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).^{Reference Boriskina, Green, Catchpole, Yablonovitch, Beard, Okada, Lany, Gershon, Zakutayev, Tahersima, Sorger, Naughton, Kempa, Dagenais, Yao, Xu, Sheng, Bronstein, Rogers, Alivisatos, Nuzzo, Gordon, Wu, Wisser, Salleo, Dionne, Bermel, Greffet, Celanovic, Soljacic, Manor, Rotschild, Raman, Zhu, Fan and Chen7,Reference Boriskina, Tong, Hsu, Liao, Huang, Chiloyan and Chen34–Reference Mojiri, Taylor, Thomsen and Rosengarten38} 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.^{Reference Cao, McEnaney, Chen and Ren39} 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.^{Reference Ni, Zandavi, Javid, Boriskina, Cooper and Chen40–Reference Siva Reddy, Kaushik, Ranjan and Tyagi42} 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,^{Reference Ni, Zandavi, Javid, Boriskina, Cooper and Chen40,Reference Ni, Miljkovic, Ghasemi, Huang, Boriskina, Lin, Wang, Xu, Rahman, Zhang and Chen43,Reference Cooper44} or applications relying on concentrating sunlight to small areas with lenses and reflectors.^{Reference Weinstein, Loomis, Bhatia, Bierman, Wang and Chen6,Reference Boriskina, Tong, Hsu, Liao, Huang, Chiloyan and Chen34}

Figure 3.

(a) Wavelength spectra of solar radiation (red) and atmospheric transparency (yellow). Terrestrial objects emit thermal radiation in the mid to far-infrared spectral range, which overlaps with the transparency window in the atmospheric spectrum, allowing for their passive cooling via radiation into the ultra-cold of outer space. (b) Local electromagnetic field intensity distribution at two separate wavelengths within the visible spectrum in a broadband solar absorber that combines thin-film interference and localized surface plasmon resonance effects in an Ag-SiO₂ nanocomposite structure.^{Reference Lu, Raza, Noorulla, Alketbi, Fang, Chen and Zhang57} (c) Local temperature distribution in the internally hot, externally cool solar absorber utilizing carbon foam as the blackbody absorber and a transparent silica aerogel^{Reference Weinstein, McEnaney, Strobach, Yang, Bhatia, Zhao, Huang, Loomis, Cao, Boriskina, Ren, Wang and Chen60} as thermal insulation material. (d) Optical (left) and infrared (right) images of a human hand covered with wearable fabrics, which are either blocking infrared emission from the skin (cotton, bottom) or allowing it to pass through (polyethylene, top).^{Reference Zandavi, Huang, Ni, Pang, Osgood, Kamal, Jain, Chen and Boriskina68}

Solar absorbers can be engineered by introducing metal nanostructures, such as nanoparticles, nanopores, and nanodisks, either on the absorber surface or inside its volume.^{Reference Bermel, Yazawa, Gray, Xu and Shakouri8,Reference Bermel, Lee, Joannopoulos, Celanovic and Soljacic35,Reference Mojiri, Taylor, Thomsen and Rosengarten38,Reference Cao, McEnaney, Chen and Ren39,Reference Chou, Xiang, Yoonkyung, Lee, Lenert, Rinnerbauer, Celanovic, Soljačić, Fang, Wang and Kim45–Reference Fei Guo, Sun, Cao, Liu and Ren48} Surface plasmon modes excited by incident sunlight on these nanostructures facilitate efficient absorption of solar photons and conversion of their energy into heat. The nanostructured solar absorbers can be tailored to simultaneously offer high reflectance (i.e., low emittance) at longer wavelengths, thus facilitating heat trapping.^{Reference Weinstein, Loomis, Bhatia, Bierman, Wang and Chen6,Reference Boriskina, Green, Catchpole, Yablonovitch, Beard, Okada, Lany, Gershon, Zakutayev, Tahersima, Sorger, Naughton, Kempa, Dagenais, Yao, Xu, Sheng, Bronstein, Rogers, Alivisatos, Nuzzo, Gordon, Wu, Wisser, Salleo, Dionne, Bermel, Greffet, Celanovic, Soljacic, Manor, Rotschild, Raman, Zhu, Fan and Chen7,Reference Boriskina, Tong, Hsu, Liao, Huang, Chiloyan and Chen34,Reference Peters, Goldschmidt, Löper, Groß, Üpping, Dimroth, Wehrspohn and Bläsi49–Reference Kraemer, Poudel, Feng, Caylor, Yu, Yan, Ma, Wang, Wang, Muto, McEnaney, Chiesa, Ren and Chen51}

