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.
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 power being 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.