- Title
- Atmospheric water generation using desiccant coated heat exchangers
- Creator
- Lovis, Lucas
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2023
- Description
- Research Doctorate - Doctor of Philosphy (PhD)
- Description
- Water is a precious resource and the need for water is increasing with population growth, aquifer depletion and the impacts of climate change. Water scarcity is especially significant in developing countries and remote locations. From this, improving and developing alternative water generation technologies is important. Atmospheric water vapour is a widely available and yet underutilised water reservoir. Desiccant based atmospheric water generators (AWG) are an emerging technology based on the cyclic sorption and desorption of water vapour in the air. Desiccant coated heat exchangers (DCHE) are a potential sorption reactor for multicyclic AWG technologies, as the heat and mass transfer rates to the desiccant are high. In this thesis, DCHE-AWG systems were analysed from a material, theoretical and experimental perspective. The specific water production (SWP) for multicyclic desiccant based AWG systems primarily depends on the sorption and desorption rates of the desiccant. The mechanisms governing the equilibrium uptake capacity of desiccants and the interparticle diffusion rate of water vapour are well known. However, the mechanisms governing the intraparticle diffusion rate of water vapour within desiccants are not well understood. Methods for the enhancement of intraparticle water sorption and desorption kinetics were identified, including the influence of pore microstructure, surface hydrophilicity and composites. Additionally, desiccants with the highest potential SWP and lowest potential specific energy consumption (SEC) were identified. Furthermore, the suitability of various empirical kinetic models for the investigated desiccants was determined. To determine the feasibility of a plate-fin DCHE-AWG system, a transient one-dimensional mathematical model was used to simulate performance. A heat and mass transfer analysis and a parametric study were conducted to determine the impact of the operational and geometric parameters on the SWP and SEC. The heat and mass transfer analysis found that the coating at the inlet and outlet regions of the channels was underutilised. The parametric study found that the adsorption and desorption cycle times should be optimised independently, the primary air velocity should be high during adsorption and low during desorption, and secondary channel cooling during adsorption did not significantly improve performance. Following this, a global optimisation was conducted using the genetic algorithm to determine the maximum performance for various process arrangements and desiccant coatings. Heating the inlet air to both the primary and secondary channels during desorption increased the SWP and decreased the SEC. The relative benefit of dual channel heating was more significant for low relative humidity conditions. Additionally, the Type A silica gel coating performed similarly to the MIL-101(Cr) coating and outperformed the SAP-LiCl coating in terms of SWP. To further improve performance, the process arrangement was modified by closing the primary and secondary channel loops to conserve heat and water vapour within the system. Dual channel heating with dual channel closed loops led to an increase in SWP by up to 76% and a decrease in SEC by up to 85% compared to the same system without closed loops. The primary channel closed loop design led to a decrease in SEC by up to 20% compared to the dual channel closed loop design, however, this design also led to a decrease in SWP by up to 25%. In 100% RH conditions, the primary closed loop design achieved a SEC of 3.9 MJ/L and is a promising method to reduce the energy consumption of AWG systems. For more accurate modelling of the behaviour of silica gel based DCHE-AWG systems, the kinetic and equilibrium properties of silica gel were measured for varying coating thicknesses, hydroxyethyl cellulose binder concentrations, temperatures and relative humidity conditions using a thermogravimetric analyser. Additionally, the particle size, pore textural properties and density of the samples were measured using scanning electron microscopy, nitrogen adsorption and pycnometry respectively. The equilibrium uptake was lower for the coating samples compared to the powder sample and was not strongly dependent on the adsorption temperature. The kinetic constants and maximum ideal SWP were inversely related to the coating thickness. The adsorption order and kinetic constant varied depending on whether monolayer adsorption, multilayer adsorption, pore filling or capillary condensation were occurring. To experimentally verify the mathematical model, an intercooler was coated with silica gel and a parametric study was conducted to determine the impact of varying the cycle times, channel velocities, relative humidity conditions and desorption temperatures on the SWP and SEC. The model was extended to two-dimensions and the results matched well with the calculated primary outlet humidity, condensation rate, SWP and SEC for most of the tested conditions. The maximum measured SWP was 5.2 L/kg/day and the minimum measured SEC was 8.6 MJ/L. The measured SWP was found to be higher than most adsorption reactor types investigated in the literature. However, further reductions in the SEC are required so that AWG can become more competitive with other water generation technologies. Based on simulated results, the SEC was minimised within the coating thickness range of 400-800 um. This work demonstrated the feasibility of multicyclic DCHE-AWG systems for addressing water scarcity, demonstrated the improvement in performance compared to other AWG designs and provided insight into methods to improve design and operation from both a theoretical and practical perspective. Future work can include the experimental testing of other promising desiccants combined with the primary channel closed loop design.
- Subject
- atmospheric water generation; desiccant coated heat exchanger; adsorption; water
- Identifier
- http://hdl.handle.net/1959.13/1511608
- Identifier
- uon:56514
- Rights
- Copyright 2023 Lucas Lovis
- Language
- eng
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