Self-optimizing control and mode switching for multi-functional Variable Refrigerant Flow air conditioning systems via extremum seeking

This paper proposes extremum seeking control (ESC) schemes for different operational modes of a Multi-functional Variable Refrigerant Flow (MFVRF) system as well as a mode switching scheme, in order to optimize energy efficiency in real time without using detailed plant models. Five operational modes are considered: Cooling Only, Heating Only, Cooling Dominated, Heating Dominated, and Heat Recovery. The zone temperature setpoint is achieved by controlling the respective indoor unit (IDU) fan speed, and the feedback required is the total power consumption of the compressor motor, outdoor unit (ODU) condenser fan and IDU evaporator fans. For different operational modes, the ESC inputs are chosen to be different combinations of compressor suction pressure setpoint, ODU fan speed, bypass flow valve opening, superheat setpoints for IDU's and ODU. Also, a set of switching logic has been developed for changing the operation modes of ODU and IDU, along with bumpless transfer schemes. To evaluate the proposed control and mode-switching strategies, a Modelica based dynamic simulation model is developed for the MFVRF system. Simulation results demonstrate the capability for the ESC to achieve the optimal operation in model-free fashion, as well as the effectiveness of the proposed mode switching scheme.