COMPARISON OF DIFFERENT REFRIGERANTS FOR A TWO-STAGE TRANSPORT REFRIGERATION SYSTEM

The single-stage vapour compression cycle with the synthetic refrigerants R-404A and R-410A is state of the art in transport refrigeration. Both named refrigerants have a high Global Warming Potential (GWP). Current research activities focus, amongst others, on new energy efficient vapour compression cycles and the application of low GWP-refrigerants. A two-stage vapour compression cycle with an internal heat exchanger at intermediate pressure is a widely used inter-stage system at low and medium evaporating temperatures in commercial refrigeration. Such a cycle leads to a considerable increase of the system's energy efficiency compared to a single-stage unit, especially for the refrigerant R-744 in combination with low temperature applications. Due to its thermodynamic properties, the natural refrigerant R-744 with GWP(100)=1 might be an eco-friendly and competitive alternative to the commonly used synthetic high-GWP-refrigerants R-404A and R-410A. In this paper, a numerical comparison of a R-410A and a R-744 two-stage vapour compression cycle with an internal heat exchanger at intermediate pressure for transport refrigeration is presented. The object oriented modelling language Modelica and the component library TIL are used to model the refrigerant cycle. This model is validated against previously published experimental data of a R-744 two-stage test unit for commercial transport refrigeration. Thermo-physical effects such as pressure drop and heat transfer in the different components of the refrigeration cycle are considered in the model by general correlations. The applied model predicts the measured cooling capacities and the Coefficients of Performance (COP) within an uncertainty of 4 and 8%, respectively. A simulative comparison of the R-410A and the R-744 two-stage vapour compression cycle is presented and discussed for two different applications: The COP of both refrigerants is compared for a constant cooling capacity of 15kW and varying ambient temperature. The average energy consumption per year of a transport refrigeration system in Germany, Spain and the USA is estimated for both refrigerants.