Theoretical and numerical studies on effect of silica gel bed thickness for atmospheric water harvesting application

Solid desiccant based atmospheric water harvesting (AWH) systems provide a sustainable method to mitigate water scarcity by extracting potable water from ambient air. This paper investigates the influence of adsorber bed thickness on water production through 2-dimensional CFD study with silica gel as adsorbent. Domains with fixed width of 500 mm and varying bed thicknesses of 50, 150, and 300 mm are analyzed. Results indicate that the bed with 50 mm thickness achieved the highest specific water production (SWP) of 0.25 L/kg, compared to 0.072 L/kg and 0.04 L/kg for thicknesses of 150 and 300 mm, respectively for equal adsorption/desorption time period of 8 hrs. A theoretical lumped analysis is also carried out to deduce a mathematical expression for moisture penetration depth during adsorption/desorption processes. Theoretical results predict a penetration depth of 70 mm for adsorption and 160 mm for desorption. This provides a bed sizing criterion for maximizing SWP. Furthermore, the effects of increasing the adsorption to desorption time ratio (t_ads/t_des) and airflow reversal during desorption are also examined through CFD simulations. Using the combination of these techniques, enhancement in SWP ranging from 15 % to 148 % can be obtained depending on the adsorbent thickness.