by Mariette DiChristina
The world's largest solar collector absorbs an awesome amount of the sun's energy: equal to 37 trillion kilowatts annually - or 4,000 times the amount of electricity used by all humans on the planet. A typical square mile of that collector - otherwise known as the surface waters of Earth's vast oceans - contains more energy than 7,000 barrels of oil.
From the earliest water wheels, humans have sought to tap sea power, expressed in waves, currents, and tides. But a more promising idea extracts that stored heat energy directly: Ocean Thermal Energy Conversion, or OTEC, generates electricity by using the temperature differences between tropical waters drawn from the sun-warmed surface, and those from the chilly 2,500-foot depths below. Near lush Kailua-Kona, on an old black-lava bed on Hawaii's west coast, a test plant produces up to 100 kilowatts net. Rather than creating air pollutants or spent radioactive fuel, OTEC's by-product is not only harmless, it's downright useful: 7,000 gallons per day of desalinated ocean water with a crisp taste that rivals the best bottled offerings.
Using largely conventional components, OTEC plants built on coasts or moored offshore could provide enough power and water to make tropical areas, including the Hawaiian islands, independent of costly fuel imports, say proponents. On drawing boards are plans by Sea Solar Power of York, Pa., for a 100-megawatt floating OTEC plant off the Indian state of Tamil Nadu. Other proposals include smaller plants in the Marshall and Virgin Islands. Some 98 tropical nations and territories could benefit from the technology, according to one study.
OTEC has advantages over other ocean-energy schemes. The largest wave-powered devices have produced only a few kilowatts, for example. Waves and currents have low energy potential - that is, they are not consistently vigorous enough to provide much power to run generators. Tides have greater power potential, but the technology to tap them is costly and limited to a few coastal spots where the tide regularly rises and falls at least 16 feet and can be harnessed. One, built across an estuary in Brittany, can generate 240 megawatts. The only North American demonstration project, on Nova Scotia's Annapolis River, can produce 50 megawatts.
OTEC isn't affected by capricious tides and waves, however. The solar energy stored in the seas is always available. Better yet, that 'fuel' is free as long as the sun hits the ocean," adds Luis Vega, the shorts- clad director of the Kailua-Kona demonstration project.
That turns out to be a necessary bit of good fortune for OTEC. Tropical-ocean surface waters are typically some 80 F, while those far below hover several degrees above freezing. That temperature gradient gives OTEC a typical energy conversion of 3 or 4 percent. As any engineer knows, the greater the temperature difference between a heat source (in this case, the warm water) and a heat sink (cold water), the greater the efficiency of an energy-conversion system. In comparison, conventional oil- or coal-fired steam plants, which may have temperature differentials 500 F, have thermal efficiencies around 30 to 35 percent.
To compensate for its low thermal efficiency, OTEC has to move a lot of water. That means OTEC-generated electricity has a glut of work to do at the plant before any of it can be made available to the community power grid. Some 20 to 40 percent of the power, in fact, goes to pump the water through intake pipes in and around an OTEC system. While it takes roughly 150 kilowatts of juice to run the Kailua-Kona test plant, larger commercial plants would use a lower percentage of the total energy produced, says Vega.
That's why, a century after the idea was first conceived, OTEC researchers are still striving to develop plants that consistently produce more energy than is needed to run the pumps, and that operate well enough in the corrosive marine climate to justify the development and construction. "It's a beautiful process," says Vega. "But it needs large, costly components." During the 1970s the U.S. government invested $260 million in OTEC research. After the 1980 election, federal support fizzled.
One thing is not in doubt: The theory works. Georges Claude, a Frenchman who also invented the neon sign, proved it. In 1930, Claude designed and tested an OTEC plant on Cuba's north coast. His patented invention, a version of OTEC called open cycle, generated 22 kilowatts of power - but consumed more than that in operating, partly because of the poor site choice. Claude's next attempt, a floating plant off Brazil, was thwarted by storm damage to an intake pipe; the luckless inventor died virtually bankrupt from his OTEC efforts.
It's been smoother sailing for the Kailua-Kona plant, operated by the Pacific International Center for High Technology Research of Honolulu. Last September, the Kailua-Kona project took Claude's open-cycle concept to an OTEC world record, generating 255 kilowatts gross of electricity, and 104 net. Operated in a $12-million, five-year project, the plant's power is used by neighboring enterprises at the Natural Energy Laboratory, a Hawaiian facility devoted to developing solar and ocean resources.
Imagine a boiling hurricane. That's essentially what you see through the circular viewing portal when the Kailua-Kona OTEC plant is running. Inside a chamber, air froths from ocean water, forming whitecaps on the turbulent surface. More seawater - 9,000 gallons a minute - pours in from 13 upright white plastic pipes. As the pressure inside drops to that of the atmosphere at 70,000 feet, the water abruptly goes ballistic, an72 degree F steam shoots about. "That steam is cool enough to touch," a technician advises, "but in that vacuum your hand would blow apart."
