Alternative to ACs: IIT develops radiative cooling system which needs no electricity

Researchers at the Indian Institute of Technology, Guwahati have designed an affordable radiative cooler coating material, an alternative to air-conditioning system that needs no electricity to operate.According to officials, this 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 alternative to 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.

Researchers at the Indian Institute of Technology, Guwahati have designed an affordable 'radiative cooler' coating material, an alternative to an air-conditioning system that needs no electricity to operate.

According to officials, this 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 alternative to 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 the 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 the daytime, the material should reflect the solar and atmospheric radiations falling on it. Since the materials used in conventional coolers absorb more solar radiation and emit less during the day, they do not work during the 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 aluminum 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. 

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