Most of the low temperature systems such as superconducting magnets require the efficient cooling system. The irreversible loss due to the entropy generation at the low temperature must be minimized to overcome this requirement. The concept of exergy gives a useful method to know where is the main source of this entropy generation. This paper describes the fundamentals of exergy analysis for cryocoolers or simple cryogenic systems. It includes the definition of exergy, exergy loss and thermodynamic efficiency of the system. Several examples of calculation method for the simple cryogenic system are also given.
This paper describes a new calorimetric method for AC loss measurement of superconductors. In this method, AC losses of superconductors can be detected as the capacity change. The bubbles generated by the AC losses move into the parallel electrodes and cause the capacity change of it, because the dielectric constant of helium gas is different from that of liquid helium. Then this capacity change is converted to the frequency change by a RC-oscillation circuit. Consequently, AC losses can be calculated by the frequency change, which is calibrated by a reference heater. This measurement system has some advantages, such as the shorter measuring time compared with that of other calorimetric ones and the less affection by the recondensation of bubbles. The resolution of this measurement is 4mW, however, it can be improved by the optimization of the electrode structure and the application of higher oscillation frequencies.