Comparison of CO ₂ and R134a Two-Phase Ejector Performance for Use in Automotive Air Conditioning Applications

Two-phase ejectors are devices capable of recovering the expansion power that is lost by the throttling process in air conditioning cycles, resulting in improved system performance. High-pressure fluids such as CO₂ have received the majority of attention in two-phase ejector studies in recent years due to the fluid's high throttling loss and high potential for improvement. However, low-pressure working fluids such as R134a, commonly used in automotive applications, have received considerably less attention owing to their lower recovery potential. While the two fluids have very different properties, both offer the potential for noticeable COP improvement with ejector cycles. Thus, understanding the operation and performance of ejectors with both fluids can be important to the design of ejector air conditioning cycles.

This paper compares available experimental data for the performance of two-phase ejectors using CO₂ and R134a. CO₂ ejectors are capable of recovering a greater amount of power than R134a due to CO₂'s larger throttling loss as well as the ability of CO₂ ejectors to recover a larger portion of the available power. Possible explanations for the difference in ejector performance between the two fluids are discussed. The actual amount of expansion power recovered by ejectors with the two fluids is used to obtain realistic estimates of the COP improvement that ejector cycles with the two fluids can achieve. These estimates show good agreement with the reported actual COP improvements of two-phase ejector cycles.