H2GO: Automakers gear up for clean fuelH2GO: Automakers gear up for clean fuel

The 14^th World Hydrogen Energy Conference here in June showcased hydrogen-based power technologies, especially the latest developments in automotive applications on the verge of production. Highlighting the latter were BMW's CleanEnergy World Tour, which displayed the dual-fuel liquid-hydrogen/ gasoline powered 7 Series, and Ford's Focus Fuel Cell Vehicle (FCV). Hydrogen cars produce theoretically only water for emissions, eliminating pollution and global warming concerns only if their fuel is produced from energy resources, such as wind, hydro, solar, or nuclear.

BMW hopes to have its car in production in two to four years, and certainly within the seven-year design lifetime of the current 7 Series introduced in the U.S. last year. Ford will begin low-volume production of the gaseous hydrogen fuel-cell/battery hybrid Focus in 2004. Both will depend on a hydrogen infrastructure growing sufficiently (and, ideally, eventually based on renewable energy) to extend beyond limited-range and central-fueling fleet operations.

Ultimate clean machine. BMW's 745h (for hydrogen) is powered by a dual fuel V8 internal combustion engine (ICE). Dual-fuel operation was selected for the initial production cars because hydrogen fueling stations will not be common at the time of the introduction, says Christoph Huss, BMW senior VP for Science and Traffic Policy. The car has a roughly 170-liter liquid hydrogen (LH₂) tank, which holds about 9.5 kg of fuel (the energy equivalent of 50 liters of gasoline because of LH₂'s 3.7 times lower energy density). Its range on hydrogen is 180 miles. Gasoline operation can run the car an additional 400 miles.

Now before you get out a calculator to see that for hydrogen this is a gas mileage equivalent of less than 14 mpg, there are some factors to keep in mind. With essentially two fuel supply systems, the dual-fuel engine is optimized for gasoline operation. Huss notes hydrogen has an equivalent octane rating of more than 110. Thus, if the engine were designed for a higher compression ratio to better use hydrogen, combustion efficiency would go up and the 30% loss in horsepower and torque from being able to burn two fuels would be recovered. Once hydrogen fueling stations become more prevalent, BMW plans to make this design change. Then the catalytic converter could be eliminated and reduction of nitrogen oxide emissions further optimized.

BMW chose a hydrogen ICE for propulsion, as opposed to a fuel-cell powerplant, to optimize power, according to Franz-Josef Wetzel, head of Future Powertrain Technology. "Each system should do what it can do best," he emphasizes. Thus the strategy to use the ICE to deliver power where, he says, it is most efficient at levels more than 100 kW (134 hp), while a fuel cell is efficient in the single kW range. In the hydrogen mode, the 4.4-liter engine produces 181 hp-a level only possible with an ICE and offering the prospect of performance for which BMW is noted.

In the 745h, a 5-kW UTC (United Technologies Corp.) Fuel Cells (South Windsor, CT) unit replaces a battery. Wetzel says electricity generation with the fuel cell is upwards of 50% efficient, more than double that of generation by an engine driven alternator, which in turn absorbs about 8% of ICE power. The fuel cell supplies three times the power of an alternator for the car's 42V systems.

BMW engineers chose liquid rather than gaseous hydrogen to pack more energy in a given volume tank. A double-walled vacuum-jacket (at 10^-9 bar) tank built by Magna Steyr (Graz, Austria) holds the cryogenic fuel at -253C. In addition, within the tank walls are 100 layers of metallized Mylar that function similar to multiple Space Blankets, producing an effective thickness of 100 ft of styrofoam. Pressure in the tank is a maximum of 5-7 bar. Such low pressures offer the prospect of non-cylindrical free-form tanks to better conform to the volume available and maximize the quantity of fuel. Magna Steyr's Assistant Executive VP for R&D, Jorg Buchholz, notes the European Integrated Hydrogen Program is looking to rewrite regulations to permit low-pressure storage in free-form tanks.

For handling LH₂ and filling the 745h tank, BMW and Reis Robotics (Obernburg, Germany) engineers have developed a robotic refueling system now in operation at a station servicing a shuttle-car test fleet at the Munich airport. An infrared-sensing-based system locates the filler door and fitting, then couples the refueling hose. But a robot system isn't necessary (as evidenced at the station established for testing in Oxnard, CA), says Huss, rather just provisions for a driver to mate the fitting without skin being exposed to supercold temperatures.

Design duel. Ford engineers elected to use gaseous storage for the Focus FCV. The arguments for this are given by Kevin Casey, VP of Stuart Energy Systems (Mississauga, Ontario, Canada), a maker of gaseous H₂ fueling stations. "Liquefying hydrogen is in-efficient and expensive. We don't have to drill down into the arctic but into the infrastructure to produce hydrogen on-site economically with off-peak power," he emphasizes. But vehicle gas storage requires robust cylindrical-shaped tanks to contain up to several-hundred-bar-pressure hydrogen in order to provide acceptable range. BMW's Huss says hydrogen won't be produced at each fueling site since it is more efficient to ship LH₂. On-site liquid supplies can be vaporized to provide fuel for gaseous-based vehicles, he adds. While not agreeing with the automaker, Casey adds, "It's good that BMW is spreading the word on hydrogen."

In the future, to further leverage hydrogen's advantages in an ICE, Huss sees direct injection of liquid hydrogen (rather than using the current gaseous inlet mixture) being possible by the end of the decade. This would boost combustion efficiency by increasing the change in temperature (DT) in the cylinders, similar to the effect of today's intake intercoolers. But he cautions before this can be realized, "High pressure pumps and valves for liquid hydrogen use have to be developed for low temperature operation." Other future developments could see using a slurry of solid and liquid hydrogen to increase energy density carried in the storage tank, use of more aluminum in the engine to reduce weight, and perhaps even a diesel-cycle engine to increase mileage.