Researchers at IIT Guwahati develop an electricity-free radiative cooler

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• Jul 4, 2022 | Dhupali Das

The researchers thought of employing silicon dioxide and aluminium nitride thin films to tackle this problem.

Digital Desk: Researchers at the Indian Institute of Technology (IIT) Guwahati have created a low-cost, effective "passive" radiative cooling system that runs without electricity and can serve as a substitute for traditional air conditioners, the institute announced on Monday.

This 'Radiative Cooler' coating material functions both during the day and at night and is an electricity-free cooling system that can be applied on rooftops.

Systems that use passive radiative cooling work by releasing heat that has been absorbed from the environment as infrared radiation that can pass through the atmosphere before being released into cold outer space.

Prof. Debabrata Sikdar, Assistant Professor at the Department of Electronics and Electrical Engineering, IIT Guwahati, said, "These radiative coolers 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."

"Radiative cooling is a unique method that cools an object on the earth by sending excess heat directly into the incredibly cold universe. Traditional cooling technologies release the waste heat into the surroundings," he added. 

However, the majority of passive radiative coolers only work at night. These coolers must also reflect the entire sun's energy to function during the day. These cooling solutions have not yet been able to cool the room adequately during the daytime.

"The material must reflect the solar and atmospheric radiations striking it for a radiative cooler to function throughout the day. Conventional coolers do not work during the day because their materials absorb more solar energy and reflect less of it. While polymer-based passive radiative coolers can be used to accomplish daytime cooling, oxidation destroys the polymers, resulting in a limited lifespan," said IIT Guwahati research student Ashish Kumar Chowdhary.

The researchers thought of employing silicon dioxide and aluminium nitride thin films to tackle this problem. However, the wavelength range of solar and atmospheric radiations is matched by the low optical density of these materials.

But they have a high optical density at wavelengths where there is an atmospheric transmission. When optical density is high, radiations penetrate a medium more slowly and are absorbed more fully. The substance emits all of the radiation it has absorbed to maintain thermal equilibrium, much like a black body. 

Instead of using a single layer, the researchers applied silicon dioxide and aluminium nitride thin film layers in a cascade over a silicon substrate. 

The cooler they developed had an approximate reflectance of 97% for solar and atmospheric radiation and emissivity of 80% for wavelengths of atmospheric transmission.

The net cooling power is estimated to be 115 watts per square metre, with the ability to lower the ambient temperature by up to 15 degrees below the outside temperature.

Their innovation was recently featured in the Current Science Report after its first publication in the Journal of Physics D: Applied Physics.

The team reported that this cooler provides roughly 1.6 times more cooling power than a recent cooler design achieving a comparable reduction in ambient temperature. They added that this cooler would hit the market once the large-scale prototypes are developed and tested for operational stability and durability under various climatic conditions.