What the world needs is a more energy efficient window. Unfortunately, that sometimes means we want to let lots of light in but keep heat out, such as on a cloudy summer day. Allowing more natural light in reduces the need for artificial lighting. Keeping heat out reduces the thermal load on air conditioning equipment.
Other times, we want to let the sun’s rays, called near infrared radiation (NIR), to warm the interior while reducing how much sunlight gets inside, such as on a brilliant winter day when the sun is low in the sky. Letting more thermal energy in reduces the load on heating equipment. Cutting down on the amount of light that gets through reduces glare.
Researchers at the Cockrell School of Engineering at The University of Texas at Austin are one step closer to delivering smart windows that have a new level of energy efficiency. Their research has created new materials that allow windows to transmit light without transferring heat, or conversely to block light while allowing heat transmission, as described in two new research papers.
The materials are controlled by a small voltage applied by the building occupants. Look at the illustration above to see how this enhanced selectability of features would work. By allowing indoor occupants to more precisely control the energy and sunlight passing through a window, the new materials could significantly reduce costs for heating and cooling buildings.
In a 2013, engineering professor Delia Milliron and her research group published a paper in Nature demonstrated how, with a small jolt of electricity, a nanocrystal material could be switched back and forth, enabling independent control of light and energy. The team now has engineered two new advancements in electrochromic materials — a highly selective cool mode and a warm mode — not thought possible several years ago.
The cool mode material is a major step toward a commercialized product because it enables control of 90 percent of NIR and 80 percent of the visible light from the sun and takes only minutes to switch between modes. The previously reported material could require hours.
In a second research paper, Milliron and her team demonstrate how to achieve optical control properties in windows from a single component film. The simple coating creates a new warm mode in which visible light can be blocked while near-infrared light can enter. This would be useful on a sunny winter day, when an occupant would want infrared radiation to pass into a building for warmth, but the glare from sunlight to be reduced. Using only doped titania nanocrystals, dynamic control over the transmittance of solar radiation is possible. At different voltages, this material can selectively block visible or infrared radiation.
“These two advancements show that sophisticated dynamic control of sunlight is possible,” Milliron says. “We believe our deliberately crafted nanocrystal-based materials could meet the performance and cost targets needed to progress toward commercialization of smart windows.” Both studies received funding from the U.S. Department of Energy and the Welch Foundation, as well as the NSF Graduate Fellowship Program.