“Our way was the easiest to get to the goal of making a switchable energy-generating window,” said Lance Wheeler, a research scientist at the US Department of Energy’s (DOE’s) National Renewable Energy Laboratory (NREL). “Now, we’re stepping back to determine the best way to do this – instead of the easiest or most convenient way.”
The DOE’s Building Technologies Office is providing three years of funding for Wheeler and his newly constituted team to perfect the technology behind the window. The heart of the technology is a heat-triggered perovskite material that can be cheaper to use in window applications. “I was working on making a different material that was inspired by the material we’re working on now,” he said. “I found that if you took it off of the hot plate it would turn clear.”
The initial version also used single-walled carbon nanotubes to capture the light, along with layers of titanium dioxide and tin oxide. When light hits this transparent glass, it prompts the release of molecules of methylamine. In turn, the glass darkens and as that happens, electricity is generated. When the sun isn’t shining, the molecules are reabsorbed, and the glass becomes clear again. Part of the continuing research will center on better ways to make the switch.
The Building Technologies Office provided funding for Wheeler and Rob Tenent to participate in a two-month “entrepreneurial boot camp,” working with industry mentors to develop viable ways to bring their SwitchGlaze technology to market. “It rolls off the tongue better than switchable photovoltaic window,” said Wheeler.
From Wheeler’s initial discovery to integrating it into a photovoltaic (PV) device took about 18 months. Now, with three years of funding, the focus is on making sure that the device is the best it can be. And while the typical path from the laboratory involves taking a small sample and seeing if it works on a larger scale, this time the idea is to again think small.
“That’s really what we’re working on—making it work better small before we make it bigger,” said Wheeler. “There are still more fundamental scientific questions that have to be answered,” said Tenent. “One of the challenges we’ve got is that skyscrapers don’t have abundant roof space to put PV on,” Tenent said. Incorporating PV into the windows is one answer. “The big goal is definitely to get it into your downtown skyline where you have lots of glass and no roof space,” said Wheeler.
“If you buy a nice skylight, usually it’s going to be electronically controlled to have a rain sensor and actuate if it’s open or closed,” Wheeler said. “The big problem is you have to tear a wall apart to wire it. If you can put the power on board, that’s what this switchable energy generation gives you.”
The durability of the switchable component is key to ensuring the adoption of the technology. The device must be able to switch back and forth between transparent and tinted many more times than has been demonstrated in the lab so far. “Eventually, it’s got to switch 50,000 times,” said Wheeler. “We’ve shown it can switch around 20–30 times in the lab right now.”
“The durability piece of the equation—we actually do a lot of that,” said Tenent, an electrochemist. “That’s what our core windows program at NREL is based on. We developed the standard for evaluating the durability of electrochromics. The 50,000-cycle threshold is based on the electrochromic standard. It’s become kind of a de facto accepted metric.”
The initial technology had a power conversion efficiency of 11.3 per cent, aiming to maintain that figure over the thousands of switching cycles. “Right now there are things specific to a window and not to a solar panel, like how much light is let through and what sort of solar heat gain you get when you let it through,” said Wheeler. “As visible light goes through, what colour does it come out on the other side? All those things you can’t measure simultaneously with the infrastructure we have now. So, we’re building a system that can do all those things: measure its electrical output as well as its window characteristics.”
“Ideally, what we can do in three years is get it to the point where industry is comfortable taking it on themselves,” said Wheeler. “That’s where we want to be.”
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