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Twin Scientists Advance Understanding of PV Windows

Aug. 19, 2019

To say twin brothers Lance and Vincent Wheeler share interests is an oversimplification. As adults, they played on the same hockey team, attended the same college, graduate school, and then went to work at the National Renewable Energy Laboratory (NREL).

Now, only one twin remains at NREL, but their collaboration continues. The dynamic duo co-authored a newly published article in ACS Energy Letters, titled “Detailed Balance Analysis of Photovoltaic Windows.” The sole authors are listed as Lance M. Wheeler and Vincent M. Wheeler—they also share a middle name, Michael.

The Wheeler brothers.
Vincent (left) and Lance Wheeler in 2015, when both were working at NREL. The two grew up in Minnesota where the enjoyed hockey and curling but eventually turned to science after a professional hockey career didn't work out. Photo by Dennis Schroeder/NREL
“Wheeler and Wheeler. I like the sound of that,” said Vincent, who’s older by 4 minutes. “There are two types of twins. There are the kind that push away from each other, to really differentiate themselves. Then there is the kind that do everything together. We were that kind for sure.”

Born the day after Christmas 1984, the oldest Wheeler boys—two other brothers arrived years later—grew up outside Minneapolis. The frigid winters lent themselves to two ice-based sports: curling and hockey. After high school, both signed on to play for the Bancroft Hawks, part of the Ontario Junior Hockey League. For two seasons, from 2003-2005, Lance played defense while Vince handled goaltending duties.

“Both of us thought we were going to be pro hockey players,” Vincent said. “When that didn’t happen, we turned to science.”

Although Vincent started college elsewhere, he wound up joining Lance at Saint John’s University. “It only took a year of him not enjoying himself where he was to convince him to join me where I was,” said Lance. Vincent graduated with a degree in mathematics. Lance’s degree was in physics. In graduate school at the University of Minnesota-Twin Cities, the brothers went on to earn master’s and doctoral degrees in mechanical engineering.

“We always wanted to work together,” Vincent said. “Once we were at graduate school, we had two supervisors who did different topics. It didn’t make sense until now.”

NREL hired the Wheelers in 2014 as postdoctoral researchers, with Lance joining the staff in June and Vincent following a couple of months later. Lance focused on materials science and Vincent on computational science. But Vincent’s research exposed him to solar ideation before he left NREL at the end of 2015.

For most of the next three years, he conducted research into solar thermal engineering and solar thermochemistry at Australian National University. Now he’s an assistant professor at the University of Wisconsin at Stout where he primarily teaches undergraduate engineering students thermal sciences, computational methods for engineers, and solar thermal engineering, among other topics.

The younger Wheeler remains at NREL, where he has made headlines for developing a switchable photovoltaic (PV) window. When sunlight hits the window, it darkens and generates electricity. The idea holds the potential to change city skylines from glass-fronted buildings to structures capable of capitalizing on solar energy.

The lives of the fraternal twins have diverged over the years, and not just geographically. Vincent is married, with two children younger than 5. But the brothers get together several times a year. One such occasion happened about a year ago, when Vincent visited Lance’s house. The visit was his first time back in the United States after living in Australia for almost three years. A short bike ride from NREL, the house had a wall Lance intended to knock down. Before that happened, he used special paint to turn much of that into a chalkboard.

Lance began writing out the math involved in modeling a PV window, considering such factors as solar heat gain and efficiency. “I wrote it all up and he said, ‘You’ve got this part wrong.’ We went back and forth that way.” Lance contributed an understanding of PV energy while Vincent provided his knowledge of thermal science. “I was moving home,” Vincent said. “I was traveling with essentially everything I owned sitting in a suitcase on Lance’s floor. We were working on it while I was still jet-lagged.”

The paper that sprang from their chalkboard provides a look into the future of PV windows, which ­­the brothers describe as “a transformative technology for the energy landscape of the future.” Their analysis describes the theoretical performance of two types of PV windows: semitransparent and transparent. A transparent PV window allows more visible light to pass through, but that type isn’t as efficient at capturing solar energy as one that is semitransparent.

“I’m not saying either one is better,” said Lance. “The message is this: Unlike conventional solar panels, which only aim to maximize sunlight-to-electricity efficiency, solar windows must balance a number of difference performance metrics. One size does not fit all, and this work lays out the design space.”

The Wheelers noted it’s important to understand the tradeoffs that must occur in selecting a PV window. In a hot climate, a PV window should have a darker tint to help avoid solar heat gain. In a cooler climate, solar heat gain is a plus because that offsets the cost of heating a building.

The idea of PV windows has been around for a while, but the technology hasn’t been adopted. The time for semitransparent PV windows may be coming because the price of solar power has been declining while architects are designing more glass-covered buildings, Lance said.

Further behind commercialization is the transparent PV window, which is based on materials that only absorb light that can’t be seen—ultraviolet and infrared. The technology is less mature because it’s difficult to make materials that absorb those wavelengths but stop before the visible light spectrum. Also, the ultraviolet portion of the spectrum is poor in photons. While the infrared part is rich in photons, researchers haven’t been able to demonstrate efficiency greater than 5%.

“It’s not that simple to make a good PV window,” Lance said. The paper “hopefully will be a useful roadmap for researchers doing work on photovoltaic windows.”

With one paper jointly written, the Wheelers may decide to continue this working relationship. Lance, for one, wouldn’t mind seeing his brother’s students become interns at NREL. The twins also are considering expanding their analysis of PV windows to produce more papers.

“I’ve seen twins that don’t like each other, that don’t get along, but Vince and I have done everything together and enjoyed every second of it, I would say,” said Lance. “We’ve never been competitive with each other—mostly just collaborative.”