Thermochromic Solar Control Coatings on SiO2-Coated Float Glass for Energy-Efficient Smart Windows
YEUNG C. 1,2, HABETS R. 1,2, LEUFKENS L. 1,2, VERHEIJEN M. 4,5, VROON Z. 1,2,3, MANN D. 1,2, BUSKENS P. 1,2,6
1 The Netherlands Organisation for Applied Scientific Research (TNO), Eindhoven, Netherlands; 2 Brightlands Materials Center, Geleen, Netherlands; 3 Zuyd University of Applied Sciences, Heerlen, Netherlands; 4 Department of Applied Physics, Eindhoven University of Technology, Eindhoven, Netherlands; 5 Eurofins Materials Science, Eindhoven, Netherlands; 6 Design and Synthesis of Inorganic Materials (DESINe), Institute for Materials Research, Hasselt University, Diepenbeek, Belgium
Heating and cooling of buildings is a major contributor to the energy consumption of the built environment, therefore there is a clear need for energy-efficient windows to maintain a comfortable indoor environment whilst limiting energy usage. For intermediate climates with hot summers and cold winters, smart windows with switchable solar control properties are ideal. Thermochromic materials are of interest for application in smart windows since their optical properties change based on temperature as external stimulus. In our developments, we used VO2 as thermochromic material, utilizing its reversible structural phase transition from a monoclinic, infrared transparent to a rutile, infrared blocking state. We showed that the material can be used for highly transparent coatings with large modulation in infrared transparency and that the transition temperature can be tuned in a wide range between 0°C – 68°C via W doping. Which makes the material a perfect candidate to optimize for use in smart windows in various climate zones.[2,3]
Here, we report a nanocomposite single-layer coating comprising VO2 and SiO2. We prepared these coatings by dip coating of a SiO2-coated float glass in an alcoholic solution of a vanadium(IV)oxalate complex and pre-oligomerized tetraethoxysilane, and thermally annealed the dried xerocoat in a two-step process. During thermal anneal of the xerocoat, phase separation occurred which resulted in Si and V rich domains forming a composite coating with randomly distributed VO2 nanodomains, incorporated in an SiO2 matrix. This process and the impact of varying VO2/SiO2 ratio and coating thickness on the size and shape of VO2 domains was studied in detail, using high resolution transmission electron microscopy. Furthermore, the influence of coating thickness and composition on the optical properties was investigated. By optimizing the size, shape, loading and dispersion of VO2 domains in the SiO2 matrix, highest combination of Tvis (67%) and ΔTsol (22.9%) were obtained, surpassing all other VO2 based coatings and films reported in literature to date.[3,4]
Additionally, we investigated the theoretical maximum performance of thermochromic coatings in a real insulated glazing unit (IGU) and analyzed several material combinations to reach highest solar modulation potential, whilst complying to window regulations. Here we could show that our best performing coating is currently at 59% of the theoretical maximum performance within an IGU and can adapt the windows g value by 13% at a Tvis > 60%. Via building energy simulations, we could further show that this new smart window can lead to energy savings of up to 8.8% on top of current high performance glazing and annual cost savings of approximately 500 € for an average household in the Netherlands can be reached.
 Mann et al. Energies 2020, 13, 2842.
 Calvi et al. Sol. Energy Mater. Sol. Cells 2021, 224, 110977.
 Yeung et al. Solar Energy Materials and Solar Cells 2021, 230, 111238.
 Mann et al. submitted manuscript to Energy and Buildings.
 Mann et al. IOP Conf. Ser. Earth Environ. Sci. 2022, 1085, 012060.