Optimal Operating Conditions and Cost-Effectiveness of a Single-Stage Ammonia/Water Absorption Refrigerator Based on Exergy Analysis

In this paper, optimal design/operating conditions are presented by considering the cost-effectiveness and operability of a single-stage ammonia/water absorption refrigerator (AAR) via exergy analysis. Chemical exergy change constitutes complexity with respect to exergy analysis of absorption systems. In the study, Gibbs free energy is considered in the exergy analysis to precisely evaluate the absorption and rectification processes including chemical exergy change. The theoretical maximum exergy efficiency of AAR and the influence of its design/operating conditions on exergy efficiency/destructions are investigated under an ideal condition. The analysis indicates the importance of the evaporator outlet liquid (bleed) ammonia mass fraction and the desorber temperature. A condition of bleed mass fraction control is illustrated. In addition, the study involves performing a sensitivity analysis of design parameters (pinch temperatures) with respect to exergy efficiency and optimal desorber temperature. Finally, design conditions that maximize exergy efficiency per cost are derived relative to the sum of thermal conductance as a cost parameter. The study demonstrates the potential for downsizing the AAR without reducing exergy efficiency. The results indicate that approximately 39% total thermal conductance reduction, maintaining nominal efficiency, or 19% total thermal conductance reduction with an exergy efficiency increase of 16% are expected when compared to those in a commercial AAR.