The pace of advancement in electric machine technology in the multi-megawatt scale has not kept up in recent years with the large strides made in smaller machines and in adjacent areas like power electronics. Rapidly advancing enabling technologies and exciting new applications offer the opportunity for potentially transformational electric machine designs. Superconducting machines, for example, can lead to a step change in power density and efficiency over conventional designs, making them attractive for applications ranging from renewable energy generation to transportation. Most superconducting machines developed to-date are partially superconducting, with either the stator or the rotor made with conventional (copper or aluminum) conductors. Fully superconducting machines can further improve specific power if the increased losses at cryogenic temperatures can be accommodated. The losses introduced in fully superconducting machines are a strong function of the electrical frequency, and are particularly difficult to manage. This project is focused on a machine concept that can retain high power density at relatively low frequencies, making fully superconducting designs more practical. A lightweight direct-drive generator that can help reduce the cost of electricity from offshore wind will be designed and risk reduced. The project is also expected to help reinvigorate research on large electrical machines in US schools, and train the next generation of electrical machine experts to support the growing 'electrification' trend in the industry.
A fully superconducting generator employing active shielding for field containment and enhancement will be pursued. Significantly higher electromagnetic shear force can now be obtained in the airgap, leading to substantially higher torque to weight ratio. The active shielding concept has been applied to relatively high speed, high frequency aerospace machines, with significant weight reduction. At the frequencies encountered in the aerospace application, 'ac' losses in the armature winding would be prohibitively large, and thus only partially superconducting machines with just 'dc' superconducting excitation coils have been considered thus far. In this project, superconducting coils will be utilized in the armature winding as well. An iron core has been employed in such machines in the past, with relatively modest flux densities experienced by the superconductor, leading to manageable 'ac' losses. Maintaining the high flux densities that can be obtained with the shielded 'air-core' topology and accommodating the resulting 'ac' losses will be challenging, but key to obtaining the promised reduction in size and weight of the generators. Preliminary studies indicate that the mitigating effects of the ultra-low frequency can compensate for the increased field, enabling a significant reduction in tower top weight (5-10X compared to state-of-the-art). Detailed multi-physics models incorporating electromagnetic, cryogenic, mechanical and materials interactions used in the partially superconducting machine projects will be extended to include physics-based 'ac' loss computations in the superconducting windings. A scalable (10-50 MW) fully superconducting machine will then be designed and used in system level assessment.
This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.
