Experimentation of a film rejecting 70% of solar heat
In an article online since November 8, 20181, a team of researchers from the University of Hong Kong and from the Massachusetts Institute of Technology (MIT) explored the ability of a phase-change material set on windows to reject incoming solar heat.
A significant portion of a building's heat comes from windows, because of sunlight. Even if smart windows already exist on the market, they are either not very efficient, or, like some electrochromic windows, they may need more power to drive them, according to one of the authors, Nicholas Fang. The aim of the study was therefore to find an efficient material that does not consume energy. The researchers also wanted to find a material that remained quite transparent.
They searched among different thermochromic materials (that is temperature-sensitive materials that temporarily change phase or color in response to heat). They found that poly (N-isopropylacrylamide)-2-Aminoethylmethacrylate hydrochloride had interesting properties. The microparticles are filled with water, and at a temperature close to 29.5°C or higher, they essentially squeeze out all their water and shrink into tight bundles of fibres that reflect light in a different way, turning the material translucent.
In order to conduct the experiment, a solution composed of those particles was applied between two sheets of approximately 30 cm-by-30 cm glass to create a film-coated window. The researchers shone light from a solar simulator onto the window, and they found that the film turned frosty in response to the heat. The solar irradiance transmitted through the other side of the window was measured. It showed that 70% of the heat produced by the lamp was rejected.
One of the parameters that allowed this experience to succeed was the size of the microparticles. Indeed, in previous studies, similar materials were used but they did not reject heat as well. The limitation came from the size of the particles: their diameter shrank to about 100 nanometres. The particles used by the team of Hong-Kong University and the MIT shrank to a diameter of about 500 nanometers, a size "more compatible to the infrared spectrum of solar light", according to Fang.
In the future, the researchers plan to conduct more tests of the film to see whether tweaking its formula and applying it in other ways might improve its heat-shielding properties.
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1 LI X.-H., LIU C., FENG S.-P., et al. Broadband Light Management with Thermochromic Hydrogel Microparticles for Smart Windows. Joule. 2018, November 8. DOI: 10.1016/j.joule.2018.10.019