Coordinated optimization of the variable refrigerant flow and variable air volume combined air-conditioning system in heating conditions

Although variable refrigerant flow systems have become attractive due to good energy performance in part-load conditions, the shortcoming of the variable refrigerant flow system in outdoor air ventilation has not been well solved. A variable refrigerant flow and variable air volume combined air-conditioning system was proposed to solve this problem. The variable air volume part of the combined system consists of an outdoor air processing unit and air supply and distribution devices. The combined system was found having potential to obtain high-level energy efficiency without compromising thermal comfort by an intelligent control system. Therefore, a so-called global coordinated optimization strategy based on a local coordinated optimization strategy is proposed in this study. The local coordinated optimization strategy decreases energy consumption of the combined system through optimizing load allocation between the variable refrigerant flow unit and the outdoor air processing unit. The global coordinated optimization strategy has a much deeper coordination of the variable refrigerant flow and outdoor air processing units, as it combines the local coordinated optimization strategy with stopping and restarting of the outdoor air processing unit. Performances of the combined system in heating conditions under this strategy are evaluated in the simulation platform. Results show that the global coordinated optimization strategy can effectively reduce the energy consumption of the combined system.