Optimal subcooling in vapor compression systems via extremum seeking control: Theory and experiments

•COP is maximized by an optimal condenser subcooling.•Optimal subcooling is a convex function of many variables.•Extremum seeking control is used to achieve optimal subcooling.•A novel VCS system architecture is used to enable direct subcooling control.•Simulation and experimental results demonstrate a 9–11% increase in COP.

With vapor compression systems consuming a significant portion of the overall U.S. energy consumption each year, extensive efforts have been made toward the development of control strategies which aim to maximize system efficiency while providing the desired cooling. However, previous control strategies under-utilize a degree of freedom corresponding to the amount of refrigerant in the system, which is related to condenser subcooling and can significantly affect system efficiency. In this paper, an alternative system architecture, which utilizes a receiver and an additional electronic expansion valve, is used to provide independent control of condenser subcooling. Simulation and experimental results show there exists an optimal subcooling which maximizes system efficiency; however, this optimal subcooling changes with operating conditions. Therefore, extremum seeking control is implemented to find the optimal subcooling in an adaptive, model-free manner. Experimental results demonstrate a 9% increase in efficiency using the alternative architecture and extremum seeking control.