Smart windows can save up to 40% on heating and cooling costs in commercial buildings and high-rise residence towers. In addition, they can save on electricity by providing more natural light for the interior of buildings during the day.
Smart Windows Save Energy
Lots of companies are working on bringing this technology to market, but there is one significant drawback to smart windows that is keeping them from wide acceptance. They need to draw power from the building’s electrical system in order to function. Small changes in voltage can make big differences in opacity and heat rejection but even limited amounts of power have to come from someplace.
Making Smart Windows Smarter
Researchers at Princeton think they have come up with the solution — smart windows that also act as solar panels to generate the small voltages they need to operate. With no need to connect them to the local electrical system, such windows would be inexpensive to install. In the alternative, the process could be added to existing windows quickly and economically.
“Sunlight is a mixture of electromagnetic radiation made up of near-UV rays, visible light, and infrared energy, or heat,” said Yueh-Lin Loo, director of the Andlinger Center for Energy and the Environment at the Princeton School of 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.”
Near-UV Solar Cell Is Invisible
The secret is to use daylight that is near the UV end of the electromagnetic spectrum to power the solar cell. The human eye does not react to light in that part of the spectrum so the solar cell makes no difference to the appearance of the window. “This new technology is actually smart management of the entire spectrum of sunlight,” says Loo, who is one of the authors of a research paper published on June 30 in the journal Nature Energy.
“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 writes. “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.”
New Technology Uses Proven Production Techniques
The researchers applied organic semiconductors to the surface of window glass using the same production method utilized by manufacturers of light-emitting diodes (LEDs). Making use of manufacturing techniques that are now common and well understood helps keep the cost of creating the self-powered smart windows low.
The window itself is coated with electrochromic polymers that control the tint and can be operated by using the power produced by the near-UV light solar cell. When the sun shines, the window generates a charge that can turn the glass from clear to dark blue. In its darkened state, the window blocks 80% of the ambient light from outside the building.
Nicholas Davy, a doctoral student in the chemical and biological engineering department and lead author of the report, says other researchers have developed transparent solar cells that run on infrared light. But infrared is the portion of the spectrum that also transmits heat, which conflicts with the window’s role of keeping heat from being transmitted into or out of the interior of a building.
Flexibility & Scalability
Davy writes that his team’s primary goal was 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,” he says. “Then you could control the sunlight passing into your home using an app on your phone, thereby instantly improving energy efficiency, comfort, and privacy.”
Joseph Berry is a senior research scientist at the National Renewable Energy Laboratory who studies solar cells. Berry was not involved in the Princeton research but finds the project interesting because the system 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,” says 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.”
Other Applications For Near UV Solar Cells
Davy and Loo are exploring other applications for their transparent solar cells, which they say could be ideal for powering IoT sensors and 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 says. The pair have formed a startup business called Andluca Technologies to commercialize their invention.
Photos by David Kelly Crow