Ultra High-Field, High-Efficiency Superconducting Machines for Offshore Wind Turbines

Abstract

Offshore wind has gained traction worldwide as an alternative to fossil-fuel based power plants. Coastal US states have started incentivizing the development of offshore wind farms, with at least 24,135 MW currently planned for deployment. To make offshore wind more cost-competitive, major turbine OEMs like GE, Siemens, and MHI Vestas have been steadily increasing the power produced per turbine from 1.6 MW in 2000 to 12 MW in 2020. Increasing the turbine power reduces balance-of-plant costs, increases electromagnetic efficiency, and boosts capacity factor, leading to lower levelized cost of electricity (LCOE) for power purchasers [1]. However, even at 12 MW per turbine, the cost of shipping, installing, and maintaining thousands of wind turbines makes the price of electricity prohibitively expensive. Traditional permanent-magnet turbine technologies make it pointless to increase the power rating beyond 15 MW since there size and weight are significantly increases their installation cost. Low temperature superconducting (SC) generators were explored for a long time for high-power application where volume and wight requirements are strict [2]-[3]. Recent technology development in lowe temperature and high temperature SC cables renewed SC machines interest in wind turbine applications [3]-[4].Achieving ambitious goals of harvesting 24,135 MW of offshore wind energy hinges on the successful development of superconducting generator technology. Superconducting generators can produce extremely high specific power and run at higher efficiency than commercial, permanent-magnet generators because they can produce up to 10X more magnetic flux density which reduces the weight significantly. This shows that with 10X power per turbine is possible at same PM generator specification. To demonstrate these benefits, a 12 MW superconducting wind turbine has been investigated for 3 different generator topologies: fully active, hybrid and fully passive. 12 MW power was chosen instead of the theoretical limit of 120MW to shows that partial SC machines can outperform the best commercial 12 MW turbines while maintaining a 10T flux densities which have already been demonstrated in MRI applications.Low temperature NB3Sn [2] cables and high temperature VIPER [3] cables are analyzed for field windings with Litz wire armature windings. A multi objective optimization is performed on machine designs to achieve less than 60 Ton machine weight while reaching greater than 98% efficiency. Two studies are conducted at outer diameter (OD) of 4.2 m and 7.2 m to analyze the reduction in LCOE due to transportation and installation cost. Fig. 1 shows the obtained Pareto-front for motor active weight Vs motor efficiency at 7.2 m OD. Results shows that targeted weight and efficiency can be achieved at ultra-high-field designs. The best designs have 10-12 T peak field at field windings and 3-5 T peak field at airgap. The optimization performed here equally weighted the mass and efficiency of the generator to reach the final design. However, LCOE is sensitive to superconductor length than the mass and efficiency-related cost penalties. This will be considered to further refine design optimization study presented here. The thermal design studies will also be refined using temperature-dependent thermal conductivities ranging all the way from room temperature to the proposed 4K operating temperature. Along the same lines, there is an outstanding need to identify cryocoolers which can reach 4K while satisfying the heat load requirements from ambient conduction and radiation. Upon completion of the electromagnetic, thermal, and mechanical design, the weight estimates for the generator will be refined to include both the generator and its supporting systems for comparison with the weight of commercial turbines. This is an important research study which also translates to a concrete value proposition for the feasibility of SC offshore wind turbines.An integration scheme was explored by mounting the proposed generator inside of a commercial turbine with the associated aerodynamic, mechanical, electrical, and thermal systems needed for operation in an offshore wind farm. Fig. 2 shows the cross-section and full turbine structure and the detail winding arrangements of a selected 7.2 m OD design. The proposed turbine would output 200% of the rated power for a commercial turbine of the same size. As it can be seen in the figure, the volume of the turbine nacelle is not fully occupied, leaving additional room to increase the turbine power even further with larger-diameter SC machines. In conclusion, this paper will produce electromagnetic, thermal, mechanical and cost analysis to support the feasibility of using ultra-high-field superconducting generators for offshore wind applications. **