| Author: | Zhao, Xiaomeng |
| Title: | Multiscale engineering of three-dimensional composites for solar thermal energy applications |
| Advisors: | Shen, Xi (AAE) Hsu, Li-ta (AAE) |
| Degree: | Ph.D. |
| Year: | 2025 |
| Subject: | Water-supply Solar energy Water resources development Energy harvesting -- Materials Porous materials Hong Kong Polytechnic University -- Dissertations |
| Department: | Department of Aeronautical and Aviation Engineering |
| Pages: | xviii, 138 pages : color illustrations, map |
| Language: | English |
| Abstract: | Global water scarcity urgently demands sustainable, low-cost, and off-grid solutions for generating clean water. Two sustainable sources of water are seawater and atmosphere. However, existing desalination and atmospheric water harvesting (AWH) methods often rely on electricity from fossil fuels, resulting in high energy consumption and limited applicability in rural or remote regions. Solar-driven water harvesting is a promising technology that has gained widespread attention. To solve the low energy efficiency problems existing in the solar-driven interfacial evaporation or sorption-based atmospheric water harvesting (SAWH) materials, such as salt accumulation, high vapor diffusion resistance, and slow sorption/desorption kinetics, this thesis is focused on the multiscale engineering design of materials to optimize the thermal, mass (water and salt), and vapor transport, thereby overcoming the limitations of the coupling of transport properties in porous materials and significantly enhancing overall energy efficiency. Chapter 1 provides the foundational context of solar-driven water generation, explaining how solar-driven evaporation and atmospheric water harvesting can be optimized through material design. Key performance-influencing parameters, including thermal transport, mass transport, and vapor transport, are discussed to establish the critical factors in effective solar-driven processes. This chapter lays the groundwork for subsequent multiscale engineering design of materials, forming the systematic basis of the research. Chapter 2 addresses the fundamental trade-off between salt rejection and heat localization in high salinity by developing a structural gradient aerogel (SGA). The fan-shaped, tapered channels in SGA facilitated two-way water and salt transport, while horizontally aligned surface pore channels localized heat. This innovative design achieved a stable evaporation rate of 2.09 kg m⁻² h⁻¹ over seven days in 3.5 wt% NaCl solution under one-sun irradiation and maintained performance in high-salinity brine (20 wt% NaCl). These findings underscore the significant potential of SGA for solar desalination and the purification of high-salinity brines. Chapter 3 focuses on overcoming the high vapor diffusion resistance with increasing size to address the challenge of scalability. Using additive freeze printing (AFP), the aerogel lattice incorporated macroscopic interconnected pores and microscopic vertical channels, significantly enhancing vapor diffusion while efficiently managing water and thermal transport. This innovative design enabled AFP aerogel lattice to maintain an excellent evaporation rate of 2.08 kg m⁻² h⁻¹ and an energy efficiency of 84.6% at a lateral size of 0.08 m, significantly better than conventional aerogels of the same size, overcoming the scalability challenges faced by conventional aerogels. Chapter 4 solves the problems of long salt-loading time caused by salt transport and low sorption kinetics caused by high vapor diffusion resistance in hygroscopic hydrogels, realizing fast and efficient solar-driven SAWH. The SA/CNT/HPC/LiCl (SCHL) hydrogels with interconnected structures was fabricated using phase-change-assisted (PCA) salt-loading method and AFP techniques. The PCA method used phase transitions to rapidly incorporate LiCl, significantly accelerating salt transport and reducing the salt-loading time while maintaining high water sorption capacities of 0.78, 1.60, and 3.25 g g⁻¹ at 30%, 60%, and 90% relative humidity, respectively. The millimeter-scale interconnected pores of AFP-SCHL hydrogel enabled it to reach 80% of equilibrium sorption capacity within 70 minutes, compared to 205 minutes for bulk-SCHL hydrogel, demonstrating significantly enhanced sorption kinetics. Under natural outdoor sunlight, AFP-SCHL hydrogel achieved a water production rate of 2.5 g g⁻¹ day⁻¹, showcasing the practical potential for efficient atmospheric water harvesting. Chapter 5 summarizes the research works and outlines future research directions based on the progress achieved. While previous studies have primarily focused on enhancing evaporation efficiency, the ultimate goal of our research is freshwater collection. Notably, the water collection rate often lags significantly behind the evaporation rate. Therefore, future efforts will prioritize improving freshwater collection by optimizing the condensing device. Furthermore, although the performance of solar-driven evaporation and AWH has been enhanced through multiscale material engineering, the amount of water collected remains limited. Integrating these two methods to enable all-day freshwater collection represents a key focus of future research. This thesis overcomes the problems existing in solar-driven water harvesting materials through innovative multiscale material engineering design, including the development of structurally graded aerogel to solve the trade-off between salt rejection and heat localization under high salinity conditions, the design of size-insensitive hierarchical porous aerogel lattice to overcome the high vapor diffusion resistance that increases with size, and the preparation of hydrogels with interconnected structures shortens salt loading time and improves sorption kinetics. These efforts have promoted solar-driven water harvesting technology to better practical applications. |
| Rights: | All rights reserved |
| Access: | open access |
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