Spotlight on Research: The Smart Window

Spotlight on Research: The Smart Window

An Elegant Solution for Improving the Energy Efficiency of Buildings
April 26, 2015

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.


Illustration of a full-size Smart Window. Photo courtesy of Jenny Sabin of Sabin Design Lab at Cornell University (architecture).

A short while ago, I sat down with Dr. Shu Yang, Professor in the Materials Science and Engineering (MSE) and Chemical and Biomolecular Engineering (MSE) Departments, to discuss her research using material science to solve energy challenges in buildings. Professor Yang's group has developed a Smart Window that can address these challenges simply and elegantly. Here's what I learned in that conversation.

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 its native state, the glass beads and silicone sheet have the same 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.

The Smart Window needs no external energy source to make it work. It's easy to use, constructed from low-cost materials and the glass beads can be arranged in customized patterns to suit any design requirement.

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. Watch the demonstration below.

How does the Smart Window work?

Watch a demonstration of the Smart Window: Demo 1Demo2.

When you need to block light, you simply stretch the Smart Window, just like you would stretch a blind. 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

Read this Smithsonian article to learn more about how cephalods change colour and watch a fascinating video of an octopus showing off these abilities.

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.

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.


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.

The Smart Window work is part of the eSkin Project - a collaboration between Yang, Jenny Sabin (Cornell), Jan Van de Spiegel, Nader Engheta, and Kaori Ihida-Stansbury (Penn).