Development of stirling-type pulse tube refrigerator (PTR) with superconducting cold linear compressor (SCLC)

Abstract

This dissertation proposes a ‘cold compressor, colder expander’ concept. As a proof of the concept, a Stirling-type pulse tube refrigerator (PTR) is developed and tested in conjunction with two types of linear compressor (made of normal metal and superconductor, respectively) that produce a mass flow at ‘cold state’. The ‘cold state’ means colder than atmospheric temperature below 120 K. The mass flow generated at the ‘cold state’ by the cold compressor can be directly fed to a refrigeration system without undergoing an inevitable precooling process and it rejects compression heat at the ‘cold state’, simultaneously. This concept extends the temperature range of the refrigeration system and can readily create a ‘colder state’. In this dissertation, as the first step, the performance of the PTR system is estimated by the numerical model which considers the dynamic characteristics of the linear compressor with thermo-hydraulic governing equations for each sub component of the PTR. The experimental validation as a proof of the concept is carried out to demonstrate the capability of the PTR operating between 80 K and 20 K. For the linear compressor which is submerged in liquid nitrogen ( $LN_2$ ), the electrical to mechanical work conversion efficiency is attained to exceed 85% for all the relevant frequency domain. The no-load temperature of 18.7 K is recorded and the nominal heat lift of 0.4 W is measured at the cold-end temperature of 20 K. This PTR including the linear compressor should be optimized and it is expected to produce higher thermal performance than the current PTR does. This dissertation also presents further parametric studies on the PTR system with the pre-developed numerical model. Next, the experimental validation of the superconducting cold linear compressor (SCLC) unit as an innovative concept is carried out for the pre-developed Stirling-type PTR system above. The SCLC stably operates without creating over-heat problem under $8.5 A_{rms}$. The compression efficiency is attained to be 70%, and this value shows lower than that of the non-superconducting linear compressor above. The loss mechanisms in the SCLC unit are furthermore investigated and it is found that the AC loss in the cooling structure for the HTS winding in the SCLC occupies over 50% of the total SCLC loss. The key features of this dissertation are summarized as follows

(i) Development of a Stirling-type PTR system (including a cold linear compressor) - Constructing and demonstrating of the Stirling-type PTR system - Understanding major loss mechanisms in the Stirling-type PTR system - Establishing a design methodology of the Stirling-type PTR system with reflecting dynamic characteristics of the cold linear compressor - Parametric studies on the Stirling-type PTR system with the pre-mentioned design mythology (ii) Development of a SCLC unit with a high temperature superconductor (HTS) coil - Constructing the SCLC unit - Integrating and demonstrating the SCLC unit with the pre-developed Stirling-type PTR - Understanding major loss mechanisms in the SCLC unit