Solar Power: The Slow Revolution
SOLAR CELLS POWER EVERY SATELLITE, AND SATELLITES ARE indispensable. In the military arena they direct battle operations for America; they collect and send GPS (Global Positioning System) data and other intelligence used to plan precision bombing and Special Forces missions; they carry high-speed communications between troops in the field and those in command. In the words of Gen. Lester Lyles, commander of the Air Force Materiel Command, “No American military force could fight without the use of space-based assets.” They run much of civilian life as well. Satellites form and interconnect networks for wireless communication; they facilitate electronic money handling by allowing businesses to bypass slow land-based phone links; they coordinate the transportation of people and goods in the air, on land, and by sea; they keep operators at fixed sites in touch with mobile resources and companies in intimate contact with their far-flung holdings throughout the globe. Live television from across the sea didn’t exist until communication satellites came of age, and they will soon speed up the Internet by unclogging portions of the information highway prone to gridlock.
When you see a picture of a satellite, look for extended flat blue surfaces radiating from its payload. They are solar modules, consisting of many solar cells wired together. Solar cells convert sunlight directly into electricity. The solar material ranges from several to a few hundred microns thick, and within that slice, photons, packets of energy from the sun, push electrons out of the cell to generate electricity. There are no moving parts. These are the first true quantum power devices.
A New Yorker named Charles Fritts built the world’s first solar electric module in 1883, using selenium coated with a thin layer of gold. He optimistically believed that his “photoelectric plate” would soon compete with Edison’s new coalfired electrical generating plants. A hundred and nineteen years later, solar cells have yet to come near competing economically with electricity from centralized power plants. Yet Fritts’s lone module has grown into a multibillion-dollar business, producing the world’s most versatile means of generating electricity.
On learning of Fritts’s discovery, Werner von Siemens, whose reputation ranked alongside Edison’s among those studying electricity, remarked that the direct conversion of sunlight by selenium ranked as “scientifically of the most far-reaching importance.” James Clerk Maxwell, one of the great scientists of all time, wondered, “Is the radiation the immediate cause or does it act by producing some change in the chemical state?”
Their bafflement helped lead to a new realm of physics, quantum mechanics. Its discoveries included the novel description of light as containing packets of energy called photons, and elucidation of the nature of electrons and their behavior. When these concepts explained how solar cells worked, scientists called the phenomenon the photovoltaic effect, and the technology behind it photovoltaics. When Albert Einstein won his Nobel Prize in 1921, the committee cited his work on the photovoltaic effect as his most important achievement.
More thorough investigations in the 1920s and 1930s led experts like E. D. Wilson, of Westinghouse Electric’s photo-electric division, to dismiss selenium as a power converter because it could change only about a tenth of a percent of the sunlight it received into electricity. “The photovoltaic cell will not prove interesting to the practical engineer until the efficiency has been increased at least fifty times,” Wilson wrote.
A solar cell meeting Wilson’s criterion emerged in 1953 from research at Bell Telephone Laboratories. Calvin Fuller, a Bell chemist, had developed the first working silicon transistor by carefully introducing impurities to transform silicon from an insulator to a conductor. Working on a hunch, a colleague, Gerald Pearson, hooked one of Fuller’s silicon devices to an ammeter and then shone lamplight on it. The needle jumped. Fuller had unknowingly constructed a very good solar cell.
Pearson rushed down the hall to where his good friend Daryl Chapin was struggling to wring more power out of selenium. Handing Chapin the cell he had just tested, Pearson said, “Don’t waste another moment on selenium.” Tests confirmed silicon’s superiority, and Chapin dropped selenium to concentrate on improving the new material’s efficiency.
Silicon worked better than selenium for several reasons. Its internal structure was better ordered, giving the liberated electrons more of a chance to reach the contacts and make electricity; its internal structure also allowed it to use more of the solar spectrum than selenium could; the p-n junction, which is an integral part of a junction transistor, could permit higher voltages in silicon; the ability to bring the p-n junction closer to the surface meant more current; and it had better contacts, lowering internal resistance.
