Investigation into optimal control of terminal unit of air conditioning system for reducing energy consumption

•The model of heat transfer of terminal unit under wet condition was established and validated.•Air- and water- linkage control is reported and compared with the chilled water flow rate control.•Air- and water- linkage control can improve system stability and decreases energy consumption of the fan.•The control strategy performance was validated by experiments. In the operation of central air conditioning systems, the phenomenon of large flow and small temperature difference is often seen. Due to great differences in user cooling loadings, the terminal temperature difference between the supply and return water is considerable. Thus, the performance of the system is insignificant when using a variable frequency to control the chilled water pump. In order to find a more effective approach, this paper presents a deep analysis of the terminal surface heat exchange performance under wet conditions. By building a simulation model, the effects of the chilled water flow and air flow on the surface heat exchange performance were determined, and an optimal energy-saving control strategy based on a fixed-temperature-difference surface cooler was proposed. This strategy not only maintains the good alignment of the temperature difference between the supply and return chilled water, but also improves system stability and decreases chilled water transfer loss. Based on this strategy, the system prioritises the utilisation of air flow to meet the cooling load disturbance and reduce energy consumption of the fan. The total energy saved is up to 12.5%. The control strategy was validated by experimental results. This strategy is benefit for reducing energy consumption.