Innovative devices ready to energize portable systems.
Miniature fuel cells are poised to replace batteries as the power source of choice for handheld communications and electronics equipment. Tests with prototypes indicate that these devices can generate more power, last longer and remain more environmentally friendly than existing batteries.
U.S. warfighters are carrying a growing array of communications equipment and sensors. Technologies developed for programs such as Land Warrior will add even more weight and power concerns to forces in the field. An alternative energy source such as fuel cells could efficiently power equipment for several days, allowing more operational flexibility by extending patrol times and simplifying logistics requirements, experts say.
Fuel cells have been used in spacecraft for decades, but the devices remained too expensive for widespread use. Recent advances coupled with renewed interest from the U.S. government and private industry have placed the technology on the verge of entering commercial and government markets.
Researchers at the Los Alamos National Laboratory, Los Alamos, New Mexico, are studying methanol and direct hydrogen technologies for use in electronic devices. Fuel cells use hydrogen to generate electricity. This process is usually accomplished by sending gas through a membrane that separates the hydrogen atom’s protons and electrons. The electrons accumulate within the device, then pass through an electrode as current. Direct hydrogen systems draw their power either from liquid hydrogen or from a metallic substance containing the gas. Methanol-powered devices use methanol diluted with water to create energy.
According to Dr. Kenneth Stroh, program manager for transportation and fuel cell programs, Energy and Sustainable Systems Program Office at Los Alamos, direct hydrogen devices are less complicated than methanol-based systems because they do not require pumps and baffles to keep fluids circulating. He notes that direct hydrogen devices need only a fuel source, a passive spring-loaded pressure regulator and the fuel cell stack itself. These devices have no moving parts and can operate under a broad range of conditions.
However, the goal for both methanol- and hydrogen-based systems for portable electronics is to create an air-breathing device that passively operates without any moving parts, he says.
Methanol-based systems are attractive because they potentially could provide large amounts of power. Current experiments with this technology range from commercial systems operating at levels as low as 100 milliwatts to military power supply programs seeking to develop portable devices capable of delivering up to 250 watts.
Stroh relates that Los Alamos engineers joke about a laptop computer that could run for 1,000 hours on a fuel cell. This may not be possible; however, it is feasible to power a device up to 100 hours, depending on its fuel source, he says.
Motorola is interested in using methanol to power small devices such as cellular telephones with a plastic cartridge similar in size to a fountain pen’s ink container. “The idea is that you can go to your local store and buy a box of these cartridges. They would contain methanol diluted in water that would run your cell phone for a week or more,” Stroh explains.
Richard N. Silver, group leader for electronic and electrochemical materials and devices at Los Alamos, notes that the laboratory demonstrated a complete 70-watt methanol fuel cell system last year. The device ran reliably, managing its fuel, fluids and gases under a variety of environmental conditions. However, Silver warns that the fuel cell is still a laboratory prototype and that more science and engineering work must be conducted to create greater performance efficiencies.
Los Alamos has been researching proton exchange membrane fuel cells since the late 1970s, but the program remained small until the mid-1990s when the automotive industry became interested in using fuel cells to power vehicles. In the short term, the U.S. Department of Energy and the automobile industry believe that small electronic devices will represent the first significant market penetration for fuel cells, Stroh maintains. This commercial entry provides manufacturers with an opportunity to develop methods to produce fuel cells in commercial quantities efficiently.
Unlike traditional batteries, the only disposable part of a fuel cell is the fuel container. This simplicity addresses Defense Department concerns about the logistics, cost and environmental impact of large-scale military battery use. “If you’re out on an exercise, you can’t just throw them on the ground,” Stroh says.
Los Alamos is investigating various types of fuel cell packaging and storage methods. Development goals include applications capable of providing 400 to 500 watt-hours per kilogram. By comparison, lithium ion batteries perform in the area of 100 watt-hours per kilogram. Stroh cautions that this difference depends on how much fuel can be loaded into the device to maintain power. “If you can’t get much fuel in there, you’re much worse off than [with] a lithium ion battery.”
