Experimental and Analytical Investigation of Automotive Ejector Air-Conditioning Cycles Using Low-Pressure Refrigerants

In recent years, ejectors have received much attention because of their ability to reduce throttling losses and increase the efficiency of stationary and mobile air-conditioning systems. While much of the initial research was carried out with high pressure fluids, such as carbon dioxide, it was soon discovered that ejectors can also offer significant advantages in systems that utilize low pressure working fluids. Because throttling losses are generally less significant for low pressure refrigerants, the dominant improvement mechanism in these systems comes mostly from the system’s low pressure side, where reduced mass flow rates and higher suction pressures cause less pressure drop. Systems with microchannel evaporators can benefit the most because using an ejector can significantly reduce refrigerant mal-distribution and yield better thermodynamic utilization of the available surface areas. This paper presents experimental and numerical results obtained with a realistic air-conditioning system for a small size vehicle. Numerical simulations with R134a and R1234yf were performed to determine the effect that inefficiency of the liquid-vapor separator in the standard two-phase ejector cycle could have on the performance of the cycle. The results show that a realistic amount of separator inefficiency can have a significant impact on the performance of the two-phase ejector cycle, and that R1234yf gains 1 to 3 % greater benefit from the ejector than R134a but is impacted more severely by separator inefficiency. A two-phase ejector system concept was realized in which evaporation occurred on two different temperature levels without the use of a liquid-vapor separator. Better matched temperature glides between air and refrigerant streams result, which drives the reduction of exergetic losses and further increases system efficiency. Conventional and ejector system tests were carried out with R134a refrigerant. The ejector cycle used was able to improve the COP by as much as 10 % over a conventional cycle. However, this relatively large improvement is partly due to the fact that the cycle setup and operating conditions were slightly more favorable to the ejector cycle.