Executive Summary : | Water and Energy are two inextricably linked, indispensable components of modern life. Industrial fog harvesting can play a major role in providing sustainable solution for both freshwater and energy. For example, wet type cooling towers (CT) in power plants produce dense fog plumes that arise during evaporative cooling of the circulating water (CW). Besides entailing technical hazards for outdoor electrical installations (e.g., the plant switchyard) and causing environmental concerns, these CT plumes are frowned upon as a major sources of fresh water loss. Our earlier study on harvesting CT fog using metal mesh structures has shown a feasibility of recovering nearly 10.5 m3/h water from a 500 MW unit. The study has also identified the salient factors that affect the fog-capture efficiency. We hypothesized that the fog collection efficiency could be significantly improved by intelligently tuning the metal mesh surface wettability and mesh design that would (i) create minimum flow resistance to the fog stream, (ii) ensure maximum possible deposition of fog droplets from the stream passing through the mesh onto the mesh fibers, and (iii) allow continuous drainage of the deposited water along the mesh fiber without mesh-pore clogging and avoiding any significant loss due to carryover and premature dripping. The proposed work stems from this hypothesis. We aim at developing metal mesh-based fog collection system that offers high aerodynamic efficiency and fog droplet deposition efficiency in the regimes of flow and thermodynamic parameters pertinent to cooling tower cells; the mesh pore design and surface wettability will be tuned to produces minimal mesh pore clogging by adequate drainage of the deposited water, and offers enough adhesion so that the collected water does not get re-entrained into the fog stream or drip prematurely down into the oncoming stream. The proposed work aims at developing, through extensive experiments and CFD analyses, strategies to address what we refer to as the mesh conundrum. We will develop facile and scalable methods of fabricating wettability-engineered metal mesh to achieve high collection efficiency. Effects of flow parameters and mesh inclination with respect to gravity and the oncoming fog stream velocity will be characterized through experiments conducted inside an in-house fog tunnel. Fog droplet deposition will be studied through high-speed Mie-scattering on different mesh designs. Different mesh weaving patterns and surface modification strategies and their efficacy in improving water collection will also be evaluated through rigorous water-collection tests. Optimum mesh design will be identified over the practical operating conditions of cooling towers. The design bases for field-deployable cooling tower fog harvesters will be drawn from the results of the proposed study. |