Multilayer Optical Films for Passive Radiative Cooling and Thermal Management Across Diverse Applications

Background

Passive radiative cooling is an emerging field within thermal management that leverages the natural process of heat emission from surfaces to the cold of outer space, particularly through the atmospheric transparency window in the long-wave infrared (LWIR) range (8–13 μm). This approach is attractive for reducing energy consumption in buildings, vehicles, and storage systems, as it can provide cooling without electricity, making it especially valuable in regions with limited access to power or in applications where conventional air conditioning is impractical. The growing demand for sustainable, low-cost cooling solutions—driven by climate change, urbanization, and the increasing energy burden of traditional cooling—has intensified research into materials and systems that can efficiently radiate heat while minimizing unwanted solar and atmospheric heat gain.

However, current radiative cooling technologies face significant limitations. Many existing materials either absorb solar energy or emit heat inefficiently outside the LWIR atmospheric window, leading to suboptimal cooling performance. Conventional radiative coolers often lack the spectral selectivity needed to maximize heat loss through the atmospheric window while blocking heat gain from other wavelengths, resulting in limited sub-ambient cooling, especially under direct sunlight. Additionally, scalable fabrication of high-performance, spectrally selective materials remains challenging, with some designs relying on expensive or mechanically fragile components. These shortcomings hinder widespread adoption and limit the effectiveness of passive radiative cooling in real-world applications, underscoring the need for new solutions that combine spectral precision, durability, and cost-effective manufacturing.

Technology

The selective long-wave infrared transmitter is an advanced multilayer optical film engineered for high-performance thermal management and passive radiative cooling. Its structure consists of alternating ultrathin layers of low-refractive-index polymers (such as polypropylene or polyethylene) and high-refractive-index materials (like germanium or silicon), forming multiple Fabry-Pérot cavities. This configuration is meticulously designed to achieve strong transmission (at least 60%) within the 8–13 μm atmospheric window—where Earth’s thermal radiation can escape to space—while reflecting infrared radiation outside this range (2–8 μm and 13–25 μm). A porous polymer top layer further enhances the film by providing solar reflectance and infrared transparency, allowing the film to act as both a radiative window and a barrier to convection. The technology is fabricated using scalable methods such as physical vapor deposition, hot pressing, and coextrusion, making it suitable for large-area applications in passive cooling for vehicles, buildings, data centers, food storage, dew harvesting, and more.

What differentiates this technology is its unique combination of high LWIR transmittance and strong out-of-band reflectance, which is not achieved by existing solutions that typically suffer from inefficiencies due to absorption or emission outside the optimal window. The multilayer design, requiring as few as three or four layers, offers exceptional spectral selectivity while remaining compatible with a wide range of everyday materials. Its scalable fabrication processes and the use of cost-effective materials (with silicon as a lower-cost alternative to germanium) make it practical for widespread deployment. Experimental validation has demonstrated that the film achieves deep sub-ambient cooling—up to 30°C below ambient temperatures—by efficiently transmitting thermal radiation to the sky while blocking unwanted heat gain. This capability, along with its role as a convection barrier and its adaptability for various cooling and sensing applications, positions the technology as a transformative solution for reducing reliance on conventional air conditioning and improving energy efficiency across multiple sectors.

Benefits

• Enables deep sub-ambient passive cooling up to 30°C below ambient temperature
• High spectral selectivity with ≥60% transmission in the 8–13 μm LWIR atmospheric window and ≥60% reflection outside this range, optimizing radiative cooling efficiency
• Combines solar reflectance and infrared transparency via a porous polymer top layer, reducing solar heat gain while allowing heat radiation
• Acts as both a radiative window and a convection barrier, minimizing parasitic heat transfer and enhancing cooling performance
• Compatible with a wide range of emissive materials and applicable to diverse environments including buildings, vehicles, data centers, and shelters
• Fabricated using scalable, cost-effective manufacturing methods suitable for large-area production
• Offers lower-cost material alternatives (e.g., silicon instead of germanium) without significant performance loss
• Improves thermal management by shortening the thermal resistance path between the cooled surface and the sky, enabling faster cooling

Applications

• Building passive cooling panels
• Vehicle rooftop cooling films
• Data center thermal management
• Cold storage container insulation
• Dew harvesting system covers

Additional info

This multilayer structure utilizes alternating ultrathin layers of low-refractive-index polymers and high-refractive-index materials like germanium or silicon to form Fabry-Pérot cavities. It achieves at least 60% transmission within the 8–13 μm atmospheric window while reflecting at least 60% in the 2–8 μm and 13–25 μm ranges. Featuring a porous polymer top layer for solar reflectance, the film enables sub-ambient cooling by functioning as a radiative window and convection barrier.