[Image above] A new CaCO3-acrylic paint looks very similar to commercial white paint, but infrared images reveal that the newly developed paint stays much cooler in direct sunlight. Credit: (top) Li et al., Cell Reports Physical Science (CC BY-NC-ND 4.0); (bottom) Joseph Peoples, Purdue University
Almost every week this summer felt like one natural disaster after another—hurricanes, wildfires, a derecho. But one factor underlying many of these events that stayed consistent throughout the summer was the elevated temperatures.
Summer 2020 ranked as one of the hottest on record for the United States, and the month of July tied for the second-hottest July on record for the globe. And even though people do not typically worry about extreme heat as much as they do other natural disasters, extreme heat is actually more deadly than your typical hurricanes and fires—an average 12,000 heat-related premature deaths occur in the contiguous United States each year, according to a recent study led by Duke University.
As extreme summer heat becomes more common and intense in the coming decades, people will need more access to temperature-regulated living spaces. However, relying on air conditioning to achieve cooler temperatures is not an ideal approach.
Most air conditioning units run on electricity, which means installing more air conditioners will increase electric power demands and place greater strain on electric systems, potentially requiring costly repairs and maintenance efforts in the years to come. In addition, potent greenhouse gases called hydrofluorocarbons are used as refrigerants in many air conditioning units. These gases are considerably more potent as heat-trapping gases than carbon dioxide, which is bad news for the environment if they leak out.
Of course, designing more energy-efficient units and alternative refrigerants can help address these concerns. However, there are other ways besides air conditioning to regulate temperature as well—such as passive design.
Passive design relies on using insulation, orientation, thermal mass, and natural ventilation to achieve optimal temperatures without air conditioning. An example of a passive design element is thermochromic glass windows, which naturally turn opaque in response to heat and thus limit the amount of thermal radiation entering a building.
Radiative cooling paint is another example of a passive design element. Such paint reflects sunlight and offsets heat gains inside a building.
Titanium dioxide pigment is the main ingredient in established radiative cooling paints on the market, as it effectively reflects most visible and near-infrared light. However, titanium dioxide absorbs ultraviolet rays, which can lead to heating under intense sunlight—thus complicating efforts to keep a building as cool as possible.
Scientists have investigated other materials with higher band gaps to reduce ultraviolet absorption, such as SiO2, ZnS, and BaSO4. But these materials tend to have lower refractive indices compared to titanium dioxide, meaning they do not scatter light as efficiently.
In short, “none of the [existing commercial heat-reflective] paints have achieved full daytime sub-ambient cooling,” researchers write in a recent open-access paper.
The researchers come from Purdue University in Indiana, though some of the postdoctoral researchers are now associate professors at universities in China. The recent paper is the culmination of a six-year study that builds on attempts going back to the 1970s to develop radiative cooling paint for passive building design.
In their paper, the researchers explain that both high solar reflectance (wavelength 0–3 μm) and high sky window emissivity (wavelength 8–13 μm) is needed to create a paint capable of full daytime sub-ambient cooling. To achieve these properties, the researchers used a systematic approach that considered multiple factors.
First, to minimize solar absorption in the ultraviolet region, they investigated materials with higher electron band gaps. According to a Purdue press release, they considered more than 100 different material combinations and narrowed them down to 10. After testing about 50 different formulations for each material, they settled on a calcium carbonate (CaCO3)-acrylic formulation.
Like other materials with higher band gaps, calcium carbonate has a low refractive index. To compensate for this limitation, they adopted a particle volume concentration of 60%. While higher particle concentration typically leads to decreased reflectance, “as the particle concentration continues to increase and pass the critical particle volume concentration (CPVC), the reflectance will increase again.”
Finally, the researchers designed their calcium carbonate filler with a broad particle size distribution instead of a single size “to efficiently scatter all of the wavelengths in the solar spectrum, and hence enhance the solar reflectance, as predicted in our previous simulations.” In addition, “the acrylic matrix introduces vibrational resonance peaks in the [infrared] band, thus ensuring a high sky window emissivity.”
In lab testing, the researchers showed their CaCO3-acrylic paint reaches 95.5% reflectance in the solar spectrum, substantially higher than solar reflectance values of commercial radiative cooling paints, which are about 80%–90%. And in field testing, the paint demonstrated full daytime cooling. For example, in a field test in West Lafayette, Indiana, on March 21–23, 2018, the sample stayed 10°C below the ambient temperature at night and at least 1.7°C below the ambient temperature at a peak solar irradiation of about 963 W/m2.
“Our radiative cooling paint showed a cooling performance that was among the best of the reported state-of-the-art approaches, while offering unprecedented combined benefits, including convenient paint form, low cost, and compatibility with commercial paint fabrication processes,” they write in the conclusion.
The team filed an international patent application on this paint formulation and is currently conducting further comparisons between its CaCO3-acrylic paint with a wide range of commercial paints. The team is also working to develop other paint colors that could have cooling benefits.
The open-access paper, published in Cell Reports Physical Science, is “Full daytime sub-ambient radiative cooling in commercial-like paints with high figure of merit” (DOI: 10.1016/j.xcrp.2020.100221).