As temperatures rise during hot weather, keeping buildings and other structures cool can require large amounts of energy. Air conditioners are a common way to deal with the heat, but they consume electricity and need access to cooling systems and, in some cases, coolants that can affect the environment.Researchers at Columbia Engineering have created a polymer coating that can be applied like paint and uses microscopic air voids instead of white pigments. According to the University’s engineering department, the coating can reflect sunlight while also releasing heat towards the sky, allowing the surface to cool below the surrounding air temperature.The researchers tested the coating in different environments. Under the warm, dry conditions of the Arizona desert, the coated surface became about 6°C cooler than the ambient air. On the ther hand, in a foggy, tropical environment in Bangladesh, it achieved cooling of about 3°C below the surrounding air.The work is based on a process called passive daytime radiative cooling (PDRC). It allows a surface to cool without using an external energy source by reflecting sunlight and radiating heat towards the colder sky. The study, titled ‘Hierarchically Porous Polymer Coatings for Highly Efficient Passive Daytime Radiative Cooling’, was published in journal Science.
How coating works
For PDRC to work effectively, a surface needs to do two things. It must reflect a large amount of sunlight so that it does not gain too much heat, and it must have high thermal emittance so that it can release heat towards the sky.The Columbia Engineering team designed the polymer coating to achieve both. The researchers used a solution-based process known as phase-inversion to create a porous, foam-like structure inside the polymer.The structure contains air voids ranging from the nano to microscale. These tiny spaces help the material scatter and reflect sunlight because air and the surrounding polymer have different refractive indices.This approach also changes how the polymer looks. The polymers used by the researchers can normally be transparent, but the air voids make the material appear white. The researchers therefore used the air voids in place of the pigments normally used to make white paint.The coating has a solar reflectance of more than 96% and a thermal emittance of about 97%. According to the researchers, these properties allowed it to remain significantly cooler than its surroundings under different sky conditions.
Air voids replace white pigments
The researchers built on earlier work showing that simple polymers such as acrylic, silicone and PET can be effective heat radiators and could be used for PDRC.The main challenge was that these polymers are normally transparent. A transparent material does not reflect enough sunlight for effective daytime radiative cooling. Existing approaches could use reflectors such as silver mirrors, but the researchers wanted a method that would be easier to make and apply.They chose phase-inversion because it is a simple, solution-based method for producing light-scattering air voids inside polymers. Since polymers and solvents are already used in paints, the process could produce a coating that is close to paint in how it can be applied.The researchers said the air voids can reflect sunlight across wavelengths ranging from ultraviolet to infrared. This allows the coating to avoid absorbing much of the incoming solar energy while its thermal properties help it release heat towards the sky.“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,” lead author Jyotirmoy Mandal said.
Impact in desert and tropical conditions
The researchers tested the material under different environmental conditions to see whether its cooling ability could work beyond a single climate.In the warm, arid desert environment in Arizona, the coating cooled to about 6°C below the ambient temperature. In Bangladesh’s foggy, tropical environment, it cooled to about 3°C below ambient temperature.The difference was important because PDRC can be affected by atmospheric conditions. “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 said.The coating was designed as an exterior material. The researchers said it could be applied to rooftops and buildings, as well as water tanks, vehicles and spacecraft. In principle, it can be used on surfaces that can be painted.The team also worked on color limitation that can affect cooling. Conventional cooling surfaces often rely on white because white materials can reflect sunlight, but exterior surfaces are not always required or preferred to be white.
Researchers made coloured versions
The Columbia team added dyes to create coloured versions of the polymer coating while retaining cooling capabilities. Nanfang Yu said balancing colour and cooling performance was an important part of the work because the colour of an exterior coating can be a matter of choice. He noted that paint manufacturers had been working for years on coloured coatings for uses such as roofs.The researchers also considered environmental and operational factors, including recyclability, biocompatibility and high-temperature operation. Their tests showed that the technique could be applied to different polymers to achieve these properties.Mandal said the wide range of polymers available could allow other properties to be incorporated into PDRC coatings when suitable polymers and solvents are available.
Using sky to release heat
The idea behind radiative cooling is that the sky can act as a heat sink. A surface exposed to the sky can lose heat through thermal radiation while reflecting incoming sunlight.Claes-Goran Granqvist, a physics professor at Uppsala University and a pioneer in radiative cooling, who was not involved in the study, said radiative cooling had received less attention from materials scientists than some other methods of heating and cooling.“Nature offers many ways for heating and cooling, some of which are extremely well known and widely studied and others that are poorly known,” Granqvist said.He described radiative cooling as one of the less well-known approaches and said the study demonstrated the potential of hierarchically porous polymer coatings.The researchers said their work showed that the coating could provide cooling even in full sunlight. Its porous structure allows sunlight to be reflected, while its thermal emittance allows heat to be radiated towards the sky.
Further applications
After demonstrating the cooling effect, Yang, Yu and Mandal continued refining the design for practical use. They also explored the use of completely biocompatible polymers and solvents. The researchers said they were in talks with industry about possible next steps.Yang linked the work to the need for cooling technologies as heat waves and high temperatures affect different parts of the world.“Now is a critical time to develop promising solutions for sustainable humanity,” Yang said. “This year, we witnessed heat waves and record-breaking temperatures in North America, Europe, Asia, and Australia.”For Yu, the material also offered an unusual explanation for why the coating appears white. He said he once considered white to be the hardest colour to create in paint. “When I studied watercolor painting years ago, white paints were the most expensive,” Yu said.The researchers found that the colour could instead be produced through properly sized air voids inside a transparent material. Yu pointed to examples in nature where air-filled structures create white or silvery appearances.“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,” he said.