IIT Guwahati Researchers Develop 'Radiative Cooler', An 'Electricity-Free' Alternative To ACs

The coating material of the radiative cooler is an electricity-free cooling system that can be applied in the rooftops and functions both during the day and night time. It is an alternative to ACs.

The radiative cooler is an electricity-free system which absorbs heat from the surroundings, and incident solar radiation, and emits heat into outer space and reflects the solar radiation. The coating is applied on rooftops and the cooling system acts as an alternative to ACs ( Image Source :ABP Live (with special arrangement with IIT Guwahati) )

Researchers at the Indian Institute of Technology Guwahati have developed a 'Radiative Cooler' which does not require electricity to operate. This is an affordable and efficient 'passive' radiative cooling system that can act as an alternative to air-conditioners. 

The coating material of the radiative cooler is an electricity-free cooling system that can be applied in the rooftops and functions both during the day and night time. The cooler can act as an alternative to the conventional air-conditioners.

How Passive Radiative Coolers Work

Passive radiative Coolers operate by emitting the heat absorbed from the surroundings in the form of infrared radiations which can pass through the atmosphere before being released into the cold outer space. Most passive radiative coolers were able to operate only at night so far. In order to operate during the day, the coolers need to reflect entire solar radiation. Till date, passive radiative coolers have not been able to provide sufficient cooling at daytime. Therefore, researchers at IIT Guwahati set out to resolve these issues and present an affordable and more efficient radiative cooling system that can operate during the day as well as night.

The researchers who designed and modelled the passive radiative cooler include Ashish Kumar Chowdhary and Professor Debabrata Sikdar, among others. The study describing their innovation was recently covered in the Current Science Reports, a column to report scientific research in India. 

Why Is It Challenging To Design A Passive Radiative Cooler For Daytime Operation?

In a statement released by IIT Guwahati, Professor Debabrata Sikdar said designing a passive radiative cooler for daytime operation is more challenging due to the simultaneous requirement of high reflectance in the entire solar spectral regime and high emissivity in the atmospheric transmittance window. The solar spectral regime ranges from 0.3 to 2.5 micrometre wavelengths. Meanwhile, the atmospheric transmittance window ranges from eight to 13 micrometre wavelengths.

Solar spectrum consists of radio waves, microwaves, visible light, ultraviolet rays, X-rays and gamma rays. Atmospheric transmission is the process by which radiation is propagated through a medium. Atmospheric transmittance is the ratio of the transmitted radiation to the total radiation incident upon the medium. 

Why Are Radiative Coolers The Best Alternatives To ACs?

Sikdar added that these radiative coolers requiring no external sources for their operations 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. Traditional cooling technologies dump the waste into the surroundings. Meanwhile, radiative cooling is a unique process that cools an object on Earth by sending excessive heat directly into the extremely cold universe.

Design Of The New Radiative Cooler

According to the study, the theoretical design of the radiative cooling system is tested and verified against rigorous computer-based simulations. The radiative cooler is less prone to imperfections during the fabrication process. Therefore, the cooling power to be obtained after the construction of the cooler is expected to closely match the calculations. 

This innovation is believed to encourage cooler manufacturers to explore radiative cooling to design electricity-free cooling systems. The researchers hope that the radiative cooler will reach the market once the large-scale prototypes are developed and tested for operational stability and durability under different climatic conditions.

Chowdhary explained the technology necessary to overcome the limitations of conventional radiative coolers. He said that for a radiative cooler to work during the day, the material should reflect the solar and atmospheric radiations falling on it. The materials used in conventional coolers do not work during daytime because they absorb more solar radiation and emit less during the day.

How Daytime Cooling Is Achieved Using The New Cooler

Chowdhary said that daytime cooling can be achieved using polymer-based passive radiative coolers. However, oxidation degrades the polymers resulting in a limited lifespan.

The researchers tried to address this issue using thin films of silicon dioxide and aluminium nitride, which have low optical density corresponding to the wavelength range of solar and atmospheric radiations. However, these materials have high optical density at atmospheric transmittance wavelengths. A material with high optical density emits all the absorbed radiations like a black body, in order to remain at thermal equilibrium. A black body is an object that absorbs all the electromagnetic radiation which falls on it. A black body must emit radiation at the same rate as it absorbs the radiation to stay in equilibrium. 

The researchers did not use a single layer as a ground metal. Instead, they placed silicon dioxide and aluminium nitride thin layers on a silver layer, which was used as a ground metal. The silver layer was placed over a silicon substrate. 

The radiative cooler designed by the researchers at IIT Guwahati achieved 97 per cent reflectance for solar and atmospheric radiations and 80 per cent emissivity for radiations in atmospheric transmittance wavelengths. According to the study, the net cooling power is estimated to be 115 Watts per metre square. This cooling power could reduce ambient temperatures up to 15 degrees below the outside temperature.

What Are The Key Advantages Of The System Compared To Existing Technologies?

The radiative cooling system is more advantageous than existing cooking technologies. These advantages include the fact that the design is lithography-free and large-area compatible. Lithography is a printing process that involves using a stone or metal block on which an image has been drawn with a thick substance that attracts ink. 

Large-area compatibility is important to develop affordable and efficient large-scale radiative coolers. 

The design of the radiative cooler will ensure effective cooling during daytime without any need to adjust the angle or position or the cooler towards the Sun. 

Compared to a recent cooler design which achieves a comparable reduction in ambient temperatures, the radiative cooler provides around 1.6 times more cooling power.