Mitigation of Effects of Pulsed Heat Loads in Helium Refrigerators for Fusion Devices Using Supercritical Helium Storage

Supercritical fluid is increasingly getting attention for storing mass and energy in energy systems. In a fusion device such as a tokamak, which is one of the futuristic energy generation systems, the heat loads generated from superconducting magnets and nuclear heating are pulsating in nature. Large-scale helium refrigerators are employed for cooling of these magnets. The immediate effect of pulsed load in such refrigerators is the fluctuation of the return stream mass flow rate, which needs to be mitigated to an acceptable level, to avoid instability in plant operation and to reduce loss of refrigeration. In this paper, a novel technique to mitigate flow fluctuation is suggested, where a part of high-pressure helium at supercritical state from the inlet of a Joule-Thomson valve of a modified Claude-cycle-based refrigerator is stored in a storage vessel during high heat load condition and released during low load condition. Through thermodynamic analysis, the advantages and concept of supercritical storage for mitigation of mass fluctuation have been discussed. Verification of the concept through dynamic simulation in Aspen HYSYS has revealed that mitigations of about 78% and 74% of fluctuations in return stream during high and low heat load conditions, respectively, could be achieved by utilizing the scheme. Advantages and feasibility of this scheme vis-a-vis helium inventory, buffer size, and control for its implementation are discussed.