The design study of 15-MW offshore wind turbine generators was carried out with RE1Ba2Cu3O7δ (RE denotes rare earth; RE=Y, Gd, Eu, henceforth REBCO) superconducting tapes. In Japan, the sea becomes deep abruptly with distance from the shore. Therefore, offshore wind farms need to be built on floating bodies, and then, the output power should be over 15 MW per wind turbine from the viewpoint of economical efficiency. However, the conventional generators composed of copper wire and iron cores for low-speed revolution are huge and heavy compared with the generators for commercial frequency. Enhancement of the magnetic field at the gap by using a superconducting rotor should lead to the drastic reduction of weight and size of generators. In this paper, the critical current and ac loss properties of REBCO superconducting tapes were first investigated. By taking them into account, several kinds of 15-MW fully superconducting generators were designed. For each case, the properties of the generator were numerically simulated. Then, the induced ac losses in the field and armature windings, the iron loss, the efficiency, the required REBCO tape length, the electrical weight of the generators, and so on were evaluated. As a result, it was shown that fully superconducting generators can be much reduced in electrical weight against conventional ones, and magnetic field at the gap should be around 2 T from the viewpoint of cost and total weight.
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