Investigations on Air-cooled Air Gap Membrane Distillation and Radial Waveguides for Desalination

This thesis presents investigations on air-cooled air gap membrane distillation for desalination and the application of radial waveguides based on total internal reflection for solar thermal desalination.

Using an air-cooled design for an air gap membrane distillation (AGMD) process may result in significantly lower energy requirements for desalination. Experiments were conducted on AGMD module to study the effect of air gap, support mesh conductivity and hydrophobicity, condensing surface hydrophobicity. A novel modular design was used in which modules could be used in a series configuration to increase the flux value for the distillate. The output from the series configuration was found to have about three times the production from a single pass water-cooled system with the same temperature difference between the saline and clear water streams. The results also indicated that the mesh conductivity had a favorable effect on the flux value whereas the hydrophobicity of the mesh had no significant effect. The hydrophobicity of the condensing surface was favorable on two accounts: first, it led to an increase in the flux of the distillate at temperatures below 60 °C and second, the temperature difference of the saline feed when it enters and leaves the module is lower which can lead to energy savings and higher yields when used in a series configuration.

The second part of the thesis considers use of low-cost radial waveguides to collect and concentrate solar energy for use in thermal desalination processes. The optical-waveguide-based solar energy concentrators are based on total internal reflection and minimize/eliminate moving parts, tracking structures and cost. The use of optical waveguides for thermal desalination is explored using an analytical closed-form solution for the coupled optical and thermal transport of solar irradiation through a radial planar waveguide concentrator integrated with a central receiver. The analytical model is verified against and supported by computational optical ray tracing simulations. The effects of various design and operating parameters are systematically investigated on the system performance, which is quantified in terms of net thermal power delivered, aperture area required and collection efficiency. Design constraints like thermal stress, maximum continuous operation temperature and structural constraints have been considered to identify realistic waveguide configurations which are suitable for real world applications. The study provides realistic estimates for the performance achievable with radial planar waveguide concentrator-receiver configuration. In addition to this, a cost analysis has been conducted to determine the preferred design configurations that minimize the cost per unit area of the planar waveguide concentrator coupled to the receiver. Considering applications to thermal desalination which is a low temperature application, optimal design configuration of waveguide concentrator-receiver system is identified that result in the minimum levelized cost of power (LCOP).