Smart Windows Using Thermochromic VO₂ Coatings for Autonomous Adaptation of Solar Heat Gain for Optimized Energy Efficiency in Buildings

Crystalline vanadium dioxide is a naturally occurring smart material that reacts on a number of different stimuli with a structural phase transition (STP) that changes the optical, electrical and physical properties of the material. This phase and property change can be used in smart applications, such as thermochromic glass coatings for smart windows. Here a thermal and optical stimulus is used to switch the solar infrared transmission of a coated glass plate between a transparent and a blocking state. Integrated into a window this enables the autonomous adaptation of its solar heat gain based on a buildings energy and comfort needs. This significantly reduces the energy demand of a building on heating and cooling simultaneously and increases comfort levels for occupants. Furthermore, smart windows using thermochromic VO₂ coatings can reduce the CO₂ emissions of buildings and add to the transition towards a climate neutral built environment.<br/>In the past years, we have developed a process to synthesize doped thermochromic VO₂ nanoparticles at high purity, crystallinity and precise size and shape.¹ We integrated this functional smart material into a coating on glass, resulting in thermochroic smart window coatings with optimized optical properties, combining high visible transmission &gt;60% with high solar modulation &gt;20%.² Here we present the importance of the functional materials characteristics on the smart properties. Various parameters, such as crystallinity, size and compatibility between functional nanoparticles and coating matrix, are crucial to optimize functional performance. To influence the STP towards application oriented requirements, we used precise meatal ion doping to reduce the phase transition temperature from 68°C for regular VO₂ to application oriented region between 20 – 30°C.³ The technology has been scaled up to 1 m² sized glass plates using an industrial roller coater and the coated glass plates have been integrated into insulating glass units. We further investigated the combination of the smart window coating with industrial low emissivity coatings, adding insulating properties to the adaptive solar heat gain properties of the smart window coating. Synergistic and parasitic effects of both coatings have been investigated to find the correct balance between both effects.² Full sized window demonstrators have been installed in test buildings, monitoring the thermochromic effect dependent on the glass and outdoor temperature, as well as the solar irradiance.⁴ Using the gained insight into the smart window behavior in a real life setup, the thermochromic properties and phase transition temperature could be further optimized. Finally, we present the energy and CO₂ emission savings potential of the new smart window via building energy simulations. Here we show that in a regular Dutch household energy savings of approximately 9% in comparison to existing highly energy efficient windows can be achieved and that the Dutch built environment may save up to 5.8 Mt in CO₂ emissions per year using widespread implementation of the new smart window.^2,3 Overarching, we present the development of a new smart product, from conceptualization, to lab scale optimization, scale up, system integration, real life testing and impact analysis. The knowledge gained during this multi-year development can be transferred towards other smart material developments.<br/>¹L. Calvi <i>et al. Sol. Energy Mater. </i><i>Sol. Cells</i> <b>2023</b>, <i>257</i>, 112350;<br/>²D. Mann <i>et al. </i><i>IOP Conf. Ser. Earth Environ. Sci.</i> <b>2022</b>, <i>1085</i>, 012060; D. Mann <i>et al. Energies</i> <b>2023</b>, <i>16</i>, 4984.<br/>³L. Calvi <i>et al. Sol. Energy Mater. Sol. Cells</i> <b>2021</b>, <i>224</i>, 110977.<br/>⁴D. Mann <i>et al. </i><i>IOP Conf. Ser. Earth Environ. Sci.</i> <b>2021</b>, <i>855</i>, 012001.