Air conditioners are electric devices that consume electricity from the power grid consistently during the hot season. A research group working in association with the Penn State researchers invented a porous plastic sheet that reflects sunlight and emits thermal radiation into outer space. There’s no compressor, there’s no fan and there’s no plug, either. When left in a sunny environment, the box made from such a plastic sheet maintained its internal air much cooler than the ambient air using only light and porous plastic.
The technology is called passive daytime radiative cooling, and researchers at Penn State and the Dalian University of Technology note it has attracted attention for its energy-saving potential, since it can supplement or reduce reliance on electricity-hungry air conditioning. Unlike traditional cooling technologies, the current technology can reflect the incoming sunlight and dissipate any remaining thermal energy through the atmospheric window into outer space. The difficult task for the Penn State researchers was designing a plastic film capable of reflecting the incoming light while emitting the thermal energy through the atmospheric window.
A box made from the material beat cardboard by nearly 10 degrees
Based on the results from a recent study conducted by Akhlesh Lakhtakia of Pennsylvania State University and his colleagues at Dalian University of Technology, test boxes enclosed within porous PMMA sheets kept their contents cool even when temperatures outside were high. In the experiment, one test box was placed outside under sunlight when the surrounding temperature was 80°F. The inside temperature was recorded as 65.3°F, 14.7°F (8.2°C) cooler than outside. On the other hand, a box made of cardboard, which was also placed in the same condition, recorded a temperature of 75.2°F, as stated by Penn State College of Engineering. The test was also conducted indoors, using a solar simulator in place of direct sunlight to control for the effect of wind. (The solar simulator was lab equipment used to run the test, not part of the cooling material itself. The PMMA sheets remain unpowered in either setting.) The cooling effect was less efficient indoors, since a room's ceiling is much warmer than the night sky the outdoor sheets can radiate heat toward.
The sheets reflect sunlight and radiate heat separately
The scientific paper upon which this demonstration is based, Powder-Sintered Hierarchically Porous PMMA with Optimal Pore Parameters for Passive Daytime Radiative Cooling (Li et al.) , was published in Advanced Materials Technologies in 2025. The researchers manufactured the PMMA films via a single-stage dry powder-sintering technique, in which plastic particles were compressed into a film with hierarchically porous structures that had pores smaller than 5 micrometers in size. The pores deflect the sun’s rays due to the creation of numerous interfaces.
