Exergy-based analysis of irreversibilities for heat pump working fluids and cycle layouts

Abstract

The use of zeotropic mixtures in heat pumps in well-designed cycles enables considerable performance improvements beyond the limitations of conventional systems. A beneficial working fluid choice enables matching the temperature profiles of the working fluid and the secondary media. The reduced temperature differences in the heat exchangers decrease the exergy destruction and yield higher cycle performance. Further improvements may be obtained from installing internal heat exchangers and reducing the superheating inside the evaporator. Conducting an advanced exergy analysis for the evaluation of different cycle layouts and working fluids yields varying amounts of unavoidable exergy destruction depending on the scenario. These differences in unavoidable exergy destruction may be considered as the performance improvement potential obtainable by an optimization of the working fluid and the cycle layout. This study therefore suggests splitting the exergy destruction into two contributions, of which one is associated with the heat exchanger operating with an ideal fluid and the other with the fluid being non-ideal. Furthermore, the exergy destruction was related to deviations from the Lorenz cycle in terms of COP. The suggested concept was demonstrated by the case of a heat pump with different mixtures of propane/butane at different cycle layouts. The results were found to be a suitable measure of the potential performance improvement obtainable by working fluid and cycle layout optimization. Furthermore, relating the exergy destruction to changes in COP enabled an intuitive interpretation of the results.