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Chicago Landmark to Start Saving Energy With Innovative Solar Windows

Willis Tower, a Chicago landmark formerly known as the Sears Tower, is taking a big step toward energy efficiency with the help of a California company that has developed revolutionary transparent glass window panes that double as powerful solar panels.

Officials for Willis Tower were so impressed with Pythagoras Solar’s innovative window panes that they chose the San Mateo-based company to collaborate on a pilot project to meet the building’s new goals for saving energy and saving money on utility bills by incorporating energy efficiency and renewable energy products and services.

“We are excited to launch this pilot with Pythagoras Solar’s leading-edge solar window solutions as a test for not only the energy savings that can be achieved but the potential they represent to actually generate power through the sun,” said John Huston, Executive Vice President of American Landmark Properties, one of the ownership partners of Willis Tower, in a statement.

Willis Tower to be Transformed into “Vertical Solar Farm”

The solar window panels, known as photovoltaic glass units, or PVGUs, are the first of their kind to strike a meaningful balance between solar power density, energy efficiency, and transparency.

The window panes’ three layers consist of a combined layer of alternating photovoltaic (PV) cells and optics sandwiched between two panes of insulated glass. The first layer of insulated glass allows sunlight to pass to the middle layer while radiating heat away from the glass and back outside. When direct sunlight strikes the optics, it’s reflected down onto horizontal layers of PV cells where it can be transformed into electricity. The remaining diffused visual light is allowed to pass through to the second layer of insulated glass where its brightness is diminished slightly by a shading effect before entering the office space inside.

The result is a fully-operational solar PV replacement for insulated glass that has both a low U-value and a low solar heat gain coefficient “ideal for providing greater comfort for workers in glass towers” while generating up to four times more electricity than any other building-integrated PV solution currently on the market, according to Pythagoras. In fact, the Pythagoras website claims that the company’s PVGUs can produce the same amount of electricity as rooftop-mounted solar panels.

The pilot project’s PVGUs will be installed on the south side of the Willis Tower’s 56th floor. The project could grow to 2 megawatts in size, which is the electricity-generating equivalent of a 10-acre field of conventional solar panels, and transform the tower into a vertical solar farm, according to Inhabitat, a website that follows technology trends and developments in architecture design.

“We are incredibly proud to be considered to contribute our part for the “greening” of the tallest building in the Western Hemisphere, the iconic Willis Tower,” said Gonen Fink, Pythagoras co-founder and CEO, in a statement. “It is inspiring to see a team not waver in its dedication to making a true and lasting change through smart investments in the right solutions. With its combined benefits, our technology is set to provide Willis Tower with a valuable tool that will help move it toward its energy efficiency goals.”


Solar Window Pioneer Announces Pilot Installment of Solar Windows in Willis Tower, Pythagoras Solar press release, March 21, 2011.

Willis Tower Harvests Sun Energy, WLS-TV (ABC) Chicago, March 22, 2011.

Jetson Green website, Ingenious PV Glass Window Hits Chicago.

Green Building Elements website, Chicago Skyscraper to Generate Solar Electricity.

Inhabitat website, Chicago’s Willis Tower to Become a Vertical Solar Farm.

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