Traditional cooling systems for buildings use refrigerants and electricity, which contribute to the atmospheric greenhouse effect that exacerbates more extreme weather events. In response, materials scientists have turned to unconventional methods for cooling down buildings. An international team of researchers co-led by Akhlesh Lakhtakia, Penn State Evan Pugh University Professor of Engineering Science and Mechanics, has developed porous plastic sheets that can lower building temperatures through radiative cooling.
When applied to an enclosed space, the sheets, which are made of powdered polymethyl methacrylate (PMMA) and are about one-twelfth of an inch thick, can decrease the temperature of an enclosed space by 8.4 degrees Celsius, or about 14 degrees Fahrenheit. The researchers have published their findings in Advanced Materials Technologies.
“While other passive radiators reflect short-wave infrared light back into space, our passive radiator reflects both visible light and short-wave infrared light, which results in high daytime cooling,” Lakhtakia said. “The sheets would be an inexpensive, effective addition to homeowners’ siding and roofs to passively cool down a home and to supplement air conditioning units.”
To fabricate the sheets, Lakhtakia’s collaborators at the Dalian University of Technology in China used one-step powder sintering to fuse the PMMA powder into flat sheets with various sizes of air pockets, or pores, within them. Like the pores in our skin, the pores in the PMMA sheets cause light to scatter and radiate at different angles, thereby cooling down interior spaces. At night, the porous sheets radiate medium-wave infrared light through the atmosphere into deep space, again cooling down interior spaces.
“Powder sintering is a powerful method to prepare the porous materials with desired porosity and superior macroscopic properties,” said corresponding author Mingkai Lei, professor of materials science and engineering at the Dalian University of Technology. “We look forward to working with Lakhtakia and Penn State on additional applications of the material.”
To test the materials, Lakhtakia’s collaborators created a box out of the porous PMMA sheets, placed a thermometer within the box, then placed it in the sun and measured the temperature. By reflecting, on average, 96% of infrared and visible light, the porous PMMA sheets cooled down the sunny 80 degree Fahrenheit outdoor air temperature to 65.3 degrees Fahrenheit inside the box. By contrast, an ordinary cardboard box with the same dimensions cooled down the space to 75.2 degrees Fahrenheit.
They then repeated the experiment in the lab, using a solar simulator in place of the sun, which allowed the researchers to control for conditions like wind. Since room temperature has a significantly higher temperature than cold outer space, the radiative cooling effect was lessened inside the lab than in open air.
When exposed to the sun on a building, the PMMA sheets will degrade after a few years and lose their effectiveness, Lakhtakia said, but he envisions new businesses starting up to install and replace them, in turn creating new jobs in communities.
“The sheets can be easily ground up, recycled and powder sintered on an industrial scale to reuse on another building,” Lakhtakia said. “The passive daytime cooling requires no energy or electricity and can be applied to buildings in communities experiencing increased daytime temperatures year over year due to climate change.”
In addition to Lakhtakia and Lei, the co-authors include first author Yupeng Li, Hui Zhao, Xiangren Meng, Yishu Zhao and Haodong Liu, all from the Dalian University of Technology.
