Calculation of AC loss in an HTS wind turbine generator
In order to achieve lower cost of energy the typical power rating of large wind turbine generators has steadily increased over recent years. However, with the mast-top weight of multi-megawatt generators exceeding 100 tonnes, installation becomes increasingly costly and difficult. Direct drive turbine designs can give significant weight and cost reductions. We have developed a design for a lightweight direct drive transverse flux generator with a rating of 2 MW. The design features a multi-pole permanent magnet rotor with a single global HTS stator coil of between 4 and 6 m diameter for each phase. As one consequence of this design approach, the HTS conductor is exposed to leakage field from the magnets and the self-field of the generated current. The magnitude of the loss associated with these time-varying fields is crucial to the viability of the HTS generator concept. The stator design seeks to minimize exposure of the HTS tape to alternating magnetic fields perpendicular to the face of the tape in order to reduce the AC loss in the stator coils to an acceptable level. For a coil operating at 50 Hz, the total AC loss is calculated as 15.1 W/m. Thus, AC losses within each of the three 6 m diameter HTS global coils of a 2 MW generator would be 285 W. The thermal load for the cryogenic system of the 2 MW generator is estimated to total 936 W, with the majority (90%) due to AC loss. Assuming a cryogenic specific power of 20, the energy required to cool the 2 MW generator represents less than 1% of total output.