IIT Guwahati Designs Affordable Alternative to Air-Conditioning that Needs No Electricity, Here are the Details

With the IIT Guwahati innovation, manufacturers can now explore ‘Radiative Cooling’ technology to make electricity-free cooling systems

IIT Guwahati researchers have designed an affordable and efficient ‘passive’ radiative cooling system that does not require electricity to operate. 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 the 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. Till now, these cooling systems are not able to provide sufficient cooling at daytime.  IIT Guwahati Researchers set out to resolve these issues and bring out an afforfable and more efficient radiative cooling system that can operate round the clock.

Ashish Kumar Chowdhary, Research Scholar at IIT Guwahati under the supervision of Prof. Debabrata Sikdar, Assistant Professor, Department of Electronics and Electrical Engineering, IIT Guwahati, along with his research team has designed and modelled such a passive radiative cooler. Their innovation has been recently covered in the Current Science Report, which was initially published in Journal of Physics D: Applied Physics by IOP Publishing, United Kingdom.

The theoretical design of the radiative cooling system are tested and verified against rigorous computer-based simulations. This patterning-free design of radiative cooler is large-area compatible and hence, also less prone to imperfections during fabrication process. Therefore, it is expected that the cooling power to be obtained after construction of the cooler is going to closely match the calculations. With this innovation, cooler manufacturers can now explore radiative cooling to make electricity-free cooling systems. The team hopes 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.

To address this, the innovators 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.

Instead of a single layer, the team cascaded silicon dioxide and aluminium nitride thin film layers on a silver layer, used as a ground metal, placed over a silicon substrate. The cooler they designed achieved about 97 per cent reflectance for solar and atmospheric radiations and 80 per cent emissivity for radiations in atmospheric transmittance wavelengths. The net cooling power is estimated to be 115 Wm⁻² which could reduce ambient temperature up to 15 degrees below the outside temperature.

Key Advantages to IIT Guwahati Innovation

• The design is lithography-free and large-area compatible, which is vital for developing affordable and efficient large-scale radiative coolers.
• This design will ensure effective cooling during entire daytime without any need to adjust the angle or position of the cooler towards sun.
• When compared to a recent cooler design achieving a comparable reduction in ambient temperature, this cooler is found to provide around 1.6 times more cooling power.
• The research team is also responsible for developing smart window materials that can automatically control a building’s climate.