GEOGRAPHY

Ask for fresh water from the ocean! In-depth interpretation of solar-powered evaporative desalination!


On 1 June 2022, Swee Ching Tan, assistant professor at the National University of Singapore, hosted the international academic journal Nano Research Energy at Tsinghua University.https://doi.org/10.26599/NRE.2022.9120014Published a review article titled “Towards Highly Salt-Rejecting Solar Interfacial Evaporation: Photothermal Materials Selection, Structural Designs, and Energy Management.”

Figure 1. (a) Schematic diagram of the design concept and structure of the solar absorber and the structural diagram of the solar-driven interface steam generator. (b) Schematic diagram of a solar steam generator based on “mushrooms”. (c) Schematic diagram of the expanded equipment manufacturing process. (d) Two schemes for the preparation of carbon materials using fallen leaves. (e) CTGS/paper manufacturing programme.

Environmental pollution and freshwater shortages caused by the consumption of fossil resources have become global problems. Therefore, exploring environmentally friendly energy sources to alleviate freshwater shortages is essential for humanity. In various energy sources in the world (such as wind energy, tidal energy, geothermal energy, solar energy, etc.), solar energy has the characteristics of clean, large amount and easy availability. The efficient use of solar energy is of great significance to alleviate environmental pollution and global energy pressure. Solar thermal conversion technology is one of the easiest ways to utilize solar energy. For the problem of freshwater shortages, although the earth is rich in water resources, 97.5% of the total water volume is undrinkable seawater. Therefore, the researchers are trying to explore efficient desalination techniques. Due to the poor light absorption performance of water, the natural evaporation efficiency is too low to efficiently supply fresh water. Inspired by natural evaporation to obtain fresh water, solar-powered evaporation systems based on CSP conversion technology are considered promising desalination technologies to alleviate freshwater shortages.

Solar-powered evaporation uses photothermal conversion materials to convert solar energy into heat, which can then be transferred to seawater to accelerate freshwater production. Due to the low efficiency of photothermal conversion and low heat loss, the fresh water efficiency of traditional volumetric systems is relatively low. From current work, the researchers found that the location of the solar absorber affects the evaporation rate. Once the solar absorber is located at the interface between seawater and the air described above, it can reduce heat loss to water and provide more evaporation areas to release steam. We can improve sunlight capture and photothermal efficiency, and reduce heat loss to achieve efficient solar-powered evaporation rates. Therefore, the researchers explored a variety of photothermal conversion materials with broadband light absorption to maximize thermal energy conversion efficiency. Solar-driven evaporation devices generally have the following characteristics: (1) containing a wide absorption spectrum and a high solar absorber with high photothermal conversion efficiency; (2) Excellent thermal management performance; (3) Contains a hydrophilic structure, which is easy to continuously supply water to the heating zone and release steam. Solar energy can be trapped by solar absorbers and confined to the water vapor interface, effectively using sunlight to generate heat to produce fresh water. It is worth noting that the salt accumulation, heat conduction loss, convection energy loss and radiation energy loss in the process of seawater desalination have a negative impact on the evaporation efficiency of solar energy. In recent years, researchers have developed many light-thermal conversion materials such as carbon-based sheets, semiconductors, metal-based plasma materials, and conjugated polymers. In addition, the development of efficient solar-powered evaporation technology is of great significance to alleviate fresh water shortages and energy crises.

In this review, the authors introduce design principles and strategies for efficient solar-powered evaporation. Key components of solar-powered evaporators include solar absorbers and substrates. The author introduces four photothermal conversion materials (carbon-based sheets, semiconductors, metal-based plasma materials, and conjugated polymers) and their photothermal conversion mechanisms and the structure of substrate design. In solar-powered evaporation systems, solar absorbers play a key role in heat generation, and substrates can continuously deliver water to produce steam. The authors discuss ways to reduce the light reflection of solar absorbers for better light absorption, as well as ways to reduce enthalpy of evaporation to obtain a high evaporation rate. The authors also discuss various effective desalination techniques. Finally, the authors discuss the conclusions and perspectives on solar-powered evaporation, as well as the next challenges in the field.