Other approaches to enhance solar absorptance rely on the use of thin-film, photonic-crystal, and graded-index coatings as well as mesoscale structures combining photonic crystals and thin films with nanoparticles.^{Reference Cao, Huang, Tang, Sun, Boriskina, Chen and Ren37,Reference Chou, Xiang, Yoonkyung, Lee, Lenert, Rinnerbauer, Celanovic, Soljačić, Fang, Wang and Kim45,Reference Kats, Byrnes, Blanchard, Kolle, Genevet, Aizenberg and Capasso52–Reference Zhou, Tan, Ji, Zhu, Zhang, Xu, Gan, Yu and Zhu56} Lithography-free fabrication techniques yielding nanocomposite films and coatings are especially attractive for solar-thermal applications owing to their cost effectiveness and scalability.^{Reference Cao, McEnaney, Chen and Ren39} An example of such scalable ultrathin nanocomposite film composed of silver (Ag) and glass (SiO₂) materials is shown in Figure 3b.^{Reference Lu, Raza, Noorulla, Alketbi, Fang, Chen and Zhang57} This nanocomposite absorber traps sunlight in a broad frequency range via excitation of multiple plasmonic resonances. These resonances are excited at several frequencies overlapping with the solar spectrum, and exhibit different spatial distributions of electromagnetic field, as shown in Figure 3b. Multifrequency response of this absorber stems from its complex mesosale internal structure. As surface plasmon resonances decay, their energy is dissipated as heat, elevating the absorber temperature. Furthermore, both sunlight and heat can be trapped inside internally hot, externally cool solar absorbers capped with optically transparent yet thermally insulating materials (Figure 3c). Highly porous aerogels and foams are excellent candidates for thermally insulating absorber coatings due to their extra-low thermal conductivity values. They can be made optically transparent for the solar light by reducing the size of the pores to the nanoscale,^{Reference Ni, Miljkovic, Ghasemi, Huang, Boriskina, Lin, Wang, Xu, Rahman, Zhang and Chen43,Reference Tillotson and Hrubesh58,Reference Zhao, Yang, Bhatia, Strobach and Wang59} and can increase efficiency of blackbody absorbers such as carbon black particles. The infrared camera image shown in Figure 3c illustrates the use of optically transparent silica aerogel insulation to trap heat inside a solar-thermal receiver illuminated by artificial sunlight from a solar simulator.^{Reference Weinstein, McEnaney, Strobach, Yang, Bhatia, Zhao, Huang, Loomis, Cao, Boriskina, Ren, Wang and Chen60,Reference McEnaney, Weinstein, Kraemer, Ghasemi and Chen61}

Spectrally selective coatings are also finding use in daytime radiative cooling applications.^{Reference Chan, Riffat and Zhu62–Reference Hossain and Gu64} Thermal spectra of terrestrial emitters peak in the mid-infrared range (∼7–15 μm). The earth’s atmosphere is transparent within this range, known as the “atmospheric transparency window” (Figure 3a). Mid-infrared photons can escape through this window into outer space, thus causing cooling. Most materials, including vegetation and commercial paints, have high emittance in the mid to far-infrared range, allowing for nighttime radiative cooling.^{Reference Eriksson and Granqvist12,Reference Eriksson, Lushiku and Granqvist65}

Daytime radiative cooling is more challenging, as the thermal radiation process has to compete with heating via sunlight absorption. Recently, this challenge has been met by the development of spectrally selective surfaces that efficiently reflect sunlight and simultaneously emit efficiently in the mid-infrared. The refractive index engineering of multilayered structures consisting of hafnia (HfO₂) and silica (SiO₂) glasses resulted in daytime radiative cooling below the ambient temperature.^{Reference Raman, Anoma, Zhu, Rephaeli and Fan10,Reference Zhu, Raman, Wang, Anoma and Fan66,Reference Chen, Zhu, Raman and Fan67} Glass-polymer hybrid metamaterials as well as metal-lined polymer films have also been developed for daytime radiative cooling.^{Reference Zhai, Ma, David, Zhao, Lou, Tan, Yang and Yin13,Reference Kou, Jurado, Chen, Fan and Minnich14,Reference Beysens, Muselli, Milimouk, Ohayon, Berkowicz, Soyeux, Mileta and Ortega16} Optically transparent polymers such as polyethylene and poly(methyl methacrylate) (PMMA) offer opportunities to create inexpensive, lightweight, and large-scale films for practical applications.

