A fuel-cell power unit that can use natural gas, propane or diesel may in a couple of years provide on-site electricity to factories, computer-server farms and even your home. The solid oxide fuel cell, or SOFC, is also set to go mobile, with new systems providing auxiliary or "hotel" power to long-haul trucks. They may also keep a solar-powered surveillance drone in the sky for what could be years at a time. The latter's "two-way" fuel cell system could in addition electrolyze water to store backup energy as hydrogen to supplement intermittent solar and wind power. In time, say researchers, SOFCs might show up as range extenders—power units that augment batteries to extend distance driven electrically—in hybrid vehicles.
"Compared to any other device that converts chemical energy into electricity, the fuel cell, and in particular the solid oxide or ceramic fuel cell, is hands down the most efficient," says veteran fuel cell researcher Eric Wachsman, director of the University of Maryland Energy Research Center, who published research pointing the way to lower temperature SOFCs on November 18 in Science. That's why SOFCs can be tallied as green technology, even if their use of hydrocarbon fuels entails releases of carbon dioxide.
SOFCs have long been seen as second-string to the more well-known hydrogen-based fuel cells. That's because SOFCs run hot, too hot for cars. In the mid-1990s when the U.S. Department of Energy (DoE) was selecting the technologies that would go into the green car of the future, Wachsman recalls, it chose the 80-degree Celsius polymer electrolyte membrane (PEM) fuel cells over the 1,000-degree C SOFC. But the continued lack of a hydrogen fuel distribution system means that "we placed our eggs in the wrong basket by investing billions in hydrogen PEMs instead of the type of fuel cell that runs off the fuels that we have today," Wachsman says.
That flexible-fueling advantage has, however, enabled Sunnyvale, Calif.-based Bloom Energy to sell some 120 natural gas–fuel SOFCs, stand-alone heat and power units that produce both electricity and heat for a local site to green-minded Fortune 500 corporate plants and state university facilities—notably, subsidized distributed power demonstration projects in California. The company is even building a new plant in Delaware and will sell 30 megawatts of its Bloom Box fuel-cell units to the local utility, Delmarva Power. Unfortunately, Bloom Boxes use a traditional ceramic fuel-cell design that tends to be relatively expensive to operate, a competitive disadvantage that Bloom hopes to address with a new, lower-cost power-leasing program.
Hot power box
A SOFC converts a fuel's chemical energy into electricity, says Bob Stokes, a longtime fuel-cell researcher and CEO of Versa Power Systems in Littleton, Colo., one of the up-and-coming developers of SOFCs. In general, the system consists of two electrodes sandwiching a solid oxide or ceramic membrane (or electrolyte). The electrochemical device produces electricity directly by oxidizing—read, slow burning—fuels.
"Unlike other fuel cells which transport positively charged [hydrogen, or H+,] ions through a membrane, solid oxide types use a ceramic oxide—through which negatively charged oxygen ions pass," Stokes explains. The oxygen sensor in your car is based on the same yttria-stabilized zirconium oxide ceramic. The oxygen (O–) ions, react with hydrogen from the fuel to create water, electricity and, if the fuel contains hydrocarbons, carbon dioxide.
Fuel-flexible SOFCs don't have to use hydrogen as fuel; they can run just fine on hydrocarbon fuels, such as natural gas, propane or diesel. A system can either break down a carbon-containing fuel into hydrogen and carbon with a pretreatment steam reformer or do it internally, using its own heat and design.
And although SOFCs operate hotter than most other common fuel cell types, they can convert as much as 60 percent of the fuel into usable electricity, Stokes says. "This means that the amount of carbon dioxide it releases per unit of usable energy that it produces is half that of what a conventional engine emits." The heat also allows SOFC to run without the costly platinum-based catalysts that current polymer electrolyte membrane systems need.
Making membranes
Design and manufacturing innovations, funded in part by DoE programs, are bringing down the cost of the technology as well. Older SOFC designs use the electrolyte layer as a structural support, but the thicker component has a higher electrical resistance, which entails higher operating temperatures to avoid power losses, Wachsman explains.
Engineers have lowered operating temperatures by using electrode-supported designs with thin, more conductive electrolytes, but the new techniques needed to make the dense, gas-impermeable electrolyte layers can be problematic. "The thinner the membrane, the more unstable it is," he says. Developers manage the trade-off between thickness and conductivity by supplementing the ceramic with scandium, a transition metal rare earth that boosts conductivity, albeit at a high cost.
Many manufacturers have adopted (or adapted) a tubular configuration, which enables relatively easy and thus low-cost assembly. Reduced temperatures in addition mean cheaper steels can be used elsewhere.
Next-gen products
As a result of the design improvements, prospects for the technology are on the rise, says Brian Warshay, an analyst at Boston-based Lux Research who follows power grid–related technologies. "We see the main application for SOFCs in natural gas–fueled stationary power supplies for industrial users and those who need continuous, on-site distributed power such as Web-server farms—high-reliability base-load power systems of 100 kilowatts or larger," he says.
Heat and power units for homes may also become more common, such SOFCs can be 85 percent efficient. The fuel cell not only supplies electricity but heats the house and the hot water. These, Warshay notes, are particularly popular outside the U.S., "where energy usage is significantly lower than here," as the outsize electricity demands by American users would generally overtax the capacities of the first round of home-size heat and power models being marketed in Asia and Europe.
Stokes and other industry observers also expect even larger, megawatt-size distributed power units that are composed of modular 250-kilowatt stacks to hit the market within two to three years, having recently watched large multinational corporations such as General Electric and Rolls Royce sign supply deals with SOFC cell- and stack-makers.
Then there's trucks. Delphi engineers, using the newer electrode-supported design, have developed a five-kilowatt (maximum) SOFC auxiliary power unit (APU) for long-haul diesel rigs. The APU, which could arrive next year, would provide "hotel load power" for parked trucks.
Two-way fuel cells
Meanwhile, Versa, a solid oxide fuel cell stack supplier, is working with Boeing and "a large European company" on an innovative reversible SOFC that cycles back and forth between providing power and electrolyzing water into hydrogen and oxygen, Stokes says. The two-way system could store energy as hydrogen to back up intermittent solar or wind power installations and even the Solar Eagle, a dragonflylike unmanned aerial vehicle that is to fly multiyear missions.
Wachsman and his research colleagues have also published details in Science on a potential path toward SOFCs that operate at temperatures as low as 350 degrees Cwith a new design that features high-conductivity electrolytes and a specially nanostructured electrode.
SOFC technology capable of lower "intermediate" temperatures ranging of 600 to 800 degrees C is the goal of a recent half-million-dollar National Science Foundation project at Argonne National Laboratory and the University of Illinois at Chicago. Christos G. Takoudis's interdisciplinary team plans to wield a unique atomic layer deposition/chemical vapor deposition (ALD/CVD) hybrid reactor that can lay down novel thin-film cell materials and structures that run cooler by design.
But before that research group makes its final project report three years from now, second-generation improved ceramic SOFCs should have begun to augment theBloom Box's initial success in occupying and developing a small but key niche of the energy market.
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