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.
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 often 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.
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.