Though great on a hot day, an air conditioner is expensive, consumes a lot of energy, and often requires coolants that deplete ozone.

Researchers at Columbia Engineering have invented a kind of “paint-on” alternative: A high-performance exterior coating. The polymer can be fabricated, dyed, and applied like paint to cool down down rooftops, buildings, water tanks, vehicles, and even spacecraft.

Columbia’s idea centers on a phenomenon known as passive daytime radiative cooling (PDRC), where a surface spontaneously cools by reflecting sunlight and radiating heat to the colder atmosphere.

The university's surface-coating solution contains a polymer, a solvent, and water. When the solvent evaporates, water droplets form within the polymer. Eventually the water evaporates, leaving a foam-like film that contains air voids.

The air voids 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 efficiently sends heat skyward.

The study was published this month in Science.

Polymers and solvents are already used in paints, and the Columbia Engineering method essentially replaces the white paint's pigments 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,” said lead researcher Jyotirmoy Mandal.

The researchers found that their polymer coating reflects more than 96 percent of sunlight.

Mandal spoke with Tech Briefs, via email, about why his team’s PDRC approach is such a promising sustainable solution. His written responses are below.

Tech Briefs: What inspired this work?

Jyotirmoy Mandal: Cooling human-made structures is a major challenge we face today. Electrical cooling methods, such as air conditioners, are less than ideal because:

  • They are expensive and use large amounts of energy.
  • They require ready access to electricity – which is not readily available in resource-poor settings.
  • They only move heat from inside the building to outside, and they need energy to do this. So they actually have a net heating effect, and this leads to urban heat islands.
  • They often cause CO­2 and greenhouse gas emissions.

So, to mitigate these effects, passive cooling methods with a net heating effect are needed.

PDRC, which is eco-friendly, passive, and has a net cooling effect, can reduce AC usage. More importantly to me, in developing countries where electrical cooling is unavailable or unaffordable, it can provide relief.

Tech Briefs: What is passive daytime radiative cooling (PDRC) exactly?

Mandal: Passive daytime radiative cooling is a process where a surface efficiently reflects sunlight to avoid solar heating, and radiates heat into the sky through the atmosphere’s infrared window. Due to these combined effects, the surface has a net heat loss even under sunlight, and spontaneously cools buildings down. Under open skies, the cooling effect can be so strong that sub-ambient temperatures can be reached. The process of PDRC is schematically depicted in the picture below.

A PDRC schematic.

Tech Briefs: How is the polymer coating made?

Mandal: The process we employ to make our coating – a simple, solution-based process called phase inversion – makes the polymer porous, with micro- and nano-sizes air voids. Polymers and solvents are already used in paints, so this method essentially replaces the pigments in white paint with air voids. Unlike the pigments, however, the air voids have no absorption and efficiently reflect all wavelengths of sunlight from UV to IR, resulting in a superb solar reflectance. The pores also enhance the thermal radiation from the polymer. The resulting performance makes it highly promising for PDRC.

Tech Briefs: How do you apply the coating?

Mandal: A precursor solution of the polymer (e.g., P(VdF-HFP)) and non-solvent (e.g., water) in a solvent (e.g. acetone) is prepared. We apply a film of the solution onto a substrate and dry it in air. The rapid evaporation of the volatile solvent causes the polymer to phase-separate from the non-solvent, which forms micro- and nanodroplets. Eventually, the non-solvent also evaporates, leaving a porous polymer coating that acts as the cooling film. The process is illustrated in the YouTube video below.

Given the paint-like applicability, it can be put on any paintable structure. We can also spray or dip-coat the porous coating, or make it into sheets. Potential areas of application include roofs, water tanks, vehicles, and any industry that uses super-white surfaces.

Tech Briefs: How effective are the coatings?

Mandal: The notable outdoor cooling performance test experiments took place in Phoenix, AZ, and Chittagong, Bangladesh.

Our coatings show a remarkable PDRC capability. In the arid conditions of Phoenix, for example, we see a 6˚C sub-ambient temperature drop, whereas in the humid, tropical weather (which impedes radiative cooling) of Chittagong, Bangladesh, we see a drop of 3˚C. Depending on the weather, even larger drops (~ 10˚C) can occur.

Tech Briefs: Why have PDRC designs been challenging, and how does your polymer coating address those challenges?

Mandal: Researchers have developed several highly effective PDRC designs over the years, but such designs rely on silver mirrors to reflect sunlight, and are rather sophisticated. Furthermore, they come as premade sheets or in devices. All these make widespread deployment on roofs difficult – in developing countries, they are particularly so.

On the other hand, you have white paints, which are inexpensive, and easily applicable on roofs. Although they look white, however, their pigments usually absorb UV light of the sun and do not reflect the infrared part of sunlight (which has ~50% of solar energy) well enough. As a result, even the best white paints only reflect ~90% of sunlight. Most reflect only around 80%. The rest is absorbed, and under strong sunlight, causes significant heating.

In contrast, our coating reflects 96-99% sunlight regardless of the underlying roofing material, so no silver mirrors are needed. Also, it can radiate 97% of the theoretical maximum heat that can be emitted through the atmosphere’s infrared window. These performances are among the best we know, but what makes our coating promising is that it can be applied either as a sheet or painted in situ on roofs as a paint.

Tech Briefs: What is most exciting to you about this coating and its possibilities?

Mandal: The cooling performance, which is comparable to or better than state-of-the-art designs, and the simplicity of the process, which is almost paint-like.

Furthermore, since the process is applicable to a very large range of polymers, solvents, and non-solvents, we can use different polymers to incorporate different properties suitable for particular applications. For example, polystyrene can be used for high-temperature coatings on engines, since it has a high melting point ~250˚C. Ethyl-cellulose, which is biofriendly and works with green solvents, can be used as a completely biocompatible alternative.

We have also shown that we can add dyes to the coating to get colored coatings with cooling capability. The cooling performance is lesser than our white coating, but still significantly better than commercial colored paints. This is crucial, given that human choice of colors are subjective. The paint industry has been trying to develop cool-colors for many years now.

In its current form, our coating is already applicable like a paint that can cool buildings, vehicles, and water-tanks. We hope to improve it further to make it into a market-ready alternative to traditional paints.

What do you think? Will polymer coatings cool down buildings? Share your comments and questions below.