Super-white paint developed by a team at the University of California Los Angeles can reflect almost all of the incident heat from the sun and so could be used to reducing cooling system demands and bills for buildings in hot climes by painting roofs and areas of exterior walls to keep the building cool. The researchers describe the passive approach to daytime cooling of buildings in the journal Joule. (Mandal, J. et al. Joule (2020); DOI: 10.1016/j.joule.2020.04.010].
Anyone who has swapped between dark and light-colored clothes while out in the sun will know that they feel much cooler in the brighter fabrics, at least while the sun is shining on them. It is different in the shade where dark clothes will radiate body heat faster. Obviously, it would be impractical to create white fabric covers for buildings but a lick of white paint would do the same trick. Indeed, many buildings are painted white in the sunnier parts of the world for this very reason. However, there is a limit to how much of the sun's heat energy can be reflected by conventional paints even when infrared wavelengths are taken into account.
Indeed, UCLA's Aaswath Raman points out that the best "cooling" paints manage up to 85 percent reflectivity by using titanium dioxide as one of their ingredients. Unfortunately, titania also absorb ultraviolet, which leads to heating of the painted surface. Raman and his colleagues at UCLA and at Columbia University wanted to cancel this effect too, so they began looking for alternative additives.
They investigated the effects of barite, barium sulfate, a well-known artistic pigment and polytetrafluoroethylene, PTFE, well known as the non-stick material Teflon. These two materials can reflect ultraviolet. The researchers also looked to reformulating their paint to reduce the amount of polymer binders used as these too absorb heat negating the cooling effect to a degree.
"The potential cooling benefits this [new formulation] can yield may be realized in the near future because the modifications we propose are within the capabilities of the paint and coatings industry," explains UCLA's Jyotirmoy Mandal. In parallel with reformulating cool paints, the team also suggests that research needs to map the parts of a building where the application of such paints might have the greatest impact. They also need to consider the effect of pollution. Moreover, such materials might perhaps fancifully have a global effect if used on a sufficiently large scale allowing us to engineer the Earth's albedo and radiative cooling in the context of global warming. Many places, including the state of California and New York City, are already encouraging developers to incorporate cool-roof technologies into new buildings.
"We hope that the work will spur future initiatives in super-white coatings for not only energy savings in buildings, but also mitigating the heat island effects of cities, and perhaps even showing a practical way that, if applied on a massive, global scale could affect climate change," explains Mandal. "This would require a collaboration among experts in diverse fields like optics, materials science and meteorology, and experts from the industry and policy sectors."