Simulation and experiment on direct contact membrane distillation regenerator in the liquid dehumidification air-conditioning system

Abstract

Liquid dehumidification air-conditioning (LDAC) systems are a promising dehumidification technology that reduce energy consumption. However, in hot and humid climates, liquid regeneration deteriorates the performance of regenerators and limits their applications. Therefore, developing methods to ensure regeneration performance in harsh climates as well as to reduce the droplet splatter is a challenging task that must be undertaken to advance the technology and make it viable. In this study, a novel regeneration method using the direct contact membrane distillation (DCMD) regenerator was applied to the LDAC system. The mathematical model of the DCMD solution regeneration process was developed and verified through PTFE hollow fiber membrane liquid regeneration experiments. The impacts of membrane length, temperature, concentration, and solution volume flow on the temperature and concentration of the resultant solution at the outlet, regeneration capacity mass transfer coefficient, membrane water flux, and heat flow were analyzed. The results indicate that the concentration of the solution at the outlet linearly increases from 34.9% to 35.9% when the solution is heated from 40 to 85 °C. However, when the temperature of the solution is less than 45 °C, the solution concentration at the outlet is lower than that at the inlet, which indicates that the regeneration capacity is negative. Thus, the solution temperature at the DCMD regenerator inlet should be maintained at >65 °C to provide a higher and stable regeneration efficiency. Finally, it was established that a flow rate that is either extremely high or low would impair the DCMD regeneration, whilst a volume flow of 1800 ml min−1 is still most appropriate for the regenerator to regenerate high-concentration solutions. Research on the cold side indicates that when the temperature of the water does not exceed 30 °C, the regenerator still has the ability to regenerate, and increasing the volume flow of water can enhance the regeneration effect.