PDF - Kleinman Center for Energy Policy

Spotlight on Research: A recap of an interview with Dr. Shu Yang on her research into using material
science to solve energy challenges in buildings. Professor Yang teaches in the Materials Science and
Engineering (MSE) and Chemical and Biomolecular Engineering (MSE) Departments. She is Principal
Investigator of the Shu Yang Group. Yang’s group has developed a Smart Window that is the focus of this
interview.
This is the first in a series of interviews to be conducted by the Kleinman Center for Energy Policy. These
interviews will feature areas of energy research being explored by members of the Penn community and
their implications for energy policy.
What Problem is the Smart Window trying to solve?
The Smart Window is trying to balance several competing
requirements for windows. Architects prefer transparency in the
design of buildings for their aesthetic appeal. However, transparent
windows may let in too much sunlight on a scorching summer day,
causing interiors to become over-heated and then require artificial
cooling. This is energy inefficient and leads to a bigger ecological
footprint of a building.
How does the Smart Window alleviate these concerns?
The Smart Window offers an on-demand solution that balances the
need for transparency and the need for minimal energy
consumption with an elegant and simple solution. A Smart Window
consists of a silicon sheet containing tiny glass beads that are
sandwiched between the two glass panes of a standard window. In Illustration of a full-size Smart Window.
Photo courtesy of Jenny Sabin of
its native state, the glass beads and silicone sheet have the same
PennCurrent
refractive index, rendering them invisible and making the window
appear transparent. However, when the material in the window is stretched, air pockets appear around
the beads, changing the way light is reflected and creating opaque areas that block light, preventing
over-heating of the building interior.
How does the Smart Window differ from current technologies?
Current technologies to block light include the use of pigments, particles, and liquid crystals that can
change from a translucent or opaque state to a transparent state when electric current is applied
continuously. So staying in a transparent state is costly. In addition, these windows are difficult to
implement. The chemicals may not last long and often don’t work well in practical applications.
Yang’s Smart Window leverages the innate properties of materials to allow or block light in different
material states. No external energy source is required. This improves the applicability of the product,
making it easier and more cost efficient to construct, implement and use. Furthermore, in Professor
Yang’s solution, the starting point of the technology is transparent, which is aesthetically important.
Simply stretching the material turns off the transparency and makes the window appear opaque.
Interestingly, making the material opaque doesn’t darken the room. When the Smart Window is
stretched to block light, it appears translucent from the inside but opaque from the outside. This
minimizes the need to use artificial lighting indoors. The glass beads can also be arranged in customized
patterns, allowing for several different aesthetic design options for the window. And last but not least,
when the Smart Window becomes opaque it can function as an extension of the wall and can be used as
a projection surface. Yang demonstrated this concept in the lab, using a Smart Window to project the
movie Frozen!
How does the Smart Window work?
When you need to block light, you simply stretch the Smart Window, just like you would stretch a blind.
Watch a demonstration here and here. This manual method is easy to do and cost-effective. A motor
can also be used to activate the window. Using a motor isn’t a new concept, it’s frequently used in
building designs and could be easily incorporated into the design of a Smart Window.
An elegant solution inspired by nature
The Smart Window draws inspiration from creatures in nature with color-changing abilities.
Cephalopods such as octopi and squid have thousands of cells containing elastic sacs of pigments in
various colors just beneath their skin. Cephalopods expand and contract their skin using a complex array
of nerves and muscles, causing these pigments to appear and disappear, and instantly changing their
appearance. [This Smithsonian article offers more details on how this works and includes a fascinating
video of an octopus showing off these abilities].
Like the pigmented cells embedded in the skin of cephalopods, the glass beads embedded in the Smart
Window remain invisible until the material is stretched. However, no pigments are necessary. A simple
change in optical properties allows the Smart Window to solve a complex problem using an elegant
design.
Application of the Smart Window
The Smart Window uses inexpensive, commercially available low-end materials in its construction. Thus,
Yang does not see cost as a barrier to the implementation of the Smart Window. The larger challenge
will be to ensure the materials’ robustness for long-term usage, as well as integration with existing
building components. Further testing in the field to measure performance and efficiencies will help to
determine the precise costs.
What’s next for the Smart Window?
Professor Yang is interested in exploring additional options for the Smart Window. Currently, the
material works in the visible spectrum, the window changes from transparent to observably opaque.
Yang would like to conduct further research to determine whether the material could be modified to
block infrared light, which would allow it to remain transparent yet block incoming heat at the same
time. Another interesting application would be the prevention of bird collisions. Currently, there appear
to be no ideal solutions to solve this problem. People use markers or stickers on windows, which don’t
exactly sit well with the aesthetic values of building designers and architects. Yang is interested in
exploring if the material properties could be modified to reflect ultraviolet light which would make the
window visible to birds, while remaining transparent to people.
To move forward, the Smart Window needs research funding, partnerships and support from external
stakeholders. The Smart Window is currently in the prototype phase. Yang has developed working
prototypes that are able to successfully demonstrate the concepts in her lab. She is interested in
working with architects to put the prototype through its paces and use these practical test cases to
quantify energy savings. Yang would also like to leverage on-campus resources, specifically Facilities and
Real Estate Services (FRES) to install the window and begin to gather data on energy efficiency. The
Smart Window would also benefit from further research funding. This funding would be used to test the
already proven concepts for different applications, explore additional applications, define the
constraints of the material, test the durability and longevity of the product and identify its robustness
and life-time when used in practical applications. Once the product is ready to be implemented, venture
capital investment would be necessary to commercialize it for large-scale applications.