Optimisation of a Cascade Refrigeration System with Natural Refrigerants, Based on Nature-Inspired Algorithms

Refrigeration at low temperatures is an important requirement for long storage duration where compression cascade systems can offer very efficient and eco-friendly solution opportunities in the context of the current environmental preoccupations, by using innocuous refrigerants pairs as working fluids. In the present work, energy- and exergy-based modelling is performed for fluid pairs of natural refrigerants with low or no GWP and ODP effects. R744/R717, R744/R290, R744/R600, R744/R600a, R744/R1270 pairs are considered, where the low-temperature loop (LT) is filled with carbon dioxide, while the high-temperature loop (HT) employs either ammonia, propane, butane, isobutene or propylene. Optimised system performance is assessed for the effect of temperature variations in the evaporator–condenser. To this aim, several optimisation methods are evaluated to best determine the evaporation temperature in the HT loop that maximises the system’s COP. With the suitable optimising method selected, optimal operating conditions are successively computed for each refrigerant pair, and the resulting performances are determined for condensation and evaporation temperatures in the respective ranges of 25–50 °C and − 55 to − 30 °C. It can be observed that for these conditions, the highest optimised performances are achieved with the R744/R1270 and R744/R717 pairs.