SolarWindows: Fifty Times More Energy than Rooftop Photovoltaics?
Tom Lombardo posted on August 22, 2015 |

A year ago I first heard about the SolarWindowTM, a transparent photovoltaic panel that claimed to deliver fifty times as much energy compared to traditional rooftop PV panels. I was skeptical, to say the least, but I decided to look into it. The company website didn’t provide a lot of detail, and my attempt to get more information through email was unsuccessful. They kept me on their mailing list, though, so when they announced an informative webcast to be delivered on August 20th, I signed up. Here's a synopsis of the webcast...

Fifty Times More than Rooftop PV?

Let’s put things into perspective. The SolarWindow is designed for steel-and-glass skyscrapers. A fifty story building has nearly six acres (24,000 square meters) of window area; its rooftop could have one-tenth as much space, some of which is taken up by HVAC equipment and access doors. Still, that gives SolarWindows a little more than ten times the area, not fifty. And the rooftop array would consist of very efficient PV panels that are pointed for optimal production. SolarWindows are mounted vertically on all four sides of the building; at best, one or two sides would receive direct sunlight. Can a massive increase in surface area overcome the lower light levels? 

Normally I’d do a few calculations to determine the theoretical and practical energy that the array could produce, but a city building surrounded by who-knows-what is a tough model to create. Fortunately the folks at SolarWindow sought out independent experts to build a model and run the numbers. (Ha! I’m off the hook!) To start, the National Renewable Energy Laboratory (NREL) independently measured and verified the amount of power that a SolarWindow could generate under various conditions. NREL developed the PV power curves for the module. Next, the engineers at SolarWindows examined factors such as sun angles throughout the day, climate, building footprint, shading, rooftop area, amount of glass in a typical building, etc. All told, more than thirty variables were used in the model, which was  independently verified by engineers at the University of North Carolina’s Energy Production and Infrastructure Center. Engineers then calculated the typical power production and derated the result to create a more conservative estimate. Applying that model to a hypothetical fifty-story building in various cities yielded these results:

The Technology

SolarWindows consist of a thin film organic PV coating that’s designed to be inserted between two layers of glass in a double-pane window. Being the “meat” in that sandwich protects the material from degradation caused by weathering. The coating is mostly carbon, nitrogen, hydrogen, and oxygen - all safe and abundant materials, making the SolarWindow inexpensive to produce. It’s transparent to visible light, but converts infrared and ultraviolet light into electricity. Transparent conductors carry the current to the edges, where a custom-designed quick-connect interface allows an electrician to connect the SolarWindow to the building’s electrical wiring.

In bright sunlight organic PV (OPV) cells are less efficient than semiconductor PV, but OPVs outperform traditional PV under low light, indirect light, and artificial lighting conditions. Silicon PV cells require a certain threshold of light in order to generate any power at all (sort of like overcoming a diode's knee voltage), where OPVs are more linear, generating power in proportion to the light received, even if it’s just a trickle. Since SolarWindows will be exposed to indirect light, that’s a necessity. Also, OPVs can generate electricity from artificial light coming from inside the building itself - a bonus.

Manufacturing and Installation

SolarWindows are manufactured using standard glass coating processes - the same methods that are currently used to add coatings that reduce a window’s emissivity or increase its thermal insulation. According to glass industry expert Patrick Sargent, “SolarWindows can be seamlessly integrated into existing processes, utilizing secondary coating lines that will not impact existing methodologies or supply chains.”

A glazier will install SolarWindows using the exact same process that’s used to install skyscraper windows today. Using the quick-connect interface, an electrician will then make the final connections between each window and the building wiring, using standard techniques.

It would appear, then, that the manufacturers and installers require no additional training or changes in techniques or materials. If that’s really true, then it’s a huge step towards making SolarWindows a part of all new tall buildings.

Payback Period and Expected Life

Since the materials, manufacturing, and installation are far less costly than traditional PV, SolarWindow accountants calculated a payback period of less than one year. That takes into consideration the (US) federal renewable energy incentive, but not state incentives or solar renewable energy credits. The payback model was developed by CPA and renewable energy tax incentive expert Ken Schuckers, who stated that the one year estimate was on the conservative side. The model was also validated by engineers at the University of North Carolina’s Energy Production and Infrastructure Center.

How long will a SolarWindow last? While silicon-based PV modules are guaranteed for at least twenty years (and most will go forty years or more), OPVs don’t have the same longevity. Part of that is due to their flexibility - the mechanical stress degrades their performance. Exposure to elements also reduces the life of an OPV panel. The SolarWindow takes care of both those factors thanks to its being sandwiched between layers of glass. Will that be enough to keep these windows generating power for a few decades? That’s the big unknown in the equation, but Dr Scott Hammond, Principal Scientist at SolarWindow Technologies, is confident that their proprietary coating combined with the glass surrounding will be up to the task. The good news is that a one year payback period makes the risk easier to take.  

SolarWindow Technologies is currently seeking investors and building industry partnerships. If all goes well, SolarWindows could be commercially available around the end of 2017.

You can watch a short video introducing the SolarWindow here.

Images courtesy of SolarWindow Technologies

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