Penn State and Dalian University of Technology researchers developed hierarchically porous PMMA sheets that reflect sunlight and radiate heat to space, enabling cooling without electricity.

The porous PMMA sheets are created by a single-stage dry powder-sintering process and contain pores smaller than 5 micrometers, with the most porous sample reaching about 43% porosity that enhances reflective and emissive properties.

In outdoor box tests, the PMMA setup cooled interior temperatures to around 65.3°F when ambient conditions were 80°F, outperforming a similar cardboard box and demonstrating a substantial cooling effect under solar exposure.

The material achieves high solar reflectivity of about 96% in the 0.3–2.5 micrometer range and strong mid-infrared emissivity around 96.7% in the 8–13 micrometer window, enabling heat rejection to the sky.

Overall performance in the best samples resulted in sub-ambient temperatures by radiating heat to space while reflecting solar energy, contributing to passive cooling without energy input.

Scale estimates suggest cooling of up to roughly 8.4°C (about 14°F) for larger applied areas, indicating meaningful potential for buildings beyond small test boxes.

Such cooling could be realized in real-world applications like siding or roofing to reduce cooling loads and complement conventional air conditioning.

Outdoor tests show the PMMA box achieved cooler interior temperatures than a cardboard box under identical solar conditions, reinforcing the material’s practical cooling potential.

The cooling mechanism relies on scattering within microscopic pores to reflect solar radiation and, at night, emitting long-wave infrared radiation (8–13 μm) to the night sky.

Durability and outdoor longevity remain uncertain, with concerns that continuous sunlight exposure could degrade PMMA, suggesting a need for maintenance or reprocessing considerations.

Funding from national Chinese science foundations supported the work, and results were published in Advanced Materials Technologies, highlighting the study’s scientific basis and peer-reviewed validation.

The broader significance is advancing passive cooling technologies to reduce dependence on air conditioning, aiming to lower energy use and greenhouse gas emissions.