Performances of Stirling Type Pulse Tube Cryocooler for Sub‐cooled Nitrogen System Available to Purchase

This paper describes performances of a Stirling type pulse tube cryocooler as a nitrogen sub‐cooler. The main objective of this work is a demonstration of a cooling system for High Tc Superconducting (HTS) power applications such as fault current limiters, cables and transformers. Cooling capacity necessary for these applications is more than several hundred watts at 77 K level. High efficiency and high reliability are also required. Developments of several hundred watts class compact cryocooler such as Stirling‐type pulse tube cryocooler are expected for the HTS power applications. A liner‐motor‐driven compressor (model:2S241K) and coldhead, which were developed by CFIC Inc., were used in our system to evaluate efficiency and reliability for our application. A cooling stage of the cold head was designed as a heat exchanger to cool a sub‐cooled nitrogen flow to reduce a temperature difference between working gas of cryocooler (helium) and the sub‐cooled nitrogen. A cooling capacity of 207 W at 77 K was obtained with an input power of 4.33 kW. The Carnot efficiency was 13.8 %, which is about 1.7 times higher than that of commercial GM cryocooler (for reference). The long‐term operation over 0.5 year (4414 hours) without performance degradation was obtained.