New design could open door to small-scale fuel
cells.
In a breakthrough that could make fuel cells
practical for such small machines as lawnmowers and chainsaws,
researchers have developed a new mechanism to efficiently control
hydrogen fuel cell power.
Many standard fuel cell designs use electronics to
control power output, but such designs require complex systems to
manage humidity and fuel recovery and recycling systems to achieve
acceptable efficiency.
The new process controls the hydrogen feed to match
the required power output, just as one controls the feed of gasoline
into an internal combustion engine. The system functions as a closed
system that uses the waste water to regulate the size of the reaction
chamber, the site where the gasses combine to form water, heat and
electricity.
National Science Foundation (NSF) awardee Jay
Benziger of Princeton University developed the new technique with his
student Claire Woo, a recipient of an NSF Research Experiences for
Undergraduates award and now a doctoral candidate at the University of
California, Berkeley. Woo and Benziger published their findings in the
February 2007 Chemical Engineering Science, now available online.
The researchers believe the first applications for
their technology will be in smaller engines. Fuel cells are currently
inefficient on such scales due to the need for fuel recycling and
excess hydrogen in standard designs. The researchers' new design is
closed, so 100 percent of the fuel is used and there is no need for a
costly fuel recycling system.
"The system is ideal for small internal combustion
engines that lack emissions controls and are highly polluting," said
Benziger. "There is also no need for an extensive hydrogen
distribution system for these small motors; the hydrogen could be
supplied in returnable tanks such as the propane tanks used for gas
grills."
Benziger's next goal is to connect several of the
new fuel cells together to increase power, a system that could
potentially compete with cells now being tested in the automotive
industry.
Source / Further
information:
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Publishing date: 24-Jan-2007
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Feb. 2007
Chemical Engineering Science article abstract: We
demonstrate that the power output from a PEM fuel cell can be
directly regulated by limiting the hydrogen feed to the fuel cell.
Regulation is accomplished by varying the internal resistance of
the membrane–electrode assembly in a self-draining fuel cell with
the effluents connected to water reservoirs. The fuel cell
functionally operates as a dead-end design where no gas flows out
of the cell and water is permitted to flow in and out of the gas
flow channel. The variable water level in the flow channel
regulates the internal resistance of the fuel cell. The hydrogen
and oxygen (or air) feeds are set directly to stoichiometrically
match the current, which then control the water level internal to
the fuel cell. Standard PID feedback control of the reactant feeds
has been incorporated to speed up the system response to changes
in load. With dry feeds of hydrogen and oxygen, 100% hydrogen
utilization is achieved with 130% stoichiometric feed on the
oxygen. When air was substituted for oxygen, 100% hydrogen
utilization was achieved with stoichiometric air feed. Current
regulation is limited by the size of the fuel cell (which sets a
minimum internal impedance), and the dynamic range of the mass
flow controllers. This type of regulation could be beneficial for
small fuel cell systems where recycling unreacted hydrogen may be
impractical.
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