Thermodynamic modelling of humid-air expansion within an R717/R729 cascade refrigeration system for cold store applications.

This work focuses on the thermodynamic behaviour of the humid air from a refrigerated space as the working fluid of a cascade refrigeration system with an inverse Brayton cycle as the bottom sub-cycle. The integration of this technology with a conventional NH3 compression cycle for a cold store application overcomes some of their respective drawbacks. The cold heat exchanger is replaced by an inlet port delivering the cold air from the turbine and the heat load due to the air fan is therefore eliminated. The effect of temperature and pressure drop within the turbine brings the fluid to the vapour content saturation and ice formation. In this study, the fluid’s transformation is analyzed as a series of equilibrium states for a realistic estimation of the outlet air conditions. Finally, a first screening of the relevant design parameters for an overall system optimization is presented, showing that the efficiency gap of the Brayton cycle can be reduced in this cascade configuration.