Gallery: Chicago’s Willis Tower to Become a Vertical Solar Farm


Chicago’s iconic Willis Tower (formally the Sears Tower) is set to become a massive solar electric plant with the installation of a pilot solar electric glass project. The high-profile project on the south side of the 56th floor will replace the windows with a new type of photovoltaic glass developed by Pythagoras Solar which preserves daylighting and views while reducing heat gain and producing the same energy as a conventional solar panel. The project could grow to 2 MW in size — which is comparable to a 10 acre field of solar panels — turning North America’s tallest building into a huge urban vertical solar farm.

The project is a collaboration between the tower’s owner and the manufacturer to prove the viability of the building integrated photovoltaic (BIPV) system, which will also save energy by reducing heat gain and cooling costs. The new windows, dubbed high power density photovoltaic glass units (PVGU), are a clever hybrid technology that lays typical monocrystalline silicon solar cell horizontally between two layers of glass to form an individual tile. An internal plastic reflective prism directs angled sunlight onto the solar cells but allows diffuse daylight and horizontal light through. Think of it as a louvered shade which allows for views but cuts out the harsh direct sun.

The manufacturer claims that the vertically integrated solar cells will produce the same amount of energy as normal rooftop-mounted solar panels. This is great news for cities that have precious little rooftop space and towering walls of glass. The product is also a potential breakthrough in energy efficiency in glass towers, where solar heat gain is the bane of energy-efficient design.

+ Pythagoras Solar


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  1. Kevin Pearson July 28, 2011 at 11:14 am

    What a cool Idea if you could retrofit all buildings….. Well we have the product that will do that. at proactive energy concepts and it will change the way we power buildings. I love the idea of knocking down the glare on windows so all of you that don’t know the lie’s the window companies and tint companies have been telling you. Understand heat needs to be reduced not concentrated by inefficient window coverings. The 20 years of research we have on this issue tells me that were finally moving in the right direction. Kudos to these guys and what a great market to develop.

  2. ahinalu April 18, 2011 at 12:33 pm

    Every high-rise in the US should have these on every sun facing side and a wind farm on their roofs. People worry about me leaving my phone charger plugged in when I’m not charging my phone yet there’s so much wasted potential energy not being used and so much wasted energy being used to cool these buildings.

    Two rocks one stone, that’s change I can believe in. LOL

  3. electric38 March 30, 2011 at 5:29 pm

    The interesting item is the heat block. I live in Nevada where just that aspect of it presents a significant cost factor. If energy is created, while blocking heat, much the better. If they incorporated LED’s in the interior lighting, a very low energy use should result, as they do not generate more heat (that AC must counteract).

  4. craigjohnson March 27, 2011 at 4:46 am

    Very interesting! @msrp—if that side of the building is so sunny, it’s likely that people had their shades closed during the day anyhow. But you’re right: BIPV doesn’t make any difference, really, if the overall energy use in the building isn’t taken into account, so I hope they’re doing a thorough audit of lighting anyway.

    The article should make it clear that these windows don’t produce the same amount of power as a solar panel that’s the same size as the window, and they’re not converting indirect (diffuse) light. Pythagoras is probably trying to say that the PV elements within the windows are very efficient and do absorb the entire visible spectrum. Clearly some light gets through because of the optical design of the window, so this is light that’s not being absorbed. But standard definitions of efficiency are based on absorbance over an area. Per unit area they might indeed be more efficient than windows made with a thin active layer in between the glazing that’s transparent to some visible light, but they’re only as efficient as standard commercial solar panels for light that’s directly incident on the active area. To me this is an important distinction. To some it’s boring as hell.

    Craig Johnson
    UNSW, Sydney

  5. msrp March 23, 2011 at 9:58 am

    The cynic in me wonders how many more lamps will be running because of the dimmer light.

    But in all seriousness, it’s a really neat idea, and what a great location for a trial run. The 90-plus summer days and the lake-borne ice storms (sometimes in the same day…) will certainly put it through its paces.

  6. poyntek March 22, 2011 at 9:48 am

    Nice idea, I wonder if it will all be grid tied solar power, or if they will create some storage of solar power for emergency use in power outages? I made a much smaller solar backup system for my home office, that works great when I lose power. In fact, I used it last night when a storm knocked out the power. I still had lights, laptop, cell charger, FIOS connection and phone. It was cheap to make and pretty easy to setup. You can look at how I made it if you want…

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