AC without electricity? IIT Guwahati researchers develop radiative cooling system

New Delhi, July 06: Ever imagined an AC that runs without electricity? Well, researchers at Indian Institute of Technology (IIT), Guwahati have claimed to have developed a substitute for an air conditioner that is not only reasonably priced and effective, but also runs without power!

This 'Radiative Cooler' coating material is an 'electricity-free' cooling system as it can be applied on the rooftops and functions both during day and night time to provide an alternate to the conventional air-conditioners.

"Passive radiative cooling systems operate by emitting heat absorbed from the surrounding in the form of infrared radiations that can pass through the atmosphere before getting dumped into the cold outer space. Most passive radiative coolers operate only at night. For daytime operation, these coolers need to reflect entire solar radiation as well," Ashish Kumar Chowdhary, research scholar at IIT Guwahati, said.

"Till now, these cooling systems are not able to provide sufficient cooling at daytime. We are set to resolve these issues and bring out an affordable and more efficient radiative cooling system that can operate round-the-clock," he added.

Their innovation has been published in the 'Journal of Physics D: Applied Physics' by IOP Publishing, United Kingdom.

"Designing a passive radiative cooler for daytime operation is more challenging due to the simultaneous requirement of high reflectance in entire solar spectral regime and high emissivity in the atmospheric transmittance window," said Debabrata Sikdar, Assistant Professor, Department of Electronics and Electrical Engineering, IIT Guwahati.

"These radiative coolers requiring no external energy sources for their operation could be one of the best alternatives to replace the conventional air conditioning systems used to cool buildings and automobiles in countries experiencing hot weather, such as India. Unlike traditional cooling technologies that dump the waste heat into the surroundings, radiative cooling is a unique process that cools an object on the earth sending excessive heat directly into the extremely cold universe," he added.

According to the team, they hope this will reach the market once the large-scale prototypes are developed and tested for operational stability and durability under different climatic conditions. They are now working towards this.

"For a radiative cooler to work during daytime, the material should reflect the solar and atmospheric radiations falling on it. Since the materials used in conventional coolers absorb more solar radiations and emit less during the day, those do not work during daytime. While daytime cooling can be achieved using polymer-based passive radiative coolers, oxidation degrades the polymers resulting in a limited lifespan," Chowdhary said.

"To address this, we considered using thin films of silicon dioxide and aluminium nitride. These materials have low optical density corresponding to the wavelength range of solar and atmospheric radiations. But at atmospheric transmittance wavelengths, they have high optical density. When optical density is high, radiations travel slower through a medium and get absorbed more. To remain at thermal equilibrium, the material emits all the absorbed radiations like a black body," he added.