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Self-powered System Makes Smart Windows Smarter


Smart windows equipped with controllable glazing can augment lighting, cooling and heating systems by varying their tint, saving up to 40 percent in an average building's energy costs.

These smart windows require power for operation, so they are relatively complicated to install in existing buildings. But by applying a new solar cell technology, researchers at Princeton University have developed a different type of smart window: a self-powered version that promises to be inexpensive and easy to apply to existing windows. This system features solar cells that selectively absorb near-ultraviolet (near-UV) light, so the new windows are 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 the Theodora D. '78 and William H. Walton III '74 Professor in 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 smart window controls the transmission of visible light and infrared heat into the building, while the new type of solar cell uses near-UV light to power the system.

"This new technology is actually smart management of the entire spectrum of sunlight," said Loo, who is a professor of chemical and biological engineering. Loo is one of the authors of a paper, published June 30, that describes this technology, which was developed in her lab.

Because near-UV light is invisible to the human eye, the researchers set out to harness it for the electrical energy needed to activate the 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."

Princeton engineers invented a window system that could simultaneously generate electricity and lower heating and cooling costs. The team, led by Professor Yueh-Lin (Lynn) Loo, center, includes graduate students Nicholas Davy, left, and Melda Sezen-Edmonds, right. Behind them is a cleanroom at the Andlinger Center for Energy and the Environment, where Loo is the director.

In the paper published in Nature Energy, the researchers described
how they used organic semiconductors - contorted hexabenzocoronene
(cHBC) derivatives - for constructing the solar cells. The researchers
chose the material because its chemical structure could be modified to
absorb a narrow range of wavelengths - in this case, near-UV light. To
construct the solar cell, the semiconductor molecules are deposited as
thin films on glass with the same production methods used by organic
light-emitting diode manufacturers. When the solar cell is operational,
sunlight excites the cHBC semiconductors to produce electricity.

At the same time, the researchers constructed a smart window
consisting of electrochromic polymers, which control the tint, and can
be operated solely using power produced by the solar cell. When near-UV
light from the sun generates an electrical charge in the solar cell, the
charge triggers a reaction in the electrochromic window, causing it to
change from clear to dark blue. When darkened, the window can block more
than 80 percent of light.

Nicholas Davy, a doctoral student in the chemical and biological
engineering department and the paper's lead author, said other
researchers have already developed transparent solar cells, but those
target infrared energy. However, infrared energy carries heat, so using
it to generate electricity can conflict with a smart window's function
of controlling the flow of heat in or out of a building. Transparent
near-UV solar cells, on the other hand, don't generate as much power as
the infrared version, but don't impede the transmission of infrared
radiation, so they complement the smart window's task.

Davy said that the Princeton team's aim is to create a flexible
version of the solar-powered smart window system that can be applied to
existing windows via lamination.

"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 Davy. "Then
you could control the sunlight passing into your home using an app on
your phone, thereby instantly improving energy efficiency, comfort, and

Joseph Berry, senior research scientist at the National Renewable
Energy Laboratory, who studies solar cells but was not involved in the
research, said the research project is interesting because the device
scales well and targets 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."

Davy and Loo have started a new company, called Andluca Technologies,
based on the technology described in the paper, and are already
exploring other applications for the transparent solar cells. They
explained that the near-UV solar cell technology can also power
internet-of-things sensors and other low-power consumer products.

"It does not generate enough power for a car, but it can provide
auxiliary power for smaller devices, for example, a fan to cool the car
while it's parked in the hot sun," Loo said.

Besides Loo and Davy, Melda Sezen-Edmonds, a graduate student in
chemical and biological engineering, is the co-author responsible for
the electrochromic portion of the paper. Other authors are Jia Gao, a
postdoctoral researcher in Loo's group then, now with Enablence
Technologies in California; Xin Lin, a graduate student in electrical
engineering; Amy Liu, an undergraduate in computer science; Nan Yao,
director of Princeton's Imaging and Analysis Center; and Antoine Kahn,
the Stephen C. Macaleer '63 Professor in Engineering and Applied Science
and vice dean of Princeton's School of Engineering and Applied Science.
Support for the project was provided in part by the National Science
Foundation, and the Wilke Family Fund administered by the School of
Engineering and Applied Science at Princeton.

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