Nuclear plants may be clean hydrogen source
For more than 100 years, visionaries have periodically espoused the dream of an economy driven by hydrogen - an efficient fuel that emits only water when burned. Today, their vision may be on the verge of reality: Energy policymakers around the world are increasingly recognizing the potential of hydrogen as a fuel for transportation, which accounts for more than one third of the nation's annual energy consumption.
One key problem that must be solved first is to develop a source and distribution system for the massive amount of new hydrogen that will be needed. A number of new and existing technologies are under study, but one strong candidate for providing hydrogen in future decades is nuclear energy.
Heightened interest in hydrogen as a widespread fuel is driven by environmental, political and technological factors:
Fuel cells combine hydrogen and oxygen to produce electricity. Their only waste product is water vapor. In the last five years, their power density - the ratio of power output to size - has increased ten-fold while their costs have decreased ten-fold. Every major automobile manufacturer has a program to develop fuel-cell-powered vehicles, and many experts predict that hydrogen-powered electric cars will appear on American roads in a few years.
But in the longer term, full conversion to hydrogen-based transportation will take decades, if only because of the enormous quantities of hydrogen required to fuel the dream.
"Americans drive nearly three trillion miles a year," said David Lewis, director of Argonne's Chemical Technology Division. "Even if you assume that electric cars will be twice as efficient as today's internal combustion engines, you'd still need 34 million metric tons of hydrogen to cover that many miles. That's a 70 percent increase in worldwide production just to handle this nation's current transportation needs. Add in the rest of the world, and the numbers become truly daunting."
Hydrogen is the most abundant element in the universe, but hydrogen gas, the form needed to power fuel cells, is rare in nature and must be manufactured. Current world production is about 50 metric tons per year, mainly as a feedstock for the oil and fertilizer industries.
About 95 percent of hydrogen is manufactured with an efficient, economical steam-reforming process that releases hydrogen from methane or natural gas. But a key goal for hydrogen power is to reduce carbon dioxide emissions, and here steam reforming has a problem: to create steam, the plants burn natural gas, which emits carbon dioxide.
"Using steam reforming to produce hydrogen for transportation," said Argonne engineer Leon Walters, "would eliminate some carbon-dioxide. But wouldn't it be better to manufacture hydrogen without making any greenhouse gas?"
One possibility is electrolysis, the use of electricity to split water into hydrogen and oxygen. Electrolysis has been used for more than 100 years to manufacture pure hydrogen and oxygen.
"If electric cars will be twice as efficient as cars with internal combustion engines, then electrolytically produced hydrogen is already close to competitive with dollar-fifty-a-gallon gasoline," said Walters. "Centralized hydrogen electrolyzers could be installed at corner gas stations and a home refueling station could soon be as close as the electrical outlet in your garage."
Large electrolysis units are operating around the world in demonstrations of central fueling for public transportation and auxiliary energy for large buildings.
But to displace the nation's automobile transportation fuel with electrolytically generated hydrogen would require 241 gigawatts of new generating capacity. "That's the equivalent of 241 modern 1,000-megawatt power plants," Walters said. "Clearly, it won't happen in only five or 10 years."
Where would all this additional electricity come from?
"Renewable energy technologies - wind, solar and geothermal - can make an important contribution," he said. "These technologies tend to be too intermittent to provide reliable base-load electricity, but they can generate hydrogen and store it when the wind is blowing or the sun is out. On the other hand, they are too diffuse to generate 241 gigawatts of new capacity. You'd need 640,000 windmills, for example, which would occupy a total land area of 71,000 square miles - nearly the size of Ohio and Indiana combined.
Nuclear power as a hydrogen source
"The only energy technology that can generate that much additional electricity without producing greenhouse gases is nuclear power," Walters said.
But no one expects electrolysis to do it all. "Together with steam reforming, electrolysis," he said, "is more likely a near-term hydrogen source as the market gets going. In the long term, there's greater potential for developing advanced nuclear power plants to provide the heat for centralized hydrogen production on the scale needed."
Ultimately, nuclear power could become the vital link in the energy supply chain. This vision has emerged as one element in the U.S. Department of Energy's Generation-IV Nuclear deliberations. In collaboration with 10 other nations, the Generation-IV program is developing an international consensus on research and development for the next phase of nuclear energy. Nuclear power is no longer viewed as solely a source of electricity.
"Future advanced reactors can provide heat for manufacturing hydrogen," said Argonne nuclear engineer David Wade. "Nuclear energy is the only way we know to generate large amounts of heat without burning large amounts of fossil fuel. But today's nuclear power plants don't produce enough heat."
Coolant outlet temperatures from today's reactors are about 400 degrees Celsius (750 degrees Fahrenheit). But a number of advanced reactors types with outlet temperatures as high as 900 degrees C (1,650 degrees F) are being developed for deployment in 20 or 30 years.
One technology that the Generation IV program recommends for further development is the liquid-metal-cooled fast reactor. These reactors can not only provide heat for steam reforming of methane, but they also can create new nuclear fuel by converting the non-fissile portion of natural uranium into fissile plutonium.
"If nuclear power is to provide the bulk of energy to manufacture hydrogen on a global scale," Wade said, "we'll need to create new fuel, because existing natural supplies will be exhausted in 50 years."
Sodium-cooled fast reactors, he said, can produce peak outlet temperatures of about 600 degrees C (1,100 degrees F), hot enough for steam reforming. Operating experience with them has been excellent. Argonne operated Experimental Breeder Reactor II safely and reliably for 30 years at Argonne-West in Idaho.
The Russian nuclear program, he said, is developing a lead-cooled fast reactor that could provide temperatures of 850 to 900 degrees C (1,560 to 1,650 degrees F), hot enough to support a third hydrogen-manufacturing technology, the thermochemical cracking of water.
Cracking water for hydrogen
Thermochemical cracking is being developed around the world, Wade said. Typically, this process uses temperatures of 700 to 900 degrees C (1,290 to 1,650 degrees F) in combination with chemical reagents to break water into hydrogen and oxygen. The chemicals are recycled. The only input is water, and the only outputs are hydrogen and oxygen.
Gas-cooled, high-temperature reactors that operate at about 900 degrees C (1,650 degrees F) are a likely heat source for thermochemical water splitting for hydrogen. A few are currently operating in Great Britain and France, but they are being phased out because they are not economically competitive. A second class of gas-cooled reactors was developed in the United States and Germany. South Africa expects to have a commercial prototype ready for the market within a decade, and Japan is planning a pilot-scale demonstration with a high temperature helium-cooled nuclear reactor as the heat source.
Thermochemical processes have not yet been commercialized, Wade said, mainly because steam reforming is more economical and can supply the existing market. But recent concerns about the environment and energy independence have revived interest in greenhouse-gas-free processes.
"Transition to a hydrogen economy created by nuclear power could take three or more decades", Wade said. "But, it could provide a clean, abundant and affordable fuel supply for transportation and homes and industry."