The cryogenic system plays a vital role in the development of superconductivity. In a sense, superconductivity would be in more widespread use now if it were not for the problems associated with the cryocoolers needed to cool the superconducting devices or facilities. For a variety of high-Tc superconducting applications such as transformers, fault current limiters, motors, generators, power cables and synchronous compensators, the technology itself is relatively mature. However, the problems associated with the used cryocoolers have hampered the advancement of their practical applications. An ideal cryocooler for the applications should have the following features: low maintenance, high reliability, long operation life, high capacity and high thermodynamic efficiency.
In the authors’ laboratory, a long-life, high-capacity and high-efficiency cryogenic system based on the Stirling-type pulse tube cryocooler (SPTC) is being developing. The pulse tube cryocooler (PTC) without any moving component at the cold end has the intrinsic merits of long life at the coldhead, and the SPTC driven by the linear compressor also achieves the high reliability at the warm end. The developing SPTC is inherited from a series of ones developed for aerospace applications and thus keeps the merits of high reliability and long life. Its mean-time-to-failure (MTTF) can reach 10 years, which is over ten times the average MTTF of most existing cryocoolers for the similar applications. Another formidable challenge for the aimed applications is often from the required huge cooling powers. The developing high-capacity SPTC can achieve 1.1 kW of cooling powers at 77 K for each unit. And multiple units can also be combined together to provide more than 20 kW at 77 K. Another advantage of the cryogenic system is that it can vary freely between 20 K and 90 K. The SPTC has also achieved the high efficiency with a relative COP of 20% of Carnot at 77 K.
The application background and design approaches will be described and the performance characteristics of the developed cryogenic system presented and discussed.