Circulatory System Keeps Sunny Windows Cool

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Big windows in the home are great. They provide a good view and let in lots of light. But they can also let in lots of heat from the sun and then trap it inside, turning your home into a greenhouse. In the summer, that can translate into higher energy bills from overuse of the air conditioner.

Materials scientists think they have a simple solution: add a water-based circulatory system to the windows to cool them down. If the technology can be mass-produced, such windows would reduce the amount of energy needed to keep a home cool. The research team, led by Donald Ingber, founding director of the Wyss Institute, and Benjamin Hatton, an assistant professor of materials science and engineering at the University of Toronto, reported their results in the July 29 online edition of Solar Energy Materials and Solar Cells.

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The circulatory system isn’t in the glass itself. Instead, the scientists used silicone rubber. They poured liquid silicone into a mold with a tiny waffle pattern of channels, and then when it solidified, peeled it off. They then stretched a thin sheet of transparent silicone rubber over a four-inch-square sheet of glass. After exposing the glass to a bright light to simulate the sun and letting it warm up to about 100 degrees — they pumped water through the channels in the silicone. The temperature of the glass dropped by 12 to 14 degrees.

In a real system, the silicone would be stretched over window glass, and even though it takes energy to pump the water, it isn’t as much as it would take to cool a room with an air conditioner. To get an idea, a fish tank pump that moves 500 gallons per hour uses the energy of a 40-Watt light bulb. A one-room air conditioner uses 20 times that amount.

After the water exits the window pane it could be air-cooled to “room” temperature, perhaps piped under the floor before being recirculated.

This kind of cooling could do more than just keep air conditioning bills down — it could also boost the efficiency of solar cells. Conventional solar cells lose efficiency if they get too hot, and existing cooling systems rely on bulky equipment. A lightweight, transparent cheap coating would solve that problem.

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Silicone rubber is a good choice because along with being inexpensive, easy to cut and stretch, it also stays transparent when the water pumps though the channels. In fact, it’s actually harder to see when the water is flowing, as the channels are faintly visible when they are empty. And because manufacturing the channeled rubber required no exotic or expensive techniques, the process could be scaled up for a reasonable cost.

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