Building-Integrated Photovoltaics: Windows that Generate Electricity and Save Energy
Tom Lombardo posted on December 17, 2017 |

The ideal solar panel is efficient, durable, affordable, and inconspicuous. While most photovoltaics meet two or three of those criteria, the quest to achieve all four - especially the inconspicuous part - in a single product continues to confound engineers. Nevertheless, they persist in developing windows and roofs that generate electricity with minimal obtrusiveness. Today I'll look at photoelectric windows, and in a future article, I'll examine some rooftop technologies.

Heat-Activated Solar Windows

Engineers at the National Renewable Energy Laboratory (NREL) have developed a smart window that automatically darkens when exposed to sunlight, much like eyeglasses with photochromic lenses that automatically turn into sunglasses. What's special about this glass? It also generates electricity.

Other "solar windows" are permanently transparent to visible light; their photovoltaic properties are tuned to the infrared and ultraviolet parts of the electromagnetic spectrum. Those devices have relatively low efficiencies, since they only absorb a small fraction of the available light. NREL's solar glass remains transparent until it's activated, at which point it absorbs 97% of the visible light and converts it to electricity with 11% efficiency. That may not sound great, but it's on par with organic photovoltaics and about half as efficient as the best monocrystalline silicon PV panels.

Unlike photochromic lenses, which darken when exposed to ultraviolet light, NREL's perovskite-based smart windows use heat to trigger the darkening mechanism. When a window's temperature approaches 45oC, the glass absorbs 97% of the sunlight. The darkening effect helps cool the interior of the building, reducing the need for air-conditioning. At the same time, the integrated photovoltaics generate electricity. Under ideal conditions, one square meter of this glass could generate about 80 Watts of power. In realistic applications, I'd estimate about 30 to 40 Watts per square meter.

Of course, there's a trade-off. By absorbing 97% of the sunlight, the glass increases the need for artificial illumination. On the other hand, it generates enough electricity to power LED lighting. By itself, that would be a net loss; the gain, however, is in the reduction of power needed for cooling, which is more energy-intensive than lighting. (Since the material is heat-activated, its effect will be more pronounced in the summer months.) Reducing power consumption ("Nega-Watts") is almost always less costly than generating energy. These windows do some of each.

Regular Glass (left) vs Photothermal Glass (right). Image courtesy of NREL.
Regular Glass (left) vs Photothermal Glass (right). Image courtesy of NREL.

At this point, the NREL study is a proof of concept. The experiments showed successful darkening and lightening, but the material loses its efficacy after about 20 cycles. NREL engineers are working to improve the stability before attempting to bring the technology to market.

Double-Pane Windows with Photoelectric Properties

Dr. Gavin Bell and Dr. Yorck Ramachers, researchers at the University of Warwick, have developed a double-pane window that produces electricity through the photoemission effect. In a traditional semiconductor photovoltaic cell, a photon collides with an electron, and if it has enough energy, it knocks the electron free, allowing it to jump over the semiconductor band-gap. Bell and Ramachers' device, based on early work by Nikola Tesla and Albert Einstein, uses double-pane glass instead of a semiconductor PN junction. Tesla demonstrated the effect by separating the two panes with a vacuum; Bell and Ramachers showed that it can also work with an inert gas like argon, which is already used in energy-efficient windows, making commercial development of the technology much less costly.

Double-Pane Photoemissive Glass. Image courtesy of Joule.
Double-Pane Photoemissive Glass. Image courtesy of Joule.

Even better, photoemissive glass has the potential to outshine silicon photovoltaics in power generation. The theoretical maximum efficiency of a single-junction silicon PV cell - the kind found in most solar panels today - is about 33%. Bell and Ramachers calculated the theoretical maximum efficiency of their photoelectric glass and found it to be almost 57%. Of course, both of those are best-case scenarios. I always tell my students that "theoretical maximum" translates to "it'll never be that good in practice." Nonetheless, a higher ceiling offers more potential.

The researchers are still trying to find an optimal photosensitive cathode material. They believe that different substances can be chosen based on the desired amount of tinting, allowing architects to tailor each type of window with its position on the building.

The vision of a sustainable future isn't always crystal clear. The purpose of research is to shed light on a problem. Sometimes the solution to that problem includes a little opacity; other times, it's a modern reboot of a century-old concept.

"Once in awhile, you get shown the light
in the strangest of places, if you look at it right." - Robert Hunter


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