1997 Discover Awards: Environment: Solar Power Incognito

Tuesday, July 01, 1997
RELATED TAGS: ALTERNATIVE ENERGY
WINNER: United Solar Systems’ Flexible Solar Shingles

INNOVATOR: Subhendu Guha

Back in 1994, while he was lecturing on the virtues of solar energy in the home, Subhendu Guha was showing a slide of solar cells arrayed on a roof. But it’s so ugly, said an architect in the audience. Who would want that on their house? It was just a flip remark, but it got Guha thinking.

When Guha, who is the executive vice president of United Solar Systems, got back to his office in Troy, Michigan, he went to work immediately. I got together this group of people in a brainstorming session and asked them, ‘How can we make the photovoltaics look more like the roof?’ Perhaps, he suggested, solar cells need not be mounted to a metal frame that juts skyward but could be nailed directly onto the roof. Maybe, in other words, they could make a shingle that not only protects a house from rain and snow but also produces electricity. I am amazed nobody thought of it before, Guha says.

Last July, Guha put the finishing touches on his solar shingles. They operate on the same principles as conventional solar cells: sunlight falls on thin layers of silicon and stimulates an electric current. The real innovation is in the way they are manufactured. Giant rolls of stainless-steel sheeting, each about a foot wide and half a mile long, are fed through a series of machines that lay down a nine-layer coating of silicon. The silicon layers are in turn coated with a semiconducting film that tints them to match the color of roofing shingles. Then the sheets are cut into eight-inch strips that are wired together into seven-foot shingles. Finally, the shingles are coated with protective plastic. Roofers install the solar shingles just as they do normal ones, except that for each they must drill a hole in the roof for electrical leads. An electrician wires the leads to the home’s electrical system.

It takes 20 square feet of shingles to light a 100-watt bulb, which means that, on average, one-third of a home’s roof should provide all the home’s electrical requirements when the sun is shining. Guha thinks that electric rates are high enough to make his shingles economical in some parts of the United States. In Japan, high energy prices and government subsidies for solar energy should make the shingles more promising.

Finalists

Metal-Munching Plants

Rutgers University’s Phytoremediation

INNOVATOR: Ilya Raskin

Last year in Ukraine, not far from the site of the 1986 Chernobyl disaster, an unusual garden bloomed. On a pond contaminated with the highly radioactive metals cesium and strontium, sunflowers floated on small Styrofoam rafts and sent their roots into the deadly waters. Despite the poisons, the plants thrived, and as they grew, they absorbed large amounts of cesium and strontium into their roots and stems.

These sunflowers herald a new and cheaper way to clean up water and soil contaminated by heavy metals. Phytoremediation--from the Greek word phyton, for plant--is the brainchild of Ilya Raskin, a plant biologist at Rutgers University in New Brunswick, New Jersey. In 1989, Raskin worked down the hall from a scientist studying the use of bacteria to break down contaminants in soil and water, as was done after the Exxon Valdez oil spill. His colleague complained about the limitations of bacterial cleanup--There’s nothing they can do with metals, Raskin recalls him saying. But Raskin remembered reading an article about plants that naturally accumulate metals from the soil, up to 3 or 4 percent of their total weight. If he could find the right plants and grow them under the right conditions, he thought, he’d have a natural heavy-metal cleanup crew.

We ordered seeds and started screening them, Raskin says. The sunflower, he found, is particularly good at accumulating heavy metals, but unfortunately much of the metal it absorbs stays in its roots. To make them easier to harvest, Raskin grows the plants in water. Indian mustard is even more promising for cleaning up contaminated soil because it sucks lead into its leaves, which can then be harvested normally.

After harvesting, the metal-saturated plants can be incinerated and put in a toxic-waste dump, where their ashes will take up much less space than the original contaminated soil. So far Raskin has used the technique in six waste sites--three in New Jersey, and one each in Massachusetts, Connecticut, and Chernobyl. Depending on the type of contamination, Raskin says, phytoremediation may cut costs by a factor of 50 to 80 percent over traditional cleanup methods.

City of Life

Southern California Edison et al.’s

Solar Two Power Plant

INNOVATOR: Thomas Brumleve

It took nearly 25 years for Thomas Brumleve to see his invention come to life. In June 1996, in California’s Mojave Desert, the Solar Two power plant began sending 10 megawatts of power--enough for 10,000 homes-- into Southern California Edison’s electric grid. And Brumleve, who took early retirement from Sandia National Laboratories in Livermore, California, in 1984, was invited to the dedication as an honored guest.

