August 2005 Mechanical Engineering Departments: Tech Focus, Pt. 2

This section was edited by
Associate Editor Gayle Ehrenman.

Fluid Handling and Fluid Power

Cutting the Contami- nation
by Gayle Ehrenman

Fuel cell vehicles require specific conditions to operate properly. Tamper with the unique chemistry that allows them to run, and the output of the fuel cells suffers.

FlexFab Horizons International of Hastings, Mich., has developed a custom silicone hose, which it claims will help prevent such output drop-offs. The hose, which will be used by a "major Asian carmaker" for critical fuel cell cooling water in hybrid engines, has solved leaching problems that plagued earlier hoses, according to Rod Ward, director of sales engineering for FlexFab. Previous-generation hoses, which were made from EPDM (a cured synthetic rubber), nitrile, and standard rubber formulations, introduced harmful contamination over time.

"The challenge was to develop a hose construction that wouldn't contribute any contaminants to the fuel cell cooling water, even under elevated temperatures," Ward said. "Any contamination that leaches from the hose will tend to collect on the polymer electrolyte membrane in the fuel stacks, where electrons are stripped from hydrogen gas as part of the process of generating electricity," he said. "Even tiny amounts of contamination can cause the kilowatt output per cell to drop off significantly."

The automaker's criterion for a successful hose design in this application is six months in service, with no discernible amperage drop. This would indicate sufficient stability to attain the company's target of a five-year minimum lifespan on fuel cells, according to Ward.

"The previous material formulations all leached trace amounts of contamination into the deionized cooling water," Ward said. "In some cases, it was detectable within a few hours or days."

FlexFab field-tested the hoses for six months. "Laboratory testing for leachables wasn't enough to ensure performance in actual service," Ward said. "There are so many possible compounds that could end up in the water that it's impractical to screen for all of them. The only way to be sure of their performance in the field is to put them into service and monitor any amperage loss."

Deep
Space Deep Freeze
by Jeffrey Winters

The vacuum of space is unfathomably cold; in the shade, temperatures are as cold as minus 250°F. So keeping material samples aboard the International Space Station at a mere minus 300 ought to be not much more complicated than sitting them on the windowsill to cool.

But it's not that simple, says engineer David Ray of the University of Alabama at Birmingham Center for Biophysical Sciences and Engineering. "You need to have crew interaction with the samples in the freezer," Ray said. "It's got to be part of a human-friendly environment."

NASA recently selected a design created by the UAB team for a high-tech freezer for the space station and the shuttle, which would be used for storing biological and materials sciences samples.

A new design for a cryogenic freezer uses only 240 watts, the same as
two light bulbs.

"We'd been thinking about how to do this for a while," said UAB engineer William Crysel, the manager of the freezer project. "We'd built refrigerators and incubators for space since the 1990s, and we had talked about how to go about making a freezer."

The result is something quite unlike the standard kitchen refrigerator. For one thing, the freezer—called the General Laboratory Active Cryogenic ISS Experiment Refrigerator in order to get the acronym Glacier—doesn't use Freon (which is too dangerous for use on the space station, Crysel says) or a compressor. Instead, the cooler uses a Stirling engine to lower the temperature. Stirling engines are most commonly used as machines that convert heat into mechanical energy, but reconfigured, they can draw heat from a volume of air.

A Stirling engine uses much less electricity than other potential solutions, such as acoustic freezers, which is a big plus for use on the space station where every watt is precious. A single unit draws only 240 watts for cooling—about the same as two standard light bulbs. The units also have to be compact, fitting within a compartment measuring 22 by 18 by 21 inches.

To conserve space, the UAB engineers have designed insulating panels made from aerogel—a lightweight material that's more than 99 percent air.

Three of the UAB freezers are scheduled to be delivered to NASA by January 2007.