Scientists exploring better ways to tap the sun`s energy are seeking guidance from what may be an expert in the field: the ferocious polar bear.
Although polar bears look white, they actually have transparent, pigmentless hair and black skin. And that, two scientists at Boston`s Northeastern University now believe, makes the bears remarkably efficient solar energy collectors.
The bears have pulled off a ”fantastic engineering feat,” said Richard E. Grojean, a physicist and professor emeritus at Northeastern, who got the whole ball of fur rolling about 10 years ago. His interest was piqued by the odd results of a wildlife census in Canada.
When the Canadian government tried to take a count of baby harp seals in the mid-1970s, traditional aerial photography didn`t work, because the white seals blended in with their snowy environment. They next tried infrared film, which can usually detect warm-blooded animals. This failed also; the seals`
insulation was just too good. Finally, they tried ultraviolet photography.
The harp seals, as well as polar bears and almost all other warm-blooded arctic animals, showed up completely black on the ultraviolet film. While the snow reflected ultraviolet rays, the animals absorbed them.
At the time, Grojean was a consultant to the U.S. Army Research and Development Command in Natick. Applying the solar polar bear phenomenon to the design of cold-weather shelters and uniforms seemed like a natural idea to him, and to colleagues Malcolm Henry and John Sousa.
The research was dropped in 1981, Grojean said, for lack of funding. And though Grojean has retired, Gregory J. Kowalski, an associate professor of mechanical engineering, is continuing the work. (Northeastern, through its graduate programs, is providing support.)
The polar bear has two important needs: whiteness and warmth. It must appear white so it can stalk prey in the gleaming snow of the Arctic, and it has to be able to stay warm in a climate where the temperature often reaches 20 degrees below zero.
Based on their research, Grojean and Kowalski say the secret to the bear`s success seems to be that its pigmentless hair traps and transmits to the skin 90 percent of sunlight in the invisible ultraviolet portion of the spectrum, but only 10 percent of the light in the visible spectrum. The bear collects abundant energy for warmth from the invisible ultraviolet light while reflecting the visible light, making it appear white.
Polar bears know how to stay warm. Their hair is better at trapping the sun`s rays than the flat-plate solar collectors used on rooftops to heat water, whose efficiency rating averages about 35 percent. Polar bears, according to Grojean, have a 90 percent efficiency in trapping ultraviolet light. Through thermal balance equations, Grojean calculated that the bears trap 90 percent of ultraviolet light and 17 percent from the entire solar spectrum.
So how do the bears do it? Grojean doesn`t know exactly, but he said he believes the smooth, hollow hairs, with their labyrinthine, rough cores, work like optical fibers. In an optical fiber, light enters at one end and bounces along the inside. The outer wall acts like a mirror, so that once the light bounces in, it can`t get back out until it reaches the other end. The polar bear`s hair acts in much the same way, but the light enters from the sides of the hair, as if the hair`s outer layer were a one-way mirror.
The energy of the light travels to the skin, is absorbed and is converted to heat. Because the hair is also a great insulator, the heat doesn`t escape, making the energy transfer a one-way process.
But Grojean said polar bear hair has something an optical fiber doesn`t have: a core or medulla running down the center of the hair. The medulla is hollow and is crisscrossed by an elaborate system of membranes. The researchers believe the core is involved in transmitting light, in addition to providing structural strength and rigidity without weight, much like a bird`s bones.
Light can be absorbed, transmitted or scattered. Kowalski said he knows the hair is not absorbing the light, because the hair`s temperature is the same as the bear`s surroundings. He knows the light is not transmitted. If it were, the hair would act like a pane of glass and the bear would appear black because we would be looking through the hair at the black skin.
So the light must be scattered when it hits the fur. The question is how. If scattering is at work in the polar bear hair, the researchers say, it means that ultraviolet light enters the hair, scatters off the medulla and is trapped internally, making its way to the skin. Visible light, Grojean and Kowalski believe, is scattered off the outside of the bear`s hair, making him appear white.
Ultimately, Grojean believes, solving the mystery of polar bear fur could have direct application to designing better solar panels for homes and industry. Kowalski said his calculations suggest that applying the principles of polar bear fur might make it possible to increase the efficiency of solar panels by 50 percent.