Wearable technologies can also be adapted to incorporate passive radiative cooling functionality. Since human skin is an almost ideal blackbody emitter in the infrared spectral range, fabrics that exhibit a transparency window in the same spectral range can help skin cool via the thermal radiation mechanism (Figure 3d).^{Reference Zandavi, Huang, Ni, Pang, Osgood, Kamal, Jain, Chen and Boriskina68} Polyethylene is a polymer that exhibits uniquely low mid-infrared absorptance and high transmittance for thermal radiation from the skin. Polyethylene fabrics that combine visible opaqueness with infrared transparency have been theoretically predicted^{Reference Tong, Huang, Boriskina, Loomis, Xu and Chen15} and later demonstrated to achieve skin temperature reduction by 1–2^°C relative to conventional textiles.^{Reference Hsu, Song, Catrysse, Liu, Peng, Xie, Fan and Cui69–Reference Jafar-Zanjani, Salary and Mosallaei73} Visible opaqueness of the PE fabrics stems from light scattering by their internal microstructure comprised of either fibers or pores of 1–20 μm in size.

Water purification

Solar heat trapped by selective absorbers can be used for solar-driven water purification.^{Reference Raza, Lu, Alzaim, Li and Zhang74} Vapor generation for water distillation in solar stills is an ancient technology^{Reference Tiwari, Singh and Tripathi5} whose commercial adoption for large-scale applications has long been stymied by higher cost relative to membrane-based water purification techniques. However, passive solar technology offers an attractive solution for small-scale off-grid applications, especially in economically disadvantaged geographical locations or disaster zones. Recently, the interfacial solar vapor generation approach revived interest in solar distillation.^{Reference Neumann, Urban, Day, Lal, Nordlander and Halas75–Reference Liu, Huang, Hsiung, Tian, Wang, Han and Fratalocchi83}

Interfacial evaporation occurs on the surface of water and can be achieved by using a solar absorber floating on the water surface (Figure 4a).^{Reference Ni, Zandavi, Javid, Boriskina, Cooper and Chen40,Reference Wang84–Reference Ni, Li, Boriskina, Li, Yang, Zhang and Chen86} To maintain high efficiency of the evaporation process, parasitic heat losses from the absorber should be minimized. These include optical loss (reflection) as well as thermal loss (heat conduction, convection, and radiation). To reduce all the losses, the absorber needs to be engineered to provide not only spectral selectivity, but also low thermal conductivity for vertical heat localization; a porous microstructure with high wettability for water transport; thermal, humidity and chemical stability; and opportunities for low-cost and scalable fabrication.

Figure 4.

(a) Thermodynamic processes and energy balance in a floating solar still.^{Reference Wang84} (b, c) Thermal management and water path design strategies can increase solar still efficiency. (b) Vertical thermal isolation in mushroom structures reduces thermal losses.^{Reference Xu, Hu, Xu, Li, Zhou, Zhu and Zhu85} (c) Lateral thermal concentration in perforated floating stills enables water boiling under illumination by sunlight without any external optical concentrators.^{Reference Ni, Li, Boriskina, Li, Yang, Zhang and Chen86} (d) Optimizing the ratio of wicking and insulating surfaces mitigates effects of salt fouling and yields self-cleaning solar stills.^{Reference Ni, Zandavi, Javid, Boriskina, Cooper and Chen40}

After demonstrations of the concept of heat localization for interfacial solar evaporation in high concentration plasmonic opto-nanofluids (i.e., suspensions of silica-core gold-coated nanoshells in water) and floating porous carbon foam absorbers,^{Reference Neumann, Urban, Day, Lal, Nordlander and Halas75,Reference Ghasemi, Ni, Marconnet, Loomis, Yerci, Miljkovic and Chen76,Reference Hogan, Urban, Ayala-Orozco, Pimpinelli, Nordlander and Halas87} many solar still designs and materials were reported. These included noble-metal nanospheres, nanoshells and nanorods,^{Reference Zhou, Tan, Wang, Xu, Yuan, Cai, Zhu and Zhu55,Reference Chang, Yang, Liu, Tao, Song, Shang, Wu and Deng78,Reference Wang, He, Liu, Cheng and Zhu81,Reference Wang, Liu, Tao, Shen, Yi, Zhang, Liu, Song, Zhang, Shang and Deng88–Reference Boriskina, Ghasemi and Chen92} less expensive carbon-based black absorbers,^{Reference Jin, Chang, Shi, Shi, Hong and Wang21,Reference Ni, Miljkovic, Ghasemi, Huang, Boriskina, Lin, Wang, Xu, Rahman, Zhang and Chen43,Reference Nafey, Abdelkader, Abdelmotalip and Mabrouk93–Reference Ito, Tanabe, Han, Fujita, Tanigaki and Chen95} and other exotic and nature-inspired materials, including paper, carbonized wood, leaves, and mushrooms.^{Reference Zhuang, Zhou, Xu, Xu, Hu, Li, Lv, Zheng, Zhu, Wang and Zhu79,Reference Liu, Jiang, Tadepalli, Raliya, Biswas, Naik and Singamaneni96–Reference Hong, Shi, Li, Zhang, Jin and Wang98} For example, the structure of mushrooms offers a restricted vertical water pathway, which was utilized in the development of efficient solar evaporators based on natural carbonized mushrooms (Figure 4b).^{Reference Xu, Hu, Xu, Li, Zhou, Zhu and Zhu85} High-temperature solar steam is of special interest for solar sterilization of food, waste, or medical equipment in off-grid locations,^{Reference Neumann, Feronti, Neumann, Dong, Schell, Lu, Kim, Quinn, Thompson, Grady, Nordlander, Oden and Halas99} and can typically only be achieved under concentrated sunlight and in pressurized systems. However, lateral thermal concentration^{Reference Ni, Li, Boriskina, Li, Yang, Zhang and Chen86} (Figure 4c) can increase vapor temperature up to the water boiling point by enlarging the ratio of solar absorption area to the evaporation area. Even higher steam temperatures can be reached in contactless solar stills, where a porous solar absorber is separated from the water surface by an air gap; it heats the water and the water vapor radiatively by emitting infrared photons.^{Reference Cooper, Zandavi, Ni, Tsurimaki, Huang, Boriskina and Chen100}

