Windows play multiple crucial roles in our homes. They illuminate, insulate and ventilate our spaces while providing views of—and protection from—the outdoors. Smart windows, or windows that use solar cell technology to convert sunlight into electricity, present the additional opportunity to leverage windows as energy sources.

However, incorporating solar cells into windows while balancing the other complex, and often conflicting, roles of windows proves challenging. For example, juggling luminosity preferences and energy harvesting goals throughout changing seasons requires complex and strategic approaches to material design.

Scientists from the U.S. Department of Energy’s (DOE) Argonne National Laboratory, Northwestern University, the University of Chicago and University of Wisconsin-Milwaukee recently combined solar cell technology with a novel optimization approach to develop a smart window prototype that maximizes design across a wide range of criteria.

The optimization algorithm uses comprehensive physical models and advanced computational techniques to maximize overall energy usage while balancing building temperature demands and lighting requirements across locations and throughout changing seasons.

To demonstrate the feasibility of a smart window capable of this level of customization, the scientists produced a small prototype of the window with an area of a few square centimeters.

The prototype consists of dozens of layers of varying materials that control the amount and frequency of light passing through, as well as the amount of solar energy converted into electricity.

One group of layers, made of a type of material called a perovskite, comprises the window’s solar cell, which harvests sunlight for energy conversion. The window prototype also includes a set of layers called a nanophotonic coating, developed by associate professor of mechanical engineering Cheng Sun and his research group at Northwestern’s McCormick School of Engineering. The coating tunes the frequencies of light that can pass through the window.

Each layer is tens of microns thick—thinner than the diameter of a grain of sand. The scientists chose an aperiodic design for the layers, meaning each layer varies in thickness. As the angle of the sun’s rays against the window changes throughout the day and year, the aperiodic design enables the performance of the window to vary in accordance with the user’s preferences.

“The variation in layer thickness is optimized for a wide spectrum of change in the nature of the sunlight that reaches the window,” said Sun. “This enables us to systematically allow less infrared transmission in the summertime and more in the wintertime to save energy consumption for temperature regulation, while optimizing the visible transmission for the purpose of indoor lighting and energy harvesting.”

The scientists optimized the prototype used in this study for a 2,000 square foot, single-story home in Phoenix. Based on experimental characterization of the window prototype, the scientists calculated significant annual energy savings over leading commercially available window technologies. The calculations used the EnergyPlus building model, a software developed at the National Renewable Energy Laboratory, a DOE Office of Energy Efficiency and Renewable Energy laboratory, that estimates realistic power consumption over time.

Source: “Customizable Smart Window Technology Could Improve Energy Efficiency Of Buildings”,

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