Model-free control and staging for real-time energy efficient operation of a variable refrigerant flow system with multiple outdoor units

Abstract

This paper presents a model-free control and automatic staging strategy for the operation of a variable refrigerant flow system with multiple outdoor units, which maximizes energy efficiency in real time and handles the outdoor-unit operation during load changes. An extremum seeking controller is used to minimize the total power consumption in real time while the thermal load is regulated with inner loop controllers. Minimization of power with extremum seeking control leads naturally to manipulated input behavior that can be utilized to develop control logic for two aspects of multi-outdoor-unit operation in model-free manner: (1) automatically stage off the outdoor unit with least efficient compressor, and (2) automatically switch the operation of outdoor-unit heat exchanger between the evaporating and condensing mode for simultaneous heating and cooling. As the extremum seeking controller can drive the least-efficient compressor to a lower capacity, the outdoor unit with the least efficient compressor can thus be identified and turned off in a model-free manner under a decreasing load. Under simultaneous heating and cooling operation, the fan speed of the least efficient outdoor-unit heat exchanger will be lower than all other outdoor units, which indicates that the heat exchanger can be switched between operation as a condenser and evaporator to achieve thermal balance for higher efficiency. The proposed strategy is evaluated using a Modelica model of a variable-refrigerant-flow system that consists of three outdoor units and 12 indoor units. The effectiveness of the proposed framework is validated with the simulation study.