A novel propane – CO2 refrigeration system for mobile insulated boxes in last mile delivery

The transport refrigeration industry predominantly relies on mechanical vapor compression systems, which still use refrigerants with high environmental impact, for both long and short distance transportation. In short distance application, the integration of thermal energy storage (TES) and the use of natural refrigerants represent a promising path in reducing the environmental impact of the sector. This paper presents an innovative indirect expansion refrigeration system utilizing natural refrigerants (R290 and R744) to freeze eutectic plates, located inside an insulated box, designed for last-mile delivery of frozen food. A numerical model is developed and first validated against experimental data available for the baseline R452A solution, achieving a -1.2% error on the estimated energy consumption over the entire pulldown. When considering the new system, numerical simulations of a 36-hours pulldown show that, despite delivering the same cooling energy to the eutectic plates (-0.6 % compared to the baseline), the R290-R744 system can achieve a better distribution of the cooling effect in the eutectic plates, avoiding subcooling. During a 36-hours pulldown, the R290-R744 can perform with the same overall COP (0.79). However, on an annual basis, the use of natural refrigerants instead of synthetics allows achieving a reduction of the system overall carbon footprint equal to -69.6%.