Glass façades characterize modern architecture. While in winter the sunlight is used to support the heating, in summer the inside of the building heats up and requires active cooling. Smart windows with special surface coatings can regulate solar radiation according to the weather situation - a forward-looking solution in times of energy saving.
The Fraunhofer FEP has now succeeded in producing the world's first thermochromic coating on ultra-thin glass using a roll-to-roll process. The results can make mechanical blinds superfluous and at the same time reduce a building's cooling and heating energy requirements.
Office complexes, public buildings and new buildings are usually architecturally characterized by large, south-facing windows and glass façades. While in winter the solar radiation is used to support the heating, in summer the building interior heats up and requires active cooling. Shading by blinds, for example, reduces comfort and does not contribute to the use of the heat input in the building in winter. Smart windows offer a highly attractive solution here, particularly in view of the coming fall and winter periods, combined with the current government requirements for energy saving and the energy crisis. Such windows can regulate the heat input of solar radiation according to the weather situation.
Coating technologies for thermochromic and electrochromic layers
The Fraunhofer FEP is researching surface coatings that can make a major contribution here and enable a reduction in heat radiation through window glass into the building. Together with project partners, the researchers are working on active, smart coating systems in the EU project "Switch2Save", for example, which utilize the effects of electrochromism (switching of energy transmission by applying a voltage) and thermochromism (switching of energy transmission by exceeding/falling below a temperature). Such electrochromic films can be used in insulating glazing and are not only used in new buildings. Retrofitting existing buildings is also possible and is the subject of the recently launched "FLEX-G 4.0" project.
Some passive technologies, such as SolarControl systems and low-E coatings (low emissivity), are already commercially available on the market. However, these thin layers produced on film or glass only lead to a permanent adjustment of the energy transmittance. They therefore only work in one setting, e.g. to prevent heat radiation in summer. In winter, however, this is also kept out. They also use valuable resources such as silver in the manufacturing process. Fraunhofer researchers are therefore focusing on optimizing the properties and replacing such scarce materials.
With all technologies - whether passive (low-E; SolarControl) or active (electrochromic; thermochromic) - the challenge is to master the balancing act between the various properties that need to be effective at the same time: Do the optical impression and optical effectiveness in different wavelength ranges play the greater role or is this negligible compared to a large energy transmittance. The range of switching temperatures for thermochromic coatings and, of course, the manufacturing costs must also be taken into account.
In order to find versatile and new solutions for this, researchers at the Fraunhofer FEP are currently developing coating technologies for thermochromic elements on ultra-thin glass. The substrate material with a thickness of approx. 100 µm places high demands on handling and scaling to larger areas has so far proved very difficult. At the same time, the use of a polymer film as an alternative substrate, which could make handling easier, is not readily possible. The reason for this is the high temperatures of several 100 °C in the manufacturing process.
First thermochromic layers on ultra-thin glass in the roll-to-roll process worldwide
Visual comparison between a thermochromic coating (center) and uncoated glass At the beginning of 2022, researchers at Fraunhofer FEP succeeded in producing the world's first thermochromic coating based on vanadium dioxide on ultra-thin glass using efficient roll-to-roll technology. Dr. Cindy Steiner, group leader at Fraunhofer FEP, is delighted: "We have thus taken an important step in scaling up the technology from laboratory to pilot scale with our roll-to-roll systems! The thermochromic layers change their transmission in the infrared range when a certain temperature is exceeded. The transmission in the visible range remains unchanged. This means that the user does not notice any visual change to the window and has no restrictions in terms of light comfort or visibility. This effectively blocks heat radiation in summer, reducing the need for air conditioning. In winter, the sun's heat radiation is let through, which leads to savings in heating energy consumption."
The switching temperature is around 20 °C, which means that the thermochromic ultra-thin glass mounted on buildings switches between a translucent and reflective state when it gets warmer than 20 °C. "This switching temperature can be adjusted according to the climatic requirements through the composition, process control and structure of the coating system," adds Dr. Steiner.
The next step is to scale up the technology and bring it to market maturity. Research will focus in particular on optimizing substrate handling, long-term stability and setting the required switching temperature.
The combination of the technologies presented here will make mechanical blinds superfluous in future and can reduce a building's cooling and heating energy requirements by between 10% and, in extreme cases, up to 60%.
Research into energy-saving window layers is currently being carried out in the Switch2Save research project under the project name "Lightweight switchable smart solutions for energy-saving large windows and glass facades" until the end of September 2023 and FLEX-G 4.0 under the project name "Technologies for innovative switchable films as a retrofit solution for energy-saving windows and glass facades" until the end of July 2026. FLEX-G 4.0 is funded by the EU and the Federal Ministry for Economic Affairs and Climate Protection.
