Development and Analysis of a Solar Humidification Dehumidification Desalination System
In this thesis, an investigation was performed in order to understand the performance of a solar humidification dehumidification (HDH) desalination system. Initially, a mathematical model of the system, including solar water heater, condenser, economizer and long duct humidifier was developed. Using a sensitivity analysis, it was found that improving the intensities of heat and mass transfer in the humidifier would significantly enhance the yield of the system. This led to the development of a novel cascading humidifier, in which air was directed through a series of falling water sheets. An experiment was performed to first identify and characterise flow regimes in the crossflow interactions, and from this, to develop correlations to describe the heat and mass transfer for such interactions. Four flow regimes were identified and mapped based on the Reynolds number of the air and the Weber number of the water. Subsequently, Buckingham’s π theorem and a least squares analysis was employed to develop a series of empirical relations for Nusselt and Sherwood numbers. This led to the proposal of three new dimensionless numbers named the Prandtl Number of Evaporation, the Schmidt Number of Evaporation and the Lewis Number of Evaporation. These describe the transfer phenomena in low temperature evaporation processes with crossflow. Finally, the new correlations for Nusselt and Sherwood numbers were used to develop a model of a cascading humidifier, incorporated in a solar HDH system. It was found that a cascading humidifier enhances the yield of the HDH system by approximately 15%, while reducing the evaporation area to approximately a quarter of that required in a long channel humidifier.