Illustrations of light transmission through a VO2-based glazing prototype, which harnesses power while blocking unwanted heat.
Courtesy of "VO2 thermochromic smart window for energy savings and generation," published in Scientific Reports Illustrations of light transmission through a VO2-based glazing prototype, which harnesses power while blocking unwanted heat.


For years, scientists and manufacturers have struggled to produce an effective solar-harvesting window. Although several preliminary technologies exist, they typically suffer from at least one fatal flaw: the obstruction of view and/or reduction of transparency based on the inclusion of opaque photovoltaic materials. Although these materials can provide effective shading in windows that receive unwanted quantities of direct sunlight—effectively replacing frits or other shading devices—they are not welcome in all situations.

I recently wrote about a company that had developed transparent windows that blocked infrared radiation. Recently, a team of scientists at Shanghai University, Chinese Academy of Sciences (CAS), and Zhengzhou University unveiled a solution to both harness energy  and reduce building energy loss using windows that do not diminish light or view transmission. The key is the utilization of vanadium oxide (VO2), a solid compound applied in optical and storage technologies for its rapid phase transition between transparent and reflective states.
The researchers created a prototype window with a layer of VO2 thin film encapsulated between two sheets of clear polycarbonate. The film not only allowed the transmission of visible light, but also scattered the light towards the perimeter of the glazing unit. The scientists placed an edge-facing photovoltaic cell on one side, and were able to power a 1.5 volt lamp as a result.
In addition, the researchers noted that VO2 also acts as a light wavelength regulator. The film in their prototype permitted infrared light transmission up to 154°F, but above this temperature became reflective to this portion of the light spectrum. As a result, although the window still appeared transparent, it was thermochromically blocking heat without the need for added power or switching controls.
Although the VO2 film technology will require more time for development, it already exhibits features superior to many other contemporary power-harnessing and energy-saving window strategies.
Blaine Brownell, AIA, is a regularly featured columnist whose stories appear on this website each week. His views and conclusions are not necessarily those of ARCHITECT magazine nor of the American Institute of Architects.