Willis Tower takes Solar Windows to New Heights

By Matt Baker

Two summers ago, the owners of the Willis Tower announced an ambitious, five-year, $350 million renovation plan for the nation’s tallest building. The project called for wind turbines, green roofs and solar panels. Every elevator and escalator was to be updated and fuel cells would join next generation boilers to heat and cool the skyscraper.

Building management is still trying to acquire funding to move that project forward. In the meantime, two square meters on the 56th floor are the test site for another bold initiative: solar windows. The pilot project, deployed last November, uses building integrated photovoltaic (BIPV) technology by Pythagoras Solar. This novel approach reclaims a building’s façade by embedding solar cell technology in the windows themselves.

The double-glazed windows are very similar to any insulated glass unit common in the construction of very tall buildings. They conform to industry standard sizes and have a low SHGC of 0.14, dissipating much of the sun’s heat before it passes into the occupied space.

The difference comes with the introduction of monocrystalline solar cells embedded in the glass. “Having the optical device built into the cavity of the double glazed window allows us to manipulate light in various ways to achieve a combined benefit that otherwise we can only get from other kinds of products,” said Udi Paret, Vice President of Marketing for Pythagoras Solar.

Situated perpendicular to the window, the photovoltaic cells gather the sun’s energy while allowing diffuse light to enter the building. Seeing one of these window installations is akin to looking out a window with open Venetian blinds. With the diffuse light still coming in, offices can harvest daylight and reduce the need for artificial lighting—all while generating power in the photovoltaic cells and maintaining a comfortable environment.

This BIPV system has a measured efficiency of approximately 13%, comparable to traditional fixed rooftop installations. The difference is the use of a building’s façade, which in the case of buildings like the Willis, is a significantly larger area. A building owner could install BIPV windows as the glass curtain and give the rooftop over to mechanical installations or vegetated roofs.

The Willis Tower pilot project was installed last November in the bottom and top thirds of one window in an untenanted space. If the technology were to be deployed further, a similar strategy would be used elsewhere to preserve views. Even so, BIPV installed partially in every window of the east, south and west facades (and entirely on mechanical floors), would generate roughly 2.5 mW of electricity. This would offset about 20% of the Willis Tower’s future power needs.

“If everything goes well,” Paret said, “the idea is to move to a next phase and test it on a larger area.” Windows on lower floors would be more susceptible to shading and further trials are probably needed to configure the best strategy for a building-wide install.

One advantage of integrated photovoltaics over orthodox solar panels is that the initial cost can be offset by a reduction in building materials and labor. BIPV modules aren’t add-ons; they replace part of the building’s structure.

This argument is best suited to new construction. Even so, Pythagoras expects the units to deliver a return on investment for a retrofit of between three and five years.

After taking into account current federal incentives, the Pythagoras BIPV modules are priced between $100 and $125 per square foot. As the system relies on standard, commonplace manufacturing techniques, an increase in production is quite feasible and costs could come down over the next few years.

Pythagoras Solar was created in 2007 and has operations in the U.S., Israel and China. The solar windows are currently manufactured in Israel in cooperation with Flextronics, though they hope to move to facilities in the U.S. sometime next year.

Photo: C. William Brubaker, courtesy University of Illinois at Chicago Library and College of Architecture and the Arts.
Diagram: courtesy Pythagoras Solar

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One Response to “Willis Tower takes Solar Windows to New Heights”
  1. I hope they are also keeping the birds in mind. As a major migratory route Chicago enjoys hundreds of thousands of fly by visitors every spring and fall, many of whom mistake glass windows downtown for sky. If you want to know more about helping migratory birds you can contact Chicago Bird Collision Monitors, http://www.birdmonitors.net/

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