Extremum seeking control for efficient operation of an air-source heat pump water heater with internal heat exchanger cycle vapor injection

The vapor-injection (VI) technique is known effective for improving the performance of air-source heat pump (ASHP) in cold climates. Optimizing the intermediate pressure is critical for the efficiency of VI-ASHP under varying operating conditions, while model based control is cost prohibitive in practice. We propose an extremum seeking control (ESC) framework as a model-free optimal control strategy for internal heat exchanger (IHXC) based VI-ASHP, minimizing the total power consumption in real time. The ESC takes the total power as the only feedback, and the intermediate pressure setpoint as the manipulated input. A Modelica-based dynamic simulation model of an IHXC-VI ASHP water heater is developed. The water outlet temperature is regulated by the compressor capacity, while the injection-loop expansion valve regulates the intermediate pressure. Under fixed, staircase and realistic ambient temperature profiles, simulation results show that ESC can optimize the intermediate pressure for system efficiency with good steady-state and transient performance.