Solar windows could turn skyscrapers into power plants

Source: By Miranda Willson, E&E News reporter • Posted: Sunday, August 23, 2020

Solar windows hold the promise of producing an unlimited supply of electricity while being integrated into building design, but they have struggled to achieve efficiency while remaining transparent.

Now, researchers at the University of Michigan say they have overcome that barrier, producing a transparent solar panel from a new material that achieves a record-breaking 8.1% energy efficiency and 43.3% transparency, according to a new study.

Advances in solar windows have the potential to make solar energy more cost-effective overall, said Lance Wheeler, a materials scientist at the National Renewable Energy Laboratory, who did not participate in the research. That’s because the concept integrates the costs of glass used in nontransparent solar photovoltaics — which is the most expensive part of PV modules — with the costs of installing windows in buildings.

“I’m personally excited to see where PV windows can go. They’re at the cusp of being everywhere,” he said.

Most solar windows on the market are made of inorganic, silicon-based materials, said Yongxi Li, an assistant research scientist in electrical engineering and computer science at the University of Michigan. By comparison, Li and other researchers leading the study say their record panel was made of organic, carbon- and hydrogen-based materials.

“There are several advantages of organic compared to inorganic materials,” said Li, first author on the study, which was published yesterday in Proceedings of the National Academy of Sciences. “The silicon material is very heavy, and in general, the organic is very low in density, so it’s light-weight.”

The organic material also rendered a more neutral-colored window as compared to a silicon version, which improves aesthetic appeal, Li said.

The researchers’ organic solar windows also achieved something that other solar window models have been unable to do as effectively: absorb invisible infrared and ultraviolet light from the sun for electricity generation, while letting visible light pass through the window in order to light the building, said Wheeler.

“You have to construct these materials that selectively grab the infrared and UV portions of the spectrum, and avoid absorbing the visible spectrum. That’s what these folks have done very successfully,” said Wheeler, who was not involved in the study.

In addition to developing solar windows with 8% efficiency and 43% transparency, Li and his team produced an alternative solar window with 10.8% efficiency and 45.8% transparency, the study said. Because that alternative version has a slight green tint, however, it might not be usable as a window, Li said.

“It is important to develop power generating windows with aesthetically acceptable neutral colors that are easily applied in the broadest possible applications,” Li said in an email.

As they continue to work on their model, the researchers aim to increase their solar window transparency to 50% and obtain 14% to 15% energy efficiency, while minimizing tint or color, Li said. By comparison, the most effective solar panels have about 25% efficiency, according to Wheeler, and regular windows are between 50% and 80% transparent, said Stephen Forrest, a professor of materials science and engineering at the University of Michigan and lead author of the study.

The researchers also hope to lower the costs of producing their solar windows, Li said. While their organic model costs about $5 per watt of energy produced, silicon models typically cost about 30 cents per watt, he said.

“I think [our model’s] price will decrease at an industrial scale,” Li said.

‘Fantastic’ or overhyped?

Although Li and his team have achieved what Wheeler described as “fantastic” efficiency and transparency, others questioned whether there are solar windows on the market that have reached similar milestones. For example, Silicon Valley-based technology startup Ubiquitous Energy Inc. says it developed a transparent solar panel last year with 9.8% efficiency and 38.3% transparency, according to a press release from the company.

“The one thing I will note is that this is not a record-breaking performance,” Richard Lunt, co-founder of Ubiquitous Energy and a professor at Michigan State University, said in an email.

Xiaoting Wang, an energy specialist with Bloomberg New Energy Finance’s solar team, similarly questioned the significance of the researchers’ organic solar windows. Although they appear to be energy efficient compared with other organic models, organic solar PV products generally have “the lowest performance among all available PV products,” Wang said.

All solar windows — both organic and inorganic — still need to overcome limitations when it comes to their durability, as well, Wheeler said. Compared to replacing nontransparent rooftop solar panels, the costs of replacing solar windows after they reach their end of life could be very high, he said.

“If you’re going to decorate the entire facade of a high-rise building covered in [solar-generating] glass, you better make sure that lasts a long time, because to replace that solar panel is going to be a big endeavor,” Wheeler said.