|Almost there: a commercially viable fuel cell|
|Contact: Dan Krotz, [email protected]|
Berkeley Lab researchers have developed a solid oxide fuel cell (SOFC) that promises to generate electricity as cheaply as the most efficient gas turbine.
Their innovation, which paves the way for pollution-free power generators that serve neighborhoods and industrial sites, lies in replacing ceramic electrodes with stainless-steel-supported electrodes that are stronger, easier to manufacture, and, most importantly, cheaper. This latter advantage marks a turning point in the push to develop commercially viable fuel cells.
"We're closer to breaking the cost barrier than ever before," says Steve Visco, who developed the SOFC technology with fellow Materials Sciences Division researchers Craig Jacobson and Lutgard De Jonghe.
That barrier is $400 per kilowatt, a stringent bar set by the Department of Energy's Solid State Energy Conversion Alliance, a government, industry, and scientific group tasked with developing affordable fuel cell-based power generators. The $400 target -- nearly one-tenth the cost of today's fuel cells -- is equivalent to the most efficient gas turbines and diesel generators, and is based on the premise that a fuel cell's success hinges on its competitiveness.
"Green is great for marketing, but people won't buy an environmentally friendly product if it's twice as expensive," says Visco.
Fuel cells work by converting chemical energy to electrical energy, capitalizing on hydrogen and oxygen's strong propensity to bond and form water. Unlike gas turbines, this process doesn't emit air pollutants such as nitrous oxide and sulfur dioxide. And because fuel cells are more efficient than gas turbines, they emit far less carbon dioxide, a greenhouse gas.
An SOFC is composed of a gas-tight electrolyte layer sandwiched between porous cathode and anode layers. Oxygen from the air flows through the cathode, and a fuel gas containing hydrogen, such as methane, flows past the anode. Negatively charged oxygen ions migrate through the electrolyte membrane and react with the hydrogen to form water, which reacts with the methane fuel to form carbon dioxide and hydrogen. This electrochemical reaction generates electrons, which flow from the anode to an external load and back to the cathode, a final step that both completes the circuit and supplies electric power. To increase voltage output, several fuel cells are stacked together, a configuration called a fuel cell stack that forms the heart of a clean power generator.
Visco and colleagues' foray into affordable fuel cell design began several years ago when they developed a way to lower a fuel cell's operating temperature to 800 degrees Celsius without sacrificing efficiency. Until then, fuel cells worked most efficiently at 1,000 degrees Celsius, a high temperature that decreases the cell's life span and precludes the use of metal components.
They fabricated extremely thin ceramic electrodes that conduct ions at 800 degrees Celsius as readily as thicker electrodes do at 1,000 degrees Celsius. Lowering the temperature also allowed them to use metal components, instead of ceramic, to connect several ceramic cells into a stack. Their design didn't hit the $400 per kilowatt target, but it allowed them to reduce the cell's operating temperature without sacrificing performance -- and it got them thinking.
"Craig Jacobson wondered if the electrode support itself could be made of metal, which is strong, cheap, and readily available," Visco says.
With this in mind, they developed a fuel cell that features 10 to 15 microns of a zirconia-based electrolyte layered onto 10 to 20 microns of a nickel-based electrode. Together these are supported by and bonded to approximately two millimeters of porous high-strength commercial alloy.
The alloy is manufactured using the same process used to make metal filters that work in high temperature applications. Powdered steel is fired in an oxygen-free environment, which creates a porous metal. This stainless steel alloy is much stronger than ceramic, and unlike ceramic, it can be welded, brazed, hammered, and crimp-sealed. This translates to increased design flexibility and reduced manufacturing costs. Furthermore, the cost of stainless steel is approximately $2 per pound, while zirconia is between $30 and $60 per pound.
Alloy construction offers other advantages. A stable, high performance cathode can be operated at between 600 and 800 degrees Celsius. Efficiency loss due to current collection is minimized. And the alloy increases a fuel cell's strength as well as its electronic and thermal conductivity.
But does the design meet the $400 per kilowatt target? First, there's more to a fuel-cell-based generator than fuel cells. Roughly speaking, one-third of a generator's cost lies in the actual fuel cell stack, the other two-thirds lies in external "plumbing" such as insulation and a DC-to-AC inverter. This means the fuel cell stack can't exceed $130 per kilowatt if the entire unit is to meet the $400 per kilowatt target. No problem there: the raw materials for the Berkeley Lab stainless steel-based fuel cell are only $37 per kilowatt.
"The low cost of a metal-based SOFC's raw materials, and its design flexibility, should allow a stack to be manufactured below the $130 fuel cell target," Visco says.
To meet the $400 generator target, the Berkeley Lab fuel cell must now be developed into planar and tubular stack designs, and paired with a low-cost inverter and other supporting technology.
Ultimately, such technology could play a key role in meeting the nation's growing demand for power without incurring a proportional jump in air pollution. According to a recent Department of Energy report, annual energy demand will increase from a current capacity of 363 million kilowatts to 750 million kilowatts by 2020.
Distributed power generators that use fuel cells offer one solution. In this scenario, small-scale power generators serve hospitals, neighborhoods, and manufacturing plants. This stands in contrast to today's sprawling power grids, in which large power plants feed miles of power lines that lose one-third of the generated electricity before it reaches the customer.
Instead, power generators that produce between three and ten megawatts of electricity could be located at or near the customer. This is an ideal fuel cell application, because while both large gas turbines and the best fuel cell boast an energy conversion efficiency rate of 50 percent, gas turbines become less efficient as they become smaller. Fuel cell generators, however, retain their 50 percent efficiency rating regardless of size, meaning they're ideally suited for small-scale work like distributed and portable power generation.
"Instead of building a large, fuel cell-based power plant, which is expensive and therefore risky, it makes sense to start smaller," Visco says. "The big question is not if fuel cells will enter the market, but when."
More information about the Department
of Energy's Solid State Energy Conversion Alliance