A new approach for estimation of avoidable exergy destruction: A case study of a heat pump unit

Estimation of avoidable endogenous/exogenous parts of exergy destruction in the components of an energy conversion system is provided within the methodology of advanced exergy-based analysis. According to this methodology the unavoidable, endogenous and unavoidable endogenous parts of the exergy destruction are calculated assuming special operating conditions of components. The remaining ones (i.e. avoidable, exogenous, unavoidable exogenous, avoidable exogenous, avoidable endogenous) are then calculated as differences between appropriate parts of the exergy destruction. From the viewpoint of the system improvement only avoidable parts are of crucial interest. This work proposes a new approach to estimate the avoidable exergy destruction rates of system components. The avoidable (internally caused) exergy destruction of the selected component is suggested to be computed as the difference between its exergy destruction under real operation conditions and its exergy destruction under conditions at which its irreversibilities are minimized, while the remaining components are operated with their current efficiency. The avoidable exergy destruction within the k-th component caused by the removable irreversibilities occurring within the r-th component (externally caused) is estimated by subtracting the exergy destruction within the k-th component under conditions at which only the r-th component is working with reduced irreversibilities from the exergy destruction rate taking place within the k-th component under its real operation. An example of application of the proposed approach for an air-source R290 heat pump unit is provided.
The results obtained on the base of the new approach revealed that the highest total exergy destruction decrease (28 %) within the system can be obtained by reducing the evaporator pinch point temperature from 12 K to 3 K. Increasing compressor efficiency to 0.88 could provide 19 % decrease of the total exergy destruction within the investigated heat pump unit. Decrement in condenser pinch point temperature from 5 K to 1 K could offer 11 % reduction of the total exergy destruction. Finally, it was found that the existing methodology of advanced exergy analysis leads the investigator to focus firstly on the evaporator, secondly on the condenser, thirdly on the compressor and then on the throttling valve. However, the proposed approach identified the evaporator as the main component for the heat pump performance enhancement, followed by the compressor.