Glass windows equipped with an lifelike vascular system can buffer heat from passing through them and could one day help people save money on air conditioning, according to engineers at Harvard University. Details on a prototype version of the glass as published this week in Solar Energy Materials and Solar Cells.

Although floor-to-ceiling glass windows are a lovely feature in any apartment, they can be costly. Windows are poor insulators, allowing heat to readily pass into a home during summer when you're trying to stay cool, and vice versa during the winter when you want to stay warm.

If smarter, more efficient windows could be engineered, they could cut total energy costs for a building by up to 40 percent.

The new lifelike glass is based on cooling systems found in the people's skin that helps them regulate body temperature. When humans are hot, blood vessels near the surface of the skin dilate or expand. This extra surface area allows more heat to diffuse from the bloodstream into the air blowing against skin, which cools the body. The reverse occurs when we're cold; the blood vessels constrict, meaning less heat can escape from them.

The vascular system in the glass is comprised of microscopic troughs carved into the surface of a silicone sheet. This sheet is then pressed and sealed against a piece of glass, creating a network of channels through which water can be pumped, which is known as a microfluidics system.

When heat hits the glass, some of it is transferred into the water flowing through the channels and is carried away. The channels become transparent when water is running through them, so the aesthetics of the windowpane remain intact

"The water comes in at a low temperature, runs next to a hot window, and carries that thermal energy away," said lead author Dr. Benjamin Hatton. Hatton, who conducted the research while earning his PhD at Harvard's Wyss Institute, is now an assistant professor of materials science and engineering at the University of Toronto.

The Wyss Institute is dedicated to creating new technologies based on the design principles found in nature.

To test the cooling system, the glass was heated to 100 °F to simulate the hottest temperature a window might face during a given year. When cold water was pumped through the silcone capillaries, the glass' temperature rapidly dropped [video].

The channels that make up the artificial circulatory system are visible when they're empty (left), but transparent when they're filled with water (right). Windows with this system installed would remain transparent. (Credit: Wyss Institute)

Pumping just a small amount of water through an artificial circulatory system can cool a hot window pane within seconds.

Water-cooled windowpane, from the Wyss Institute on Vimeo.

The current prototype is small — measuring 4 inches by 4 inches — so the next step is to scale up the technology for use in a full pane of glass.

Researchers used math models to estimate that pumping just half a soda can of water through the system could cool a full-sized window pane by 14 °F. The energy needed to pump the water would be far less than the energy absorbed, generating a big net win in terms of efficiency.

The vascular glass could also be used in solar panels, which lose a fraction of their ability to generate energy when they become too hot.

Source: Hatton BD, Wheeldon I, Hancock MJ, Kolle M, Aizenberg J, Ingber DE. An artificial vasculature for adaptive thermal control of windows. Solar Energy Materials and Solar Cells. 2013.