Solar Windows: The Next Step Toward Net Zero?

Power-generating fenestration already on the market offers just a hint at what’s to come.

Rooftop solar panels are one thing, but imagine if every window in every skyscraper were a tiny power plant. Startups worldwide are scrambling to make it possible.

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You might say that competition to invent the best way to turn the solar energy that hits glass into electricity has heated up. Some solutions already have come to market. But it’s anybody’s guess which technological approach will come to dominate.

While solar windows will never be as efficient as solar panels because they need to be at least somewhat see-through, consider that by one estimate, nearly 90 million square feet of “flat glass” will be installed annually in buildings worldwide by 2020. That’s a lot of windows — and a lot of potential power generation.

At the heart of challenge: How to make a window serve its main purpose — to let natural light into a building and provide views to the outside — while generating enough electricity to justify the additional expense.

Already on the market

Onyx Solar makes use of conventional technology by embedding silicon photovoltaic cells in windows and skylights. Crystalline silicon cells, the kind used in most rooftop arrays, aren’t transparent, so they are most appropriate in skylights and in windows that don’t need to be transparent. Amorphous silicon cells, a type of “thin PV,” allow more light through, so the company uses them for partially transparent windows.

Brite Solar, a Greek company with a U.S. division, comes at the problem from the solar panel angle: Its Solar Glass product is more of a solar panel that can serve as a window than a window that generates energy. The company intersperses bars of translucent thin-film PV cells with clear glass. It developed its product line for energy-hungry greenhouses but is attempting to expand to commercial and residential building.

Dutch company Physee applies silicon PV cells in a more unique configuration. Physee’s PowerWindows line the rims of double-paned windows with small cells. The units include USB ports that allow users to plug in phones and computers right at the source of electricity.

More revolutionary approaches are less proven but could turn out to be more transformative. Here’s a look at a few.

Energy-producing coatings

SolarWindow Technologies Inc. has been working on its solution for nearly a decade. The Maryland startup makes polymer coatings to be sprayed in layers on the outside surface of windows before installation. The coatings work together to collect ultraviolet and infrared light, convert it into electricity via tiny “organic photovoltaic” cells and conduct it through transparent glass “wiring.”

The company claims to have generated more electricity while remaining see-though than any similar product. Last year it reached an agreement with California window manufacturer Tristar Glass Industries for the fabrication of its products.

Silicon Valley company Ubiquitous Energy is developing its own clear, energy-producing window coating. Like SolarWindow, it relies on organic photovoltaics in the middle layer of coatings. But its application method and the materials it uses are somewhat different.

Quantum dots and perovskites

New Mexico company UbiQD Energy is banking on a very different approach: It’s developing a low-cost method to produce “quantum dots,” which are similar to the components that enhance the color and efficiency of LEDs. The dots tint the glass and absorb a “glow” that they then direct toward solar cells on the edge of the window.

More recently, researchers have begun to focus on a class of complex synthetic materials called perovskites as a potential coating. Perovskites have properties that allow them to convert energy to electricity even more efficiently than silicon cells. In lab conditions, they’ve topped 30 percent efficiency — they convert nearly a third of the energy from the sun into electricity. (The upper range for commercial PV cells is in the low 20s).

One challenge with perovskites is their lack of durability and their susceptibility to degradation by moisture. Another is that some perovskites can contain toxic materials. Nevertheless, perovskite PV cells pioneered by researchers at universities and publicly funded laboratories such as the National Renewable Energy Laboratory in Colorado are attracting capital. One company working to resolve the durability and toxicity issues is Oxford Photovoltaics in Great Britain.

Follow the money

Which approach will prove scalable enough to keep the promise of turning the world’s windows into power plants? The amount of money being bet on various technologies testifies to the perceived opportunity.

SolarWindow Technologies is a publicly traded company with a market capitalization of more than $270 million. Ubiquitous Energy has raised more the $25 million in private financing. Oxford PV has raised more than $40 million in just three years. With so much capital chasing an idea that seems on the cusp of breakthroughs, don’t be surprised by watts to come.


Ken Edelstein is editor of the Kendeda Living Building Chronicle, which covers green design and construction in the Southeast.

Image Credit: SolarWindow Technologies, Inc.

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