Design and operation of a robust cryostat for commercial superconducting energy storage systems

Cryogenic systems for commercial superconducting magnetic energy storage systems must operate unattended at remote locations for extended periods of time. The magnet cryostat is a central element of these cryogenic systems. Superconductivity, Inc.’s (SI’s) energy storage system, called the SSD®, is also designed to be mobile so the cryostat must be able to support the magnet against dynamic transportation loading in addition to static and seismic loads. This paper reports on the design and operation of SI’s newest cryostat, which contains the superconducting energy storage magnet, current leads and liquid helium inventory. During normal operation the cryostat functions as part of a closed system connected to a helium refrigerator-liquefier. The refrigeration system collects and reliquefies helium boiloff and transfers the liquid back to the cryostat to maintain a constant liquid level. Due to its low heat leak and relatively large helium inventory the cryostat is capable of maintaining operation of the superconducting energy storage magnet for 55 to 60 hours in open cycle mode without refrigerator-liquefier assistance. This allows the energy storage system to continue to operate during planned semi-annual refrigerator maintenance and in the event of an unscheduled refrigerator outage. This has significantly improved overall system reliability.