Daryl Chapin, Calvin Fuller, and Gerald Pearson spent more than a year surmounting hurdles, and on April 25, 1954, they presented to the world the first solar cell that could generate useful power. The New York Times rhapsodized that their work “may mark the beginning of a new era, leading eventually to the realization of one of mankind’s most cherished dreams—the harnessing of the almost limitless energy of the sun for the uses of civilization.” U.S. News & World Report speculated that the silicon solar cell discovered at Bell might “provide more power than all the world’s coal, oil and uranium.” The Bell team doubled the cell’s efficiency over the next 18 months, but nothing of commercial significance emerged. And small wonder. Chapin calculated that with a one-watt cell costing $286, a homeowner would have to pay $1,430,000 for enough of them to power the average American house.
Chapin despaired over the fate of the new device, but a colleague, Gordon Raisbeck, saw grounds for optimism. It was wrong to write off solar cells, he wrote in a 1955 article in Scientific American . Bell’s invention, he insisted, would find use “in inaccessible places where no lines go” and “in doing jobs the need for which we have not yet felt.”
Soon enough, Air Force and Army scientists began to eye the cells to power a new top-secret technology, an earth-orbiting satellite, something more remote from the power grid than ever thought of before. But when the Navy got the nod to launch Vanguard, it eschewed solar power as “unconventional and not fully established” and chose relatively conventional chemical batteries instead (as had been used in Explorer 1 , America’s first satellite). The Navy’s decision enraged the Army’s lead researcher on power devices, Hans Ziegler, and he won the support of the nation’s most eminent civilian space scientists for a power source that would last indefinitely and permit meaningful experiments in space. Under pressure from these scientists, the Navy relented but still insisted on a dual power supply. After 19 days in space the chemical batteries died and the solar plant took over. Its long life enabled geophysicists to discover our planet’s true shape and broke down the prejudice against solar energy in space.
By the 1960s both the Americans and the Soviets had come to regard the solar cell as critically important for their space programs. From milliwatts on Vanguard to kilowatts for the International Space Station, photovoltaics have powered almost every satellite ever launched. The urgent demand for cells above the earth opened an unexpected and relatively large business for companies manufacturing the devices. And their success in space led people to ask why they couldn’t help on earth.
High price remained the primary obstacle to terrestrial applications. Despite a drop from nearly $300 a watt in 1956 to $100 a watt in 1970, a further plunge to $20 a watt would be needed before solar cells could compete against chemical batteries and generators for the off-grid market.
Elliot Herman, an American industrial chemist, found with the help of funding from the Exxon Corporation that he could drastically reduce the cost of modules for earthbound use without any major breakthroughs. Instead of competing with the semiconductor industry for expensive high-grade silicon, he and his team chose to purchase much cheaper reject wafers, which they found worked perfectly well for generating power. Nor, he also found, did they have to trim the cylindrical wafers into rectangles, as they did for satellites, where a tight fit and light weight were paramount. They could save a great deal of expensive silicon by maintaining the wafer’s original form. Furthermore, the cells didn’t have to be packaged as rigorously as for space, where they contended with meteorites and radiation. Changes such as these had reduced the cost of solar cells by early 1973 to the magical $20 a watt for large orders.
In 1973 Berman’s company, Solar Power Corporation, began selling modules to offshore oil rigs in the Gulf of Mexico. Berman had learned that the rigs required small amounts of electricity for warning lights and horns to prevent boats and ships from running into them and relied on huge nonrechargeable batteries for the job. Maintaining and replacing the batteries was expensive and time-consuming. Lighter, longer-lasting solar panels, running power into smaller rechargeable batteries, saved the industry time and money, and by 1980 solar-powered navigation aids were standard in the Gulf of Mexico. They soon became common on offshore rigs worldwide.
