New light-transmissive metamaterial cools rooms and self-cleans

Maximizing natural light in buildings is a popular trend due to its potential to save on energy costs. However, traditional glass roofs and walls often lead to issues such as glare, lack of privacy, and overheating. While alternative solutions like coatings and light-diffusing materials have been explored, they haven’t fully addressed these problems.

Now, researchers at the Institute for Microstructure Technology (IMT) and the Light Technology Institute (LTI) at KIT have developed a novel polymer-based metamaterial that is not only more transparent to light but adds privacy, cools the room inside, and automatically cleans itself. It could potentially have significant implications for the future of building design and energy efficiency.

This Polymer-based Micro-Photonic Multi-Functional Metamaterial (PMMM) consists of microscopic pyramids made of silicone, each measuring approximately ten micrometers, which is about one-tenth the diameter of a hair. This unique design provides the PMMM film with multiple functions, including light diffusion, self-cleaning, and radiative cooling, all while maintaining a high level of transparency.

“A key feature is the ability to efficiently radiate heat through the Earth’s atmosphere’s long-wave infrared transmission window, releasing heat into the cold expanse of the universe. This allows for passive radiative cooling without electricity consumption,” explains Bryce S. Richards, Professor at IMT and LTI. 

In the lab and in experiments conducted under real outdoor conditions, the researchers rigorously tested the material’s properties. They measured its light transmittance, light scattering, reflection properties, self-cleaning ability, and cooling performance using modern spectrophotometry. 

The results were quite impressive. The tests showed that the material achieved a cooling effect of 6 °C compared to the ambient temperature. Additionally, the material exhibited a high spectral transmittance of 95%, compared to the usual 91% of most glass. At the same time, the micro-pyramid structure scattered 73% of the incoming sunlight, resulting in a diffused appearance and creating a more comfortable and private indoor environment. 

“When the material is used in roofs and walls, it allows for bright yet glare-free and privacy-protected indoor spaces for work and living. In greenhouses, the high light transmittance could increase yields because the photosynthesis efficiency is estimated to be nine percent higher than in greenhouses with glass roofs,” says RichardsGan Huang, a Group Leader at IMT.

The PMMM film’s micro-pyramids provide superhydrophobic properties, akin to a lotus leaf, causing water to form droplets that effortlessly remove dirt and dust. This self-cleaning feature ensures easy maintenance and durability of the material.

“Our newly developed material has the potential to be used in various areas and makes a significant contribution to sustainable and energy-efficient architecture,” explains Richards. 

“The material can simultaneously optimize the use of sunlight indoors, provide passive cooling, and reduce reliance on air conditioning. The solution is scalable and can be seamlessly integrated into plans for environmentally friendly building construction and urban development,” says Huang.

Journal reference:

Gan Huang, Ashok R. Yengannagari, Kishin Matsumori, Prit Patel, Anurag Datla, Karina Trindade, Enkhlen Amarsanaa, Tonghan Zhao, Uwe Köhler, Dmitry Busko & Bryce S. Richards. Radiative cooling and indoor light management enabled by a transparent and self-cleaning polymer-based metamaterial. Nature Communications, 2024; DOI: 10.1038/s41467-024-48150-2

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