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

The use of zeotropic mixtures in well-designed heat pump cycles enables considerable performance improvements. Temperature glide matching of the heat exchange processes and recovery of expansion losses by internal heat exchange decrease the exergy destruction yield higher cycle performances. A case study for supply to district heating is presented, indicating increases in COP from 4.3 for a pure fluid in a standard cycle to 5.2 for a zeotropic mixture in a cycle with optimised internal heat exchange. As these improvement potentials are difficult to quantify by conventional and advanced exergy analysis, a new approach was suggested. For deriving a meaningful benchmark for cycle optimisation, the exergy destruction was distinguished into a contribution describing the irreversibilities from the respective components operating with an ideal working fluid and cycle design and another contribution resulting from the cycle and working fluid being non-ideal. In addition, the exergy destruction was related to the COP, enabling an intuitive interpretation of the exergy-based analyses.