Advanced Refrigeration Systems for Cryogenic Applications

R. Radebaugh, P.E. Bradley, E.D. Marquardt, M.A. Lewis, and J.D. Siegwarth; I. Ruelich and H. Quack (Technical Univ. of Dresden); M. Hill (Univ. of Colorado); and J. Gary and A. O'Gallagher (891)

Objective: To use measurement and modeling techniques for evaluating and improving performance of cryocooler components, such as heat exchangers and pulse tubes and to develop new and improved refrigeration and heat transfer processes for the temperature range below about 230 K.

Problem: Cryocoolers are required for many technology areas, including the cooling of infrared sensors for surveillance and atmospheric studies, the cooling of superconducting electronics and magnets, the cooling of cryopumps for clean vacuums in semiconductor fabrication processes, and the liquefaction of natural gas. The use of these technologies has been hampered because of problems with existing cryocoolers. These problems include short lifetimes, inefficiency, high cost, and excessive vibration. Improved cryocoolers would stimulate the growth of all these technology areas. Proper measurements need to be identified that will characterize losses within these cryocoolers, and models need to be developed to optimize the design of such systems.

Approach: Precision moving parts in existing cryocoolers are a source of wear, vibration, and high cost. Our approach in the development of improved refrigeration processes has been to focus our measurements and modeling on processes that eliminate most, or all, moving parts while still maintaining high efficiency. Much of our research has been on pulse tube refrigerators, which have no cold moving parts. Our studies encompass measurements and modeling of losses to further improve efficiencies of these cryocoolers while increasing their lifetimes. NIST research in this area has much industry and other government agency support to aid in the transfer of this technology to industry.

Results and Future Plans: During FY99, measurements were made on the thermal conductance of packed stainless steel spheres. The thermal conductance degradation factor was found to be 0.11, in good agreement with the value of 0.10 for stainless steel screen. From measurements at various helium filling pressures we have determined that most of the heat is transported by the helium gas a distance of about 4 µm across the boundary rather than by the direct metallic contact. Almost no prior data existed for the thermal conductances of these packed materials, but they are needed for the optimum design of regenerators in many types of cryocoolers. A new in situ measurement technique was developed this past year that gave nearly the same result for a regenerator with packed stainless steel screen.

A pulse tube liquefier prototype was completed for NASA/Johnson as part of a program for developing the technology of liquefying oxygen on Mars. NASA expects to have a satellite sent to Mars in the year 2007 that would convert the carbon dioxide atmosphere of Mars into oxygen and then be liquefied using our pulse tube liquefier technology. After two years, a sufficient quantity of liquid oxygen would be collected to fire rockets for lifting off of Mars and returning to Earth with Mars rock samples. Extensive measurements with this liquefier prototype were completed and the results were used to update our models of pulse tube refrigerators. Several types of losses were identified and measured to explain the very high efficiency. The Carnot efficiency of 20 % with respect to input PV power is among the best ever achieved in a cryocooler of this size. This excellent performance indicates the power of the NIST modeling and optimization tools to advance cryocooler technology.