New capacity control technique for two-phase ejectors in transcritical CO₂ condensing units: Preliminary experimental outcomes

With the ever-growing need for climate change mitigation, CO₂ as the sole refrigerant (R744) has become an appealing future-proof option for several refrigeration and heat pump solutions. Currently transcritical CO₂ systems offer, by far, the best energy and environmental performance in various medium- and large-capacity applications, provided that some of the available expansion work is recovered via two-phase ejectors. However, although similar benefits are expected for small-scale units (e.g. condensing units), at present no appropriate capacity control strategy is available for two-phase ejectors in these solutions. The aim of this work is to bridge this knowledge gap by presenting the first experimental results related to an innovative capacity control technique for two-phase ejectors in transcritical CO₂ condensing units. The novel capacity control methodology is based on the pulse-width modulation (PWM) of the refrigerant flow through the ejector. At the compressor speed of 40 Hz, water temperature at the gas cooler inlet of 35 °C and ethylene glycol temperature at the evaporator inlet of 5 °C, it was found that the high pressure and the cooling capacity can be increased by up to about 20 bar and 17%, respectively. Furthermore, at the optimal operation condition the solution relying on the PWM ejector offered an increase in coefficient of performance (COP) by 14% compared to the unit with passive ejector and by about 29% over the standard system. In addition to the encouraging results obtained, the proposed strategy features low cost, simplicity, low vulnerability to clogging, no need to vary the nozzle throat area for flow control and major potential than its currently available competitors. Having no practical size or application constraints, the suggested capacity control mechanism could possibly lead to lay robust foundations for a significantly more sustainable future in the whole cooling and heating sector.