Two-phase ejectors: a high potential for energy savings

Researchers from Xian University in China ⁽¹⁾ claim that ejector-based transcritical CO₂ systems, with the advantage of low cost, no moving part, simple structure and requiring no maintenance and lubrication, performs excellently at design conditions. The introduction of ejector can save a part of compressor consumption by recovering the energy losses from the throttling process, which results in a great performance improvement. However, as no moving structure can be adjusted to catch up with any change in operating conditions, parameters such as the high pressure, low pressure or the refrigerant mass flow rate may be changed unexpectedly when working under off-design conditions. 

 

These researchers consider that the main obstacle to the popularization of two-phase ejectors is the lack of clear understanding of the involved complicated thermodynamic mechanism. Based on the observation that no exhaustive detailed synthesis of the various models of the performance of two-phase ejectors is available, their study aims to provide the state of the art of ejection technologies for researchers, developers and manufacturers.  

 

According to DTU researchers in Denmark ⁽²⁾, the adoption of a two-phase ejector leads to coefficient of performance (COP) enhancements between 10% and 30% for CO₂ vapour-compression systems at the design conditions. However, the adoption of a suitable capacity control strategy is crucial in order to maximize the COP of these systems over all the operation conditions, including off-design conditions, otherwise performance may be severely penalized. This is due to the fact that the performance of two-phase ejectors is significantly governed by their four characteristic elements, i.e. suction nozzle, motive nozzle, mixing chamber and diffuser, which need to be permanently suited to the operating conditions so as to implement an appropriate capacity,  conditioning the effective expansion work recovery. 

 

In their article, the Danish researchers give the results of an extensive review on today's most widely used capacity control mechanisms for two-phase ejectors, i.e. needle-based capacity control method, multi-ejector arrangement and vortex-based capacity control mechanism.  

 

They conclude that two-phase ejectors can be properly capacity controlled in large- and medium-scale vapour-compression units. However, they stress the dramatic lack of an appropriate capacity control method for small-capacity units, such as light commercial refrigeration equipment, automotive air conditioning systems and residential air conditioning units.  

 

These equipment require a simpler low-cost control mechanism compared to medium- and large-scale systems. Furthermore, the need to keep the ejector free of clogging is a challenge to overcome for the adoption of ejectors in small-capacity applications. At present, taking into account the low cost, the simplicity and the negligible difference in performance, expansion valve control should be preferred to both the needle-based ejector and the vortex ejector control mechanisms in these units.

 

Sources:

⁽¹⁾ Song Y. et al. Review on the simulation models of the two-phase-ejector used in the transcritical carbon dioxide systems, International Journal of Refrigeration, November 2020. Available in FRIDOC (free of charge for IIR members).

⁽²⁾ Gullo P. et al. A review on current status of capacity control techniques for two-phase ejectors, International Journal of Refrigeration, November 2020. Available in FRIDOC (free of charge for IIR members).