Sustainable freshwater production by air-gap membrane distillation system: a numerical and experimental study

Water scarcity is one of the most important issues that is characterising our times. Climate change, pollution, exploitation of water reserves and population growth are among the root causes for this situation. Today, it is estimated that at least 4 billion people live in a water-stressed condition, which leads many countries to face serious problems, even counting that fresh water needs will increase globally by 20-30% by 2050. The concomitant shortage of fresh water reserves, easy access to salt water sources and high insolation in some places in the world provide opportunities to use desalination processes that can be driven or partially driven by solar energy. However, the most developed conventional desalination methods to date, such as Reverse Osmosis (RO), requires large amounts of energy and expensive systems. Membrane distillation, a process that is still in its early stages, may be a useful alternative to mitigate this challenge, as it can treat water with high salt concentrations and be coupled with renewable energy sources, such as solar energy, to improve performances and reduce costs. In this thesis, after an overview of desalination systems developed to date and in particular membrane desalination, a model for a single-stage Air Gap Membrane Distillation (AGMD) system is proposed. This active distillation process is driven by low-temperature thermal energy, where the required heat is provided by electrical heaters and by direct solar irradiance. The scope of this thesis is to analyse the AGMD device, in different configurations, and demonstrate the achievability of this process whose performances are investigated by means of Simulink and MATLAB software. This study has been carried out developing a 2D model that simultaneously considers both heat and mass transfer in the AGMD process by coupling the continuity, momentum, and energy equations implemented through the finite difference method. After describing the physical process of membrane distillation and explaining the methods used to build the model, this thesis proceeds to analyse various configurations of the AGMD device, searching for the best operating conditions through a sensitivity analysis. The validation process of the model was done thorough an experimental campaign in laboratory, where an AGMD module was tested under several conditions. The experimental data obtained are compared with the simulation results to determine the validation by employing error bars. The model enables the evaluation of multiple parameters, including freshwater productivity, thermal and membrane efficiencies, temperature polarisation coefficient and salt concentration. The last chapter deals in detail with the behaviour of the desalination device when it is heated or partially heated, by different solar insolation values and several feed water temperatures, which allows to take advantage of very low feed in temperaures. Lastly, conclusions about the work and an overview on the potential future of this technology are presented.