Almost half of the energy in sunlight is in the infrared portion of the spectrum – with wavelengths exceeding say 780 nanometer. This explains why windows with special coatings that reflect infrared (IR) light can help keep houses and offices cooler. “A simulation that we conducted together with the Department of the Built Environment shows that such a static reflector can bring the temperature in a warm climate down by as many as five to six degrees”, says PhD candidate Hitesh Khandelwal.

In India, his native country, a simple IR coating can be extremely useful therefore, as the chemical engineer explains. “Conversely, in a more varied climate you want to allow the maximum of energy from the sunlight to come in through your windows in winter.” By means of panes that can allow infrared light to come through or reflect it as needed, you can save on air conditioning costs in summer and on fuel costs in winter. “We have conducted a simulation with the climate in Madrid, where they have hot summers and relatively cold winters, and it turned out that you can save some twelve percent in energy by means of switchable IR coatings.”

 “In Madrid you can save some twelve percent
of energy with switchable IR coatings”

In the Functional Organic Materials and Devices group of the TU/e Department of Chemical Engineering and Chemistry, where Khandelwal obtained his PhD, they are experts in the field of so-called liquid crystals: the type of material of which LCD displays are made, among other things. In his PhD research, which was funded by the Dutch Polymer Institute, the Indian managed to find a combination of liquid crystals that only affect infrared light – with a wavelength between 700 and 1400 nanometers. “When you take two glass panes with two intermediate layers of my liquid crystals, they will allow nearly as much visible light to pass through as ordinary double glazing, while most of the infrared radiation is reflected.” This gives the IR blinds the advantage that the window panes just look transparent without requiring any extra artificial lighting in summer, as is often the case when conventional blinds are used. Moreover, by means of a modest electric voltage the liquid crystals switch between a largely transparent and a reflecting state - for infrared light. And a state in between the two extremes is possible as well. Khandelwal further showed that the voltage needed for the switch can even be generated in the window pane itself - with solar energy. For this he used the principle of the Luminescent Solar Concentrator, whereby sunlight is directed to solar cells in the edge of the pane.Finally, the PhD candidate also worked on a coating that automatically becomes more transparent for infrared light when the temperature decreases. “That would be a relatively cheap solution for existing window panes. We have demonstrated that the principle works for this purpose as well - though not at the right temperature for now, but that merely seems to be a matter of time.”