Performance evaluation and optimal configuration analysis of a CO2/NH3 cascade refrigeration system with falling film evaporator–condenser

Highlights: • A falling film evaporator–condenser is used in CO2/NH3 cascade refrigeration systems. • Performance evaluation of the proposed system is conducted. • An ε-NTU method based model is developed for optimal system configuration analysis. • The configuration of the proposed system is analyzed under maximum COP conditions. - Abstract: This study presents a CO2/NH3 cascade refrigeration system, in which a falling film evaporator–condenser is used as the cascade heat exchanger. The thermodynamic analysis results of the proposed system show an improvement in the coefficient of performance (COP) due to the smaller temperature difference provided by this type of cascade heat exchanger. Furthermore, an effectiveness-NTU method based model is developed by considering the constraint of the total thermal conductance. The developed model is subsequently used to examine the influences of the main parameters on the system configurations under the maximum COP condition. Results obtained reveal that, when the overall system COP is maximized, thermal conductance allocation ratios are dominated mainly by the temperature differences of the three heat exchangers and the effectiveness factors of the condenser and the evaporator. This study could contribute to the further development and the optimal design of CO2/NH3 cascade refrigeration systems