Experimental and theoretical analysis of functional controllability for multi-condenser heat pumps

Abstract

Multi-split air conditioning systems are finding increasing applications in both residential and commercial buildings. Control of multi-split air conditioning systems have received considerable attentions. Most studies focused on control methods for the cooling mode, while very few control methods were developed for the heating mode. A widely adopted heating control strategy for a typical multi-condenser heat pump is regulating the indoor electronic expansion valve (EXV) openings to control subcooling or supply air temperature of each indoor unit and simultaneous regulating the outdoor EXV opening to control suction superheat. We found from experimental study that this control strategy may cause unstable EXV openings and poor command-following performance. By performing functional controllability analysis, we provide a theoretical insight into the uncontrollable problem and prove that the system is functionally uncontrollable when the conventional heating control strategy is used. Based on findings from functional controllability analysis, a modified system configuration is proposed, with a refrigerant receiver added between the indoor EXVs and the outdoor EXV. The modified system proves to be functionally controllable. Moreover, simulated command-following controllability tests show that the modified system has better performance. This paper verified the ability of functional controllability analysis in detecting structural control problems for refrigeration systems, which can be used as a general method for practical applications.