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- Many plant species located in northern climates can acclimate under these cold conditions by supercooling, thus these plants survive temperatures as low as −40 °C (−40 °F).
en.wikipedia.org/wiki/Supercooling
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Oct 9, 2023 · Supercool (verb, “SOOP-er-kool”) To supercool a liquid means to chill it below its freezing point without it turning solid. Supercooling can happen because liquids are loose jumbles of atoms. It’s hard for such disordered atoms to lock themselves into the crystal structure of a solid.
Supercooling, [1] also known as undercooling, [2][3] is the process of lowering the temperature of a liquid below its freezing point without it becoming a solid.
Deep supercooling in plant tissues prevents intracellular freezing while limiting the degree of cellular dehydration (Fujikawa et al., 1996). Supercooling refers to the cooling of a liquid below the freezing temperature that is expected based on the solute concentration.
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When businessman Howard Bisla was tasked with saving a local shop from financial ruin, one of his first concerns was energy efficiency. In June 2018, he approached his local electricity provider in Sacramento, California, about upgrading the lights. The provider had another idea. It offered to install an experimental cooling system: panels that could stay colder than their surroundings, even under the blazing hot sun, without consuming energy.
The aluminium-backed panels now sit on the shop’s roof, their mirrored surfaces coated with a thin cooling film and angled to the sky. They cool liquid in pipes underneath that run into the shop, and, together with new lights, have reduced electricity bills by around 15%. “Even on a hot day, they’re not hot,” Bisla says.
The panels emerged from a discovery at Stanford University in California. In 2014, researchers there announced that they had created a material that stayed colder than its surroundings in direct sunlight1. Two members of the team, Shanhui Fan and Aaswath Raman, with colleague Eli Goldstein, founded a start-up firm, SkyCool Systems, and supplied Bisla’s panels. Since then, they and other researchers have made a host of materials, including films, spray paints and treated wood, that stay cool in the heat.
These materials all rely on enhancing a natural heat-shedding effect known as passive radiative cooling. Every person, building and object on Earth radiates heat, but the planet’s blanket-like atmosphere absorbs most of it and radiates it back. Infrared rays between 8 and 13 micrometres in wavelength, however, are not captured by the atmosphere and leave Earth, escaping into cold outer space. As far back as the 1960s, scientists sought to harness this phenomenon for practical use. But passive radiative cooling is noticeable only at night: in the daytime, sunlight bathes us in much more heat energy than we can send into space.
The new materials reflect a broad spectrum of light, in much the same way as mirrors or white paint do. In the crucial 8–13-µm part of the infrared spectrum, however, they strongly absorb and then emit radiation. When the materials point at the sky, the infrared rays can pass straight through the atmosphere and into space. That effectively links the materials to an inexhaustible heat sink, into which they can keep dumping heat without it coming back. As a result, they can radiate away enough heat to consistently stay a few degrees cooler than surrounding air; research suggests that temperature differences could exceed 10 °C in hot, dry places2,3. David Sailor, who leads the Urban Climate Research Center at Arizona State University in Tempe, has termed them super-cool materials.
In 2012, Raman — who was completing his PhD with Fan on materials for harvesting solar energy — stumbled on old studies about passive radiative cooling, an effect he’d not heard of. Realizing that no one had worked out how to use it under direct sunlight, he examined the optical properties a material would need to overcome the Sun’s heat. It must reflect the solar spectrum in wavelengths from 200 nanometres to 2.5 µm even more effectively than white paint, which is already up to 94% reflective. And it must absorb and emit as close as possible to 100% of the wavelengths in the crucial 8–13-µm range (see ‘Keeping their cool’).
All this could be done by engineering materials at the nanoscale, Raman and Fan thought. Creating structures smaller than the wavelengths of light that will pass through them should enhance the absorption and emission of some wavelengths and suppress that of others.
The group came up with the idea to etch patterns into surfaces4 and published it in 2013. Then the team submitted a proposal to the US Advanced Research Projects Agency—Energy (ARPA-E) for funding to make it.
“I immediately thought, ‘Wow, I’d really like to see somebody actually do this,’” recalls Howard Branz, then a programme director at ARPA-E in Washington DC, and now a technology consultant in Boulder, Colorado. “There’d been a lot of night-time radiative-cooling work, but to do it under broad, full sunlight is quite startling.”
