A transparent metamaterial for energy-efficient light and temperature regulation

Researchers at the Karlsruhe Institute of Technology (KIT) developed a metamaterial that allows sunlight to enter, maintains a more comfortable indoor climate without additional energy, and cleans itself.

The maximising of natural light in a building is good for its users, creating a healthy living or work environment. It can also save on energy costs. However, traditional glass roofs and walls have their challenges, such as glare, lack of privacy and overheating. While there are solutions in coatings and light-diffusing materials, this is not optimised yet.

The KIT researchers developed a polymer-based metamaterial film. A metamaterial is a material engineered to have a property that is rarely observed in naturally occurring materials. The new metamaterial consists of microscopic pyramids made of silicone. These micro-pyramids measure about ten micrometres, which is about one-tenth the diameter of a hair. This design gives the metamaterial film several functions: light diffusion, self-cleaning, and radiative cooling while maintaining a high level of transparency. The micro-pyramids also give the film superhydrophobic properties, similar to a lotus leaf: water beads up in droplets and removes dirt and dust from the surface.

Tests found a cooling of 6 °C compared to the ambient temperature. Additionally, the material showed a high spectral transmittance, or transparency, of 95 percent. In comparison, glass typically has a transparency of 91 percent. At the same time, the micro-pyramid structure scatters 73 percent of the incoming sunlight, resulting in a blurry appearance. These test results mean that when the material is used in roofs and walls, it allows for bright yet glare free and privacy protected indoor spaces. The material could also be used in greenhouses, as the researchers estimate that photosynthesis of the plants would be nine per cent higher than in traditional glass greenhouses.

Photo: Gan Huang / KIT