Dimmable windows with solar panels could power zero-energy buildings

Feb. 23, 2011
Hannah Hickey
UW News & Information

Windows – typically a drain on a building’s energy consumption – are being re-imagined to help boost buildings’ energy efficiency. UW engineers and architects are collaborating on smart windows that can change transparency, depending on conditions, and actually harvest energy from the sun’s rays.

“Architects like color-changing windows such as the one developed at the UW because they don’t want to use mechanical blinds,” said principal investigator Minoru Taya, a UW professor of mechanical engineering. “Now we have developed a switchable dye that is not only blocking sunlight, but harvesting sunlight.”

The work is being funded through a $2 million grant from the National Science Foundation’s Emerging Frontiers in Research and Innovation branch, which funds interdisciplinary teams. The UW proposal was selected for last year’s competitiveScience in Energy and Environmental Design program, aimed at engineering sustainable buildings.

The window includes a compound developed at the UW that can darken on command – either automatically in response to light levels, or as the user adjusts the settings. This project will add a compound that converts incoming solar energy to electricity, to be used immediately or stored in a lithium-ion battery in the window’s frame. The solar panel can even harvest energy from artificial lights, so at night it can absorb energy from the room’s lights.

The proposed window has sensors that detect the conditions and adjust the transparency of the top portion of the window. At the same time, incoming solar rays (red) would be converted to electricity (dotted red line). Image credit: University of Washington

"In addition to being energy efficient, these windows will improve the quality of life inside these buildings," said Christopher Meek, a UW research assistant professor of architecture and member of the UW Integrated Design Lab. "One example is that the window technology will reduce glare and increase visual comfort, much like certain eyeglass lens respond to sky brightness."
Project collaborators are:

  • Taya, who will oversee the project using his technology for changing window color and capturing solar rays.
  • Meek, who will look at how to integrate the technology with building design and use local conditions to increase the comfort of the occupants.
  • Joyce Cooper, a UW associate professor of mechanical engineering, who will do a life-cycle assessment of the technology. She will consider long-term impacts of using different materials and investigate options for recycling.
  • Yasuo Kuga, a UW professor of electrical engineering, who will build sensors that detect when a person is nearby. He will also explore how the technology might affect transmission of cell phone and other wireless signals through the window – an important consideration for modern buildings.
  • Christine Luscombe, a UW assistant professor of materials science and engineering, who will develop new versions of the compound that can convert more of the incoming sunlight into electricity.

For the past decade, Taya has been developing smart windows that darken on command, similar to eyeglasses that dim in sunlight except that the user can adjust the settings. His approach to the technology is novel because it requires very little energy and uses organic materials, meaning it can be built from cheap materials at low temperatures.

Another focus of Taya’s research is using organic materials, rather than silicon, to develop solar cells. He recently led a project funded by the U.S. Air Force to develop lightweight solar cells for use in airplanes. Such cells could power the cabin without having to run heavy copper wires from the engine, or power lightweight autonomous flying vehicles.

The UW window can adjust in less than 30 seconds from an almost transparent pane to one that blocks 99 percent of incoming light. The electrochromic pane uses a small amount of electricity only during the color change, and can survive more than 100,000 transitions. Photo credit: University of Washington

“This is maybe the cheapest way of making a solar cell,” Taya said. “The principle is the same as photosynthesis – the solar ray hits, the natural dye is excited and it emits electrons. It’s very similar to photosynthesis, except the key dye is not chlorophyll, it’s a human-made dye.”

The new project is a fusion of the two technologies, where the same dye provides both color switching and solar energy harvesting functions. These organic switchable dyes still have only 2 percent to 3 percent efficiency in converting sunlight to electricity, compared to 10 percent for commercial solar panels, but are attractive because they are potentially cheap, flexible and more versatile than existing solar panels.
In the proposed window, when the colored coating is activated the organic dye would trap incoming solar energy and convert it to electricity.

Researchers will use the four-year science foundation grant to refine the design and build a prototype.  “This is still considered a dreamy project. It has a long way to go to commercialization,” Taya acknowledged. “But if it’s realized, we are toward a zero-energy building, a truly autonomous building.”

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