Thermal Analysis and Design of a Cryogenic Cooling System for 10 MW Class High-Temperature Superconducting Generators for Wind Turbine

Abstract

A cooling system is an essential part of high-temperature superconducting (HTS) rotating machine manufacturing. Moreover, thermal behavior is a crucial parameter of the cooling system that shows unique characteristics of superconductivity below a specific temperature to maintain a superconducting state. Therefore, many experiments have been performed to investigate new reliable cryogenic cooling systems for a large-scale HTS rotating machine. The motivation for this thesis is the development of a cryogenic cooling system using thermal trigger switches; it effectively minimizes non-operational downtime of the HTS machine in cases of power supply or cryocooler failure. Hence, a HTS rotating machine will not be affected by this failure because it is going to be continuously supported by "thermal battery" using solid nitrogen within a period of time. This thesis focuses on two main targets. First, the thermal design of the cooling system for the 10 MW-class HTS synchronous generator (HTSSG) is enhanced to diminish thermal absorption from room temperature whenever the cryocooler fails. This step focuses on the most major losses of (1) generator rotor losses including conduction loss of current leads, conduction loss of torque tubes, radiation loss, superconducting losses (taking account on mechanical lap joint Joule heating loss and intrinsic n-value loss), and (2) conduction loss of system supporting rods and natural convection occurring in heat exchanger, as well. To get maximum elimination these losses, the numerical method was applied to optimize the design of a current lead and a torque tube, respectively. Finally, the possibility of constructing HTS field coil can be maintained a constant low temperature without an external power source or an external supply of cryogen is demonstrated. Hence, the performance of the cooling system is observed for various cryogens to investigate the feasibility of using solid cryogen as a "thermal battery". It will optimize total volume of solid cryogens to remain in the HTS field coils, which are kept in a cryostat, and are kept below a certain temperature as long as desired.