A new type of window coating repels the heat

The new coating has up to twenty thin layers. Transparency is still maintained.

The window the sun shining through can heat up the room too much. Roasting forces buildings to cool down, and air conditioning consumes energy.

A cooling solution can be glass, which lets visible light through but reflects away heat radiation. A glass coating that works in this way has been developed by a group from the University of Notre Dame in the United States.

Coated glasses cooled the small test chamber by about 5–7 degrees compared to regular glass.

Coating the operation is based on several transparent layers that reflect different wavelengths of light.

Such multi-layered optical structures have been used in optical filters for a long time, comments an expert in the field, docent Tom Kuusela from the quantum optics laboratory of the University of Turku.

The new coating reflects the Notre Dame energy research professor Tengfei Luon and according to his colleagues, the rays heat up no matter what angle during the day the sun hits the glass.

Current glass coatings typically best reflect light that arrives at a right angle. But, for example, at noon, the angle of incidence of the sun’s rays can be far from 90 degrees. Therefore, the coatings used do not always reflect radiation as effectively.

In addition to visible light, the sun’s radiation has, first of all, invisible infrared or heat rays. They have a wavelength longer than visible light. The new coating reflects so-called near-infrared radiation, which is close to visible light.

Coating according to the researchers, it also reflects ultraviolet radiation, which also heats up.

“So the goal is an optical filter that would let visible light through as well as possible, but block infrared radiation as effectively as possible at all light incidence angles,” comments Kuusela.

According to him, ultraviolet radiation does not penetrate well even through ordinary window glass, so combating it is not such a significant achievement.

In the new the coating has up to twenty thin transparent layers. Thinness is beneficial because thick layers absorb more light.

The purpose of the films is to strengthen or weaken the wavelengths in a controlled manner to obtain the desired light filtering.

The layers alternately contain silicon oxide, titanium oxide, aluminum oxide and silicon nitride. The top is pdms silicon polymer, which is an optically clear compound and very common. It is used, for example, in contact lenses.

Since there are several optical materials and up to twenty layers, there are a huge number of possible combinations of layers. That’s why the research made use of machine learning and quantum computing.

The team tested the multi-layer coating on window glasses in a small test chamber, which were placed vertically and horizontally. Score published by Cell Reports Physical Science.

Air conditioning the costs could even be halved thanks to the coatings, the researchers concluded from their modeling. According to them, the coatings would be suitable not only for buildings but also for car roof windows and windshields.

Kuusela estimates that the structure of the coating is more functional than the current tinted glasses. However, the structure is complex and expensive.

Published in Tiede magazine 6/2024.

By Editor