Study Of The Optimal Control Of Small-scale On/off Controlled Heat Pump Integrated Central Air-conditioning System

Small-scale heat pump integrated central air-conditioning system is widely used for heating/cooling small-scale buildings. The most common control method of the heat pump in small-scale system is an on/off control. The control performance is bad, and the frequent on/off of the heat pump is easy to lead to the wear and aging of compressor. This paper presents a research of the optimal control for the small-scale on/off controlled heat pump integrated central air-conditioning system by using a method combining simulation and experiment.This paper proposes a simpilified bilinear control of space temperature for the on/off controlled heat pump integrated air-conditioning system. This control algorithm suits to the space temperature control when the supply air temperature fluctuates drastically and frequently. The simulation platform and experimental platform are built. The validation results of both simulation and experiment show that the simplified bilinear control can improve the robustness of the space temperature control efficiently, and it has a stronger anti-interference ability and more accurate temperature set-point tracking capabilities comparing with the conventional PI control.In order to reduce the on/off frequency of the heat pump, this paper develops a control method combining the bilinear control technique with a temperature set-point reset technique. When the heat pump is switched on, a smaller space temperature set-point is used, while a larger set-point is used when the heat pump is off. The bilinear control is used to track the space temperature set-point in the whole control process. The simulation results show that this combining control method can reduce the on/off frequency of the heat pump efficiently and simultaneously ensure the indoor thermal comfort. This paper also presents a study on the control performance of the combining control method in a room with phase change material (PCM) wall. In this study, the PCM wall is used to increase the indoor cooling/heating storage ability, which can enhance the control performance of the space temperature set-point reset technique. The research results show that the PCM wall can reduce the fluctuation range of the space temperature efficiently to improve its control accuracy, and simultaneously reduce the on/off frequency of the heat pump obviously. The PCM wall can enhance the control performance of the combining control method observably.To solve the out of control issue of the space temperature in light load condition, this paper develops an online optimal method of the set-point of chilled water return temperature for the air-conditioning system equiped with a water tank. This optimal control utilizes the principle of energy transfer and conservation between the air system and water system very well. The set-point of chilled water return temperature is optimized online according to the real-time cooling load of the room. The water tank is fully used to reduce the on/off frequency of the heat pump. The simulation results show that this optimal control method can improve the robustness and accuracy of the space temperature control observably under light load condition, and the water tank can reduce the on/off frequency of the heat pump efficiently.Finally, this paper proposes an energy optimization strategy aiming to reduce the total operation energy consumption of the small-scale on/off controlled heat pump integrated central air-conditioning system. In this strategy, the related system settings are optimized online to achieve an optimal energy efficient operation performance subject to the constraints of the allowed maximum start-ups number of the heat pump per hour specified by customers and the allowed variation of the chilled water return temperature. The water tank in this system is optimally designed based on the allowed maximum start-ups number of the heat pump per hour specified by customers. The simulation results show that this proposed optimization strategy can guarantee the allowed maximum start-ups number of the heat pump per hour, and reduce the energy consumption of the overall system efficiently and simultaneously improve the robustness and accuracy of the space temperature control observably.