When Princeton engineers want to increase the power output of their new
fuel cell, they just give it a little more gas -- hydrogen gas, to be
exact. Though the simple control mechanism was previously thought
impossible, Jay Benziger, a professor of chemical engineering, and Claire Woo, who graduated in 2006, showed it can work.
Fuel cells, which use hydrogen to make electricity, have attracted much
attention as a clean alternative to fossil fuel-burning energy sources.
Though the potential for fuel cell-powered vehicles is widely
recognized -- they are in development by the world's largest auto
manufacturers -- much work remains to be done before they become
commercially available.
Benziger and Woo's work is a potentially major development because,
until now, there has been no efficient way to vary the amount of power
a fuel cell produces. The conventional approach has been to use
electrical resistors to dissipate excess power, sacrificing some of the
fuel cell's efficiency. In Benziger and Woo's design, the amount of
fuel flowing into the system changes the size of the reaction chamber,
and therefore the amount of power produced.
"It's almost so simple that it shouldn't work, but it does," Benziger
said, recalling the "lively conversation" that ensued after he and Woo
presented their work at the November 2006 meeting of the American
Institute of Chemical Engineers in San Francisco.
Woo agreed: "The most surprising aspect of the research was, of course,
the fact that our control scheme worked." Benziger initially had asked
her to demonstrate that the design wouldn't work when she joined his
lab the summer between her sophomore and junior years, funded by a
Research Experience for Undergraduates grant from the National Science
Foundation.
"So, we were shocked at our initial results," Woo said. "Then, the
following weeks were really exciting as we tried to figure out the
mechanism behind the control."
She and Benziger published their findings in the February issue of the journal Chemical Engineering Science.
The first applications of their design are likely to be in small
machines such as lawn mowers, the researchers said. The machines would
be easy to use, incorporating a design similar to the familiar
acceleration systems of cars that use a pedal to increase the flow of
fuel and the power output. More importantly, Benziger said, the use of
fuel cells in lawn care equipment would cut down on a major source of
greenhouse gases, especially as emissions from these machines are
largely unregulated.
In the Princeton system, some of the water produced as a byproduct
collects in a layer at the bottom of the reaction chamber, while the
rest drains to an external tank. By varying the height of the water
level in the chamber, Benziger and Woo are able to enlarge or shrink
the reaction chamber.
For example, an increased flow of hydrogen into the chamber pushes more
water out of the system, lowering the water level and increasing the
space available for the reaction to take place. Similarly, a decreased
flow of hydrogen causes the pressure inside the chamber to drop,
drawing some of the water from the tank back into the system and
shrinking the reaction chamber.
The water at the bottom of the chamber also serves to maintain the
needed humidity for the fuel cell reaction to take place. This patented
"auto-humidifying" design demonstrates an innovative method of water
management in fuel cells, which has been one of the major obstacles to
large-scale deployment of the technology in automobiles.
Conventional fuel cells feature a complicated network of serpentine
channels to combine the gases, maintain the appropriate humidity levels
and eliminate water from the system. Often, droplets of water clog the
narrow channels, leading to inefficient and irregular power production.
The Princeton system mixes the gases via diffusion in a simple reaction
chamber and relies on gravity to drain the water produced.
Benziger and Woo's reaction chamber also eliminates the need for large
and expensive fuel recycling systems that conventional cells require in
order to make full use of the hydrogen. Their system is effectively
sealed by the water at the bottom of the tank, preventing fuel from
leaving and ensuring that the gases remain in the reaction chamber
until they combine. Most traditional fuel cells repeatedly run hydrogen
and oxygen through an open reaction chamber, converting only about 30
to 40 percent of the fuel at each pass. Since the Princeton system is
closed, 100 percent of the fuel can be used in one pass.
Benziger, whose research is supported by the National Science
Foundation, now has new students, including graduate student Erin
Kimball and senior Tamara Whitaker, working on the complexities that
arise in fuel cells due to water drop formation and motion.
