Though marvelous on a hot day, an air conditioner is expensive, consumes a lot of energy, and often requires coolants that deplete ozone. Alternative to air conditioning, Engineers have invented a kind of “paint-on” alternative: A high-performance exterior coating. The polymer is fabricated, dyed, and applied like paint to cool 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.
An Alternative to Air Conditioning: Passive Paint-like polymer
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.https://www.glewengineering.com/developments-in-solar-engineering-improves-performance/
As an alternative to air conditioning, Engineers have invented a “paint-on” alternative: A high-performance exterior coating. 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 surpasses those of state-of-the-art PDRC designs but with a convenience that is almost paint-like. The researchers found that their polymer coating reflects more than 96 percent of sunlight.
An Alternative to Air Conditioning: Passive Paint-like Polymer
Cooling human-made structures is a significant 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.
- Require ready access to electricity – which is not readily available in resource-poor settings.
- Only move heat from inside the building to outside, and they need the energy to do this. So they have a net heating effect, leading to urban heat islands.
- Often cause CO2 and greenhouse gas emissions.
Air conditioning can mitigate these effects, and 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, in developing countries where electrical cooling is unavailable or unaffordable, it can provide relief. Passive daytime radiative cooling is when 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 is so strong that sub-ambient temperatures are reached. https://www.glewengineering.com/engineers-help-with-small-business-ventilation/
The process employed to make the 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 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 superb solar reflectance. The pores also enhance the thermal radiation from the polymer.
The resulting performance makes it highly promising for PDRC. 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. Next, one applies a film of the solution onto a substrate and dries it in the 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.
Given the paint-like applicability, it is put on any paintable structure. We can 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. The notable outdoor cooling performance test experiments occurred in Phoenix, AZ, and Chittagong, Bangladesh.
Coatings show a remarkable PDRC capability. In the arid conditions of Phoenix, for example, a 6˚C sub-ambient temperature drop, whereas in the humid, tropical weather (which impedes radiative cooling) of Chittagong, Bangladesh, a drop of 3˚C is seen. Depending on the weather, even more, significant drops (~ 10˚C) can occur. 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 devices.
Roofs Could Benefit
All these make widespread deployment on roofs difficult in developing countries 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 the UV light of the sun. Does 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, causing significant heating. https://www.glewengineering.com/eco-friendly-engineering-materials-designs-roofing/ https://qz.com/1408584/a-new-polymer-paint-promises-to-cut-air-conditioning-costs/
In contrast, 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 emitted through the atmosphere’s infrared window. These performances are among the best we know, this makes our coating promising. Applied as a sheet or painted in situ on roofs as paint. The cooling performance is comparable to or better than state-of-the-art designs. The simplicity of the process is almost paint-like.
Process is Applicable
Furthermore, since the process is applicable to an extensive range of polymers, solvents, and non-solvents, we can use different polymers to incorporate different properties suitable for particular applications. For example, polystyrene is used for high-temperature coatings on engines. Since it has a high melting point of ~250˚C. Ethyl-cellulose biofriendly works with green solvents and is an entirely 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 the white coating, Still significantly better than commercial-colored paints. This is crucial, given that human choice of colors is subjective. The paint industry has wanted to develop cool colors for many years now. This passive cooling approach has great benefits with minimal cost.
The coating is already applicable in its current form, like a paint that can cool buildings, vehicles, and water tanks. Improving it further to make it into a market-ready alternative to traditional paints is around the corner.