Thermodynamic Modelling and Exergtic Analysis of Ammonia-water Refrigeration System

In this study, the first and the second laws of thermodynamics (exergetic analysis) are used to analyse the performance of a single stage ammonia-water absorption cooling system operating under different working conditions. In this analysis, the performance of the system is simulated under various operating temperatures: the condenser and absorber (30 and 40°C), evaporator (-10°C, 0°C and +10°C) and generator (90 to 140°C with 10°C steps). The aim of the study is to use the exergetic analysis to identify the best operating conditions in the temperature range analysed and to evaluate the losses (exergy destruction or irreversibilities) in each component of the cooling system. The paper also investigates the relationship between the exergetic efficiency, the concentration difference between the rich mixture that enters the generator and the lean mixture leaving the generator, and the coefficient of performance (COP). The importance of determining the chemical exergy of dissolution in the analysis of each equipment is demonstrated. For each system component, mass balance, energy balance and exergy balance are carried out based on a thermodynamic model. The simulation is performed using the software EES. It is noteworthy that the exergy of each stream is divided into physical and chemical exergy, which considers both the standard chemical exergy of pure species and the chemical exergy due to dissolution. For modelling the thermodynamic properties of ammonia-water mixture, the Gibbs free energy function is used and for the liquid phase, a three constant Margules model of the excess free enthalpy is employed. The results show that the exergetic efficiency is a good tool for determining the most favourable working conditions for the cycle.