Researchers at the Columbia University School of Engineering and Applied Science have developed porous polymer coatings (PPCs) that can be used on buildings to control both light and heat.

With more than 30 per cent of energy consumption in buildings going into heating, cooling and lighting systems, the latest finding by the Columbia Engineering researchers addresses the limitations of cool roof paints and opens up inexpensive and scalable ways to control light and heat in buildings.

The porous polymer coatings developed by the research team take advantage of the material’s optical switchability in the solar wavelengths to regulate solar heating and daylighting, and extend the concept to thermal infrared wavelengths to modulate heat radiated by objects. The study is titled ‘Porous Polymers with Switchable Optical Transmittance for Optical and Thermal Regulation’.

Jyotirmoy Mandal, lead author of the study and a former Ph.D. student in the lab of Yuan Yang, assistant professor of materials science and engineering, explains that PPCs, when wet using common liquids such as alcohols or water, will reversibly switch their optical transmittance in the solar and thermal wavelengths. When these PPCs are placed in hollow plastic or glass panels, it is possible to create building envelopes that can regulate indoor temperatures and light.

Though similar in concept to smart windows but with a higher optical switchability, the PPCs developed by the Columbia Engineering team use simpler, inexpensive materials that would allow large-scale implementation.

Mandal noticed the optical switching properties of PPCs when a few drops of alcohol spilled on a white fluoropolymer PPC, turning it transparent. Similar to the mechanism that causes paper to turn translucent when wet, the drastic switching led the research team to explore the material further.

According to Yang, the PPC, when wet, becomes optically homogenous; light passes through without scattering, much like it would through solid glass, making the porous polymer transparent.

The near-perfect refractive-index matching of alcohols and the fluoropolymer allowed the team to change the solar transmittance of their PPCs by approximately 74 percent. Although the switching is slower than in typical smart windows, the transmittance changes are considerably higher, making PPCs attractive for controlling daylight in buildings.

This optical switching could also be used for thermoregulation to reduce air-conditioning and heating costs of buildings. The researchers placed panels containing PPCs on toy houses with black roofs. One panel was dry and reflective, while the other was wet and translucent, showing the black roof underneath. Under sunlight on a summer noon, the white roof became cooler than the ambient air by approximately 3 degrees Celsius, while the black one became much hotter by approximately 21 degrees Celsius.

The team also explored switching between ‘icehouse’ and ‘greenhouse’ states in the thermal infrared wavelengths, using wet infrared-transparent polyethylene PPCs. When dry, the porous polyethylene PPCs reflect sunlight but transmit radiated heat similar to an icehouse; in the wet state, the PPCs transmit sunlight, and block radiated heat like a greenhouse. The PPCs, therefore, could be used to modulate both solar and thermal radiation, and regulate heat day and night.

The PPCs have several other potential applications such as thermal camouflage and paints that respond to rain. The next phase of research will focus on scaling up the designs, and explore opportunities for deployment.

Image: The researchers demonstrated that PPC-based roofs, which can switch between reflective and transmissive states, can be used to control the indoor temperatures of buildings. Credit: Jyotirmoy Mandal/Mingxin Jia/Columbia Engineering