The Coast Guard made the change to photovoltaics more slowly, even though replacing its buoys’ batteries cost more than the buoys themselves. Its hesitancy drove one young lieutenant commander, Lloyd Lomer, to secretly initiate a photovoltaics program. He established testing to develop criteria for panels that would withstand the pummeling of waves and immersion in seawater. He then had a prototype built and placed it in one of the least sunny areas served by the Coast Guard. It operated successfully in the most trying of conditions, but still his superiors would not budge. So he went over their heads, persuading their bosses to authorize an aboveboard photovoltaics program. By the 1980s he had the Coast Guard converting all its navigation aids to solar power. The rest of the world has followed suit.
In Australia the government ordered Telecom Australia, the quasi-public agency in charge of communication networks, to offer the most remote places access to the same high-quality telephone and television service that urban centers enjoyed. Finding a reliable off-grid power supply became the chief challenge. Telecom Australia’s power engineers looked into windmills and generators, but they proved too unreliable. Solar modules attached to batteries, in contrast, demanded neither refueling visits, like generators, nor periodic upkeep, like both windmills and generators. Their solid-state construction made them long-lasting.
The first such installation, supplying four watts, went into service in 1974. The system worked so well that in 1978 the Australian government began to run entire telecommunication networks with sun power. Multiple solar-driven repeater stations spanned thousands of miles to extend long-distance calling and television service, and the experience gave people all over the world greater confidence in photovoltaics. As one Telecom Australia engineer put it, “We showed the world how solar power could be used in a big way out in the field.”
Growing markets for photovoltaics on earth brought new companies into the business with fresh ideas for building better modules and improving production. The first ones had only a coat of silicone between the cells and their surroundings. Out in the ocean, seawater and salty air corroded the wired connections between cells, and in the desert hot daytime temperatures softened the silicone, making the material vulnerable to sand whipped up by winds. The Australians also discovered that cockatoos and parrots liked silicone so much they would tear the modules to pieces as they feasted.
Bill Yerkes, the head of Solar Technology International and former head of Spectra Labs, a leading producer of solar cells for space, probably did more than anyone to put the terrestrial photovoltaics industry on a solid footing. He solved all these problems by doing away with silicone as the top cover. Yerkes replaced it with tempered glass, which was strong, able to clean itself after a rain, and easily available. He also streamlined production by screen-printing contacts onto the cells. The materials and methods he introduced in the late 1970s became standard for the industry.
Cheaper and better solar products have made electricity available to many in rural areas not connected to power lines. Their portability delivers electricity where utilities cannot or will not go. Camels, donkeys, or porters can carry them. Connected to batteries, they provide better lighting and cheaper power for electronic devices, cleanly and reliably, than any other alternatives for the billion and a half people around the world who live beyond conventional power grids.
More than a million families in the developing world use solar electricity in the home, but the price of the modules still presents the chief obstacle to more widespread use. When purchasing them, the customer buys 25 to 30 years of electricity. At least 100 million more could afford that if they could spread their payments out over time, so some companies have sprung up in Africa, the Caribbean, and Central America that act as photovoltaic utilities. Customers pay a monthly fee, and in return the provider wires the house, installs modules and lights, and makes sure they stay in working order.
Photovoltaics has also emerged as the best way to power pumps for bringing drinking water to many villages. Cisterns hold the water for use during sunless periods, eliminating the need for batteries at night or in inclement weather. The expense of stringing telephone wires above or below ground keeps half the world’s population without telephone service, and solar energy offers many the only way to connect by phone to the rest of the world. Because solar energy does so much to better the lives of people in the poorest parts of the world, the Times of India has called it “the common man’s friend.”
The cost of solar cells remains too high for photovoltaic electricity to compete with electricity from utility lines in the richer countries, but important niches keep emerging. Mobile road signs identifying temporary roadway conditions now run solely on photovoltaics; they use light-emitting diodes so efficient that a small panel can charge batteries to keep the signage illuminated indefinitely. Not having to pay someone to come out to fuel and service fluorescent floodlights has saved construction companies and highway departments considerable sums.
