Thermodynamic analysis of a two-stage humidification–dehumidification system with reflux configuration

Humidification–dehumidification technology is an innovative desalination technology which is promising for small-scale desalination applications. The relative low energy efficiency made the early system less competitive. Multi-stage humidification–dehumidification technology is one of the approaches that effectively improve the system energy efficiency. In this study, an air extraction/injection two-stage humidification–dehumidification system with reflux configuration is proposed. The reflux configuration is introduced into the two-stage system to improve the energy efficiency by eliminating the difference between the air temperatures at the extraction and injection points. A thermodynamic model is developed to investigate the system performance. It is found that the air temperatures at the extraction and injection points are only identical when the pinch point heat capacity rate ratio of first-stage dehumidifier less than unity and that of second-stage dehumidifier greater than unity. The influence of liquid-to-air mass flow rate ratio in the second stage on the energy efficiency of the system is negligible when the pinch point heat capacity rate ratio of first-stage and second-stage dehumidifier both greater than unity. The energy efficiency peaks when the pinch point heat capacity rate ratio of first-stage or second-stage dehumidifier equals unity. Additionally, the reflux configuration can improve the energy efficiency of two-stage system by 30%.