2kW급 Reverse Brayton 극저온 냉동기의 시스템 성능 특성에 관한 실험적 연구

Abstract

With increased commercialization of high-temperature superconducting(HTS) power cables cooled using liquid nitrogen and the use of liquefied natural gas as fuel, the need for large-capacity refrigeration systems is gradually increasing. Among the many refrigeration system, there is a significant interest in reverse Brayton refrigeration system, which have demonstrated excellent energy efficiency and cooling power while requiring less installation space and long maintenance cycles. Hence, significant research and development on the process and thermodynamics of reverse Brayton refrigeration system has been carried out, but experimental research for reverse Brayton refrigeration system is more required. In this paper, the effect of main components and pressures on the performance and exergy characteristics of reverse Brayton refrigeration system with neon as a refrigerant was analyzed by using the HYSYS software. as a result, The effectiveness of heat exchanger is most important parameter to increase performance and exergy efficiency. Based on the performance and exergy efficiency analysis, the thermodynamic design of a reverse Brayton refrigeration system with a cooling power of 2 kW class at 77 K has been performed. The proposed refrigeration system uses a cryogenic turbo-expander, scroll compressor, and plate-type heat exchanger unlike conventional reverse Brayton refrigeration system. This study also described the performance test conducted on the fabricated system. When operating the 77 K for target temperature, it took 5.5 hours to cool down due to the thermal mass of the plate-type heat exchangers. The isentropic efficiency of the cryogenic turbo-expander was measured to be 86 %, which is higher than the design specification. The effectiveness of the plate-type heat exchanger and the flow rate and operating pressure of the refrigerant were found to be lower than the design specification. As a results of comparing the performance of the reverse Brayton refrigeration system while changing the target temperature from 77 K to 120 K, it could be confirmed that the system is operated stably, the refrigerant flow rate is decreased, the inlet pressure of cryogenic turbo expander is decreased by about 2 ~ 3%, and the buffer tank pressure is increased. The plate-type heat exchanger effectiveness of high pressure is decreased and the plate-type heat exchanger effectiveness of low pressure is increased according to the target temperature increasing. Consequently, the cooling power of the fabricated reverse Brayton refrigeration system was measured to be 1.23 kW at 77 K and 1.64 kW at 110K. In the future, we expect to achieve the targeted cooling power through further improvements of the scroll compressor and plate-type heat exchanger. In addition, faster commercialization of HTS power cables and more efficient storage of liquefied natural gas will be realized.