Temperature drop of heating fluid as a primary condition for effective utilization of low-grade heat using flash cycles and zeotropic mixtures in refrigeration ejector systems

The conventional refrigeration ejector cycle utilizes a constant temperature process during motive heat consumption needed for vapor generation. This may be thought of as the main restriction concerning the application of low-grade heat as a motive source by these systems since it results in required significant mass flow rate with a small temperature drop of heating fluid. The first proposed novel approach is an ejector flash cycle (EFC) in which the working fluid is heated up to the saturation temperature only at the inlet to the primary nozzle of the ejector. The second possible approach is the application of a zeotropic mixture as the working fluid for the conventional ejector cycle. The performance of an EFC is compared to the ejector refrigeration cycle (ERC) working with zeotropic refrigerant. Three motive heat source temperatures (60 degrees C, 75 degrees C, 90 degrees C) and two evaporation temperatures (0 degrees C, 6 degrees C) were tested. Eight pure low-GWP refrigerants and four low-GWP refrigerant blends were analyzed. It was demonstrated that the flash cycle can efficiently operate with low-temperature motive heat source (60 degrees C) with refrigerants R1234yf and R1234ze(E). The highest coefficient of performance (COP) is attainable for propane with a motive source temperature of 60 degrees C and R1234ze(Z) for the motive temperature of 75 degrees C and 90 degrees C. The differences of liquid pump power consumption of both discussed cycles compared to the conventional compression cycle are significant and reach up to 90 percent in the case of EFC and up to 96 percent in the case of the zeotropic cycle. The electric power savings in the case of the zeotropic cycle are slightly higher, but COP and drop of the heating water temperature are notably lower. Therefore, the EFC system is suggested to be a better solution for low-grade heat utilization in the cases when a significant decrease of the motive source fluid temperature is required.