Experimental and numerical investigations of MD-based passive and active desalination technologies

The progressive increase of environmental pollution and population are leading many countries to deal with severe water scarcity issues. Nowadays, it is estimated that at least four billion people live in a water stress condition, and despite the lack of information, there are already many alarming situations. Water desalination seems to be the most common and promising procedure to contrast this problem, thanks to several proposals of different technologies. A possible subdivision is in active and passive, where in the former external mechanical components drive the desalination process, meanwhile in the latter external mechanical parts are not present. Furthermore, both technologies necessitate of external energy to compute the desalination process, which can be provided by renewable sources. The main goal of this thesis is to analyse an active plant and a passive device, both based on Membrane Distillation, and to compare the two technologies, so to understand the most suitable applications. The first is a Direct Contact Membrane Distillation (DCMD) process-based plant, powered by solar collectors and storage tank, developed by means of Simulink software. The second is a solar driven multi-stage device, capable of recovering the latent heat of the process, developed at the Politecnico di Torino. The work is subdivided in different sections. In the introduction, the state-of-art of the most important desalination technologies are presented. Next, all the parts of the active plant model are described, including the physics behind the process and the software’s language used. Then, the design and the prototyping of the passive device are discussed, with attention in describing the chosen guidelines which are imposed by the worldwide OHDC (Oman Humanitarian Desalination Contest), used as reference for the device development. In particular, it has been required a minimum production of 3 l/day, satisfied by a reasonable number of devices. The obtained results of the experimental campaign (passive device) and numerical analysis (for both), are extensively illustrated and commented, specifying that this last, carried out for the passive device, has been done by means of COMSOL Multyphisics software, obtaining a FEM model capable of predicting the behaviour of the device and allowing a comparison with the experimental results. Furthermore, basing on the previous outcomes, a detailed comparison and conclusion are presented, considering an economical and efficiency analysis of both the technologies and outlining the potential of using Membrane Distillation also in passive devices. Lastly, further improvements of the presented work are considered.