THE DEVELOPING FIELD OF Radiative Cooling

Radiative Cooling (RC) is the process of dissipation of heat from any object through the emission of thermal energy. Although knowledge of the process is age-old, due to limitations of its application environment, earlier the process did not attract much interest for further development.

However, in this era of global warming when the cost of cooling is rapidly increasing along with growing environmental awareness among people, during the last few years, globally scientists have been enthusiastically trying to improve this process. In fact, in the last five years their research works have gained tremendous momentum. This time here, I will present a few of the notable advances in this field.

Research work at the University at Buffalo  

Around four years back, engineers from the University at Buffalo (The State University of New York) successfully lowered the temperature inside a test system in an outdoor environment under direct sunlight by more than 12⁰C. They lowered the temperature of the test box in a laboratory, meant to simulate the night, by more than 14⁰C. Simultaneously they captured enough solar power that could be used to heat water to about 60⁰C. While the system tested was only 70 square centimeters, it could eventually be scaled up to cover rooftops.

Their design consisted of two mirrors, made of 10 extremely thin layers of silver and silicon dioxide, which were placed in a V-shape. Those mirrors absorbed incoming sunlight, turning solar power from visible and near-infrared waves into heat. The mirrors also reflected mid-infrared waves from an ‘emitter’ (a vertical box in between the two mirrors), which then bounced the heat they carry into the sky.

Explaining the cooling process, Qiaoqiang Gan, Professor of Electrical Engineering, University at Buffalo School of Engineering and Applied Sciences, said, “Since the thermal emission from both surfaces of the central thermal emitter is reflected to the sky, the local cooling power density on this emitter is doubled, resulting in a record high temperature reduction.”

According to him, “Most radiative cooling systems scatter the solar energy, which limits the system’s cooling capabilities. Even with a perfect spectral selection, the upper limit for the cooling power with an ambient temperature of 25⁰C is about 160 watts per square metre. In contrast, the solar energy of about 1000 watts per square metre on top of those systems was simply wasted.”

Another recent development with mirrors

A recent research too reported in the Journal of Photonics for Energy (JPE) has suggested a practical approach to enhance radiative cooling – arranging a heat mirror structure around a radiative cooling surface to amplify the cooling effect. The mirror structure effectively guides the thermal radiation towards the most transmissive portion of the atmosphere, such that thermal radiation escapes the earth most efficiently. This stronger cooling can bring down temperatures faster and widen options for designing cooling systems.

The principle is elegantly simple – the more the cooling surface faces upwards, the more cooling power it has. The mirror structure increases this power without needing to expand the surface area. Adding the mirror structure makes the cooling device take up more space for a given area, but this added space is protected from moving air, which helps block heat gain from air flow.

Through parametric simulation, Jaesuk Hwang, (author from the Centre for Quantum Technologies, National University of Singapore) has demonstrated that the mirror structure can be particularly effective in cities, where buildings are close together and different heights can mean that not every building’s roof gets a full view of the sky. This mirror setup enhances radiative cooling by focusing the cooling surface’s view to a specific area in the sky above.

The potential benefits of using such a mirror structure appear to be significant especially for tropical areas, where cooling power may be boosted by more than 40%. According to Hwang, “Radiative cooling is possible because the atmosphere is thin enough at some angles. Arid regions allow a wider range of angles for thermal emission to transmit through the atmosphere than tropical regions, so redirecting thermal radiation upwards with a heat mirror structure is most effective in tropical climates, yet overall radiative cooling is stronger in dry climates.”

This simple approach for directing thermal energy could offer practical solutions to reduce temperatures and enhance the performance of radiative cooling for buildings, particularly in tropical regions where stagnant heat is a challenge.

New aerogels for radiative cooling

A few months back, in a pioneering effort, a research team led by Associate Professor Duong Hai-Minh from the Department of Mechanical Engineering under the NUS College of Design and Engineering (National University of Singapore), has developed aerogels for two new applications – radiative cooling and Electromagnetic Wave (EMW) absorption.

Utilising plastic waste, they have engineered thin-film aerogels that function as thermal insulators and radiative coolers. These aerogels can be applied to any surface, such as building roofs, to reduce internal temperatures, offering a scalable and sustainable solution for energy-free thermal management.

Earlier, the NUS researchers devised a simple, scalable method to produce aerogels that absorb EMWs in the X-band, characteristic of those used in weather monitoring and air traffic control. These lightweight, durable aerogels protect against electromagnetic pollution, shielding both humans and sensitive equipment in our increasingly digital world.

The researchers’ work builds on their prior successes in developing aerogels from a variety of waste materials, from plastics and paper to agricultural by-products such as pineapple leaves.

The new aerogels developed by the NUS team present a passive cooling alternative, leveraging the natural process of radiative cooling to dissipate heat into space without consuming energy.

Describing their method, the team leader, Associate Professor Duong Hai-Minh said, “This process involves using specially engineered aerogels to emit infrared radiation through the atmospheric ‘sky window’, effectively cooling surface temperatures below ambient levels. We are excited to be able to upcycle fibres from disposable Polyethylene Terephthalate (PET) bottles for the new aerogels designed for this purpose, to help address the global plastic waste crisis.”

Previously the team had worked with PET fibres to produce aerogels, but this latest method is significantly more energy-efficient, consuming about 97% less energy and reducing production time by 96%. When tested in Singapore’s warm climate, conducted in collaboration with Dr. Jaesuk Hwang from the Centre for Quantum Technologies at NUS, 0.5 centimetres of the material produced a cooling effect of 2⁰C, achieved by emitting infrared heat into the surroundings while exhibiting good heat insulation, preventing heat absorption from the surrounding environment.

Commenting on the potential of these materials, Duong said, “These aerogels could reduce energy consumption in both residential and commercial buildings, especially in tropical climates where cooling is now a necessity.”

Conclusion

The time has come when we need to think rationally. The traditional methods of cooling increase energy consumption – whereas the beauty of radiative cooling techniques lies in the fact that they need no electric power. Thus, they drastically reduce carbon footprint of the entities.

What happens in case of the traditional air-conditioning systems is – they collect the heat from inside the premises and discharge that outside. Thus, the electricity-based process ultimately adds to the process of global warming.

Under such circumstances, researchers have to focus more on improving zero-energy cooling methods. As radiative cooling methods irreversibly transfer heat to the outer space, they provide a net cooling effect to earth without causing any harm to its environment. So, research works must speed up to make radiative cooling methods a commonplace.

By P. K. Chatterjee (PK)