Automatic Mode Switching for A Multi-functional Variable Refrigerant Flow System

The Multi-functional Variable Refrigerant Flow (MFVRF) system is designed to realize simultaneous heating and cooling for temperature regulation of individual zones by allowing the heat exchangers of both indoor and outdoor units to operate as an evaporator or condenser. It is thus desirable to use measurements readily available on the MFVRF system to determine switching between different modes, which involves reversing the mode of indoor unit (IDU), and/or outdoor unit (ODU) heat exchangers (HX), as well as realizing smooth transition for such mode changes. In this study, an automatic mode switching strategy is proposed for a multi-zone MFVRF system, involving both IDU and ODU mode switching. Turning on or off an IDU is determined by the zone temperature with respect to the preset hysteresis band about the temperature setpoint. The cooling mode (CM) or heating mode (HM) can be realized by opening the CM or HM related valves within a specified time duration while closing the valves for the opposite mode. The IDU fan is off if the IDU is neither in CM or HM. For the ODU-HX, a thermodynamic analysis is performed for the air-side and refrigerant-side characteristics for load conditions that need mode switching. It reveals that the air-side temperature differential becomes significantly smaller and the system COP dramatically reduced if the ODU-HX works in an inappropriate mode. Therefore, the ODU-HX air-side temperature differential is proposed as the thresholding variable for determining ODU-HX mode switching. Furthermore, two bumpless transfer schemes are applied to realize smooth mode switching with reasonable transient duration. To evaluate the proposed cost-effective and model-free mode switching strategy, simulation study is performed with a Modelica based dynamic simulation model of a four-zone MFVRF system. Simulation results validate the effectiveness of the proposed strategy as well as the bumpless transfer performance.