HO-18-C036 -- Real-time Optimization of Vapor-Injection Air-Source Heat Pump based Water Heating with Extremum Seeking PDF

Refrigerant vapor injection has emerged as an effective method to improve the performance of air-source heat pump (ASHP) against the possible performance degradation due to high discharge temperature of compressor and/or extremely low ambient temperature. For ASHP utilizing the internal heat exchanger cycle (IHXC) or economizer vapor injection system, the critical tuning parameters for system performance include the refrigerant mass flow rate of injection stream into the compressor and the injection pressure. It is ideal to optimize the settings of these parameters in real time for practical operation. A multi-variable extremum seeking control (ESC) framework is proposed as a model-free real-time optimization strategy to minimize the power consumption of an IHXC-ASHP system in real time, with the thermal regulation requirement satisfied. The manipulated input of the ESC is the intermediate pressure of injected vapor, and the total power consumption of the system is the only feedback. A Modelica-based dynamic simulation model of an ASHP water heater is developed with Dymola and TIL Library. The outlet temperature of hot water is regulated by the compressor capacity, while the opening of an expansion valve of the vapor-injection loop is used to regulate the intermediate pressure and mass flow rate of the injected vapor. Penalty terms are augmented to the cost function of ESC such that the intermediate superheat and discharge temperature constraints can be observed. Simulation results show that ESC is able to find the optimum values of intermediate pressure and outdoor fan speed that minimizes the total power consumption, which also maximizes the system COP, under different scenarios of ambient and thermal load conditions.