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, at present no appropriate capacity control strategy is available for two-phase ejectors in these units. The aim of this work is to bridge this knowledge gap by presenting the first experimental results related to an innovative control technique for two-phase ejectors in small-capacity CO2 vapour-compression refrigeration 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 ranged by up to about 20 bar and 33%, 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 with 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 and no need to vary the nozzle throat area for flow control. 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 refrigeration sector.