Study of Adsorption-based Seawater Desalination and Atmospheric Water Harvesting Technologies

Abstract

Fresh water is extremely important for human health and sustainable development, but ~ 2/3 of the world’s population have difficulties accessing fresh water. Seawater desalination and atmospheric water harvesting based on adsorption-desorption are promising fresh water collection methods, which can be driven by low-grade heat (i.e., renewable energy and industrial waste heat) with low cost.

Adsorption cooling cum desalination (AdCD) systems can produce fresh water and cooling effect simultaneously. Previous studies only investigated seawater with a salinity of 35 g/kg, and the effect of saltwater salinity on the system performance has not been studied. A comprehensive experiment verified mathematical model was developed to investigate the heat and mass transfer processes in an AdCD system. It transpires that saltwater with higher salinities can lead to lower cooling capacity and desalination performance under the same conditions. In addition, adsorption-based atmospheric water harvesting (ABAWH) and passive radiative condensers are promising for dew water collection. I pioneered the application of passive radiative condensers to ABAWH systems and demonstrated its feasibility using MIL-101-Cr (HF) as the adsorbent and P(VdF-HFP) as the radiative condenser. It was found that an estimated water harvesting productivity of ~ 3.2 mL/g MIL-101-Cr (HF)/day or ~ 678.6 mL/m² radiative cooler/day could be achieved during clear days in Hong Kong, with almost no degradation in performance of the prototype after several outdoor experiments over 40 days. Moreover, numerical calculation was conducted, showing that ~ 29.8% more water can be harvested in mid-latitude winter compared with mid-latitude summer due to its high MIR transmittance caused by low total water column. Furthermore, a mathematical model was developed to determine the suitable relative humidity (RH) levels for the combined water harvesting system under different conditions since the desired RH for ABAWH systems and passive radiative condensers trends in opposite directions. The numerical results show that the suitable RH is in the range of 63%–70% with 200 g MWCNT embedded zeolite 13X/CaCl₂, while that for 200 g silica gel lies in the range of 53%– 65% and that for 200 g MIL-101-Cr (HF) ranges from 20% to 29%, with an adsorption phase time of 15 mins in winter of mid-latitude regions. The results provide a guideline for selecting optimal operating conditions to achieve maximum decentralized water production.

This thesis aims to take a small step towards the progress of adsorption-based seawater desalination and atmospheric water harvesting systems, which can contribute to alleviating the increasingly severe water shortage for people with difficulties accessing fresh water in water-deficient and remote areas.

Date of Award	4 Sept 2023
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Chi Yan TSO (Supervisor)