Glass has become the primary material used on the surface of our modern commercial buildings. Our skyscrapers glisten as sunlight reflects off their smooth outer surfaces. Inside the building, floor-to-ceiling windows accentuate views across cities and flood our offices with natural light. We pay a heavy price for this illuminating, shiny beauty however; with sunlight comes heat in these towering greenhouses, which means we need energy intensive cooling to keep temperatures at a human friendly level.
In a world striving for energy efficiency, big windows, especially in sunny places, may seem like an unnecessary luxury, but a recent breakthrough is promising the best of both worlds. Researchers at Princeton University have developed a self-powered smart window that is both easy and inexpensive to apply to existing windows. The system features solar cells that selectively absorb near-ultraviolet (near-UV) light, which makes the windows completely self-powered.
“Sunlight is a mixture of electromagnetic radiation made up of near-UV rays, visible light, and infrared energy, or heat,” said Yueh-Lin (Lynn) Loo, director of the Andlinger Center for Energy and the Environment and professor of chemical and biological engineering. “We wanted the smart window to dynamically control the amount of natural light and heat that can come inside, saving on energy cost and making the space more comfortable.”
The researchers created a smart window made up of electrochromic polymers that control the tint of the glass and can be powered by electricity produced by the solar cell itself. When near-UV light from the sun generates an electrical charge in the solar cell, it triggers a reaction in the electrochromic window, causing it to transform from transparent to dark blue. Once the change occurs, the window can block more than 80% of sunlight hitting the glass, leading to as much as 40% saving in an average building’s energy costs.
"This new technology is actually smart management of the entire spectrum of sunlight," said Loo, who is one of the authors of a paper that describes this technology, which was developed in her lab. Loo and her colleagues set out to harness near-UV light, which is invisible to the human eye, for the electrical energy needed to activate the windows’ tinting technology.
"Using near-UV light to power these windows means that the solar cells can be transparent and occupy the same footprint of the window without competing for the same spectral range or imposing aesthetic and design constraints," Loo added. "Typical solar cells made of silicon are black because they absorb all visible light and some infrared heat - so those would be unsuitable for this application."
Within the paper, published in Nature Energy, the researchers described how they used organic semiconductors - contorted hexabenzocoronene (cHBC) derivatives - for constructing the solar cells. They chose the material because its chemical structure could be modified to absorb a narrow range of wavelengths - in this case, near-UV light - to make a smart window that offer benefits that have been the downfall of many similar products.
Smart windows are not new but typically require a power source, making them complex and costly to retrofit. Looking forward the researchers are confident they can create a flexible version of their solar-powered smart window system that can be applied to existing windows via simple lamination process.
"Someone in their house or apartment could take these wireless smart window laminates, which could have a sticky backing that is peeled off, and install them on the interior of their windows," said Nicholas Davy, a doctoral student in the chemical and biological engineering department and the paper's lead author. "Then you could control the sunlight passing into your home using an app on your phone, thereby instantly improving energy efficiency, comfort, and privacy."
The technology has received good feedback from the scientific and engineering community. Joseph Berry, senior research scientist at the National Renewable Energy Laboratory, said the research project is interesting because the device scales well and for its ability to target a specific part of the solar spectrum.
"Integrating the solar cells into the smart windows makes them more attractive for retrofits and you don't have to deal with wiring power," said Berry. "And the voltage performance is quite good. The voltage they have been able to produce can drive electronic devices directly, which is technologically quite interesting."
The technology also has potential beyond tinting. In fact, Davy and Loo have started a new company, called Andluca Technologies, to explore other applications for the transparent solar cells. They explained that the near-UV solar cell technology could even be used to power Internet of Things sensors and other low-power consumer products that are becoming increasingly present in smart buildings.
Look out of your dumb old window at all the glass that graces our urban landscapes and consider the potential energy saving ability tinting could offer, then think of all the power generating capacity from sunlight that bombards our windows and glass structures.
While still in its infancy, this technology holds the promise of transforming our wasteful glass metropolises into efficient worlds that also facilitate our highly connected smart building ambitions.
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