterized. Moreover, a preliminary test of a superconducting coil at 77 K will be completed. In order to identify the potientials that a large scale superconducting DC wind generator could offer, a 10 MW superconducting DC generator will be designed and a comparison with conventional synchronous generators will be made. Additionally, this work will also discuss the savings of HTS tapes by optimizing outer rotor diameter, pole pair number, and superconducting coil height, which contribute to a more competitive alternative to conventional generators
