Windows get PV treatment to save energy

Aiming to improve energy efficiency to any degree, the Department of Energy's National Renewable Energy Laboratory has found that perovskite materials – the very same active component that has been implemented in some of the most efficient solar panels on the market – can be applied to windows to decrease energy costs.

According to the NREL, using thermochromic windows or dynamic windows in office buildings improves energy efficiency across all climate zones in the United States by modulating the temperature inside, leading to an impressive reduction in costs.

While dynamic windows have been installed in countless buildings throughout the country, utilizing the solar-energizing properties of perovskite materials could create much more efficient energy regulation by mitigating heating loads in warmer climates and cooling loads in more frigid regions.

"Switching to thermochromic windows saves a significant amount of energy annually, with the main source of savings coming from requiring less heating energy for highly glazed office buildings in cold or seasonal climates," said Lance Wheeler, principal investigator on the project. "If all workers in the simulated office building drove an electric vehicle the distance of an average American commute, then the annual energy could be used to fully charge every worker's car every day throughout the year."

The findings, detailed in the paper "Thermochromic Halide Perovskite Windows with Ideal Transition Temperatures," suggest that adding a thermochromic laminate based on metal halide perovskite materials onto a single or even double-pane window would yield significant savings.

H2: Greater efficiency, everywhere

As buildings account for roughly 40% of all primary energy consumption in the U.S., with heating making up the largest portion, coating technology revolutionized window energy efficiency beginning in the 1980s.

Basically, by selectively absorbing or reflecting infrared light but allowing light in the visible portion of the spectrum to pass through the glass, a massive overhaul of energy consumption took place.

However, slightly more than half of the sun's energy occurs in the visible wavelengths, so more energy is required to heat or cool buildings with significant glass facades.

With the help of modeling software designed by Wheeler for a similar project called PVwindow, the researchers simulated a 12-story structure with a window-to-wall ratio of 95%. Then, factoring the energy use of the building, they tested eight climate zones across the country in 15-minute intervals, covering Hawaii, Arizona, California, Colorado, New York, Wisconsin, Minnesota, and Alaska. From this, they determined:

• Thermochromic double-pane windows improved building energy efficiency over double-pane windows in each zone.

• Energy savings were greater in colder regions.

• Thermochromic double-pane windows outperformed even standard triple-pane windows in the hottest climate zones.

The researchers noted that in colder climates, triple-pane windows provided more energy savings than thermochromic double-pane windows, but adding a thermochromic laminate to a triple-pane provided the most annual energy savings compared to the highest efficiency double-pane windows.

All of this testing, however, was still used to specify the ideal critical transition temperature of 20 to 27.5 degrees Celsius (68 to 81.5 degrees Fahrenheit) for thermochromic windows based on metal halide perovskites.

"If the transition temperature is too high, then the window may not save energy, and buildings are better off with static windows," said Wheeler. "It Is interesting that this range is valid across many different climates, from northern Minnesota to southern California. This is because dynamic windows reduce solar heat gain in the summers to save air conditioning, and they also increase solar heat gain when heat is needed in winters."

The team coupled the modeling with experimental work and demonstrated a perovskite film sandwiched between two layers of glass. The thermochromic switching was proved durable for 200 cycles.

"We were able to significantly reduce the transition temperature in the lab to match the predicted ideal temperature," the research lead said. "The work shows off the many areas of expertise at NREL and the power of collaboration between materials science and building energy science."

With more tweaking, perhaps entire buildings with perovskite windows may be used for more than just temperature regulation and energy efficiency but to offset city grids and become truly self-sustaining – at least as far as energy use goes.