Fuel cells start to look real
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Balance of plant
Auto engineers will say that a lot more goes into developing a fuel-cell vehicle than the stacks and the fuel. The so-called "balance of plant" issues, though little spoken about, also figure strongly in the success of these next-generation vehicles. A variety of sophisticated devices are required to keep the stacks going, and the parasitic losses they can entail must be minimized.
Staley lists some of the balance of plant technologies involved: "Firstly, most fuel-cell units need pressurized air to boost the amount of oxygen in the stack for the reaction to take place, so efficient compressors are required. When air is compressed, it heats, so you need an intercooler to reduce temperatures. In addition, filters must be included to keep the air, hydrogen, and water streams clean as well as flow and recirculation devices to keep them moving. Humidifiers are needed to keep the gas stream moist so the membrane does not dry out. In a reformer vehicle, the moisture is a free byproduct of the reaction. In the nonreformer case, a supply of water must be kept on hand so water is extracted from the stack exhaust and then fed back into a reservoir to make the process self-sustaining. Meanwhile, the exhaust stream is typically run through an expander to reduce the shaft load on the compressor. Finally, various energy recovery devices are used to retain heat for several of the reforming processes."
Similar to that of a battery-electric car, Staley continued, "the high-voltage, high-power output of the fuel-cell stack must be distributed to the rest of the car - to the drivetrain, and to a dc-to-dc converter to power the auxiliary electronics systems." In general, fuel-cell vehicles also use the same kind of computer controls, sensors, multiplexers, and analog-to-digital converters as current IC-engine vehicles, though the control algorithms are different. The powertrain control is also close to that of a battery-powered vehicle; it's divided into torque-generating control for the drivetrain and a power-generating control for the fuel-cell system. Traction motor technology continues to follow the evolution of the electric drivetrain."
Generally speaking, "the balance of plant packaging is going well," said GM's McCormick. "It's getting progressively simpler as we integrate all the necessary components." Said his colleague Ruselowski, "At this stage, we're starting to get away from highly sophisticated aerospace technology, which was required at first, and moving toward more automotive-type technology."
Oversize radiators
Another rarely mentioned but important engineering issue concerning fuel-cell vehicles is the radiator system. Because fuel-cell engines are different than their IC counterparts, their radiators are also different. In particular, they are much larger. "We're going to have to find innovative ways to get the heat rejection you need in fuel-cell vehicles," said McCormick.
"PEM fuel cells run at 80°C (175°F) because the membrane needs that temperature to keep the process going," explained DaimlerChrysler's Mohrdieck, "but we would really rather have a 120°C (250°F) operating temperature like IC engines because it's difficult to get rid of low-temperature heat, particularly when exterior temperatures are high. It's actually one of the biggest technical challenges; fuel cells need a very large radiator, which makes packaging and styling difficult by adding size and large air openings that hurt the drag coefficient. It's going to be easier for American cars to accommodate these concerns because of their larger size," he noted. "In addition, fuel-cell vehicles require two radiator systems - one for cooling the electronics and the electric motor and another for cooling the stack, which makes it awfully complicated."
"Fuel cells have interesting heat-transfer properties that are significantly different than the heat properties of IC engines," said Staley. "In an IC engine, one-third of the exhaust heat goes out the tailpipe, one-third into the coolant, and one-third out the radiator. In a fuel-cell powerplant, 80% of the heat must be rejected through the radiator, which has to be sized to handle the heat produced at high power as well as be able to operate at high ambient temperatures. You need more surface area to reject lower temperature heat to the environment, particularly when it's hot out. All this makes for a big problem.
"Since the radiator is going to be bigger, it leads designers to try to divide up the heat loops into several radiator systems that can be packaged separately," Staley explained. "In addition, you've got different heats at different temperatures and your efficiency in rejecting heat to the atmosphere is different. The heat-transfer people are facing a whole new set of challenges, but they're doable - it's just going to require something other than standard automotive thinking."
Material compatibility is another issue. "With fuel cells, keeping control of the conductive paths is very important," Staley said. "Fuel cells want no ionic materials in them, so deionized water and special cooling fluids need to be used. Sophisticated ground fault-detection technology and deionizing beds in some cooling circuits are also employed to accomplish this. Unfortunately, when you run these kinds of fluids through equipment, it tends to corrode them quickly." McCormick indicated that "GM has moved away from deionized water, which freezes easily, toward nonconductive coolants more appropriate to the fuel-cell environment." Staley expects that engineers will have to use stainless steel, some aluminum alloys, specially coated aluminum, and insulating polymers to do the job.
