Assessing the exergy performance of heat pump systems without using refrigerant thermodynamic properties

Abstract

This paper tackles the assessment of the exergy performance of heat pumps. This work is based on a modeling that does not require the evaluation of refrigerant thermodynamic properties and is applied to two commercial (water–water and air–air types) units for various operating conditions (i.e., temperatures at the inlet side of the evaporator and the condenser). The total exergy destruction and the irreversibility distribution into the components are estimated and cover a broad range of values. As well, the nature of the sources of exergy destruction, divided into non-renewable (electrical) and renewable (thermal) energy flows, is also investigated. This distribution, which allows the use of electricity that is really paid by the user to be clearly identified, provides different optimal operating conditions, as compared with the total exergy destruction. Finally, the influence of the exergy reference state temperature (either constant or variable) is examined; in particular, with the air–air heat pump that permits the space heating demand of a building to be achieved. Although the effect on the exergy destruction is relatively low, varying the exergy reference state temperature strongly affects the exergy efficiency, which then appears to appropriately evaluate the use of energy in heat pumps.