Efficient cryogenic cooling method for rotating HTS applications; stationary or rotating cryocooler

Abstract

This presentation will address a unique cooling strategy for rotating HTS systems that may revolutionize the adoption of electrified aircraft with superconducting motors and generators. HTS field winding in superconducting rotor typically needs to be cooled around 50 K which is an appropriate cooling target temperature of compact Stirling cryocooler or Stirling-type pulse tube refrigerator. The size of the whole cryogenic cooling system for the rotor can be further reduced if the two-stage instead of single stage cooling method is employed with EGM (Entropy Generation Minimization). Several cooling methodologies are reviewed and discussed for rotating superconducting rotors of previous developments. The active heat sink part of the cryocooler can be located at the stationary frame or the rotating frame. With the stationary cryocooler, cryogenic fluid transfer lines and transfer coupling devices have been often used but significantly add parasitic heat ingress to the overall cryogenic load of the superconducting rotor refrigeration system. Possible elimination of the sophisticated fluid transfer coupling, therefore, should not only reduce the total cryogenic cooling load but also enhance the system reliability in a harsh environment. In many cases, the amount of actual cooling load is insignificant unless the superconducting coil is operating in a ramping or ac (alternating current) mode. The virtual cooling load which originates from the parasitic thermal conduction or radiation heat leak from room-temperature environment frequently becomes the dominant factor to determine the size of the cryocooler. If the magnitude of heat generation and parasitic heat leak is manageable, a two-stage Stirling cryocooler or Stirling-type pulse tube refrigerator shall be readily applicable for cooling superconducting rotor as a compact rotating configuration. The first stage of the cooler is primarily used for thermal anchoring and the second stage is solely dedicated for cooling HTS coil in the innovative on-board rotating cryocooler concept. The 1 MW class superconducting motor for electric airplane can be cooled by a two-stage Stirling cryocooler with the cooling capacity of 10 W at 50 K and 50 W at 100 K. This kind of manufactured Stirling cooler mass can be under 20 kg. For the efficient compact cooling system, the temperature difference between the cryocooler cold head and the superconducting field winding should be minimized by conduction-cooling, which will ultimately reduce the entropy load of cryocooler and its size as a result. The ambitious power density of 20 kW/kg for electrically propelled aircraft can be only achieved by integrating a small cryocooler in the superconducting rotor as a compact structure. As we demonstrate in this presentation, a strong collaboration of cryocooler developer and superconductivity community shall be able to result in truly competitive and decisive applications of superconducting motor and generator. An on-board rotating cryocooler enables a superconducting rotor to be invisible in high-power density electric motor or generator. As a system level approach, this feature is particularly important and provides a significant merit by hiding the cryogenic refrigerator because it will increase the possibility of accepting superconducting machines that people may consider too complex and unreliable to be useful.