Trade-offs between design complexity and performance impacts for transcritical R744 ejectors

It is well known that the transcritical CO2 cycle has higher intrinsic throttling losses than comparable subcritical cycles. Replacing the expansion valve with a two-phase ejector greatly improves cycle performance by reducing throttling losses. However, ejectors have yet to be successfully applied to low-capacity systems (approximately 1 to 5 kW), such as bottle coolers, window air conditioners, beverage dispensers, heat pump water heaters or similar. It is a real challenge to integrate ejectors into mass produced systems of small capacity in a cost-effective manner due to the relatively complex design of an ejector in comparison to simpler expansion devices. In addition to presenting system level results obtained with a transcritical CO2 bottle cooler with ejector, another aim of this paper is to systematically determine the possibilities for cost reduction by identifying performance trade-offs of less complex ejector designs. In a first step, a simplified diffuser design was investigated in combination with a conventional motive nozzle having converging-diverging design. Resulting system COPs and the impact on ejector efficiency are reported.