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, 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. Researchers 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.
