Research On Optimization Control Of The Variable Refrigerant Flow (VRF) Air-conditioning System

The diversity of variable refrigerant flow systems ('VRF','VRV'or'Multi-systems')– systems with one outdoor unit and several indoor units - commercially available is increasing every year, due to its efficient operating and improving thermal comfort of indoor environment. While, the control strategies presently used in the system almost only consider how to reach the system's stability, reliability, and controllability, the control targets of the refrigeration circle are mainly according to the system nominal conditions, which are rather conservative. Little has been done to optimize the refrigeration side of a VRF system operating, especially under the part-load conditions. In order to solve these problems, some studies are carried out in the thesis.The research work can be divided into three parts: experimental and theoretic study on the present systems; the study on the optimization refrigeration circle of the system in different conditions (control target); the study on the optimization control strategy and control algorithm realization.Firstly, experimental and theoretic study has been made to two different VRF systems with distributed control system, which are available in the market and have different structures (two-pipe and three-pipe), and have different control method for the indoor unit capacity control (On-off and continues adapting). The system control strategy and energy performance are studied in detail. Examples of testing results are also presented here, in order to illustrate the methodology and also validate a simulation model.Secondly,the relationship and difference between the multi-split VRF system and single indoor unit room air-conditioner are studied, which shows the great difference on the control strategy. The characteristics of the VRF system energy performance are studied. The optimized control variables, such as evaporating pressure, condensing pressure, outdoor unit fan speed, method for controlling compressor suction gas superheating, can be achieved based on the global optimization on the system thermodynamic efficiency and energy consumption equipment efficiency. The characteristics and the calculation method of the optimization control variables (refrigeration circle) are also studied. The comparison between the optimization operation condition and conventional operation condition shows great potential on energy saving in the part load conditions, when the load of each indoor unit changes equably.At last, the control method on realizing the system optimization operation is studied. By analysis the characteristics of the present decentralized control system and centralized control system, a new distributed control method has been established in order to realize the system optimization control. The analysis on the system operation state identification also makes its possible for the goal of the energy efficiency optimization operation.