Turbo-Brayton Refrigerator of Yokohama HTS Cable Project

The operational stability and long term reliability of a High Temperature Superconducting (HTS) cable system combined with a liquid nitrogen cooling system was verified in the demonstration test performed in a real grid. The test was successfully implemented from October 29, 2012 till December 25, 2013. The HTS cable system reliability has been verified in the test, but some problems related to the refrigerators have also been revealed. To handle some problems we develop a refrigerator with a high efficiency and a long maintenance interval. Target characteristics we set are a maintenance interval of 30,000 hours, a COP of 0.1 and a cooling capacity of 5 kW. These target values were achieved using a reverse Brayton cycle and high performance turbo compressor and turbo expander units. Our cooling system utilizing this refrigerator has proved to be stable for long operation. This Turbo-Brayton refrigerator Neon gas was chosen as coolant based on its density and speed of sound. This refrigerator includes a three stage turbo compressor and a turbo expander. The heat of compression is removed by water through a heat exchanger. The heat exchanger for regeneration is set at the front of the turbo expander, while the heat exchanger for heat exchanging between neon gas and liquid nitrogen is located at the back of the expander. The mass flow rate of neon gas is controlled via a buffer tank to maintain the pressure of the system at the rated value especially when system operation is started or stopped. In second phase of Yokohama project, we installed the cooling system including Turbo-Brayton refrigerator in TEPCO's network, Asahi S/S, and its demonstration test have started from March 31, 2017. The demonstration test will be continued for one year. Progress of the demonstration test is possible to review the web site of SEI, http://www.sei.co.jp/super/cable/jissho.html. The Turbo-Brayton refiregetaror we developed and project progress of which are discussed in detail in this presentation.