Thermodynamic analysis of cryogenic vacuum cooling of superconducting cables

This paper presents a thermodynamic analysis of cryogenic vacuum cooling of superconducting cables in a specific practical application to achieve the characteristic critical temperature where the electrical resistance of the superconducting cables becomes theoretically zero, which results in no heat generation. The characteristic critical temperature (where the electrical resistance of the superconductor becomes zero) of the superconducting cable used in the project is around -200 degrees C and it is operating with only 6% losses. To reach the necessary temperature liquid nitrogen is used in the system, the refrigerant is cooled with the use of a vacuum chamber pumped by a vacuum pump to decrease the boiling point of the liquid nitrogen (-196 degrees C at 1 atm). The vacuum chamber is kept at -209 degrees C, which is one degree less than the freezing point of liquid nitrogen (-210 degrees C at 1 atm). The refrigeration system is analyzed in terms of energy and exergy methodologies to evaluate the performance of the vacuum chamber. Also, the effects of various system parameters, such as ground depth, ambient temperature, wire diameter and ground type are examined. The energy and exergy efficiencies of the overall system are calculated as 79.37% and 66.14%, respectively.