Design of gas adsorption systems with phase change materials aided by topology optimisation method

Abstract

The use of natural gas has become increasingly attractive for the industrial sector, economically or ecologically, and its storage assumes an essential role in this process. The mechanism of mass transfer by adsorption is encountered in several areas of science and is already used for storing natural gas when applied to gas adsorption tanks. In latent heat storage applications, phase change materials significantly benefit thermal stability and storage-to-volume ratio. Work using phase change materials in adsorption tanks demonstrates improvements in the adsorption capacity of the tanks by improving the thermal behaviour of the set, and the results are influenced by the geometry and positioning of the bodies belonging to the system, signalling the opportunity to optimise the distribution of material inside the tank. A topology optimisation method is a generic tool for distributing material within a domain. In the last decades, it has presented significant progress in its implementation and the possibility of exploring the existing manufacturing methods. The present work explores the topology optimisation method in designing adsorption systems with phase change materials in their interior. Initially, the displacement of the air masses in the surroundings of an adsorption system is simulated simultaneously with the adsorption process simulation in its interior. This method allows a variable convection heat exchange through the system's external walls. The convection distribution along the external wall is evaluated to strengthen the assumption of constant external convection on the external walls of a cylindrical vessel, which is the object of study of the topology optimisation. Results show that the variable-h approach is indicated for complex-shaped vessels, whereas the conventional approach may be applied for cylindrical vessels. The second part of this work addresses the internal optimisation of adsorption tanks with phase change materials (PCM). Two approaches are presented. In the first approach, the phase change material is particulate and mixed with particulate adsorbent. A Semi-Analytic model is compared to the topology optimisation method to check the total PCM ratio and how boundary conditions affect the solution. The topology optimisation method produces a graded solution for the optimised PCM ratio in the mixture. In contrast with the first approach, the last approach optimises condensed PCM. It assumes that PCM is one or more continuous bodies inside the adsorption system. The results for all these approaches are presented and compared. The study shows that topology optimisation may produce optimised PCM distributions inside adsorption systems and be an option when mixtures are unavailable.