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carbonizing" by turning back to coal. "About 850 coal-fired power plants are slated to be built by the U.S., China and India—none of which signed the Kyoto Protocol," Hoffert says. "By 2012 the emissions of those plants will overwhelm Kyoto reductions by a factor of five."

Even if plan A works and the teenagers of today complete the first leg of the relay by the time they retire, the race will be but half won. The baton will then pass in 2056 to a new generation for the next and possibly harder part of the marathon: cutting the rate of CO2 emissions in half by 2106.

Sooner or later the world is thus going to need a plan B: one or more funda mentally new technologies that together can supply 10 to 30 terawatts without belching a single ton of carbon dioxide. Energy buffs have been kicking around many such wild ideas since the 1960s. It is time to get serious about them. "If we don't start now building the infrastructure for a revolutionary change in the energy system," Hoffert warns, "we'll never be able to do it in time."

But what to build? The survey that follows sizes up some of the most promising options, as well as a couple that are popular yet implausible. None of them is a sure thing. But from one of these ideas might emerge a new engine of human civilization.

NUCLEAR FUSION

Starry-eyed physicists point to the promise of unlimited fuel and minimal waste. But politicians blanch at fusion's price tag and worry about getting burned

Fusion reactors—which make nuclear power by joining atoms rather than splitting them—top almost everyone's list of ultimate energy technologies for humanity. By harnessing the same strong thermonuclear force that fires the sun, a fusion plant could extract a giga-watt of electricity from just a few kilograms of fuel a day. Its hydrogen-isotope fuel would come from seawater and lithium, a common metal. The reactor would produce no greenhouse gases and relatively small amounts of low-level radioactive waste, which would become harmless within a century. "Even if the plant were flattened [by an accident or attack], the radiation level one kilometer outside the fence would be so small that evacuation would not be necessary," says Farrokh Najmabadi, a fusion expert who directs the Center for Energy Research at the University of California, San Diego.

The question is whether fusion can make a large contribution to the 21st century or is a 22nd-century solution. "A decade ago some scientists questioned whether fusion was possible, even in the lab," says David E. Baldwin, who as head of the energy group at General Atomics oversees the largest fusion reactor in the U.S., the DIII-D. But the past 20 years have seen dramatic improvements in tokamaks, machines that use giant electromagnetic coils to confine the ionized fuel within a doughnut-shaped chamber as it heats the plasma to more than 100 million degrees Celsius.

"We now know that fusion will work," Baldwin says. "The question is whether it is economically practical"— and if so, how quickly fusion could move from its current experimental form into large-scale commercial reactors. "Even with a crash program," he says, "I think we would need 25 to 30 years" to develop such a design.

PLAN B: SOONER-OR LATER?

Staving off catastrophic global warming means bridging a gap between the amount of carbon emitted by business as usual and a flat path toward a stable carbon dioxide concentration. That gap may grow much more rapidly than Robert H. Socolow of Princeton and many economists typically estimate, warns N.Y.U. physicist Martin I. Hoffert. The standard "seven wedge" scenario [see box on page 54] assumes that both the energy consumed per dollar of GDP and the carbon emitted per kilowatt of energy will continue to fall. Hoffert points out, however, that China and India have begun "recarbonizing," emitting more CO2 per kilowatt every year as they build coal-fired plants. Carbon-to-energy ratios have stopped falling in the U.S. as well. Socolow acknowledges that the seven-wedge projection assumes substantial advances in efficiency and renewable energy production as part of business as usual.

Even if those assumptions all prove correct, revolutionary technologies will still be needed to knock down carbon emissions in the latter half of the 21st century.

21

Historic trend

Path to stable CO2

concentration

Socolow's

typical projection

of business as usual

Hoffert's projection

of business as usual

2006

Year

2056

2006

Year

2056

2106

Plasma chamber lined with 440 blanket modules

Pipes for liquidhelium coolant

Plasma chamber lined with 440 blanket modules

Pipes for liquidhelium coolant

Superconducting magnets

< ITER fusion reactor will be the first tokamak to generate far more energy than it consumes, once operations begin in the latter part of the next decade. Fusion experts are already planning a successor reactor, called DEMO—the first commercially viable electricity plant to run on the power source of the stars.

▲ Stellarators work much like tokamaks but use more complex magnet shapes that make it easier to confine the superhot plasma (orange). The ARIES working group is analyzing reference designs for a commercial-scale stellarator.

Superconducting magnets

So far political leaders have chosen to push fusion along much more slowly. Nearly 20 years after it was first proposed, the International Thermonuclear Experimental Reactor (ITER) is only now nearing final approval. If construction begins on schedule next year, the $10-billion reactor should begin operation in southeastern France in 2016.

Meanwhile an intermediate generation of toka-maks now nearing completion in India, China and Korea will test whether coils made of superconducting materials can swirl the burning plasma within its magnetic bottle for minutes at a time. Current reactors manage a few dozen seconds at best before their power supplies give out.

ITER aims for three principal goals. First it must demonstrate that a large tokamak can control the fusion of the hydrogen isotopes deuterium and tritium into helium long enough to generate 10 times the energy it consumes. A secondary aim is to test ways to — use the high-speed neutrons created by the reaction to breed tritium fuel—for example, by shooting them into a surrounding blanket of lithium. The third goal is to integrate the wide range of technologies needed for a commercial fusion plant.

If ITER succeeds, it will not add a single watt to the grid. But it will carry fusion past a milestone that nuclear fission energy reached in

* Estimated technical feasibility from 1 (implausible) to 5 (ready for market)

Fusion Reaction

Tritium

Deuterium yy Helium + ® Energy

O Neutron

Deuterium yy Helium + ® Energy

O Neutron

Next-Generation

Fusion Reactors

Project Place

Online

EAST China

2QQG

SST-1 India

2QQG

K-Star Korea

2QQB

NIF U.S.

2QQ9

ITER France

2Q1G

NCT Japan

?

Superconducting magnets

▲ Stellarators work much like tokamaks but use more complex magnet shapes that make it easier to confine the superhot plasma (orange). The ARIES working group is analyzing reference designs for a commercial-scale stellarator.

1942, when Enrico Fermi oversaw the first self-sustaining nuclear chain reaction. Fission reactors were powering submarines 11 years later. Fusion is an incomparably harder problem, however, and some veterans in the field predict that 20 to 30 years of experiments with ITER will be needed to refine designs for a production plant.

Najmabadi is more optimistic. He leads a working group that has already produced three rough designs for commercial fusion reactors. The latest, called ARIES-AT, would have a more compact footprint—and thus a lower capital cost—than ITER. The ARIES-AT machine would produce 1,000 megawatts at a price of roughly five cents per kilowatt-hour, competitive with today's oil-and gas-fired plants. If work on a commercial plant began in parallel with ITER, rather than decades after it goes online, fusion might be ready to scale up for production by midcentury, Najmabadi argues.

Fusion would be even more cost-competitive, Hoffert suggests, if the fast neutrons produced by tokamaks were used to transmute thorium (which is relatively abundant) into uranium (which may be scarce 50 years hence) to use as fuel in nuclear fission plants. "Fusion advocates don't want to sully its clean image," Hoffert observes, "but fusion-fission hybrids may be the way to go."

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