Solar’s window of opportunity

 By RP Siegel

Transparent photovoltaics are an exciting new technology that can allow many new possibilities for electricity generation on anything from buildings to mobile devices and automobiles. It is estimated that in NYC alone, there are 6 million windows. That could potentially produce as much area as an average size coal power plant. How does this technology work and what will it mean for the future…?

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Solar energy has two primary shortfalls: The first is that it is only available when the sun is shining. (Although, much effort is going into energy storage systems to address that issue.) The second is that solar requires a relatively large amount of space compared to fossil fuels or nuclear. Putting it on rooftops makes sense, since, in most cases, that space is not being used for anything else, plus, transmission lines are not required. But what happens when we run out of rooftops?

One idea has been to develop transparent solar panels that could be used in place of windows and skylights. If affordable, these could add substantially to the amount of area that could be added for solar production. Given the fact that the Empire State Building alone has 6,500 windows, and that New York City has an estimated 6 million windows, that’s a lot of additional area, even when you take into account the fact that only some of those windows will be in the sun at any given time. Even better are those high-rise buildings with façades made entirely of glass.

But are transparent solar panels even possible? The answer is a resounding yes, with multiple approaches being pursued, and some very impressive products already on the market.

Onyx Solar makes what they call Building Integrated Photovoltaic (BIPV) glass. This glass is being used in skylights, façades, windows, curtain walls and roofs. The glass can be produced in several thicknesses, in over 1,500 colors and three different degrees of transparency. It’s also quite durable, making it suitable for solar patios and walkways as well.

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The glass combines thermal management properties like low emissivity, high insulation value (U=0.7) and reduced solar gain (SHGC between 10-40%) with the ability to generate electricity. That’s a combination that can have a huge impact on a building’s energy bills, reducing them, in some cases, by as much as 90%. All told, the value added from the installation of these engineered glass panels includes energy generation, UV and IR filtration, thermal and acoustic insulation, natural illumination, innovative design and reduced carbon emissions. This combination of features explains why the product has received numerous accolades, including being named on Popular Science’s Best of What’s New in 2015 list, and receiving Solar international’s Turnkey Project Award and Glass Magazine’s Most Innovative Glass Product Award. As for affordability, the low-E PV glass installation in the Lucia Building, in Vallodolid, Spain expects to see a payback in one year.

Onyx Solar glass has currently been installed in well over 60 buildings around the world, including the tallest building in Singapore, Sao Paulo’s airport, the most sustainable building in the northern hemisphere and a completed installation in the southernmost city on the planet.

Projects in progress include the Apple flagship store in San Francisco which will have transparent PV flooring on the rooftop, and the iconic Dubai Frame. Onyx BIPV is made with the photovoltaic circuitry contained in a thin layer sandwiched in the middle of a multi-layer glass laminate. Either crystalline or amorphous silicon can be used, depending on the application. While crystalline silicon provides higher energy output (as much as 17.65 W/ft2) in direct sun, it is far less transparent and less productive in low light conditions.

When used with an amorphous silicon layer, the circuitry still absorbs some of the visible light, resulting in glass that ranges from 10 to 30% transparent. As light transmission increases, power output decreases from 5.6W per square foot to 3.3W. That’s because you can’t have your cake and eat it too. The light that is absorbed by the circuitry that produces electricity will not shine through the glass. Research being done under the Advanced BIPV partnership in Europe is aimed at increasing transparency levels to as much as 60% as well as panel sizes larger than the current maximum of 5 x 10 ft (1.5 x 3 m). Any effort to produce a fully transparent PV panel though, would require a different approach.

But are transparent solar panels even possible? The answer is a resounding yes, with multiple approaches being pursued, and some very impressive products already on the market...

One such approach is being pursued by researchers at Michigan State University, led by Dr. Richard Lunt. This team developed what they call a transparent luminescent solar concentrator (TLSC). The way the TLSC works is as follows: First, the material used — a plastic film — does not absorb light in the visible portion of the spectrum, which makes it highly transparent to the human eye. Then, the photovoltaic circuitry is located along the edge of the film’s support structure. The transparent material is tuned to pick up portions of the spectrum in the UV and near-IR range and convert it to another infrared wavelength, concentrating it so that it glows. This luminous glowing is then guided to the edge of the film, where it is picked up by the PV circuitry. The result is a 90% transparent low cost PV film, which forms the basis for a company, co-founded by Lunt, called Ubiquitous Energy. Its Clearview Power™ technology, can be used both as a power-producing coating for windows, or on mobile electronic devices which can use it to extend battery life. Truly invisible, it can also be used to provide power for smart window technology.

In an interview with a Michigan State publication, Lunt said that more work is needed to improve the efficiency of this material, which is currently around 1%. Their goal is around 5%. Other luminescent solar concentrators (LSC) devices have achieved efficiencies as high as 7%, but those are not transparent.

Another new player in this field is SolarWindow™. Its nearly transparent coating is applied to glass as a liquid, utilizing microgrid wires as thin as a human hair. This development, which has led to a system that is claimed capable of producing power even in shade, was done with assistance from the DOE’s National Renewable Energy Lab.

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about the author
RP Siegel
Skilled writer. Technology, sustainability, engineering, energy, renewables, solar, wind, poverty, water, food. Studied both English Lit.and Engineering at university level. Inventor.