Nanomaterials for the water-energy nexus
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Introduction Many communities worldwide suffer from a shortage of fresh water resources (Figure 1a).1 In some cases, the water shortage is caused by geography and climate, while in others, economic reasons prevail, including the high cost of industrial installation for energy production and water purification, and the absence of available credit resources to invest in new infrastructure.2,3 One of the most promising strategies to address the challenges of providing renewable energy and fresh water, especially to off-the-grid communities, is to make use of the freely available sunlight as the renewable energy source as well as the vast cold universe as the heatsink. Fortunately, most of the regions with high water shortages have a natural advantage of abundant solar-energy resources (Figure 1b).4 Sunlight can be harnessed to fuel chemical reactions, generate electricity in solar cells, disinfect water, and produce heat for terrestrial thermal engines, water desalination plants, and residential use.5–8 In turn, passive cooling of roofs, solar cells, and individuals via engineering solar absorptance and thermal radiation properties of materials can save energy through reduced use of air conditioning and other electricity-consuming active-cooling technologies.9–15 Passive cooling of surfaces can also increase the efficiency of dew collection, helping to extract fresh water from the atmosphere.16–19 Examples of
nanomaterials developed to advance emerging technologies in the energy-water nexus are shown in Figure 2.20–28 We discuss some of them in detail next, while also referring the reader to the available extensive review literature.5,7,29–33
Solar harvesting and cooling To harvest solar light and heat, materials need to be spectrally engineered in the broad range of wavelengths (Figure 3a), covering both the solar spectrum (∼0.3–2.5 μm wavelength) and the infrared emission spectrum of terrestrial emitters (∼2–15 μm, depending on the emitter temperature).7,34–38 An ideal absorber should possess high spectral absorptance in the solar spectrum range, low infrared emittance to reduce radiative heat losses, excellent durability at elevated temperature in air and water, and low cost, combining inexpensive starting materials and scalable coating processes.39 Nonselective blackbody absorbers, including black fabrics, paints, and carbon-based materials, can be relatively inexpensive and provide high light absorption in a broad wavelength range.40–42 However, they also emit thermal radiation over a broad range, which typically limits their use to either low-temperature applications such as conventional solar stills,40,43,44 or applications relying on concentrating sunlight to small areas with lenses and reflectors.6,34
Svetlana V. Boriskina, Department of Mechanical Engineering, Massachusetts Institute of Technology, USA; [email protected] Aikifa Raza, Department of Mechanical and Materials Engineering, Masdar Institute, Khalifa University of Science and Technology, United Arab Emirates; [email protected] TieJun Zhang, Dep
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