The good news about global warming is that people will use a lot less energy to keep homes and commercial buildings warm. The bad news is people will use a lot more energy to keep homes and commercial buildings cool. Materials scientists at UCLA say they have come up with a super white paint that reflects up to 98% of incoming heat from the sun. Their research was published recently in the journal Joule. If used as an exterior coating on roofs and other building surfaces, it could significantly reduce cooling costs and outperform what standard white “cool-roof” paints can do.
“When you wear a white T-shirt on a hot sunny day, you feel cooler than if you wear one that’s darker in color. That’s because the white shirt reflects more sunlight and it’s the same concept for buildings,” says Aaswath Raman, an assistant professor of materials science and engineering at UCLA Samueli School of Engineering. “A roof painted white will be cooler inside than one in a darker shade. But those paints also do something else. They reject heat at infrared wavelengths, which we humans cannot see with our eyes. This could allow buildings to cool down even more by radiative cooling.” Radiative cooling can actually cool surfaces to below ambient temperature.
The best performing white paints currently available typically reflect around 85% of incoming solar radiation. The remainder is absorbed by the chemical makeup of the paint. The researchers showed that simple modifications in a paint’s ingredients could offer a significant jump, reflecting as much as 98% of incoming radiation, according to UCLA.
Current white paints with high solar reflectance use titanium oxide. While the compound is very reflective of most visible and near-infrared light, it also absorbs ultraviolet and violet light. The compound’s UV absorption qualities make it useful in sunscreen lotions, but they also lead to heating under sunlight — which gets in the way of keeping a building as cool as possible.
The researchers replaced titanium oxide with inexpensive and readily available ingredients such as barite, a white pigment used by artists, and powdered Teflon — both of which reflect UV light. The team also made further refinements to the paint’s formula, including reducing the concentration of polymer binders, which also absorb heat.
“The potential cooling benefits this can yield may be realized in the near future because the modifications we propose are within the capabilities of the paint and coatings industry,” said UCLA postdoctoral scholar Jyotirmoy Mandal, a corresponding author of the research study.
Cooling The Built Environment
Many state and local governments, including the State of California and New York City, are encouraging the use of “cool roof” technologies for 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,” says 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.”
Protecting the built environment from solar gain — the addition of heat from the sun to interior spaces — will be an important part of making a warmer world habitable in coming years. The first imperative for addressing climate change is to stop burning stuff to create the energy we need to power our lives so that we stop adding carbon dioxide to the atmosphere. Much of that electricity is used to heat and cool the buildings we live and work in.
Infrared and ultraviolet radiation are often overlooked because those portions of sunlight are not visible to the human eye. What we can’t see, we tend to ignore. But both infrared and ultraviolet rays raise the temperature of any surface they strike. Researchers at the University of Colorado Boulder have focused their attention on the infrared portion of the spectrum to create coatings for solar panels that keep them cooler, thereby raising their efficiency by 1 to 2% — a small thing for each individual panel, but a huge gain for a solar farm developer when applied to tens or hundreds of thousands of panels.
Researchers at Columbia University have also created a polymer coating for the exterior surfaces of buildings that significantly reduces how hot they get when the sun beats down on them. These sorts of heat rejection techniques can play an important role in reducing the “heat island” effect of large cities. They get so hot during the day, they can’t fully cool down overnight, meaning they start each day hotter than they should be.
As the environment warms, such passive cooling strategies will be crucial to allowing humans to continue to live comfortably as outside temperatures continue to climb.