According to Silver, the power range on portable devices runs from approximately 100 milliwatts for a pager, as much as 2 watts for a cellular telephone and up to 20 watts for a laptop. Los Alamos has recently received funding from the Defense Advanced Research Projects Agency to develop a 20-watt fuel cell power supply for portable electronics. Military plans call for a manportable device that can function for up to 10 days and weighs roughly a kilogram.
Fuel cells also have an energy density potentially 10 times greater than batteries. This difference is especially true for direct methanol systems, which can last up to 10 times longer than batteries. “Instead of having to recharge your cell phone every day, you could recharge it every two weeks,” Silver observes.
A number of commercial firms are licensing Los Alamos’ fuel cell technology for commercial and government applications. One such company is DCH Technology Incorporated. The Valencia, California-based enterprise has developed a small direct hydrogen fuel cell system for commercial and government markets.
According to DCH’s chief scientist, Dr. Mark Daugherty, the fuel cell is roughly the size of a D-sized battery and has no moving parts. A membrane electrode assembly (MEA) forms the heart of the device. Like batteries, fuel cells have anode and cathode sides with a membrane sandwiched between them to form an electrode. The anode and cathode sides are made of small amounts of platinum distributed on carbon to ensure that the hydrogen has uniform access to the membrane. The combined membrane and catalyst structures form the MEA. Between each MEA is an area called a flow field that allows oxygen to diffuse in from the outside air. Metal separator plates isolate this section from the next cell.
Hydrogen gas enters the membrane through the anode, and the atoms are broken apart into protons and electrons. Protons flow through the electrode, but electrons are trapped in the structure where they become a source of electric current and travel on an external circuit to power a device. Closing the circuit reunites the electrons and protons. They combine with oxygen from the air to form water that evaporates into the surrounding atmosphere.
The number of cells in the stack determines the fuel cell’s voltage, Daugherty explains. Normal operating voltage is about .6 volts per cell, so a 10-cell stack will produce 6 volts, he says. DCH fuel cells are available in stacks of 6, 8, 12 and 20 cells. Because individual fuel cell stacks are assembled and sealed in the factory, cells cannot be added or removed in the field, he says.
DCH powers its fuel cells with a small canister of metal hydride. The container is 4 inches long and 1.75 inches in diameter. It holds about 100 watt-hours of electricity or a 10-watt load for 10 hours. Daugherty notes that the gas in the canister is under relatively low pressure, 150 to 200 pounds per square inch. The device is safe because if the canister were breached, the hydrogen would quickly escape, cooling the container and limiting the release rate, he says.
Daugherty notes that government and commercial organizations are evaluating the company’s fuel cells for use in applications such as wearable computers. A major market study is also underway in Iceland where significant interest exists in totally eliminating hydrocarbon emitting power sources within 20 years, he adds.
While some fuel cell systems near commercial use, a great deal of science remains to make them more efficient, Stroh concedes. Methanol-based systems require additional research because they have not been subjected to the same kind of intense development efforts as direct hydrogen devices. Methanol devices also require relatively large amounts of precious metals such as platinum for their catalysts. Handheld power markets might avoid this issue because a fuel cell would still be less expensive than a lithium ion battery on a per kilowatt basis, Stroh says.
Fuel density remains an issue with designers because, while direct hydrogen systems work well, continued research into technologies to carry the gas at higher densities remains necessary, Stroh explains. Though ideal for automotive use, compressed hydrogen requires large high-pressure tanks unsuitable for portable electronics due to weight and safety concerns.
The first commercial use for fuel cells may be as trickle chargers to maintain cellular telephone batteries, Stroh says. The telephone would rely on its own battery when in use, but when placed back in its holster, the fuel cell would continue to trickle charge the battery, he explains. Researchers are also seeking other fuel sources such as sodium and lithium borohydrate and carbon nanotubes. He notes that automotive engineers are developing ways to draw hydrogen from gasoline and diesel fuel to power automobile fuel cells.
Daugherty predicts that fuel cells will begin replacing batteries in a number of functions within five to six years. He also sees industrial niche applications in devices such as seismic sensors and air and water quality monitoring stations, which must remain active on standby power for long periods of time.