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Solar Windows – Building Integrated Photovoltaic

Willis Tower in Chicago, formerly the Sears Tower
The Willis Tower in Chicago is now generating some of it’s power with building integrated photovoltaic solar windows.

Most photovoltaic systems share a common drawback: They take space, and plenty of it. That is often space that could — or must — be allocated to other purposes. This is certainly true of ground-mounted PV systems. Those hundreds of square meters may produce quiet, clean energy, but they don’t allow much room for crops or parks.

Roof-mounted solar is a better option, but what about air conditioning systems and other equipment that shares the same roof? Is it a good idea to have HVAC workers poking through the duct work beside an active solar array?

Overcoming these challenges is building integrated photovoltaic, or BIPV. In general, this refers to photovoltaic materials that replace conventional building materials. Think of photovoltaic siding, solar singles, or even windows.

How many windows are in a typical large office building, doing nothing more than allowing light in and vision out? What if those windows could do double duty, generating renewable energy?

The obvious problem is that capturing energy from light requires blocking that light. How can you have transparent windows that allow light through, but that also capture enough light to energize photovoltaic cells?

Solar Window Options

One scheme finds small PV cells mounted between two sheets of glass parallel to the window’s surface. They capture a lot of solar energy, but also block a lot of light. Thus, they’re typically useful in areas where you want more shade than light.

Another option sees the photovoltaic cells mounted perpendicular to the window’s surface; you get more light but possibly less energy depending on the height of the sun.

Chicago Skyscraper Gets Solar Windows

The Willis Tower in Chicago (formerly the Sears Tower) is presently the tallest building in the United States and one of the tallest office buildings in the world. And it has thousands of windows.

This past November, some of those windows were replaced with photovoltaic glass units, taking the building a small step toward joining the Net Zero club. Solar windows, from Pythagoras Solar, now cover the south-face of the 56th floor and produce an average power of 140 watts per square meter (140 W/m2) while allowing 50% visible light transmission. They also serve as insulating glass units (IGU), reducing heating and cooling costs while restricting the amount of direct solar radiation entering the building, thereby prolonging the life of interior furnishings and equipment. Similar modules can be used for skylights where standard roof modules, wholly opaque, would be impractical.

The solar windows presently installed on the Willis Tower represent only a small fraction of what’s possible. Estimates from Pythagoras Solar suggest the potential for over two megawatts of solar power generation on the Willis Tower alone. Considering how many residential and commercial windows exist in the world, there’s plenty of opportunity to let the sun shine in.

Read Photovoltaics System Design and Practice from Amazon.

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