Scientists at the Department of Energy’s National Renewable Energy Laboratory (NREL) report a breakthrough in the development of a next-generation thermochromic window that not only reduces the need for air conditioning, but also generates electricity.
Heat generated by sunlight shining through windows is one of the main contributors to the need for air conditioning and cooling in buildings. Since residential and commercial buildings use 74% of all electricity and 39% of all energy in the US, the shade effect of tinting windows helps buildings use less energy.
Technology known as “thermochromic photovoltaics” allows the window to change color to block glare and reduce unwanted solar heat when the glass gets warm on a hot, sunny day. This change in color also leads to the formation of a functioning solar cell that generates electricity on board. Thermochromic photovoltaic windows can help buildings become energy producers and increase their contribution to the needs of the wider energy grid. The latest breakthrough now allows for myriad colors and a wider temperature range to drive the color change. This increases design flexibility to improve energy efficiency, as well as control over building aesthetics, which is highly desirable for both architects and end users.
The research builds on previous work at NREL on a thermochromic window that darkened as the sun warmed its surface. As the window changed from transparent to tinted, perovskites embedded in the material generated electricity. Perovskites are a crystalline structure that has been shown to have remarkable efficiency in harnessing sunlight.
“A prototype window using the technology could be developed in a year,” said Bryan Rosales, NREL postdoctoral fellow and lead author of the paper “Reversible Multicolored Chromism in Formamidinium Metal Halide Layered Perovskites,” which appears in the journal Nature Communications. His co-authors on NREL are Lance Wheeler, who developed the first thermochromic photovoltaic window, Taylor Allen, David Moore, Kevin Prince, Garry Rumbles and Laura Schelhas. Other authors are Laura Mundt from the SLAC National Accelerator Laboratory and Colin Wolden from the Colorado School of Mines.
The first-generation solar window could switch back and forth between transparent and red-brown colors and required temperatures between 150 and 175 degrees Fahrenheit to trigger the transformation. The latest iteration allows for a wide range of colors and operates at 95 to 115 degrees Fahrenheit, a glass temperature that can easily be reached on a hot day.
By using a different chemical composition and materials, the researchers were also able to speed up color transformation quickly. The time has been reduced from the three minutes required during the thermochromic photovoltaic proof-of-concept window presented in 2017 to around seven seconds.
The scientists pushed a thin film of perovskite between two layers of glass and injected steam. The steam triggers a reaction that causes the perovskite to arrange itself in various shapes, from a chain to a leaf to a cube. The colors arise with the changing shapes. Lowering the humidity will return the perovskite to its normal transparent state.
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