Multi-functional Variable Refrigerant Flow System, Extremum Seeking Control, Energy Efficiency, Real-time Optimization, Mode Switch
Multi-functional variable refrigerant flow system (MFVRF) is designed to realize simultaneous heating and cooling for individual zones. It is desirable to use existing measurements to determine switching between different modes under changes of ambient and load conditions, i.e. reversing the mode of indoor unit (IDU) and/or outdoor unit (ODU) heat exchangers (HX), as well as the bumpless transfer for controller switching. In this study, a set of mode switching logic is proposed for a four-zone MFVRF system, which involves both IDU and ODU mode switching actions. For the ODU-HX mode switching, thermodynamic analysis under different load changes reveals the qualitative trend for the air-side and refrigerant-side characteristics as the operation approaches to marginal scenarios. The ODU mode switching is thus based on the air-side temperature difference. Mode switching involving IDU action only is studied with a 1H3C (one heating three cooling) mode, in which IDU-1 is in heating mode and IDU-2, IDU-3 and IDU-4 are in cooling mode. For a given zone load conditions, when the zone temperature of IDU is higher than upper limit of a preset cooling mode hysteresis band, IDU enters the cooling mode by simultaneously opening all related cooling mode valves and closing heating-mode valves within time duration. On the other hand, the cooling mode is turned off by closing all related cooling valves when the zone temperature is lower than the lower limit of the cooling mode temperature band. Similarly, when the zone temperature is lower than lower limit of heating mode temperature band, IDU enters its heating mode. When the zone temperature for IDU is higher than the upper limit of heating mode temperature band, the heating mode is turned off. For ODU Mode Switching, it is proposed in this paper to use the temperature difference between the inlet and outlet air of the ODU HX. To justify the use of ODU air-side temperature differential as the indicator variable for ODU mode switching, several cases of 2H2C (two-heating two-cooling) mode are first simulated, in which the IDU-1 and IDU-2 are operated in heating mode and IDU-3 and IDU-4 are operated in cooling mode. A negative ramp of load change applied to IDU-3 within 1000 seconds. For the ODU-HX, the air inlet temperature is fixed at the ambient 20oC, while the air outlet temperature approaches closer and closer to 20oC under reducing cooling load in IDU-3. Simulation results have revealed the decreasing trend of COP. The T-s diagram for the refrigerant cycle of 2H2C mode is evaluated under several scenarios of reduction in IDU-3 cooling load. It reveals that a decreasing temperature difference at the air side or refrigerant side can be candidate probing variables for mode switching of ODU HX. Also, similar study is conducted when the ODU HX works as evaporator, with the MFVRF system operated in 3H1C (three heating one cooling) mode. Simulations for ODU HX mode switch case have been performed, and the results validate the effectiveness of the proposed scheme of mode switching.