Volume 64

Experimental Comparison on Performance of a CO2 Air-conditioning System Using Needle- and Pulse-width-controlled Ejectors Junan Long, Binbin Yu, Junye Shi, Jiangping Chen

Abstract

In recent years, CO2 refrigeration systems have attracted significant attention due to their eco-friendly characteristics. However, their substantial throttling losses and significant reduction in cooling efficiency at high ambient temperatures limit their adoption. Although two-phase ejectors have been proven to recover expansion energy and reduce throttling losses, conventional ejectors controlled by needle valves (Needle-EJ) experience a dramatic efficiency decline under low-to-medium load conditions. This phenomenon significantly reduces the energy-saving potential and limits the practical application of ejectors. To address this issue, this research developed a novel pulse width modulation-controlled ejector (PWM-EJ) that integrated a high-speed solenoid valve directly into the motive nozzle. This enabled the effective throat area to be rapidly modulated on and off without altering the downstream geometry. Comparative experiments were conducted under typical air-conditioning conditions (Indoor: 27°C/50% RH; Outdoor: 35 – 42°C) at 2.5 – 5 kW cooling capacity. Results show that both ejector systems achieved a 20% – 28% improvement in coefficient of performance (COP) compared to a baseline expansion valve (EXV) system under maximum load conditions. Furthermore, within the 60% – 80% nozzle opening range, the PWM-EJ exhibited 15% – 18% higher isentropic efficiency and achieved a 3% – 8.5% system COP improvement over the Needle-EJ. These findings validate the PWM regulation as a more practical, robust and efficient ejector flow control technique than needle-valve regulation, representing a critical step towards the real-world deployment of ejector CO2 refrigeration systems.

Keywords CO2 refrigeration, transcritical cycle, ejector, work-recovery, pulse-width modulation