Determination of the optimum heat rejection pressure in transcritical cycles working with R744/R290 mixture

The simulation and experimental investigations have been carried out to research the relationships between optimum heat rejection pressure and related operating parameters for a transcritical system using R744/R290 mixture as a refrigerant. The system is characterised by two expansion valves in series in order to conveniently control the heat rejection pressure. A cycle simulation code has been developed, and the simulation results reveal that for a transcritical cycle working with R744/R290 mixture in which the R744 mass fraction is greater than 0.78, there is an optimal heat rejection pressure, under which a maximum system coefficient of performance (COP) can be reached. Furthermore, the researches in this work show that the optimal heat rejection pressure mainly depends on the mass fraction, the outlet refrigerant temperature of the gas cooler, the evaporation temperature whereas the superheat degree and the performance of the given compressor have weak effects on the optimum heat rejection pressure. Based on the orthogonal experimental simulation data, a correlation of the optimal heat rejection pressure with respect to mainly involved parameters is obtained under the specific conditions. The experimental data of optimal heat rejection pressure are in a relatively good agreement with the simulation results.

► A cycle simulation code for a transcritical R744/R290 system is developed.
► A correlation of the optimum heat rejection pressure is obtained.
► The optimum pressure mainly depends on mass fraction and gas cooler outlet mixture temperature.
► The evaporation temperature influences the optimum pressure to marginally.
► The superheat degree, compressor performance have weak effects on optimum pressure.