Thermal Design of Humidification– Dehumidification Systems for Affordable Small-Scale Desalination

Abstract

Humidification-dehumidification (HDH) technology is a carrier-gas-based thermal desalination technique ideal for application in a small-scale system. However, HDH technology currently has a high cost of water production (about $30/m3 of pure water produced). This article describes changes in thermal design that make HDH systems more affordable (< $5/m3), including development of thermal design algorithms for thermodynamic balancing via mass extractions and injections and design of a bubble column dehumidifier for high-heat and mass-transfer rates in the presence of large amounts of noncondensable gas. Definition of a novel nondimensional parameter known as the modified heat capacity rate ratio has enabled designs that minimize the imbalance in local driving temperature and concentration differences. A new understanding of heat transfer in bubble column heat exchangers has led to low pressure drop designs (< 1 kPa). In addition, the concept of multistaging the uniform temperature column in several temperature steps has led to highly effective designs (about 90 percent).