Heat's Out, Energy's In

Low heat emission (Low-E) glass is a feature in the windows of many of the business and residential buildings in the United States. According to the U.S. Office of Energy Efficiency and Renewable Energy (EERE), these energy efficient windows can be found in 80 percent of homes and 50 percent of commercial buildings.

However, as popular as these Low-E windows are for keeping the heat out of living and work spaces, there's a potential energy feature that remains largely untapped and the National Renewable Energy Laboratory (NREL) might just have the technology to realize it.

Researchers from NREL have developed a prototype photovoltaic (solar powered) smart window that can keep heat out of buildings while, at the same time, providing solar energy. The smart windows, built from the energy-harvesting material perovskite, are thermochromic — meaning they can change color from clear to tinted in response to heat. “There are thermochromic technologies out there but nothing that actually converts that energy into electricity,” NREL scientist Lance Wheeler said in a press release.


The inner workings of this "smart window" technology. Image Credit: NREL 

When solar energy heats up the smart window, methylamine molecules are driven out, causing the device to darken. During this transformation, NREL's smart windows are converted into solar panels, and they're pretty efficient at generating electricity. In a proof-of-concept study published in the journal Nature Communications, the NREL team demonstrated an 11.3 percent efficiency in solar power conversion.

“There is a fundamental tradeoff between a good window and a good solar cell,” explained Wheeler, who is a lead researcher in this study. “This technology bypasses that. We have a good solar cell when there’s lots of sunshine and we have a good window when there’s not.”

Not Yet Perfect

According to Electrek, the 11.3 percent efficiency is promising, as some 80 percent of a building's typical energy costs are spent on heating, cooling, and ventilation. However, while NREL's solar-powered smart windows work, there's one problem that the researchers still need to fix.

The researchers created a 1-square-centimeter prototype of the device, and it showed a decrease in performance after 20 repeated transparent-tinted cycles. This is a huge issue that will need to be resolved before NREL's smart windows could make it to commercial use, especially since most existing smart windows can last through 50,000 cycles. Meanwhile, a standard photovoltaic panel can easily keep 80 percent energy conversion efficiency over a period of 25 years, Electrek noted. Improving the stability of the transformation cycle is the subject of further research at NREL.

The potential of smart windows that turn into solar panels is huge, particularly now that many businesses are keen on switching to 100 percent renewable energy. Instead of covering their buildings with solar panels at the cost of losing windows, commercial establishments can now have both in just one device. Furthermore, as Wheeler noted, these smart windows can just as easily be installed in cars, potentially providing tomorrow's electric vehicles an option to go solar.

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