Integration of a two-phase ejector into a compact, lightweight unitary-type air-conditioner using R744 for energy efficient operation in hot climates.

Transcritical R744 technology was used to design a compact, lightweight unitary-type air-conditioner for military shelter cooling. In the past, U.S. Army has funded a series of research projects aimed at converting existing Environmental Control Units (ECUs) that use H/C/FC refrigerants to transcritical R744. However, for the nominal cooling capacity of 35 kW no baseline system that uses a conventional refrigerant exists. Therefore, a trailer-mounted R744 ECU air-conditioner had to be designed and built from ground up. This made it possible to focus the design considerations on achieving high energy efficiencies during operation in hot ambient temperatures. R744 ECU systems show large improvement potentials due their use at very high ambient conditions that are generally difficult for the transcritical cycle. Throttling losses are large, creating an ideal application for the use of expanders or ejectors. The performance, and in particular the energy efficiency of the system was further improved by implementing a two-phase ejector designed for use in transcritical R744 systems. While two-phase ejector technology has previously been used in a variety of different mobile and stationary applications, the present study utilizes a transcritical R744 two-phase ejector that is among the largest ever reported in the open literature. Up to 7% higher COPs can be achieved in comparison to the baseline case with expansion valve. Another important finding of the study points towards the importance of the vapor-liquid separator that is required in the standard two-phase ejector cycle. Less than perfect vapor and liquid separation efficiencies can have dramatic impact on the performance of the ejector cycle. Therefore, the subject has been studied in more detail and different separator designs and separation approaches that yield high separation efficiencies are presented.