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Cheap Plastic Film Cools Whatever It Touches by Up to 10°C

posted by Fnord666 on Tuesday February 14 2017, @06:37AM   
from the that's-pretty-cool dept.

charon writes:

If heat is not your thing, rejoice: A thin plastic sheet may soon provide some relief from the intense summer sun. The film, made from transparent plastic embedded with tiny glass spheres, absorbs almost no visible light, yet pulls in heat from any surface it touches. Already, the new material, when combined with a mirrorlike silver film, has been shown to cool whatever it sits on by as much as 10°C. And because it can be made cheaply at high volumes, it could be used to passively cool buildings and electronics such as solar cells, which work more efficiently at lower temperatures.

During the day most materials—concrete, asphalt, metals, and even people—absorb visible and near-infrared (IR) light from the sun. That added energy excites molecules, which warm up and, over time, emit the energy back out as photons with longer wavelengths, typically in the midrange of the infrared spectrum. That helps the materials cool back down, particularly at night when they are no longer absorbing visible light but are still radiating IR photons.

[...] So they [Xiaobo Yin and his team] bought a batch of glass powder from a commercial supplier and mixed it with the starting material for a transparent plastic called polymethylpentene. They then formed their material into 300-millimeter-wide sheets and backed them with a thin mirrorlike coating of silver. When laid across objects in the midday sun, the bottom layer of silver reflected almost all the visible light that hit it: The film absorbed only about 4% of incoming photons. At the same time, the film sucked heat out of whatever surface it was sitting on and radiated that energy at a mid-IR frequency of 10 micrometers. Because few air molecules absorb IR at that frequency, the radiation drifts into empty space without warming the air or the surrounding materials, causing the objects below to cool by as much as 10°C. Just as important, Yin notes that the new film can be made in a roll-to-roll setup for a cost of only $0.25 to $0.50 per square meter.

An abstract of the paper is available online.

Yao Zhai, et al. Scalable-manufactured randomized glass-polymer hybrid metamaterial for daytime radiative cooling, Science, 09 Feb 2017, DOI: 10.1126/science.aai7899

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Original Submission

• Excepts (Score: 0) by Anonymous Coward on Tuesday February 14 2017, @09:58AM

  by Anonymous Coward on Tuesday February 14 2017, @09:58AM (#466905)

  Excepts from the original FA - citations are "fair use", yes?

  We experimentally demonstrate an average noon-time (11am – 2pm) radiative cooling power of 93 W/m2 under direct
  sunshine during a three-day field test, and an average cooling power > 110 W/m2 over the continuous 72-hour day and night test.

  The metamaterial was fabricated in 300-mm-wide sheets at a rate of 5 m/min, such that in the course of experiment we produced hundreds of square meters of the material. The proposed structure of the randomized, glass-polymer hybrid metamaterial contains micrometer-sized SiO2 spheres randomly distributed in the matrix material of polymethylpentene (TPX) (Fig. 1A). We used TPX due to its excellent solar transmittance. Other visibly transparent polymers such as Poly(methyl methacrylate) and polyethylene
  can be used but would slightly increase solar absorption. Because both the polymer matrix material and the encapsulated SiO2 microspheres are lossless in the solar spectrum, absorption is nearly absent and direct solar irradiance does not heat the metamaterial.

  ... The extinction peaks at a size parameter of ~ 2.5, corresponding to a microsphere radius of ~ 4μm. The size parameter of the microsphere plays a key role in designing the hybrid metamaterial for radiative cooling.... A 50-μm-thick metamaterial film containing 6% of microspheres by volume has an averaged infrared emissivity > 0.93 and reflects approximately 96% of solar irradiance when backed with a 200-nm-thick silver coating.
  ...
  The hybrid metamaterial strongly reflects solar irradiation when backed with a 200-nm-thick silver thin film (Fig. 3A) prepared by electron beam evaporation. ...
  ...

  ...We produced a roll of 300-mm-wide and 50-μm-thick hybrid metamaterial film at a rate of 5 m/min (Fig. 4A). We controlled the volume concentration of the SiO2 microspheres by using gravimetric feeders. The resultant film has a homogeneous distribution of microspheres, with fluctuations in concentration of less than 0.4% (fig. S1) (32). The hybrid metamaterial films are translucent due to the scattering of visible light from the microsphere inclusions (fig. S2) (32). Additionally, when backed with a 200-nm-thick reflective silver coating, the hybrid metamaterial has a balanced white color (fig. S2) (32). The strongly scattering and nonspecular optical response of the metamaterial will avoid back-reflected glare, which can have detrimental visual effects for humans and interfere with aircraft operations (33).