Mandal, along with Professors Yuan Yang and Nanfang Yu, built upon earlier work demonstrating that many simple plastics and polymers are excellent heat radiators that could be used for passive radiative daytime cooling (PRDC). Their challenge was to get these normally transparent polymers to reflect sunlight without using silver mirrors as reflectors.
Tech Briefs: How did you achieve a reflective surface with a heat-radiating polymer?
Jyotirmoy Mandal: It's actually a pretty simple idea. Think of ice — it is transparent, but snow is white because air gaps are trapped between the snowflakes. The difference between the refractive indexes of snow and air causes light to scatter, which makes whiteness. We mixed a polymer (poly(vinylidene fluoride-hexafluo-ropropylene, PVdF-HFP), a solvent (acetone), and a non-solvent (water). The acetone dries first so there is nothing to keep the water and the polymer together. The water forms tiny droplets within the polymer that eventually evaporate, leaving air voids in their place. In our case, the difference in refractive indexes that causes the light to scatter is between the air voids and the polymer.
Tech Briefs: How does it cool?
Mandal: The process is two parts. First, you have to reflect sunlight so that you don't gain any heat from the Sun in the first place. Then, you have to radiate your own heat to cool down. Objects around us — everything from human beings to a hot radiator — are always emitting heat. That heat is what we are trying to lose to cool things down. Not only do we maximize the reflection of sunlight, but we also improve the efficiency of radiating that heat outward. So, number one, you are not warming under the sunlight and number two, you are also losing heat efficiently to the sky by radiating it. Even under sunlight at high noon, you can lose heat and cool down below the environmental temperature by means of this two-part process. Typically, our coating can radiate about 120 watts per square meter on a hot summer day, so you would have a net heat loss of 110 watts per square meter.
Tech Briefs: You created colored polymer coatings by adding dyes. Doesn't that reduce the reflectance?
Mandal: Sunlight is composed of ultraviolet rays, visible light, and infrared. But only the visible light provides the color. Commercial blue paint reflects blue light to your eye but absorbs most everything else including the infrared. In our dye, infrared passes through unabsorbed and is scattered by the porous polymer, which reflects it.
Tech Briefs: What's next?
Mandal: Making sure dust or pollution will not affect our coating. Cities like Delhi or Beijing are big challenges. We have shown that our paint is fairly durable under month-long tests, but we would like to make sure that it's very good even after five or ten years on the roof. The polymer is actually used in paints as a weatherproofing agent.
We are currently in talks with companies interested in licensing our technology. How soon it gets out depends partly on them. We are also currently testing our paint in collaboration with partners in different parts of the world.
Tech Briefs: What excites you about this project?
Mandal: In Bangladesh, we lived in a house with a roof that would become really hot under the Sun and where electricity was intermittent. I know firsthand the need for cooling that is inexpensive, and simple to make and apply.