Sun Power in the Pyrenees

A simple magnifying glass, focusing the sun's rays, can scorch a piece of wood or set a scrap of paper on fire. Solar radiation can also be concentrated on a much more awesome scale. It can burn a hole through thick steel plate, for example, or simulate the thermal shock of a nuclear blast. It can, that is, with the aid of a super reflector of the sort that has been set up by French scientists high in the Pyrenees. Ten years in the building, the world's largest solar furnace is a complex of nearly 20,000 mirrors and can concentrate enough sunlight to create temperatures in excess of 6,000DEG F.

Harnessing solar energy is hardly a new accomplishment. Nearly 22 centuries ago, the Greek mathematician Archimedes is said to have temporarily saved Syracuse from Roman conquest by setting the invading fleet aflame with numerous large mirrors. In the 18th century, the pioneer French chemist Lavoisier produced enough heat with 52-inch-wide lenses to power his experiments. Though Lavoisier's work was cut short by the French Revolution (he was guillotined), his history has not discouraged contemporary French scientists--notably Physical Chemist Felix

Trombe. 64, a research director of France's National Center for Scientific Research and its premier experimenter with the sun's energy.

For more than 20 years, Trombe has championed solar furnaces as an ideal source of intensive heat for both industrial uses and scientific experimentation. In 1946 he fashioned his first sun stove out of a captured German antiaircraft searchlight mirror at an observatory near Paris. Moving to the old Pyrenean citadel town of Mont-Louis, where the sun shines as many as 200 days a year, he has since built five larger solar furnaces. Now, in masterly style, he has created his piece de resistance on a hillside in the nearby ski resort of Odeillo. Compared with similar devices in several other countries, such as the U.S. Army's 30-kilowatt stove at Natick, Mass., Odeillo's 1,000-kilowatt structure is easily the Mount Palomar of solar furnaces.

Delicate Adjustment. The furnace's appearance is as spectacular as its power. Its glittering eight-story-high parabolic reflector (roughly half the size of a football field) towers over Odeillo's centuries-old houses. Anchored against a reinforced concrete office and laboratory building, the huge concave mirror consists of 8,570 individual reflectors. For the furnace to operate efficiently, these small (18 inches square) mirrors must be precisely adjusted so that their light will converge exactly at the parabola's focal point 59 ft. in front of the giant reflector. Only half of the mirrors have been aligned thus far, although the structure has been finished for more than a year. Reason: the work is so delicate that technicians can usually adjust no more than a few dozen even on the sunniest of days.

Far too huge to follow the sun itself, the parabolic reflector depends on the help of 63 smaller mirrors set in eight rows on a terraced slope in front of it. Called heliostats (from the Greek helios, sun; statos, to cause to stand still), they track the solar disk across the sky, capture its light and bounce it in parallel beams into the big mirror. The system involves some ingenious engineering. Each heliostat is controlled by its own photoelectric cells. Whenever one of the hehostats (each of which is made 180 individual mirrors) loses its lock on the sun, these tiny electric eyes inform a minicomputer, which in turn controls a pair of hydraulic pumps that can rotate and tilt the heliostat into th proper position. Only one manual ad justment is needed to operate the heliostats. It is made at the end of the day, when they must be reset to face the position of the next day's sunrise.

Rotating Vats. The crucible of the furnace is located inside a smaller f-shaped building near the base of the big mirror. It is set behind large stainless-steel doors at the focal point of the parabola--where the sun's scorching rays are concentrated into a blazing circle only twelve inches wide. Target material, hoisted into place by a ten-ton lift, is placed into an inclined trough-as the target melts, it runs off into catch pans. Another, more sophisticated technique is to load the material into two aluminum vats whose outer walls are water-cooled to prevent melting. Placed with their open ends at the focal point and rotated like washing machines to distribute the heat evenly, these containers can hold up to 21 tons of molten material at one time.

Is all this elaborate effort worth the French government's $2,000,000 investment in the furnace? Trombe says yes.

For one thing, the power is almost entirely free (only 13 kilowatts of electricity are needed to operate the mirrors). More important, the furnace gives off what he calls "aristocratic" or uncontaminating heat; there is, for example, none of the adulterating carbon that is produced by the hot electrodes in ordinary high-intensity electric arc furnaces. Thus the solar oven is ideal for the production of chemically pure materials.

French industry is beginning to agree In a recent test for an electronics manufacturer, the furnace fused several tons of bauxite and ceramics to produce high-voltage insulators of unmatched purity The oven could easily fuse other highly heat-resistant materials: quartz crystals for radio transmitters, corundum for industrial grinding stones and zircon parts for nuclear reactors. It could also be used in experiments to develop new space-age alloys, such as special tungsten or cobalt steels, and even materials to withstand the searing heat of a nuclear blast.

Initial Fears. Aside from the industrial and scientific benefits, the furnace has produced an entirely unexpected dividend. At first, Odeillo's villagers thought they might be blinded by the intense light from what they call le four solaire (the solar oven). Now they know that the light is concentrated at only one point and that there is no such danger. In fact, the villagers have become quite proud of the strange, shimmering edifice in their midst. And why not? The solar furnace is not only handsome in an other-worldly way; it is also a significant tourist attraction, bringing thousands of people to gaze in awe at Odeillo's mighty mirror.

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