TAGS: Architectural Coatings
Fraunhofer researchers have developed an intelligent coating for glass windows that darkens in the sun. This uses electrochromic and thermochromic materials that react to electricity and heat. In buildings with large glass façades, it stops the rooms from getting too hot because of solar radiation, thereby reducing the demand for energy-intensive air conditioning.
Electrochromic and Thermochromic Coating
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The electrochromic coating is applied to a transparent, conductive film which can then be “switched on”. Applying an electrical voltage triggers the transfer of ions and electrons, which darkens the coating and tints the window. On the other hand, thermochromic coating works passively. When a certain ambient temperature is reached, it reflects the heat radiation of sun,” explains Dr. Marco Schott, Group Manager of Electrochromic Systems at Fraunhofer ISC.
With electrochromic elements, sensors can be used to measure factors such as brightness and temperature, sending the results to control systems. This sends a current or voltage pulse to the conductive film, triggering the window to darken.
The surface of the glass gradually darkens whenever the temperature or brightness level becomes too high. This stops rooms from becoming overheated and reduces the need for air conditioning, which is particularly useful in sunnier climes and buildings with large, glazed façades. It also serves as anti-glare protection on sunny days. On cloudy days and in the evening the windows will be kept in bright state.
Fraunhofer researchers have also thought about the suitability of the technology for everyday use. “
The windows don’t turn dark suddenly, but are gradually tinted over a few minutes,” explains Schott. Energy consumption is very low. In optimal circumstances, electrochromic film only requires electricity for the switching process and a very low voltage is enough to initiate the tinting process. Thermochromic materials do not require electricity at all, instead reacting passively to the heat generated by the sun. They can be used to complement a switchable system or as an alternative, where a switchable solution is not required.
In principle, the combination of electrochromic and thermochromic layers in a composite window offers the greatest possible flexibility. By using this, architects and developers can provide individual solutions for a variety of regions and buildings.
Roll-to-Roll Manufacturing
The researchers have also resolved challenges for manufacturing. The electrochromic coating is applied to a polymer-based film substrate. The thermochromic coating, on the other hand, uses a thin glass substrate. Wet chemical and vacuum coating processes are used in a cost-effective roll-to-roll manufacturing system.
The switchable components are then laminated under vacuum onto a 4 mm thick sheet of window glass, which is integrated to an insulating glass unit thereafter. The coating process is also economically feasible on an industrial scale. The electrochromic and thermochromic switchable elements are only a few 100 μm thick and less than 500 g per square meter. Thus, they hardly add any weight to the windows, which means that they can be retrofitted in existing buildings without needing to alter the building structure.
Curved Glass and Colorful Windows
The project consortium is currently working on improving the technology further. For example, the team of experts is researching how electrochromic and thermochromic elements can be combined in a composite window to make even better use of the technology’s potential. Further research objectives include adapting the coating to curved glass forms and adding more colors to the existing options of blue and gray.
Global warming and the objectives of the European Green Deal will significantly increase the demand for energy-efficient building technology in the next few years — and all buildings in the EU are expected to be carbon neutral by 2050. The electrochromic and thermochromic windows of the EU Switch2Save project can make an important contribution to this.
Source: Fraunhofer