Solar Windows

If you picture the glittering glass skyscrapers that dot America’s cities, it becomes clear why the idea of using that vast window space to generate solar power is gaining traction. In 2009 alone, 437 million square feet of windows were installed in non-residential buildings in the United States. That many square feet of standard solar panels would generate around 4 gigawatts of power, roughly the total installed solar capacity in the U.S. today.

Such potential is leading engineers and entrepreneurs to more intensively explore the idea of turning windows into solar-power producers. Solar windows, a subset of the growing field known as building-integrated photovoltaics, are based on the concept that a window doesn’t need to be 100 percent transparent, and a solar panel doesn’t need to be 100 percent opaque. Several ways currently exist to turn a window into a power-generating device, from thin-film silicon, to dye-sensitized solar cells, to tiny organic cells.

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Some experts think the field is poised to take off, and although the world may not see an all-solar skyscraper for a while, a number of companies are promising commercial-scale production of various solar windows in the next two years. Still, the cost and technical hurdles facing this fledgling technology could get in the way of a future filled with towering, emission-free power plants. Like other cutting edge alternative energy sources, energy-generating windows could become a mainstay of a greener future in the coming decades, or they could prove to be impractical and produce only a fraction of solar-powered electricity.

“The challenge is whether you can get the cost down and the electricity generation up,” says Sarah Kurtz, a scientist with the U.S. government’s National Renewable Energy Laboratory (NREL) in Colorado. “There are lots of different schemes and strategies, and creativity will be the name of the game. If you can get the cost to the place where those windows don’t really cost any more than conventional windows, it obviously makes sense to go ahead and have your windows generate electricity.”

Building-integrated photovoltaics, or BIPV, is moving slowly, with solar panels now doubling as walls, shingles, and other parts of buildings. MJ Shiao, a senior analyst at GTM Research, a market analysis group in Cambridge, Massachusetts, says the market still represents only around 1 percent (a few hundred megawatts last year) of solar powerbeing installed around the world, and that’s mostly rooftops or semi-opaque skylights. Windows pose a greater challenge than rooftops or walls because of the need to actually see through them. So far, very few examples of skyscrapers with solar windows exist; the highest profile site is the Willis Tower (formerly Sears Tower) in Chicago, where Pythagoras Solar installed a small prototype in 2011.

Several technologies have emerged for solar windows, though none have yet taken off in a meaningful way. But one company that says it is close to commercial deployment is New Energy Technologies, based in Columbia, Maryland. It has developed a method for spraying tiny organic solar cells onto windows in a see-through coating that lets in 40 to 80 percent of sunlight, absorbing the rest. With 10 patent filings pending and no commercial prototypes yet in the field, the company is divulging few details. But the spray-on method could reduce production costs dramatically. Recently, the company announced the development of a large solar cell — 170 square centimeters — made in collaboration with NREL, which could make adding the cells to windows even cheaper.
Despite the company’s progress, its technologies highlight one of the major obstacles to solar windows: efficiency. The rate at which a solar panel turns the sun’s energy into electricity is a concern for all types of solar power, but especially for windows. “The challenge is that the light you see, if you absorb that and use it to make electricity, that means you don’t have a window anymore,” says Kurtz.

To date, the record efficiency for an organic solar cell is 10 percent, and production line efficiencies never get up to the record levels. While traditional solar panels are now producing power with 15 to 20 percent efficiency, efficiency levels for solar windows of roughly 5 percent are unlikely to be economical.

“Look at it from a physicist’s point of view,” Kurtz says. “A solar panel that’s put out in the desert in a nice location with lots of sunshine may have something on the order of a one-year payback. If that [panel] sits out there for another 20 years, you get that much return on your investment for society.” If a solar window can only achieve one-third the efficiency of a solar panel, then it will take three times as long to pay back the investment.

But some experts think it’s just a matter of time before efficiencies rise high enough — and costs drop low enough — to make solar windows a sound investment. Andreas Athienitis, a professor of mechanical engineering at Concordia University in Montreal who is working on technologies for solar windows, says more mature technologies like thin-film silicon might represent a short- and mid-term solution for BIPV, until organic cells can catch up and meet long-term goals. “I think eventually it will be a big market,” he says, but the adoption is slow because “it’s a disruptive technology.”


Another company using organic solar cells, Heliatek, based in Germany, has panels that can achieve 8 percent efficiency. The company’s organic cells use molecules called oligomers rather than traditionally used polymers — basically, short rather than long collections of atoms — which means cheaper, more precise application of the cells. Heliatek says it expects that within five years it can manufacture solar cell windows in the 50 cents-per-watt range, making them competitive with other solar technology.

Spain-based Onyx Solar offers a number of solar glass technologies. However, its windows only allow up to 30 percent of sunlight through, so a lot of light inside the building is lost. In varying formations, though, Onyx says its amorphous-silicon solar glass — a type of thin-film silicon cell — can get up to 9-percent efficiency.

But such efficiencies don’t take into account some of the practical limitations of actually covering a skyscraper with solar windows.

“The optimal installation for solar is you want it to be facing south, you want a slight tilt to it, and you want good solar access, so you don’t want anything to shade those panels,” says Shiao, of GTM Research. “The problem with skyscrapers is suddenly you’re putting them in vertical orientation, there’s only one south side to the building, and chances are that skyscraper is next to another skyscraper, which is going to shade that side of the building.”

Such challenges have left Shiao and other experts skeptical that solar windows will have a bright future. “There are a lot of technical and design challenges, which quite honestly aren’t going to be fixed,” Shiao says. “It doesn’t make sense almost at any cost, unless you’re getting the panels for free or something, to really install that system on those big structures.”

These obstacles haven’t deterred numerous fledgling companies. Oxford Photovoltaics, spun out of research done at Oxford University, says that computer modeling of a 700-foot skyscraper in Texas suggests thatcovering it in solar windows would generate up to 5.3 megawatt-hours per day of electricity. That’s enough to power 165 homes, and it could provide a skyscraper with sufficient power for all its lighting.

Oxford’s technology involves a type of cell for solar windows called a dye-sensitized solar cell. Dye-sensitized cells use a photo-electrochemical process to generate power and can be made relatively cheaply. Oxford’s transparent panels are so far getting around 6 percent efficiency, and the company hopes to bring them to market late next year.

Nazir Kherani, a professor of engineering at the University of Toronto, believes that the economics of solar windows may be most compelling for new construction with a focus on net-zero energy buildings — not for retrofitting existing skyscrapers. “With sufficient attention to design and seamless engineering, it is conceivable that we may see such buildings gradually evolving into net-zero communities, villages, and towns,” Kherani says.

Several companies involved in solar window production say they are within a year or two of scaling up or bringing a product to market, and they maintain that cell efficiencies will continue to rise and prices continue to fall, as is the case with solar panels.

What continues to drive the inventors and entrepreneurs involved in developing solar windows is the enormous potential for energy savings. Buildings accounted for 41 percent of all electricity consumption in the U.S. in 2010, more than transportation or industry. Taking a bite out of that with power-generating windows is an alluring target.

“I wouldn’t write off the possibility,” Kurtz says. “How soon will it happen? I find it’s really dangerous to predict the future.”

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