A research team led by UCLA materials scientists has demonstrated ways to make super white paint that reflects as much as 98% of incoming heat from the sun.
The advance shows practical pathways for designing paints that, if used on rooftops and other parts of a building, could significantly reduce cooling costs, beyond what standard white ‘cool-roof’ paints can achieve.
The findings, published online in Joule, are a major and practical step towards keeping buildings cooler by passive daytime radiative cooling – a spontaneous process in which a surface reflects sunlight and radiates heat into space, cooling down to potentially sub-ambient temperatures.
This can lower indoor temperatures and help cut down on air conditioner use and associated carbon dioxide emissions.
“When you wear a white T-shirt on a hot sunny day, you feel cooler than if you wore one that’s darker in color – that’s because the white shirt reflects more sunlight and it’s the same concept for buildings,” said Aaswath Raman, an assistant professor of materials science and engineering at UCLA Samueli School of Engineering, and the principal investigator on the study.
“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.”
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.
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 examined replacing titanium oxide with inexpensive and readily available ingredients such as barite, which is an artist’s pigment, and powered polytetrafluoroethylene, better known as Teflon.
These ingredients help paints 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 Schmidt Science Fellow working in Raman’s research group and the co-corresponding author on the research.
Beyond the advance, the authors suggested several long-term implications for further study, including mapping where such paints could make a difference, studying the effect of pollution on radiative cooling technologies, and on a global scale, if they could make a dent on the earth’s own ability to reflect heat from the sun.
The researchers also noted that many municipalities and governments, including the state of California and New York City, have started to encourage 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,” said Mandal, who has studied cooling paint technologies for several years.
“This would require a collaboration among experts in diverse fields like optics, materials science and meteorology, and experts from the industry and policy sectors.”
Researchers at Columbia Engineering have invented a high-performance exterior PDRC polymer coating with nano-to-microscale air voids that acts as a spontaneous air cooler and can be fabricated, dyed, and applied like paint on rooftops, buildings, water tanks, vehicles, even spacecraft – anything that can be painted.
They used a solution-based phase-inversion technique that gives the polymer a porous foam-like structure. The air voids in the porous polymer scatter and reflect sunlight, due to the difference in the refractive index between the air voids and the surrounding polymer.
The polymer turns white and thus avoids solar heating, while its intrinsic emittance causes it to efficiently lose heat to the sky.
The team—Yuan Yang, assistant professor of materials science and engineering; Nanfang Yu, associate professor of applied physics; and Jyotirmoy Mandal, lead author of the study and a doctoral student in Yang’s group (all department of applied physics and applied mathematics)—built upon earlier work that demonstrated that simple plastics and polymers, including acrylic, silicone, and PET, are excellent heat radiators and could be used for PDRC.
The challenges were how to get these normally transparent polymers to reflect sunlight without using silver mirrors as reflectors and how to make them easily deployable.
They decided to use phase-inversion because it is a simple, solution-based method for making light-scattering air-voids in polymers. Polymers and solvents are already used in paints, and the Columbia Engineering method essentially replaces the pigments in white paint with air voids that reflect all wavelengths of sunlight, from UV to infrared.
“This simple but fundamental modification yields exceptional reflectance and emittance that equal or surpass those of state-of-the-art PDRC designs, but with a convenience that is almost paint-like,” says Mandal.
The researchers found their polymer coating’s high solar reflectance (R > 96%) and high thermal emittance (Ɛ ~ 97%) kept it significantly cooler than its environment under widely different skies, e.g. by 6˚C in the warm, arid desert in Arizona and 3˚C in the foggy, tropical environment of Bangladesh.
“The fact that cooling is achieved in both desert and tropical climates, without any thermal protection or shielding, demonstrates the utility of our design wherever cooling is required,” Yang notes.
The team also created colored polymer coatings with cooling capabilities by adding dyes. “Achieving a superior balance between color and cooling performance over current paints is one of the most important aspects of our work,” Yu notes. “For exterior coatings, the choice of color is often subjective, and paint manufacturers have been trying to make colored coatings, like those for roofs, for decades.”
The group took environmental and operational issues, such as recyclability, bio-compatibility, and high-temperature operability, into consideration, and showed that their technique can be generalized to a range of polymers to achieve these functionalities. “Polymers are an amazingly diverse class of materials, and because this technique is generic, additional desirable properties can be conveniently integrated into our PDRC coatings, if suitable polymers are available,” Mandal adds.
“Nature offers many ways for heating and cooling, some of which are extremely well known and widely studied and others that are poorly known. Radiative cooling—by using the sky as a heat sink—belongs to the latter group, and its potential has been strangely overlooked by materials scientists until a few years ago,” says Uppsala University Physics Professor Claes-Göran Granqvist, a pioneer in the field of radiative cooling, who was not involved with the study.
“The publication by Mandal et al. highlights the importance of radiative cooling and represents an important breakthrough by demonstrating that hierarchically porous polymer coatings, which can be prepared cheaply and conveniently, give excellent cooling even in full sunlight.”
Yang, Yu, and Mandal are refining their design in terms of applicability, while exploring possibilities such as the use of completely biocompatible polymers and solvents. They are in talks with industry about next steps.
“Now is a critical time to develop promising solutions for sustainable humanity,” Yang notes. “This year, we witnessed heat waves and record-breaking temperatures in North America, Europe, Asia, and Australia. It is essential that we find solutions to this climate challenge, and we are very excited to be working on this new technology that addresses it.”
Yu adds that he used to think that white was the most unattainable color: “When I studied watercolor painting years ago, white paints were the most expensive. Cremnitz white or lead white was the choice of great masters, including Rembrandt and Lucian Freud.
We have now demonstrated that white is in fact the most achievable color. It can be made using nothing more than properly sized air voids embedded in a transparent medium. Air voids are what make snow white and Saharan silver ants silvery.”
More information: Jyotirmoy Mandal et al, Paints as a Scalable and Effective Radiative Cooling Technology for Buildings, Joule (2020). DOI: 10.1016/j.joule.2020.04.010