After the resulting steam rushes through a turbine-generator, it's condensed back to liquid - desalinated water - by frigid deep-ocean water pulled from other pipes. Less than 0.5 percent of the incoming ocean water becomes steam. So large amounts of water must be pumped through the plant to create enough steam to run the large, low-pressure turbine. That limits an open-cycle system to no more than three megawatts of gross power; the bearing/support system needed for larger, heavier turbines may not be practical. Vega has a solution for this problem, however. "I was influenced by the movie The Graduate," he says, referring to the promising career path suggested to a young college graduate. "You know - plastics." Designed with new kinds of lighter-weight plastic or composite turbines, a series of open-cycle-system modules might together create ten- megawatt-size plants, he says. That's still not impressive as conventional power plants go. A large nuclear reactor, for example, can produce 1,000 megawatts.
Another type of OTEC system, called closed cycle, can more easily be scaled up to a larger industrial size; it can theoretically reach 100 megawatts. In 1881, French engineer Jacques Arsene d'Arsonval (who was later to become Claude's teacher and friend) originally conceived this version, although he never tested it.
In closed-cycle OTEC, warm surface water vaporizes pressurized ammonia via a heat exchanger. The ammonia vapor then drives a turbine-generator. The cold deep-ocean water condenses the ammonia back to liquid at another heat exchanger. Closed cycle's high-water mark to date was a floating test plant called Mini-OTEC that produced 18 kilowatts of net power in 1979.
Turbines are already commercially available for use with a pressurized-ammonia system, which gives closed cycle an advantage for installations that would require large amounts of electricity. The technology nonetheless requires large, expensive heat exchangers. New heat exchangers will begin testing next January at a 50-kilowatt (gross) closed-cycle experimental plant that is soon to be constructed at the Kailua-Kona site. The heat-exchanger will employ roll-bonded aluminum, which is less costly than the titanium previously used in OTEC experimental plants.
Researchers there will also monitor the aquaculture tanks located downstream. They want to determine the effects on marine life from any ammonia that might escape from the plant, as well as from the small amounts of chlorine added to the ocean water to prevent equipment fouling from algae and other varieties of marine creatures.
The Kailua-Kona test plants will also help reveal the answer to one of the biggest OTEC unknowns: the eventual life cycle of components, which are continuously besieged by the ocean's corrosive salt spray and biofouling. "We're discovering how to deal with rust," says Vega. Because open cycle doesn't scale up easily, and closed cycle produces no drinking water, "the jury's out on which way to go - open or closed," says Vega.
Combining the two systems may yield the best of both: A hybrid OTEC could first produce electricity by closed cycle. Then, the hybrid system could desalinate the resulting warm and cold seawater effluents using the open-cycle process. Adding such a second stage to an open-cycle plant could also double water production.
Ultimately, OTEC has great potential - along with a generous share of remaining engineering and cost issues. Futurists see OTEC as an essential part of a worldwide switch from petroleum to hydrogen fuels; ocean-based OTEC plantships could electrolyze water for hydrogen. "OTEC is environmentally benign and could provide all of humanity's energy needs," declares Tom Daniel, scientific/technical program manager of the Natural Energy Laboratory.
Funding for the Kailua-Kona open-cycle plant runs out after 1995. The next step, as Vega sees it, must be to construct a scaled commercial plant of about five megawatts, and to operate it for one to two years. Such a plant could cost about $100 million over a five- year construction and development period - a stiff price, perhaps, for the tropical locales that would most benefit from OTEC's eventual use. "We need to go through a money-losing proposition to prove the money-making one," emphasizes Daniel. "That's where I believe the government should come in."
Like other forms of renewable energy, OTEC won't play well if that government considers only the immediate bottom line. Large OTEC plants could become cost-competitive if oil doubles from its current $18 or so a barrel, says Vega. Oil prices don't include what Vega and others call "externalities," such as money spent coping with the polluting effects of burning hydrocarbons or military defense of oil fields. Factoring in oil defense alone would make oil's "true" cost $100 a barrel, says energy guru Amory Lovins. (Among the closed- cycle test plant's funders is the Department of Defense's Advanced Research Projects Agency, which considers the development of new fuel sources to be of importance to the nation's defense.)
Like every other method of generating power, OTEC is not entirely innocent of environmental consequences. The flow of water from a 100-megawatt OTEC plant would equal that of the Colorado River. And that water would also be some 6¡F above or below the temperature it was when it was originally drawn into the plant. The resulting changes in salinity and temperature could have unforeseen consequences for the local ecology. Can the tide turn for OTEC? A self-acknowledged dreamer, Vega professes to have no illusions. "Some people think I'm conservative, and some think I'm crazy," he sighs. "The truth is somewhere in between."
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gene carl feldman (email@example.com) (301) 286-9428