Numerical Modeling of air-gap membrane distillation

Fresh water supply is a problem in large parts of the world and present on every continent. Many countries facing physical water scarcity, however, have access to the sea and lie in arid zones of the earth where solar energy is plentiful available. Membrane distillation (MD) describes an emerging desalination technology which has advantages when driven by solar energy or waste heat. In MD, seawater is thermally desalinated by generating a temperature gradient between hot salt water and produced fresh water which are separated by a membrane. In air-gap membrane distillation (AGMD) an insulating air-gap is introduced between membrane and distillate in order to minimize conductive losses. Despite its advantages, the permeate stream needs to be increased for large-scale application. To improve performance and energy efficiency, a detailed understanding of the highly coupled heat and mass transfer is crucial. However, for AGMD not many models exist and the existing models simplify the heat and mass transfer processes. The goal of this thesis is therefore to increase the understanding of the AGMD process and the predictive power of numerical models. A three-dimensional (3D) macro-scale model is developed with emphasis on the heat and mass transfer. It integrates aspects from multiphase flow modeling namely energy conservation over phase-change interfaces and the thermodynamic concept of moist air in the air-gap. Thereby, it computes the condensation mass flow independently from the evaporation mass flow, allowing to study the influence of convection on the heat and mass transfer in the air-gap. The model is accelerated for computation on graphical processing units (GPU). Employing the macro-scale model, a comparative analysis of the effects of module orientation on module performance and efficiency is performed. Vortexes in the air-gap are observed when using a module configuration where the hot feed flows below air-gap and membrane and the temperature gradient is opposing gravity. These vortexes lead to a significantly increased energy utilization also at low feed velocities. As the main advantage of AGMD is the reduction of heat losses, this configuration could bring further improvement. Furthermore, membrane transport properties are determined from high-resolution 3D membrane imaging combined with Lattice-Boltzmann simulation. Thereby, the 3D structure of membrane samples is obtained and porosity, tortuosity and permeability values are computed for the investigated membranes. Following the findings in the papers, further studies are suggested employing the modeling approaches developed in this thesis.