Thermal design of humidification dehumidification systems for affordable and small-scale desalination

Abstract

The humidification dehumidification (HDH) technology is a carrier-gas-based thermal desalination technique ideal for application in a small-scale system but, currently, has a high cost of water production (about 30 $/m³ of pure water produced). The present thesis describes fundamental contributions to the thermal design of HDH systems that have made them affordable (< 5 $/m³). These contributions include: (1) the development of thermal design algorithms for thermodynamic balancing via mass extractions and injections; (2) design of a bubble column dehumidifier for high heat and mass transfer rates even in the presence of a large percentage of non-condensable gas; and (3) optimization of system design with pressure as a parameter. Definition of a novel non-dimensional parameter known as the 'modified heat capacity rate ratio' (HCR) has enabled designs that minimize the imbalance in local driving temperature and concentration differences. The design algorithm has been validated by experimental data from a pilot-scale HDH unit constructed as part of the thesis work. The energy consumption of HDH was reduced by 55% by this technique. Bubble column (BC) heat exchangers can have high heat and mass transfer rates by condensing the vapor-gas mixture in a column of liquid rather than on a cold surface. New physical understanding of heat transfer in BCs has led to low pressure drop designs (< 1 kPa) and the concept of multistaging the uniform temperature column in several temperature steps has led to high effectiveness designs (about 90%). A prototype with an order of magnitude higher heat rate compared to existing dehumidifiers operating in the film condensation regime was developed to validate the physical models. Overall cost of water production in HDH can be further reduced by treating pressure as a variable parameter. Systems operating under varied pressure consume half the energy as existing HDH systems.