Transient Exergy Destruction Analysis of a Vapor Compression System

Through online optimization and control, vapor compression systems (VCSs) can effectively respond to disturbances, such as weather or varying loads that cannot be accounted for at the design stage, while simultaneously maximizing system efficiency. However, to do so requires a mathematical characterization of efficiency for the VCS. In particular, we would like to maximize the exergetic efficiency of the VCS which characterizes system efficiency relative to the maximum achievable efficiency as postulated by the second law of thermodynamics. This is equivalent to minimizing the rate of exergy destruction during system operation. Furthermore, in applications where VCSs encounter high frequency disturbances, such as in refrigerated transport applications or passenger vehicles, optimizing efficiency at steady-state conditions alone may not lead to significant reductions in energy consumption. Therefore, it is necessary to model the transient effects of changes in control variables on the rate of exergy destruction in a given system. In this paper we derive an expression for the transient rate of exergy destruction for the refrigerant-side dynamics of a VCS. A lumped parameter moving boundary modeling framework is used to model the two heat exchangers in the VCS. Open loop simulations using a validated nonlinear model of an experimental VCS are presented to highlight how changes in individual control variables affect the componentlevel and system-level exergy destruction rates as a function of time. The results are discussed in the context of their implication for exergy destruction-based optimal control.