Researchers at Columbia Engineering (New York, New York, USA) have developed porous polymer coatings (PPCs) that provide an inexpensive and scalable means of regulating the heating, cooling, and lighting of buildings. The coating relies on the optical switchability found in porous polymers, except in this model heat is modulated by thermal infrared wavelengths.
“Our work shows that by wetting PPCs with common liquids like alcohols or water, we can reversibly switch their optical transmittance in the solar and thermal wavelengths,” says Jyotirmoy Mandal, lead author of the study recently published in the scientific journal Joule. “By putting such PPCs in hollow plastic or glass panels, we can make building envelopes that can regulate indoor temperatures and light.”
The PPC design builds on previous work by Mandal and other university researchers who developed a polymer coating capable of cooling down buildings. The main problem with that cooling coating, however, was that it was not responsive to daily or seasonal changes — a limitation not shared by the PPC model. “In places like New York, which sees warm summers and harsh winters, designs that can switch between heating and cooling modes can be more useful,” notes Yuan Yang, assistant professor of materials science and engineering at Columbia.
The PPC model originated from a seemingly innocuous occurrence: Mandal saw that a few drops of alcohol on a white fluoropolymer PPC surface turned it transparent. This discovery then led him to speculate on the “drastic switching” that occurred when the surface turned translucent when wetted and how this finding could be applied. This path of inquiry ultimately led the research team to observe that PPCs become “optically homogenous” when wetted, according to Yang. “Light is no longer scattered, and passes through — much like it would through solid glass — the porous polymer becomes transparent.”
Due to the similar refractive indexes of alcohols and fluoropolymers, a slight adjustment of the solar transmittance of PPCs could make that material a viable option for controlling daylight in buildings. From there, the research team examined how optical switch could modulate temperatures. “We imagined roofs that are white during the summer to keep buildings cool, and turn black during the winter to heat them,” says Yang, “This can greatly reduce air-conditioning and heating costs of buildings.”
The researchers tested their hypothesis by placing PPC panels, one dry and reflective and the other wet and translucent, on two distinct roofs. Under the exact same light and temperature conditions, the roof with the dry, reflective PPC became ~5 °F (3 °C) cooler than the ambient air, while the wet, translucent PPC roof became ~ 38 °F (21 °C) hotter.
The team also tested other potential applications for its PPC technology, including thermal camouflages and paints that respond to rain. Such applications could be used to cool or heat buildings in regions with dry summers and rainy winters. “Given the scalability and performance of the PPC-based designs, we are hopeful that their applications will be widespread,” says Yang,
Source: Columbia Engineering, www.engineering.columbia.edu