Thermodynamic analysis of the optimal operating conditions for a two-stage CO2 refrigeration unit in warm climates with and without ejector

• Published in: Applied thermal engineering 2021-02, Vol.185, p.116284, Article 116284
• Main Authors: Expósito-Carrillo, José Antonio, Sánchez-de La Flor, Francisco José, Perís-Pérez, Bernardo, Salmerón-Lissén, José Manuel
• Format: Article
• Language: English
• Subjects: Carbon dioxide; Compressors; Control theory; Ejector; Mass flow rate; Operating conditions; Optimisation; Optimization; Pressure; R744; Refrigeration; Retailing industry; Thermodynamics
• ISSN: 1359-4311; 1873-5606
• DOI: 10.1016/j.applthermaleng.2020.116284

•The parallel compressor combined with ejectors improve the efficiency in warm climates.•The gas cooler and flash-tank pressures are used as control variables.•The ejector requires the entrainment ratio as an extra control variable.•The optimal operating curve with ejector achieves an extra 13% of COP.•The available range of the operating conditions is reduced due to the ejector. The natural refrigerant R744 seems to be the long term solution to be imposed in the food retail industry, where low and medium refrigeration temperatures are usually required, despite the technical difficulties related to high the pressure, especially in warm climates, leading to trans-critical operation with the consequent COP reduction. To overcome such difficulties, different approaches have been proposed in literature, being very popular the use of parallel compressor or its combination with ejector expansion devices. The efficient operation of trans-critical R744 systems requires the control of the gas cooler pressure, as well as the pressure of the flash-tank. In the case of ejector expansion devices, the ratio of entrained mass flow rate must be also included in the control variables and, at the same time, its use implies additional restrictions. The current paper presents the optimization methodology for the operating conditions of a two-stage CO2 refrigeration unit for both cases, with and without ejector. The curves of optimal combinations of gas cooler and flash-tank pressures, understood as operational control laws, are provided together with the COP achieved. The operating conditions and performance of the ejector case are compared showing COP improvements of up to 13%. The results also show that the operating region of the control variables is limited due to the use of the ejector.