Solar modules run switching equipment in state-of-the-art railroad yards. Wherever America’s Navy docks, photovoltaics help protect the ships. The Navy sets up protective barriers in the water around the vessels, and solar-powered light-emitting-diode lanterns demarcate the boundaries of the netting to prevent friendly craft from getting tangled up. And a little piece of electronics attached to automobile windshields lets drivers pay tolls while passing through tollgates without stopping. A microprocessor communicates with the booth, deducting fees from prepaid accounts, and a strip of photovoltaic material enhances the device’s longevity.
In many places where power lines have to go underground, photovoltaics has become a less expensive alternative. The cost-effectiveness then comes not from the energy saved but from avoiding the expense of digging up asphalt and concrete. Solar electricity thus powers 30,000 emergency call boxes along California’s highways. Officials in Carrollton, Texas, discovered in 1990 that when installing flashing lights above speed-limit signs near schools, they could cut expenditures in half by going with solar modules instead of digging to connect to utility-generated electricity. The same economics can apply for new streetlights, for illuminated bus shelters, and sometimes even for powering whole neighborhoods where utilities would otherwise have to run cable underground.
Photovoltaic materials doubling as curtain walls, overhangs, or roofing can make solar cells cost-effective supplementary energy sources for commercial structures. The advantage lies in the panels’ low price compared to marble or granite, which can’t produce a watt of electricity. By selectively blocking out or letting in sunlight, depending on the situation, photovoltaic building materials can reduce air-conditioning and heating loads. The availability of solar fuel also tends to match the electrical needs of buildings, peaking during daytime and on summer afternoons.
Before household photovoltaics can become popular in the developed world, the price of modules and their installation will have to drop by a factor between two and five, depending on utilities’ rates and the amount of solar fuel available. At present most photovoltaic material is made from silicon grown as large cylindrical single crystals or cast in multiple-crystal blocks. Isolating cells only 300 or 400 microns thick from such bulky materials demands a lot of cutting, and half of the very expensive starting material ends up on the floor as dust.
New less costly and wasteful ways of manufacturing solar cells promise much lower prices. A number of companies, for example, have begun producing cells directly from molten silicon; the hardened material, only about 100 microns thick, is then fitted into modules. Others have developed processes to spray photovoltaic material onto supporting material. All these techniques have potential for mass production.
There are those, however, who believe that today’s materials will never reach a low enough price for mass use in the developed world. So some researchers are working with organic compounds that can absorb light and change it into electricity. They envision depositing the compounds on filmlike materials that could adhere to building surfaces. But commercialization remains at least a decade away.
No one really expects photovoltaics ever to supply electricity to the developed world the way other fuel sources have. Each building will likely remain its own power plant. But this eliminates huge capital costs in remote places, and in fully developed areas users can sell back power to the utilities. Using equipment now available, a photovoltaic system producing more electricity than needed can send it back through the power lines, running the utility’s meter backward, and the same building can then buy power from those lines when the solar plant isn’t sufficient.
On-site photovoltaic power production can have many advantages, easing the burden on the electric grid, eliminating the losses that occur in transmission between a power source and its end user, helping prevent brownouts and blackouts, decreasing air pollution and greenhouse gas emissions, reducing vulnerability to terrorism, and generally simplifying power production by dissolving the many steps from extraction, processing, and transporting fuel to electrifying an outlet in your home.
For all these reasons Science magazine has called solar cells “a space-age electronic marvel at once the most sophisticated solar technology and the simplest, most environmentally benign source of electricity yet conceived.”
At the moment, solar electricity generates less than a hundredth of a percent of the world’s power. It may be useful to remember, though, that when Edwin L. Drake drilled the first successful oil well in Pennsylvania in 1859, most people associated petroleum solely with medicinal uses. Almost no one dreamed of what the future held. The story of photovoltaics may follow a similar pattern. Twenty years ago one megawatt of solar cells existed worldwide; since then cumulative production has exceeded a gigawatt, and by 2003 annual output will equal that of the first 20 years combined. Already the world depends on solar cells in satellites, navigation aids, telecommunications, and myriad electronic devices. It takes no wild leap of imagination to see the same happening for offices and homes.