Heat generated by sunlight shining through windows is the single largest contributor to the need for air conditioning and cooling in buildings. Because residential and commercial buildings use 74% of all electricity and 39% of all energy in the United States, the shading effect from tinting windows helps buildings use less energy.
The technology, termed “thermochromic photovoltaic,” allows the window to change color to block glare and reduce unwanted solar heating when the glass gets warm on a hot, sunny day. This color change also leads to the formation of a functioning solar cell that generates onboard power. Thermo-chromic photovoltaic windows can help buildings turn into energy generators, increasing their contribution to the broader energy grid’s needs.
Researchers have developed a next-generation thermochromic window that enables myriad colors and a broader range of temperatures that drive the color switch. This increases design flexibility for improving energy efficiency as well as control over building aesthetics that is highly desirable for both architects and end users.
The research builds upon previous work into a thermochromic window that darkened as the Sun heated its surface. As the window shifted from transparent to tinted, perovskites embedded within the material generated electricity. Perovskites are a crystalline structure shown to have remarkable efficiency at harnessing sunlight.
The first-generation solar window was able to switch back and forth between transparent and a reddish-brown color, requiring temperatures between 150 and 175 °F to trigger the transformation. The latest iteration allows a broad choice of colors and works at 95 to 115 °F, a glass temperature easily achieved on a hot day.
By using a different chemical composition and materials, the researchers also were able to rapidly speed up the color transformation. The time was reduced to about seven seconds from the three minutes it took during the proof-of-concept thermochromic photovoltaic window demonstrated previously. The scientists sandwiched a thin perovskite film between two layers of glass and injected vapor. The vapor triggers a reaction that causes the perovskite to arrange itself into different shapes, from a chain to a sheet to a cube. The colors emerge with the changing shapes. Lowering the humidity returns the perovskite to its normal transparent state.
One area of additional research is the number of times the thermochromic window can be cycled into an electricity-generating operating mode and return to transparent. The efficiency of converting sunlight into electricity will also be explored.