Water extraction from atmospheric air: CFD model of an adsorption heat exchanger

In the last decades, the increasing of global population and the development of the urbanization have induced a higher consumption of resources, such as the fresh water. Forecasts estimate that the average annual rate of global water demand of 2%, will generate a total growth in 2030 of 55%. The atmosphere, containing about 12,900 km3 of fresh water,can be an alternative source for the future in order to avoid a further exploitation of conventional resources. Nowadays, different solutions are explored to develop technologies able to extract fresh water from the atmosphere. In the laboratory of the Energy Department of the Polytechnic of Turin, a prototype was assembled, with the goal of produce fresh water from the air. The system uses an adsorption heat exchanger to catch the air moisture and then condense the vapour at ambient temperature. In particular, the prototype is made of several components but the adsorption heat exchanger is the one that can be studied more to improve the performances of the system. Different configurations can be tested analysing different parameters such as the geometry or the material. In order to reduce time and cost, a CFD software has be used to simulate the specific component with different operating solutions. In this work, a theoretical model of equations that models the phenomena that occur inside the adsorption heat exchanger is presented. Then the model is implemented inside the CFD software STAR-CCM+. The final CFD model was validated starting from experimental data obtained from tests, comparing the temperature and the air moisture content at the inlet and at the outlet of the adsorption heat exchanger. The configuration used for the validation of the model is a finned heat exchanger loaded with silica gel grains. In order to simulate also different configurations, such as a finned heat exchanger with a coating of adsorption material, an extension of the CFD model is required. In fact, in the latter configuration the theoretical model cannot be applied as in the simulation of grains of desiccant due to issues related to the setting of the boundary conditions. For this reason an alternative way to model a coated fin is presented, using additional tools available in the software.