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

This paper presents an exergy modeling approach for heat pumps which does not require the evaluation of refrigerant thermodynamic properties. Exergy balance and exergy destruction are estimated by the product of energy terms and energy quality factors. Therefore, evaluating refrigerant thermodynamic properties is not essential, only energy terms and operating temperatures (those of the refrigerant), are required. This method makes exergy analyses accessible to existing units and undefined processes. Since these temperatures are not necessarily given by the manufacturers, a mathematical method is presented for their calculations. Furthermore, to better understand the behavior of the system, a graphical exergy representation is presented; it allows the exergy destruction to be localized and the reasons of irreversibility to be identified. Based on the proposed exergy model, irreversibility is also analyzed along each energy path. It permits renewable flows from non-renewable ones to be separated and thus, the proper use of energy is emphasized. This method has been applied to existing water-water and air-air heat pump units and the results were compared with those obtained by using the classical thermodynamic cycle approach. The temperatures of the refrigerant are then given for various operating conditions and exergy assessments are discussed in detail.