Can Temperature-Responsive Windows Save Energy? Yes, But Location Matters

Approximately 30% of the energy loss from heating and cooling systems comes from a building’s windows. New research from Lawrence Berkeley National Laboratory (Berkeley Lab) examines the potential of one solution: thermo-responsive (TR) dynamic windows.

The study, which was published in the journal Nature Communications, ran more than 2.8 million simulations across over 2,000 global locations to see where TR dynamic windows could be most effective. The work provides important context for building upgrades aimed at boosting energy efficiency and lowering energy-related carbon dioxide emissions.

TR dynamic windows use either special coatings or sensors to change opacity at certain temperatures, modulating the amount of heat gain from sun coming through the glass. Evaluating over 100 materials and analyzing data from the simulations, the researchers found that TR dynamic windows are more suitable for areas where both heating and cooling are needed, as opposed to hot climates that predominantly use cooling. In hot climates, TR windows with optimal transition temperatures always stay dark, resulting in energy performance equivalent to static tinted windows.

“By blocking sunlight at the right time, TR dynamic windows can help reduce energy consumption and decarbonize buildings—but they are not a one-size-fits-all solution,” said Yuan Gao, the study’s lead author and a Berkeley Lab materials research scientist/engineer. “These windows will deliver the biggest benefits to buildings where both heating and cooling are important. This conclusion departs from what previous research has suggested.”

Another conclusion that overturns conventional opinion is that room temperature (25 °C or 77 °F) is not always the best transition temperature—the point at which a TR window will start to change opacity. “We found the optimal transition temperature not only depends on the climate, type of window, and building configuration, but also the intrinsic optical properties of the TR materials, especially the solar transmittance in the clear state,” Gao said.

The researchers propose a recommendation index for TR dynamic windows that factors in both energy savings compared to double-paned clear windows and the necessity of having TR dynamic windows compared to equivalent static ones. An analysis of buildings in the United States, found TR dynamic window necessity level to be low in the hot climates of the South, whereas necessity levels are highest across the Mountain West and other places with larger temperature swings.

The researchers also offer a practical guideline and an open-source mapping tool to optimize the intrinsic properties of thermo-responsive materials and evaluate their energy performance for sustainable buildings at various geographical scales. They plan to add other dynamic windows besides thermo-responsive ones (such as electrochromic windows, microshutters, and dynamic windows based on reversible metal electrodeposition) in future iterations of the tool. 

To learn more, read the article published in Nature Communications, or check out the "Behind the Paper" post for a behind-the-scenes look.