Solar bear technology.

Solar bear technology

What's white and black and warm all over? A polar bear under the Arctic sun.

The polar bear is almost a perfect solar converter, says electrical engineer Richard Grojean of Northeastern University in Boston. The bear's fur efficiently conducts ultraviolet light to its black skin, where the energy is absorbed and helps maintain the bear's body temperature. Grojean, who has been studying polar bear physics for more than a decade, is now working with a colleague, Gregory Kowalski, to see if similar principles can be used to design efficient solar collectors for cold climates.

A polar bear's hairs are completely transparent. The bear appears white because visible light reflects from the rough inner surface of each hollow hair. However, the hairs are designed to trap ultraviolet light. Like light within an optical fiber, the radiation is conducted along the hairs to the skin. This summertime energy supplement provides up to a quarter of the bear's needs. Thus, even while actively pursuing prey, the bear can still concentrate on building up its blubber layers in preparation for winter.

Two other qualities make polar bear fur an attractive model for a solar collector. This fur collects ultraviolet light coming from any direction. And, while the skin is warm, the temperature at the pelt's outer layer is about the same as the bear's surroundings. "He loses very little heat," says Grojean.

Eskimos have long known about these properties. An Eskimo never dries a polar bear pelt with its skin against the ground; the ice under the pelt would melt and later freeze the pelt to the ice. Instead, pelts are dried with the fur against the ground. "The pelt is a sort of thermal diode," says Grojean. "The energy flows only in one direction."

What's needed now is a more detailed study of the properties of polar bear hair, says Grojean. This would suggest the characteristics that, say, hollow quartz fibers should have to duplicate the bear's system. One important need would be to find a way to shift the absorbing region from ultraviolet to visible light, where more solar energy is available.

"We're not predicting that we'll be covering the roofs of buildings with fur," says Grojean. "But if we know what the properties are, then we'll know how to structure an equivalent system."