This ‘radiative cooler’ coating material is a power free cooling system as it can be applied to roofs and works both day and night.
Passive radiative cooling systems operate by emitting absorbed heat from the surrounding in the form of infrared radiation that can pass through the atmosphere before being dumped into the cold outer space.
“The operation of these radiative coolers does not require any external energy source, which is best suited to replace the traditional air conditioning systems used to cool buildings and automobiles in countries that experience hot climates such as India. can be one of the good choices,” said Prof. Debabrata Sikdar, assistant professor at the Department of Electronics and Electrical Engineering, IIT Guwahati, in a statement.
“Unlike conventional cooling techniques, which dump waste heat into the surroundings, radiative cooling is a unique process that cools an object on Earth by sending excessive heat directly into the extremely cold universe,” he said.
However, most passive radiative coolers only operate at night. For day time operation, these coolers also need to reflect all the solar radiation. So far these cooling systems are not able to provide adequate cooling during the day.
“For a radiative cooler to function during the day, the material must reflect solar and atmospheric radiation falling on it. Since the materials used in conventional coolers absorb more solar radiation and less during the day They do not operate during the day. During day time cooling can be achieved by using polymer based passive irradiation coolers, oxidation reduces polymers resulting in limited lifetime,” says IIT Guwahati Ashish Kumar Choudhary, a research scholar explained.
To address this, the team considered using thin films of silicon dioxide and aluminum nitride. These materials have a low optical density corresponding to the wavelength range of solar and atmospheric radiations.
But at atmospheric transmittance wavelengths, they have high optical density. When the optical density is high, radiation travels slower through a medium and more is absorbed. To remain at thermal equilibrium, the material emits all absorbed radiation like a black body.
Instead of a single layer, the team cascades silicon dioxide and aluminum nitride thin film layers onto a silver layer, which was placed on top of a silicon substrate.
The cooler he designed achieved about 97 percent reflectance for solar and atmospheric radiations and 80 percent emission for radiations in atmospheric transmittance wavelengths.
The net cooling power is estimated to be 115 watts per square meter which can reduce the ambient temperature by up to 15 degrees below the outside temperature.
Their innovation has been covered in a recent Current Science report, initially published in the Journal of Physics D: Applied Physics.
Compared to recent cooler designs that achieve a comparable reduction in ambient temperature, this cooler is found to provide approximately 1.6 times more cooling power, the team said, as large-scale prototypes are developed and operational stability. It will reach the market after being tested for and durability in various climatic conditions.
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