Overall, passive solar-thermal desalination has high potential for applications in decentralized water purification and zero-liquid discharge desalination, especially for high-concentration brine treatment that presents significant challenges for membrane-based filtration technologies.^{Reference Ferrar, Michanowicz, Christen, Mulcahy, Malone and Sharma101–Reference Van der Bruggen and Vandecasteele103} Solar stills can be used for recycling valuable chemicals dissolved in brine and also produce water, and in combination with power generators^{Reference Yang, Liu, Chen, Li, Duan, Xue, Xu, Xie and Zhou104} or solar-fuel generation systems,^{Reference Gao, Connor and Ho80} they show great promise for urgent survival needs in areas with both water and energy shortages. To avoid fouling of floating solar still materials with salts left behind by the evaporation process, optimum combinations of hydrophilic wicking and hydrophobic insulating materials have been proposed (Figure 4d).^{Reference Ni, Zandavi, Javid, Boriskina, Cooper and Chen40,Reference Liu, Chen, Guo, Cao and Jiang94}

Owing to many improvements in materials and design strategies, single-stage solar still technology is gradually approaching the production rate of mature filtration-based technologies (40–400 L/m²/day for seawater filtration), with a solar vaporization rate of 18–23 L/m²/day recently demonstrated under natural sunlight with a hierarchically nanostructured gel based on poly(vinyl alcohol) (PVA) and polypyrrole (PPy).^{Reference Zhao, Zhou, Shi, Qian, Alexander, Zhao, Mendez, Yang, Qu and Yu105} This high evaporation rate exceeds the photothermal efficiency limit, indicating that the phase-change enthalpy of water in nanoscale-confined space can be reduced, which is of both fundamental and applied importance.

The efficiency of the solar evaporation process can also be increased by environmental energy harvesting^{Reference Li, Li, Lu, Xu, Chen, Min, Zhu, Li, Zhou, Zhu, Zhang and Zhu82} and radiation loss recycling^{Reference Shi, Li, Jin, Zhuo, Shi, Chang, Hong, Ng and Wang106} strategies, as well as strategies to recover the latent heat of water vaporization, which is typically released into the environment once the vapor condenses in the fresh water collector.^{Reference Dongare, Alabastri, Pedersen, Zodrow, Hogan, Neumann, Wu, Wang, Deshmukh, Elimelech, Li, Nordlander and Halas107,Reference Politano, Argurio, Di Profio, Sanna, Cupolillo, Chakraborty, Arafat and Curcio108} A multistage solar still has been recently demonstrated, which recovers and reuses the latent heat several times prior to its release to the environment at lower temperature.^{Reference Chiavazzo, Morciano, Viglino, Fasano and Asinari109} This still enjoys both advantages of thermal-based desalination and membrane-based filtration processes, and uses commercial spectrally selective coating (TiNOX) for solar absorption as well as polyethylene films for thermal insulation. Such a multistage system yields a large-scale purified water production rate of 72 L/m²/day, which is one magnitude higher than that achievable with single-stage solar stills.

