Abstract
We live in a water-rich world; about 70% of the earth's surface is covered by water. We also live in a water-poor world; available freshwater on this planet is only around 0.26% of all water. From Joseph in Egypt to emperor Chong Zhen in China, Humankind's fate is controlled by the periodical-changed quantity of freshwater. We have been searching for freshwater generation methods to release us from the shackles of fate. However, more than 75% of people still suffer from freshwater scarcity to some extent, and things are even worse for people in remote and poor areas.Photothermal effect-based freshwater generation is an old but tried method that can take abundant solar energy as the heat source for freshwater production through heat-induced phase change of water. This thesis focuses on materials modification, light spectrum adjustment, heat transfer and mass transfer control, and the device design within three photothermal effect-based freshwater generation processes toward three different water sources.
The first part is about harvesting fresh water from seawater, the primary water source on this planet. This part mainly focuses on how to increase the freshwater generation rate. Here, we utilize hydrogel for photothermal effect-based freshwater generation for its low water evaporation enthalpy. However, for such kinds of systems, there always be two trade-offs, one between the free volume for water supply and hydrogel volume for decreasing evaporation enthalpy, the other between the higher water pumping and less heat dissipation for better energy efficiency. Inspired by the sponge, we developed a cellular hydrogel-based solar vapor generator named CH-SVG. In such a device, we can always balance free water supply and evaporation enthalpy as well as mass transfer and heat transfer for operation under different conditions by choosing the freshwater generator with the proper cellular hydrogel evaporator and configuration. With the help of a selective cellular hydrogel, CH-SVG exhibited an evaporation rate of 1.61 kg m-2 h-1 with the hydrogel height of 1 mm and 2.4 kg m-2 h-1 for 10 mm under one sun irradiation, showing great potential in settling the emerging freshwater shortage in arid areas along the sea sides.
In the second part, we paid more attention to collecting freshwater from sewage, especially in dealing with residues such as microorganisms after purification, which has always been neglected in the existing photothermal effect-based freshwater generation system. This part is under the background that as COVID-19 is ranging worldwide, the fresh and sterile water shortage has become an unprecedented serious issue. Photothermal effect-based freshwater, taking solar absorbers as the cornerstone, is regarded as a promising freshwater production method for its applicability and sustainability. However, freshwater production is always accompanied by a highly concentrated residuum for the existing devices. Especially the gathering of pathogens, such as bacteria and viruses, will present a danger to people. Here, we developed a nanophotonics-enabled solar absorber (NESA) characterized by two-tier-hierarchical nano Ag crystals. Unlike previous works, our Ag crystals featured a wide anisotropicshaped and wide-size distribution, synthesized through a one-pot method. With the help of these Ag crystals, broadband light absorption up to 94.7% in the visible region can be achieved, contributing to a water evaporation rate of 1.527 kg m-2 h-1 under one sun illumination. Functionally beyond traditional solar absorbers, our NESA possess an active antimicrobial property which means pathogens will be killed instead of left behind, avoiding secondary stain on purified water and the environment. We hope our NESA provides an efficient way for fresh and sterile water production for people in remote areas.
For the last part, we aimed to collect fresh water from the atmosphere, which has been neglected but contains about 12900 trillion liters of water and is accessible worldwide. It is an emerging approach to solving the worldwide water crisis. Metalorganic frameworks and hydrogels have been extensively explored as sorbents for atmospheric water harvesting (AWH). However, they suffer from relatively low water sorption capacity in arid conditions, a feature innately owned by a common material: deliquescent sorbents. Deliquescent sorbents are, however, limited by inevitable water leakage and restricted capacity. Here, we develop an efficient AWH approach that achieves record-high water harvesting capacity of 2.62 g g-1 even in arid conditions by designing devices consisting of superhydrophilic inside matrix loaded with deliquescent sorbents for efficient water adsorption, superhydrophobic and elastic fibrous skin for adaptative expansion and water leakage prevention. The fibrous skin also exhibits a preferred radiative cooling effect and an excellent photothermal effect. It can extend effective humidity and sorption capacity at night while providing energy by absorbing sunlight for desorption during the daytime. The all-in-one design that combines heterogeneous wettability, radiative cooling, photothermal effect, and elasticity-induced adaptivity opens a new route for addressing water challenges in a wide range of working conditions.
In summary, we develop three photothermal effect-based freshwater generators for three different water sources. We solved some technical problems within these systems through elegant and specific scientific design and realized a high throughput, and safe freshwater production. Furthermore, we hope our works in this thesis can act as a little spark to ignite the fire for dealing with the emerging water shortage crisis.
| Date of Award | 26 Oct 2022 |
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| Original language | English |
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| Supervisor | Zuankai WANG (Supervisor) |