Thermochromic Materials for Optimized Modulation of Solar Thermal Radiation in Windows and Their Combination with Mid-IR Thermal Insulation

The built environment is responsible for more than 1/3 of the annual energy consumption worldwide. A major contributor to building energy demand are windows due to their transparency to thermal radiation. In general, the thermal radiation relevant for buildings can be divided into two wavelength regions of interest: i) near-infrared (IR) between 800 – 2500 nm, representing thermal radiation from sunlight; ii) mid-IR between 5 – 25 µm, representing thermal radiation from radiators. Furthermore, requirements for windows to transmit or block each of these regions depend on the climate and can change between seasons. In hot climates or summertime, windows should block solar thermal radiation to limit air conditioning demand. Whereas in cold climates and wintertime, windows should transmit solar thermal radiation to contribute to a building’s heating demand, whilst preventing mid-IR thermal radiation from leaving the building. For moderate climates these changing requirements can only be met by smart adaptive glazing.<br/>One of the most interesting materials for adaptive glazing are thermochromics (TC). These materials can change their optical properties upon a change in temperature. The most prominent representative is vanadium dioxide (VO₂). VO₂ can change its crystal structure from monoclinic to rutile at 68^○C, inducing a change in electrical and optical properties. It is perfectly suited for solar heat regulating windows because of the following properties: i) switch between near-IR transparent and blocking; ii) potential for high visible transparency by limiting the switch mainly to the near-IR; iii) possibility to adjust the switching temperature in a wide range between 0 – 68^○C; iv) cheap and abundant material; v) potential to use cheap and low energy intensive application methods; vi) autonomous switch and potential for application in regular glazing; vii) compatibility with existing energy efficient window coatings.<br/>We have developed a high quality VO₂ nanoparticle that can be added via coatings or polymer films to various types of window products adding TC properties. The pigment is optimized to enable high visible transparency (<i>T_vis</i>) in combination with a substantial solar heat modulation (Δ<i>T_sol</i>), optimizing the energy performance of a window.<br/>Here we present a detailed study on how the size and crystallinity of VO₂ nanoparticles influence the optical properties and functional performance of TC coated/laminated glass. We show that by increasing crystallinity Δ<i>T_sol</i> and <i>T_vis</i> can be increased simultaneously because of an increased fraction of functional material in the VO₂ pigment. Furthermore, we show that reducing the average particle size to &lt;80 nm leads to appearance of plasmonic properties in the metallic (rutile) state of the VO₂ pigment. This results in increased absorption in the wavelength region between 780 – 1360 nm and in a corresponding increase in Δ<i>T_sol</i>. In addition, we show that integration of small VO₂ pigments in a transparent matrix can significantly increase <i>T_vis</i> whilst retaining a high Δ<i>T_sol</i>. Using all these optimizations in one coating we obtained a maximum Δ<i>T_sol</i> of 23% at a <i>T_vis</i> of 67%.<br/>Since solar heat management is only one part of a window’s interaction with thermal radiation, we also present a study on combining our TC solar heat regulating glasses with low-emissivity glass, blocking mid-IR thermal radiation. Here we show the importance of matching properties between both types of functional glasses to retain the solar heat regulating properties of the TC glass whilst adding thermal insulation in the mid-IR region. With this combination a perfect window with optimized energy efficiency, via shgc modulation of 13%, can be produced, which we validate via building energy simulations. We show that a building’s energy efficiency in the Netherlands can be increased by up to 9% in comparison to state-of-the-art energy efficient windows, resulting in about 500€ energy cost savings per year for a single household.