Chicago's Willis Tower, formerly known as Sears Tower, was the world's tallest building from 1974 to 1998 and remains the tallest building in the U.S. to this day. Its 1451-foot (442 m) height adds up to enough window area to keep a window washer busy for life, or space for enough solar panels to be comparable to a 10 acre solar power plant. As part of a pilot project, the south facing windows of the 56th floor of the Willis Tower will be replaced with Pythagoras Solar's transparent solar windows which cut down on heat gain – and therefore cooling costs – as well as harnessing energy from the sun.

While the image that comes to mind when one thinks of solar power plants is probably one of rows upon rows of panels covering large areas of desert, replacing the windows of skyscrapers with solar windows gives cities with limited free space the opportunity to create vertical solar power plants. Pythagorus Solar's solar windows, which the company calls photovoltaic glass units (PGUs), are rectangular box-shaped units that allow diffused light to pass through, but use a prism to reflect sunlight down onto a horizontal PV cell along the bottom of the unit to generate the same amount of energy as standard rooftop-mounted solar cells.

"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.

If the pilot project proves successful, Pythagoras Solar's PGU's could be expanded beyond the 56th floor to cover enough surface area to provide over two megawatts of solar power.