In 2021, researchers built a two-layer polymer film that cools like skin; under direct sun, it stayed about 7°C below ambient with 150 W/m² cooling power

With growing needs for air conditioning and the increase in ambient temperature, it is becoming more necessary to discover methods of cooling buildings without using grid electricity in materials science. The most common method is the use of reflective and radiative films that reflect the sun and radiate heat towards the sky. But in conditions where the solar radiation is too high, there might be the possibility that radiative cooling won't be able to dissipate heat at a rate that achieves below-ambient temperatures. Like human skin, which faces the same issue, the skin uses sweating to dissipate heat by evaporation after reflection stops being efficient. In 2021, scientists from Wuhan University and Nanyang Technological University of Singapore used the same principle to create building cooling films, as published in their NTU repository.

A hydrogel bottom layer does the sweating; a porous top layer does the shielding

The film produced by the team consists of two layers stacked one above another and both layers fulfill certain roles. The first layer is a hygroscopic hydrogel, which can absorb water vapor from the atmosphere at night and release it during the day to cool down like sweating and lower the temperature of the surface underneath. The second layer is a hydrophobic polymer film with a hierarchy of pores, which scatters sunlight while remaining permeable to water vapor. The upper layer not only scatters the sunlight, but also regulates the rate of evaporation from the hygroscopic layer during the day, preventing it from drying out too quickly, and helps it accumulate water vapor at night


Stacking radiative and evaporative cooling in one film

Passive cooling materials used presently are designed using one of these approaches, such as reflection of solar radiation or release of heat into the air. In this particular study, the researchers used another strategy of integrating two cooling techniques in one film, where each of the cooling processes compensates for the disadvantages of the other. The porous layer protects the material from overheating and regulates moisture loss during the daytime, moderating the hydrogel's evaporation rate rather than blocking it.

A related hydrogel was first tested on electronics

The hydrogel chemistry behind the material is not new. Some of the same authors, including Xuejiao Hu and Kang Liu, had already explored the use of a polyacrylamide hydrogel enriched with lithium and bromine one year before in a similar context. In 'Promoting Energy Efficiency via a Self-Adaptive Evaporative Cooling Hydrogel,' they showed that the material could lower a commercial polycrystalline silicon solar cell's temperature by 17°C under one-sun illumination, raising efficiency from 14.5% to 15.5%. This same hydrogel was also able to increase the maximum power point of a simulated computer chip by 45% at a fixed operating temperature, according to the 2020 Advanced Materials paper ‘Promoting Energy Efficiency via a Self‑Adaptive Evaporative Cooling Hydrogel’

In this earlier research, a self-adaptive hydrogel cooled a hot surface through evaporative cooling and then recharged itself with ambient humidity without a pump or power supply. The bilayer film builds on the same idea but adds a radiative top layer, adapting it for building-scale use rather than a solar cell or computer chip.

Why pairing the two layers matters for real buildings

Radiative cooling by itself might not be able to release the heat fast enough due to the intensity of the sun's rays, while evaporation on its own is not sufficient to prevent drying of the material unless humidity from the environment restores it. In contrast, when using a porous radiative layer above the self-healing hydrogel, the material bypasses the limitations of both approaches at the same time, resulting in a sub-ambient temperature decrease of about 7 °C along with a cooling capacity of about 150 W/m², as shown in a paper by NTU/Wuhan University in 2021 and further investigations of the bilayer structure.