October 28 2017 by Justin Harris
Forget about drawing the shades, pulling the blinds, or covering your windows. In the future windows will be smart enough to sense the amount of sunlight, and automatically adjust to control light and heat coming into our homes, offices, cars, and more.
Smart Windows Made Possible by Thermochromic Materials
Thermochromic materials change their spectral color based on temperature. Since color is simply energy with wavelengths in the visible spectrum, thermochromism offers a unique ability to automatically tune energy absorption, reflection, and transition based on the temperature. In the case of smart windows, the temperature gradient arises from amount of direct sunlight.
Current State of the Technology
Already, this technology is being employed. After development with support from the Department of Energy Inventions and Innovation Program, in 2011 Pleotint launched Suntuitive™: Sunlight-Responsive Thermochromic Window Systems. According to commercialwindows.org the windows are made by extruding patented thermochromic materials into polyvinyl butyral (PVB) which is laminated between two pieces of heat strengthened glass. As a result, heat from direct sunlight is used to reversibly tint the windows. The more direct and intense the sunlight the darker the glass becomes.
Suntuitive® Glass windows installed at an educational facility in Keller, Texas, USA. Photo courtesy of Pleotint, LLC.
Next Gen Smart Windows
The next generation of smart windows may be even more advanced. What if windows could regulate energy without changing tint or darkening? That is challenge currently being worked on. The goal is to develop a thermochromic product that remains transparent to the visible spectrum of light but can be tuned based on temperature in the infrared spectrum alone (aka. the wavelengths emitted by heat). This would provide an ability to regulate the amount of heat energy either reflected or transmitted through the glass without a tinting effect.
Nanoparticles in your Windows
One such thermochromic material capable of this feat are vanadium oxide nanoparticles. IR Dynamics and Sandia National Laboratories are working to develop nanoparticle films of vanadium oxide. These films appear transparent and undetected to the naked eye at any temperature, but reflect heat at high temperatures and allow it to pass at lower temperatures along a gradient. The temperature at which the transition occurs can be easily tuned by doping the vanadium oxide with rare earth metals or tungsten. Through such methods transition temperatures from -40o to 200oF are accessible,1 and the ideal range for buildings and cars, 70-80oF (approximately room temperature) can easily be synthesized.
The goal for this technology is that by retrofitting windows with these coatings, both heating and cooling bills could be reduced. On cool days heat would pass easily through the windows; on hot days, heat would be reflected. Likewise, the nanoparticle film can be incorporated into architectural plastics, car windows, or performance wear with wide reaching impact.
Just imagine, someday soon your thermostat may become obsolete. Instead replaced by tunable smart windows capable of regulating temperatures. You may not even see the change, but it is likely smart window will be on the front lines controlling energy use and costs in the decades to come.
- Wang, Ning, et al. "One-step hydrothermal synthesis of rare earth/W-codoped VO 2 nanoparticles: Reduced phase transition temperature and improved thermochromic properties." Journal of Alloys and Compounds 711 (2017): 222-228. DOI: https://doi.org/10.1016/j.jallcom.2017.04.012