Thermodynamic analysis of hybrid two-stage CO2 mechanical compression–ejector cooling cycle

Abstract

In the present study, the main results of thermodynamic analysis of the hybrid two-stage carbon dioxide (CO2) transcritical mechanical compression–ejector cooling cycle using R245ca are provided. In the proposed cycle, an increase in the energy efficiency of a mechanical compression refrigeration machine (MCRM) is provided by additional supercooling of CO2 after the gas cooler due to the use of cold obtained in an ejector cooling machine (ECM). The ECM uses part of the superheated vapor heat after the high-pressure compressor and the intermediate pressure heat after the low-pressure compressor. This solution provides intermediate cooling of CO2 vapor without using an external cooling medium. The proposed method of computation makes it possible to determine the optimal parameters of the hybrid cooling cycle for the design conditions, ensuring the maximum possible MCRM efficiency. At the same time, the method considers the need to determine the optimal gas cooler pressure – a parameter that has a significant impact on efficiency growth. The effect of the intermediate pressure is extremely insignificant and is defined as the geometric mean value of the product of the gas cooler and evaporator pressures. The results show an increase in efficiency of the two-stage CO2 MCRM by up to 31.6% at high temperatures of the environment. In addition, as a result of the exergy analysis, components have been identified, the improvement of which can lead to an additional increase in the efficiency of the entire system. As follows from the data obtained, the greatest attention should be paid to improving the ejector and reducing throttle losses in the CO2 cycle.