Thermodynamic Analysis of Four Magnetic Heat-Pump Cycles

Magnetic heat pumps have been successfully used for refrigeration applications at near absolute-zero-degree temperatures. In these applications, a temperature lift of a few degrees in a cryogenic environment is sufficient and can be easily achieved by a simple magnetic heat-pump cycle. To extend magnetic heat pumping to other temperature ranges and other types of application in which the temperature lift is more than just a few degrees requires more involved cycle processes. The possible cycle applications include cooling of superconducting transmission lines, space conditioning, and industrial heating. This paper investigates the characteristics of a few better-known thermomagnetic heat-pump cycles (Carnot, Ericsson, Stirling, and regenerative) in extended ranges of temperature lift. The regenerative cycle is the most efficient one. Cycle analyses were done for gadolinium operating between 0 and 7 Tesla, and with a heat-rejection temperature of 320 K. The analysis results predicted a 42 percent reduction in coefficient of performance at 260 K cooling temperature and a 15 percent reduction in capacity at 232 K cooling temperature for the magnetic Ericsson cycle as compared with the ideal regenerative cycle. Such substantial penalties indicate that the potential irreversibilities from this one source may adversely affect the viability of certain proposed MHP concepts if the relevant loss mechanisms are not adequately addressed.