Thermodynamics-Based Optimization of the Vapor Compression Cycle

The optimal steady state operation of the vapor compression cycle (VCC) is analyzed from the perspective of the underlying physics of the cycle. Three degrees of freedom of the VCC are optimized using two different objective functions---one that is based on the first law of thermodynamics and the other which minimizes the rate of exergy destruction in the cycle. The use of exergy is motivated by its ability to capture the physics of both the first and second law of thermodynamics in a single property. The optimization variables are chosen as the three enthalpies, h1, h2, and h3=h4, of the cycle, as well as the refrigerant mass flow rate. A truck transport refrigeration system (TTRS) is considered as a case study on which the optimization is applied. The exergy-based objective function produces similar results as the energy-based objective function. The optimal set points generated by the exergy-based objective function are also shown to provide an increase of 33% in COP compared against the nominal set points regularly used for the actual TTRS being considered. The optimization results highlight the evaporator and condenser pressures and temperatures as critical parameters in improving the efficiency of the overall cycle operation.