In a 2025 study, Penn State researchers tested porous PMMA sheets that reflected 96.4% of sunlight; a test box stayed 14.7°F cooler than the outdoor air

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.

The most porous sample had 43% porosity and reflected 96.4% of the solar radiation in wavelengths between 0.3 and 2.5 micrometers, which included ultraviolet, visible and near-infrared radiation. The sample also emitted 96.7% of the thermal radiation in the wavelengths ranging from 8 to 13 micrometers, which corresponds to the atmospheric window. Reflectance and emissivity have distinct functions: reflectance prevents the sunlight from being converted to heat, while emissivity helps dissipate the heat through radiation to outer space. High reflectance and mid-infrared emissivity of the samples cooled down the air temperature within the box to below the outdoor temperature. Separately, in a larger-scale estimate of the material applied to cover an enclosed area, a different measurement from the individual test-box result above, the researchers reported the PMMA films can lower the temperature by up to roughly 8.4°C (about 14°F, per the researchers' own published figure).

Other radiative cooling films land in the same neighborhood

Such findings agree with studies conducted by other research groups working on porous PMMA materials. In another study, titled A structural polymer for highly efficient all-day passive radiative cooling (Wang et al., 2021), published in Nature Communications, researchers described a hierarchically structured PMMA film containing micropore arrays and randomly oriented nanopores. It featured 0.95 solar reflectivity and 0.98 longwave infrared emissivity and provided sub-ambient cooling of about 8.2°C at nighttime and 6.0°C-8.9°C during midday under 900 W/m² solar illumination, comparable to the 8.4°C cooling reported by the Penn State-Dalian team.

Both research groups achieved comparable reflectance and emissivity values despite using different fabrication techniques: powder sintering for the Penn State-Dalian sheets and film casting for the Nature Communications film. That suggests the micropores' optical properties depend more on their structure than on which fabrication method produced them.

From a test box to a building material

According to Lakhtakia, the material's most likely initial uses are in siding and roofing, where it could reduce the load on existing cooling systems rather than replace them, since it works by reflecting solar radiation and radiating absorbed heat back to the sky rather than by consuming power. However, sunlight is also what causes the sheets to deteriorate over time. Lakhtakia estimates that it would take several years of outdoor exposure for the sheets to become ineffective; that is a typical lifespan for paint or membrane materials. The sheets used in the outdoor box were one-twelfth of an inch thick. That thickness could be suitable for applications such as bolted or laminated wall coatings. However, it remains to be seen whether the sheets will be able to withstand outdoor conditions.