Branz gave the researchers US$400,000 and a year. With so little time, the Stanford team decided to simplify the design and try layering materials in more familiar ways. To create something highly reflective, the researchers alternated four thin layers of materials that refract light strongly (hafnium dioxide) and weakly (silicon dioxide, or glass), a commonly used motif in optical engineering that works because of how light waves interfere as they pass through different layers. They used the same principle to amplify infrared emissions, depositing three thicker layers of the same materials on top.
When they tested their material outdoors1, it stayed almost 5 °C cooler than the ambient temperature, even under direct sunlight of around 850 watts per square metre. (On a bright, clear day at sea level, the intensity of sunlight directly overhead reaches around 1,000 Wm2).
Almost all the research teams have patented their inventions and are now trying to market them. Gan is working with industry partners, which he declined to name, to commercialize the PDMS–aluminium film. Columbia University has licensed its super-cool paint to New York start-up MetaRE, founded by Mandal and Yang’s Columbia collaborator Nanfang Yu, for development. MetaRE is also working with industry to develop the paint for roofing, refrigerated transportation, storage and textile applications, says chief executive April Tian. The product is “highly competitive” with conventional paints, she says.
Other start-ups have highlighted how much electricity their products could save. Fan and Raman have developed a proprietary system for SkyCool Systems’ panels. In 2017, they predicted that the system could reduce the amount of electricity a building uses for cooling by 21% during the summer in hot, dry Las Vegas, Nevada8. Raman says the panels will pay for themselves in three to five years. Yin and Ronggui Yang have started a company in Boulder called Radi-Cool, to commercialize the glass-embedded plastic. Last January, they reported that the material could reduce electricity consumption for cooling in the summer by 32–45% if it were integrated with water chillers in commercial buildings in Phoenix, Arizona; Miami, Florida; and Houston, Texas9. Hu, meanwhile, has licensed the super-cool wood material to a Maryland-based firm he co-founded called InventWood. He predicts that it could save 20–35% of cooling energy across 16 US cities7.
But these estimates are based on experiments and models that are too limited to be extrapolated to whole buildings in cities, cautions Diana Ürge-Vorsatz, an environmental scientist at the Central European University in Budapest who specializes in climate-change mitigation. Actual energy savings and how quickly a super-cool material will pay for itself will depend on a building’s structure, location and weather conditions, adds Yin.
Location is the biggest obstacle. “There are certain geographical regions where it just won’t work because the atmosphere isn’t dry enough,” says James Klausner, a mechanical engineer at Michigan State University in East Lansing who served as an ARPA-E programme director after Branz and has funded some proposals in the field. But that’s not too off-putting, he says, because the regions where the effect works well are arid areas such as the southwestern United States or the Middle East, which have high demands for air conditioning.
Another challenge is that radiative-cooling systems might increase heating costs in winter. To address this problem, Santamouris is trying to introduce a liquid layer on top of the super-cool materials that would freeze when the temperature drops low enough. Once the liquid solidifies, radiation can no longer escape to space, so the cooling effect is cut off. And last October, Mandal and Yang reported another way to stop overcooling10. If they fill the pores of their polymer coating with isopropanol, the coating starts to trap heat rather than shed it. This can be reversed by blowing air through the pores to dry them out.
There’s another issue: the materials achieve super-cooling only if they can send their radiation directly to the cold heat sink of outer space. In an urban setting, buildings, people and other objects can get in the way, absorbing the heat and re-emitting it. The best-performing materials currently remove heat at a rate of around 100 Wm–2. Gan and Yu hope to double that by positioning their films perpendicular to the roof so that emissions can escape from both surfaces. But this will require adding materials around the films that can reflect the emissions up into the sky.
•How heat from the Sun can keep us all cool
•First sun-dimming experiment will test a way to cool Earth
- XiaoZhi Lim
- 2020
Apr 1, 2022 · Supercooling is a natural phenomenon that keeps a phase change material (PCM) in its liquid state at a temperature lower than its solidification temperature. In the field of thermal energy storage systems, entering in supercooled state is generally considered as a drawback, since it prevents the release of the latent heat.
Jun 1, 2016 · Supercooling is a metastable state of water in which it remains liquid below its freezing point. It is a freeze avoidance mechanism, which in some cases, is supported by the presence of antifreeze proteins [5] and antifreeze agents. Supercooling is disrupted by freezing, which by itself is initiated by ice nucleation.
In the course of a year, perennial plants in temperate climates are exposed to several types of freezing stress including low temperature extremes, ice encasement, and unseasonable episodes of frost. Many plants can adapt to survive freezing through a process of cold acclimation.
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