‘Smart’ windows adjust opacity to cut solar warming – and the need for costly air con

07 Sep 2021

Fraunhofer-developed system uses electrochromic and thermochromic materials that react to light, electricity and heat.

Researchers at two Fraunhofer institutes in Germany have developed an intelligent coating for glass windows that darkens in the sun. The development uses electrochromic and thermochromic materials that react to electricity and heat. In buildings with large glass façades, the coating prevents the rooms from getting too hot because of solar radiation, thereby reducing the demand for energy-intensive air conditioning.

The building sector is one of the biggest emitters of greenhouse gases. According to the German Environment Agency, buildings are responsible for around 30 percent of the country’s CO₂ emissions and 35 percent of its total energy consumption.

The Fraunhofer Institute for Silicate Research (ISC) in Würzburg and the Fraunhofer Institute for Organic Electronics, Electron Beam and Plasma Technology (FEP) in Dresden have developed a sophisticated solution to this problem.

In the European Switch2Save project, researchers have been working on transparent coatings for windows and glass façades using electrochromic and thermochromic materials. These add a variable, transparent dark tint to the outside of the windows, which keeps the rooms cool.

The Fraunhofer Institutes have partnered with universities and industrial partners across six EU countries for this EU-funded research project.

Electrochromic and thermochromic coating

“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,” commented 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 process 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,” said 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.

Demonstration buildings in Athens and Uppsala

The Switch2Save initiative is intended to achieve significant energy savings in areas where outside temperatures are high, i.e., in southern European regions, by reducing use of air conditioning systems or removing the need for them entirely.

Dr. John Fahlteich, Switch2Save Project Coordinator and Head of the Research Group at Fraunhofer FEP, explained, “In warm regions of Europe, the cooling and heating energy demands of modern buildings can be reduced by up to 70 percent.” Savings are not so extensive in colder, northerly regions, but the systems could also be used here as anti-glare protection against direct sunlight.

“We are in the process of installing the technology in the Pediatric Clinic of the second-largest hospital in Greece, in Athens, and an office building in Uppsala, Sweden. In both buildings, energy consumption will be monitored and compared for a whole year both before and after installation of the new windows. By doing this, we can demonstrate the real-life performance of the Switch2Save technology and can continue to test and refine the technology for different climate zones,” said Fahlteich.

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 switchable elements are just a few 100 μm thick and weigh 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.