Atmospheric water extraction

The atmosphere holds 12,900 billion tons of fresh water, equivalent to ∼10% of the water in all of the lakes and six times the water in all rivers on Earth.^{Reference Schneider, Root and Mastrandrea110} Harvestable atmospheric water, including water vapor and water droplets in fog, is present even in very dry desert regions.^{Reference Schneider, Root and Mastrandrea110} Atmospheric water harvesting is emerging as an alternative approach for arid regions, landlocked, and remote communities.¹¹¹ Fog harvesting is the most ancient way of collecting air water, which has been used by plants, animals, and humans worldwide to harvest fresh water. Many modern nanostructured materials used in fog harvesting actually mimic biological systems of plants and insects.^{Reference Zhang, Wu, Hedhili, Yang and Wang112–Reference Hou, Chen, Xue, Zheng and Jiang116} However, fog harvesting necessitates consistently high (close to 100%) relative humidity (RH) in air, which makes it a viable solution only in some locations.^{Reference Estrela, Valiente, Corell and Millán117,Reference Olivier and de Rautenbach118} In regions with fresh water scarcity, harvesting water vapor from air is a more meaningful approach.^{Reference McHugh, Morrissey, Reed, Hungate and Schwartz119}

Active refrigeration is currently the most popular way to extract water from the atmosphere.^{Reference Wahlgren120,Reference Wikramanayake, Ozkan and Bahadur121} The method uses an engineered cold surface to cool the adjacent air mass below the dew point to produce water droplets via condensation.^{Reference Nilsson17,Reference Guan, Sebben and Bennett18,Reference Wikramanayake, Ozkan and Bahadur121,Reference Kim, Rao, Kapustin, Zhao, Yang, Yaghi and Wang122} A sorption-based approach for atmospheric water vapor harvesting is also gaining popularity (Figure 5), in which a water sorbent, such as metal–organic framework (MOF), anhydrate salts, deliquescent salts, is used to harvest water vapor from air, and it is then heated with assistance from photothermal material to release and subsequently condense the water.^{Reference Kim, Rao, Kapustin, Zhao, Yang, Yaghi and Wang122–Reference Wang, Liu, Wang and Wang125} Effective photothermal materials, such as carbon nanotubes, carbon black particles, and graphene, have been utilized to directly tap sunlight to drive the water vapor release out of the water vapor sorbents.^{Reference Zhao, Zhou, Shi, Qian, Alexander, Zhao, Mendez, Yang, Qu and Yu105} The solar-photothermal process along with an effective water vapor sorbent has recently delivered fully solar energy-driven autonomous atmospheric water generator devices.^{Reference Kim, Rao, Kapustin, Zhao, Yang, Yaghi and Wang122,Reference Kim, Yang, Rao, Narayanan, Kapustin, Furukawa, Umans, Yaghi and Wang123,Reference Li, Shi, Shi, Alsaedi and Wang126}

Figure 5.

Schematics of the working principle of an atmospheric water generator,^{Reference Li, Shi, Shi, Alsaedi and Wang126} based on the use of water sorbent materials such as metal–organic frameworks, anhydrate salts, or deliquescent salts.

An efficient vapor sorbent should be capable of absorbing large amounts of water, even from air with reasonably low RH, and releasing most of the water at a relatively low temperature (60–80^°C) achieved under sunlight illumination.^{Reference Ghasemi, Ni, Marconnet, Loomis, Yerci, Miljkovic and Chen76,Reference Li, Zhang, Shi and Wang127,Reference Liu, Yu, Feng, Bernard, Liu, Zhang, Duan, Shang, Tao, Song and Deng128} Conventional desiccants, such as silica gel, zeolite, and activated alumina have a wide water vapor sorption window, but require high temperatures (>160^°C) to efficiently release most of the captured water.^{Reference Chua, Ng, Chakraborty, Oo and Othman129–Reference Desai, Hussain and Ruthven131} Recently, new material candidates have emerged that are capable of operation under sunlight. These include MOFs such as Zr₆O₄(OH)₄(fumarate)₆,^{Reference Kim, Rao, Kapustin, Zhao, Yang, Yaghi and Wang122,Reference Kim, Yang, Rao, Narayanan, Kapustin, Furukawa, Umans, Yaghi and Wang123} anhydrous and hydrated salt couples (CuCl₂, CuSO₄ and MgSO₄),^{Reference Li, Shi, Shi, Alsaedi and Wang126} and hydrogels.^{Reference Nandakumar, Ravi, Zhang, Guo, Zhang and Tan132} It is expected that other sorbent materials will emerge in the near future with large water sorption capacities and easy water release. As research on photothermal conversion materials progresses, higher temperatures may be produced under natural sunlight, which would broaden the water sorbent materials pool.