Functional Evaporators for Effective Solar Energy Driven Interface Water Evaporation and Contaminated Water Purification

功能性的蒸發器用於高效太陽能驅動界面水蒸發和汙水淨化

Student thesis: Doctoral Thesis

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Award date5 Oct 2021

Abstract

Water shortage is an ever-growing challenge for human being. To meet this challenge, technologies for clean water generation from seawater or contaminated water are highly desired. Many technologies like filtration, reverse osmosis, thermal distillation have been reported for water remediation and purification. Solar driven water evaporation is the recently developed technology using solar energy for clean water generation. Typical interfacial solar evaporation configuration included solar absorber on the water-air interface for receiving solar irradiation and convert incident light into heat. Generated heat is confined on air-liquid interface to drive water evaporation. Important factors that need to be considered for designing interfacial evaporators are: Solar absorbance, heat insulation, water transportation, and anti-fouling (salt or microbe).

Chapter 1 introduces the general background on reported clean water production technologies, including their fundamental principles and the recently developed solar driven interfacial water evaporation technologies. Besides, the scientific problems in solar driven interfacial water evaporation are also summarized in the terms of three aspects, light management, heat management, and water supply management. Moreover, solar thermal materials, systematic design, related applications, and limitations are also summarized. Finally, the related applications based on solar driven water evaporation are also reviewed.

Chapter 2 reports a plasmonic photothermal absorber using gold nanostructures with a shape of trepang (nano-trepang). By rationally regulating anisotropy at the single nanoparticles level, the nano-trepang shows good optical absorption over entire solar spectrum (92.9%). Evaporation rate of 2.7 kg m-2 h-1 was achieved with a solar vapor efficiency of 79.3 % under 1 Sun irradiation after a polymer aerogel network was constructed.

Although Au-based absorbers show broadband light absorption as well as high photothermal conversion efficiency, they may be not the best choice with consideration of gold’s cost. Thus, Chapter 3 reports a new 2D covalent–organic framework (COF) based water evaporator. Under 1 Sun AM1.5 G, this PVA based COF evaporator displays a high solar driven water evaporation rate of 2.5 kg m–2 h–1 with a solar-to-vapor energy conversion efficiency of 93.2 %. Water collection experiments in real application scenarios were carried out with a clean water collection rate of 10.2 L m–2 d–1 under natural solar irradiation.

Chapter 4 reports arrays of aligned mini needles for efficient solar water evaporation and controlled site-specific salt formation. Rationally separating the clean water and salt from brine by condensation and gravity assistance. This tip-preferential crystallization solar evaporator is not affected by the salt clogging compared with conventional 2D solar evaporators. The maximum solar evaporation rate achieved is 2.94 kg m-2 h-1 under one Sun irradiation in brine of high salinity (25 wt % NaCl), achieving energy conversion efficiency of 94.5%.

Although high evaporation rate was achieved, many low-boiling-point contaminants (like organic solvent, pesticides, Hg2+) can also be evaporated along with water steam which compromises the effectiveness of purification. To address this problem, we demonstrated a versatile carbon hybrid aerogel (CHA) as a solar powered water purification platform in Chapter 5. With elaborate absorber design, a maximum solar evaporation rate of 2.1 kg m-2 h-1 was achieved under one Sun AM1.5 G. More importantly, CHA can effectively suppress evaporation of low-boiling-point contaminants including common pesticides and Hg2+ via its strong adsorption and retention effect.

The thesis is summarized with a conclusion of all the studies mentioned above in Chapter 6 which also include discussions on how to further enhance their performance.