The Solar Two plant generates electricity from the sun in a very different way from conventional solar cells. On a 95-acre site, nearly 2,000 huge mirrors point toward a central 300-foot power tower. The angle and direction of each mirror is computer-controlled to follow the sun and reflect its light toward the top of the tower, bathing it in the equivalent of 500 suns. At the focus of this inferno is molten salt, which enters this zone via a series of pipes at 550 degrees but quickly zooms to 1050 degrees. This superhot salt then flows to a steam generator, which turns a turbine.

Like other forms of solar power, Solar Two is nonpolluting and renewable, but economics is still an obstacle. As a demonstration project, Solar Two was relatively expensive to build. Proponents hope that commercial versions can be made and operated cheaply enough to be competitive with other ways of generating electricity. A plant ten times as large, for instance, might be economically feasible in an area with plenty of sun and high energy costs.

If so, it will be a vindication of sorts for Brumleve, who first pushed the idea of the power tower in the early 1970s. Although he didn’t conceive the notion himself, he thought of using molten salt to conduct the heat rather than boiling water directly with sunlight. The idea of molten salt jumped out at me because I’d been doing some work on energy storage, he says. I knew that molten salt stores heat energy well. So well, in fact, that Solar Two can continue to generate electricity for 12 hours after the sun goes down.

Benign Battery

U.S. Air Force’s Rome Laboratory’s

All-Plastic Battery

INNOVATOR: Don Dylis

Most batteries are environmental bad guys. They contain lead, mercury, cadmium, or other toxic heavy metals that can leak and contaminate soil or groundwater. Don Dylis’s batteries are different. Dylis, a research manager at the U.S. Air Force’s Rome Laboratory in Rome, New York, has developed a more benign battery, made entirely of plastic. Not only is it at least equivalent to standard nickel-cadmium batteries in terms of power and rechargeability, it’s also less affected by cold temperatures and can easily be molded to fit into small, awkward spaces.

Dylis got the idea 15 years ago, when he first read about plastics that were able to conduct electricity. I thought, ‘Hey, here’s something with potential for a power supply.’ The Air Force, always interested in cutting weight and saving space on its planes, would love a lightweight, moldable battery. But the field of conducting polymers was in its infancy and not ready for such practical applications. So Dylis waited.

Over time, others had the same idea, but everyone came up against the same roadblock. The anode, or negative terminal of the battery, must be made of a material that has plenty of loosely bound electrons--the carriers of an electric current. When most plastics are chemically modified to have such electrons, they fall apart.

Five years ago Dylis got the idea to combine a conducting polymer with a nonconducting polymer that could hold an electric charge without deteriorating. He convinced several researchers at Johns Hopkins to follow up on his intuition and provided funding from the Air Force to do it. His insight proved correct. After trying a series of polymers and preparations, in April 1996 the Hopkins team finally achieved what Dylis had envisioned years before: a plastic dynamo that can do everything a metal battery does. And even though the batteries are not biodegradable, they still don’t pose nearly the hazard metal ones do.

Rhubarb to the Rescue

Yale University’s Destruction of CFC Gases with Rhubarb

INNOVATORS: Robert Crabtree and Juan Burdeniuc

Even though chlorofluorocarbons (CFCs) have been banned in the United States, more than 100 million pounds of these ozone-depleting gases linger in old refrigerators, air conditioners, and storage tanks. Getting rid of this stockpile would be expensive--breaking the tough bond between the carbon and fluoride atoms in a cfc molecule requires temperatures of more than 1000 degrees or highly reactive chemicals, or both.

Last January, Yale chemistry professor Robert Crabtree and graduate student Juan Burdeniuc unveiled a safe and inexpensive alternative: the hot tube. You take a tube, says Crabtree, pack it with some solid, push through the cfc gas, and heat the thing up. The trick, of course, is to pick a solid that will react with the cfc. One possibility was sodium oxalate, an unremarkable molecule found in the leaves of rhubarb that has two unstable electrons. Perhaps, Burdeniuc thought, if he heated the chemical, those electrons would attach to fluoride atoms in the cfc molecule, thereby breaking the carbon-fluoride bonds. Since sodium oxalate was such a mild compound, though, it seemed like a long shot. But when something has a low probability and such a high payoff, says Crabtree, you go for it.

The gamble paid off. The two compounds reacted in the hot tube to form carbon, carbon dioxide, sodium fluoride (an ingredient of toothpaste) and sodium chloride (plain old salt). Crabtree thinks his rhubarb method could be the answer to disposing of cfcs, but he isn’t taking any chances. Although he still hasn’t figured out precisely what happens in the reaction, he’s already testing other compounds for one that works in seconds, rather than the several minutes the rhubarb needs. It will be several years before he brings out a product.
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