Warming up to polar bears' solar secrets. Studies of their hollow fur may lead to more efficient solar energy technologies
Boston — BELIEVE it or not, polar bears may hold secrets to state-of-the-art solar energy research. Scientists at Northeastern University here have discovered that the shaggy fur of polar bears is 95 percent efficient in converting the sun's ultraviolet rays into usable heat. This remarkable ``polar solar'' phenomenon of converting sunlight into heat is unmatched by even the most expensive and sophisticated solar collectors, which have a maximum efficiency conversion of 65 to 70 percent (on a cold winter day, their efficiency is more like 20 percent).
The unique property about the hairs in polar bear fur is that they are hollow and therefore act as a sort of tunnel. Light passes from the tip of each hair down to the bear's black skin, where it is converted into heat. According to researchers, the hairs act in much the same way as optical fibers now used in telephone transmission lines.
Basic polar-solar principles have undergone some preliminary testing in rooftop solar panels, resulting in a dramatic 50 percent increase in collection efficiency. Such improvements could spur a major change in solar energy research while making solar heating an attractive alternative for homeowners.
Solar panels work like greenhouses. When sunlight hits a solar panel, heat is trapped between the bottom-layer collector plate and a translucent top-layer retainer plate. Then the heat is either fanned off and collected or it warms some kind of liquid. By putting peltlike fibers between the collector and retainer plates, scientists reduce heat loss dramatically.
Utilizing polar-solar principles could also eliminate the need for large storage capacity, which current solar panels need during periods of overcast weather. Polar-solar panels capture the sun's ultraviolet rays, which are not blocked by clouds, thereby allowing the panels to collect solar energy even on overcast days.
How did anyone ever get close enough to an iceberg-hopping polar bear to find out about its phenomenal fur?
An unusual report by a group of Canadian scientists aroused the curiosity of Richard Grojean, professor of electrical and computer engineering at Northeastern University and technical director of the school's new Center for Electro-Magnetic Research. The report said Canadian scientists could not get population estimates of polar bears by aerial photography or infrared photography. Traditional photography proved futile, because polar bears blended in with their snowy surroundings. Infrared cameras, which can detect warm-blooded animals, failed because the polar bears' fur insulates their body heat so effectively that almost no outside heat can be detected, according to Dr. Grojean.
The Canadians were finally able to track the bears by using ultraviolet photography, which registers shortwave, the invisible rays at the end of the light spectrum. The position of the animals could be seen because the snow reflects 90 percent of the sun's ultraviolet rays while the polar bear pelts absorb them.
Grojean was fascinated by the fact that polar bears virtually vanish under an infrared lens. He discovered that white polar bear hair is actually transparent, with an inner hollow core about one-third of the diameter of the hair. In this core, ultraviolet light is scattered and by some unknown mechanism is then converted into heat. Grojean said the hair is ``wavelength selective,'' choosing to absorb shortwave, or ultraviolet light, and reflect light with wavelengths toward the middle of the spectrum. This reflection is what makes the bear's fur appear white when it is actually clear.
In addition to solar panel improvements, Grojean's research may also yield new Arctic uniforms for the military. Instead of the traditional multilayered garb, which is bulky and retains body moisture, Grojean says wearing an outer covering that employs the energy conversion technique found in polar bear pelts could be warmer and much more comfortable.
Grojean and his colleagues at Northeastern, Gregory Kowalski and Charles DiMarzio, have launched a detailed study of the polar-solar phenomenon and are seeking funding from several groups, including the United States Department of Energy. Once adequate funding is available and the mechanics of the polar-solar effect are found, prototypes of revolutionary solar collectors could be available within five years, Grojean said.