Modified Brayton refrigeration cycles for forced-flow cooling of HTS fusion system

•Brayton refrigeration cycles are investigated for forced-flow cooling of HTS fusion system.•The cycles are modified to meet the cooling requirements of magnets, thermal shield, and current leads.•Integrated refrigeration cycles with the circulation loop for cooling are newly proposed and optimized.•It is verified that a great thermodynamic performance can be achieved with modified Brayton cycles. A thermodynamic study is performed to identify the suitable refrigeration cycles for emerging application to HTS (high-temperature superconductors) fusion system. According to the recent reports on compact and efficient fusion system, the HTS magnets are supposed to operate around at 20 K by forced-flow cooling of helium gas. In addition to the main cryogenic load for magnets, there are other cooling requirements, including the refrigeration of thermal shield and current leads. In order to compose a closed refrigeration system without liquid-nitrogen supply or any boil-off loss, modified Brayton cycles are designed to cover the cooling loads with a circulation loop of coolant for magnets, thermal shield, and current leads. Innovative design is also proposed to integrate the cooling loop with the refrigeration cycle, as helium gas is used as coolant and refrigerant. A variety of modified cycles with forced-flow cooling are optimized through iterative analysis with process simulator (Aspen HYSYS) and the real-gas properties of helium. It is verified that the integrated refrigeration cycles with cooling loop could have a great merit in thermodynamic efficiency as well as the simpler operation without any cryogenic pumping device. It is also demonstrated that a single refrigeration cycle could be designed to fully cover all the cooling requirements for HTS fusion system.