Development of Multi-scale Porous Surfaces with Enhanced Evaporation Performance for Interfacial Steam and Electricity Generation

Abstract

Evaporation is a crucial phenomenon in various thermal systems. Increasing the evaporation performance can help to enhance the performances and efficiencies of those thermal systems to reduce energy consumption and promote renewable water and power production. Surface modification is one of the methods to enhance the evaporation rate of the surface by optimizing the surface properties. A porous surface with interconnected cavities can improve evaporation by the increased heat transfer areas and nucleation sites. While considering the porous structure in different scales, the micro-scale porous surface involves more nucleation sites for bubble formation while the nano-scale porous structure can modify the surface wettability and facilitate the bubble departure. The multi-scale porous surface which consists of both micro- and nano-scale porous structures can make good use of the advantages of the porous structure in different scales. In addition to surface modification, adopting the interfacial evaporation approach by localizing the heat near the liquid-air interface can improve the evaporation efficiency and response time. A multi-scale porous surface made of copper-alumina hybrid nanoparticles deposited on the superhydrophilic copper porous surface is first developed to verify the interfacial evaporation enhancement compared to only a micro-scale porous surface. The result shows that the evaporation rate of the multi-scale porous surface is about 44% higher than that of the micro-scale porous surface. Then, the multi-scale porous surface is applied in a phase change thermal diode for passive thermal management improvement. It is the first study to apply a multi-scale porous surface to improve the thermal rectification performance of the phase change thermal diode. The experimental results show that the thermal rectification performance of the thermal diode using the multi-scale porous surface can be increased to 637.4, showing a nearly 300% improvement compared to the thermal diode using the micro-scale porous surface. Given that the photothermal conversion ability of the nanoparticle can further enhance the water evaporation under sunlight due to the photothermal conversion. Another multi-scale porous surface made of photothermal polypyrrole-reduced graphene oxide nanocomposite coated on the porous wood block is fabricated to investigate solar interfacial evaporation. Thanks to the synergistic effect of highly solar-absorbed polypyrrole and water-attracted reduced graphene oxide, and the efficient heat transfer two-dimensional pathway of the polypyrrole-reduced graphene oxide nanocomposite, the highest water interfacial evaporation rate obtained by the proposed coated wood with the reduced graphene oxide content of 60% is 1.49 kg m^-2 hr^-1, showing a nearly 126% and 38% improvement compared to pure water and uncoated wood (micro-scale porous surface), respectively. While applying the proposed coated wood to the solar steam generation system, the corresponding solar-to-vapor conversion efficiency of the proposed coated wood is 93.1% which is beyond most of the wood-based interfacial steam generators in the literature. Besides steam generation, the proposed polypyrrole-reduced graphene oxide nanocomposite coated wood is capable of generating electricity through the evaporation process based on the electrokinetic phenomenon. The highest power density (310 nW cm^-2) and current density (8.77 µA cm^-2) are obtained by the polypyrrole-reduced graphene oxide composite wood with 80% reduced graphene oxide content due to the high electrical conductivity and zeta potential of the polypyrrole-reduced graphene oxide nanocomposite. The resulting power and current densities are also above most of the wood-based electricity nanogenerators in the literature. To sum up, the remarkable results in this thesis not only demonstrate the improved interfacial evaporation performance of the multi-scale porous surface, but also reveal the application potential of the multi-scale porous surface in improving the performances in passive thermal regulation, steam generation, and power production to tackle energy and water crisis.

Date of Award	2 Sept 2024
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Chi Yan TSO (Supervisor)