So far, the research of solar-powered evaporators has made great progress, mainly including the selection of photothermal materials and the optimization of structure. Many researchers are pursuing better evaporators by focusing on evaporators’ high photothermal conversion efficiency, high light absorption efficiency, sound water management system, long-term desalination, and synergistic effects.

In terms of CSP materials, many nano-metal particles, polymers, metal oxides, semiconductor materials and carbon-based materials have been shown to have excellent photothermal conversion efficiency. The CSP material can be integrated into the substrate material or individually designed as an evaporator. For the study of improving the light absorption efficiency of evaporators, porous structures often play a key role. Many pores ensure the capture of light and the reflection of light. At this stage, the best light-trapping structures can achieve nearly 100% light absorption capacity. In the design of the water management system of the evaporator, the capillary effect is the basic principle. Micro and nanoporous structures exhibit efficient capillary action, and the areas in contact with the body water are mostly designed to be insulated. For desalination designs, washing off crystalline salts with bulk water is a convenient and effective solution. In addition, taking advantage of the easy accumulation of salt, the evaporator can be designed to collect both steam and crystalline salt, successfully achieving zero liquid discharge. With the increasing study of multi-energy synergies, solar-powered evaporators combined with wind, thermal, electrical and mechanical energy are becoming hot. Obtaining higher evaporation efficiency through energy synergy, or recycling the additional energy generated by solar-powered evaporators, will undoubtedly bring many new ideas and directions to solar-powered steam technology.

In this review, the authors summarize four aspects related to solar-powered evaporators: solar thermal materials, substrate materials, energy management, and desalination design. The diversity of materials and structures provides a good research basis for follow-up research. In follow-up studies, the use of low-cost materials needs to be enhanced. Bio-based carbon materials remain an important choice for low-cost materials; At the same time, some industrial wastes can be used, such as waste carbon fiber, black waste dyes, etc. In addition, more needs to be involved in some technologies that are convenient for industrialization, such as textile technology and other technologies that can be produced at room temperature and pressure. It should not be ignored that the mechanical strength of micro and nanoporous structures should be continuously enhanced, especially in the construction of porous materials with flexibility or compressibility. Most notably, the ultra-high evaporation efficiency brought about by the long-term desalination and pluripotent synergistic effect of the evaporator will become the main research target.

About the Author:

Tan Swee Ching (Chinese, Shran Shwe Shen) is an assistant professor at the National University of Singapore. He graduated from the National University of Singapore with a bachelor’s degree, a Ph.D. from the University of Cambridge, and conducted postdoctoral research at the Massachusetts Institute of Technology in the United States. His research interests include atmospheric water collection, desalination, and protein-based solar cells. In recent years, the first author and corresponding author in Joule, Energy Environ. Sci., Adv. Mater., Adv. Energy Mater. He has published nearly 50 papers in academic journals and obtained a number of invention patents.

Thesis Information:

Wei, Z. C.; Wang, J.; Guo, S.; Tan, S. C. Towards highly salt-rejecting solar interfacial evaporation: photothermal materials selection, structural designs, and energy management. Nano Res. Energy 2022, 1: e9120014. DOI: 10.26599/NRE.2022.9120014. https://doi.org/10.26599/NRE.2022.9120014

As a sister journal of Nano Research, Nano Research Energy (ISSN: 2791-0091; e-ISSN:2790-8119; Official website:https://www.sciopen.com/journal/2790-8119Founded by Tsinghua University in March 2022, Professor Chunyi Zhi of the City University of Hong Kong and Professor Qu Liangti of Tsinghua University co-editor-in-chief. Nano Research Energy is a bookInternational multidisciplinary, open access journal in EnglishfocusNanomaterialsandNanoscience technologyIn the frontier research and application of new energy-related fields, benchmarking against international top energy journals, committed to publishing high-level original research and review papers. Open access, by 2023No APC feesTeachers are welcome to submit articles. Please contact: NanoResearchEnergy@tup.tsinghua.edu.cn.

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