Performance analysis of a nitrogen-based Brayton cryocooler prototype

When very low temperatures are needed for industrial applications, reverse Brayton cryocoolers can be adopted. This paper reports the results of an energy analysis in which the performance of a Brayton cryocooler prototype was studied. The prototype is innovative in both the cycle configuration and the pressure and temperature levels. Moreover, nitrogen, an eco-friendly gas that is safe for people, was used as the working fluid. Simultaneous measurements of the pressure and temperature at the inlet and outlet of the main thermodynamic cycle components, nitrogen flow rate, and power consumption were taken during the experimental tests. The prototype was tested at design operating conditions (maximum and minimum pressure of 18.5 and 8 bar respectively, and minimum temperature of −120 °C), obtaining a cooling effect of 15.6 kW, a temperature reduction rate at the turbine outlet of 8 °C min−1, and a coefficient of performance of 0.29, which rises to 1.34 when including the waste heat (about 55 kW) that can be recovered at low temperatures (<100 °C). Also, a sensitivity analysis was carried out by testing the prototype at different maximum pressure and minimum temperature levels. The higher the maximum pressure, the higher the prototype performance is, and a minimum temperature of about −140 °C was reached. Our findings demonstrated that the tested prototype shows great promise for several industrial applications where low temperatures are required.