Plenty of automakers are working hard (alternative spelling: $$$$) on compressed-hydrogen fuel-cell mobility. On a contrarian path, though, is 's continued commitment to liquid hydrogen feeding conventional internal combustion.
Are these guy being bumptious Bavarians or what?
Not at all. They're just on a path to the hydrogen highway that's BMW-logical.
I recently had the opportunity to talk with BMW engineers about this technology and also to drive their , a dual-fuel gasoline/hydrogen . Here are some views on the matter.
Why internal combustion? Why H2?
"Top-down philosophy" is what BMW calls its starting with the largest and most luxurious car in the lineup.
Early on (see "Technology Update: Is There an H2 APU in Your Future?" R&T, November 1999), BMW engineers felt that fuel-cell propulsion, viewed as a complete system, was not efficient for cars of its character. However, being carbon-free, hydrogen fueling of an internal-combustion engine made excellent environmental sense, particularly in its elimination of CO and CO2. The only emissions of an H2-fueled I.C. engine are NOx - and a barely measurable amount of unburned hydrocarbons from traces of motor oil sneaking through the process.
Why liquid H2?
There are three ways to store hydrogen onboard a car: compressed, at pressures as high as 10,000 psi.; liquefied, at temperatures lower than -423 degrees Fahrenheit; or as a hydride, in tanks that are thus far considered overly heavy.
BMW's choice of liquefied hydrogen, LH2, for short, is based on energy density. For a given tank size, the super-cooled LH2 offers more than 75-percent-greater energy than even its super-compressed 10,000-psi counterpart. Driving range profits accordingly.
What's bad about LH2?
First, there's the considerable cost of liquefaction, an energy-intensive process of iterative compressing and cooling. This cost, BMW says, is mitigated by the European infrastructure, with trucks used to deliver hydrogen already in liquid form. By contrast, U.S. specialists say that our country has an extensive infrastructure of pipelines delivering less costly compressed H2. That is, optimal energy pathways in Europe don't necessarily coincide with ours.
Second, and fundamentally complex, is the challenge of keeping LH2 liquefied. BMW uses a stainless-steel tank-within-a-tank with layers of vacuum super-insulation between the two. The insulation of aluminum reflective film and glass-fiber is equivalent to 56 ft. of Styrofoam. It's said that a snowman residing within the tank would take 13 years to melt!
But There is Boil-off
A cushion of gaseous hydrogen hovers over the LH2 in the tank. And, as the hydrogen inevitably warms, the pressure of this gas increases. With ordinary operation and through a temperature/pressure controller, there's a continuous stream of gaseous hydrogen fed to the engine, and this inherent boil-off stays within bounds. If, however, the car sits idle, say for a day, then the boil-off reaches a pressure that needs to be relieved. In today's Hydrogen 7, this is handled by mixing the excess gaseous hydrogen with air, feeding it to a separate catalytic converter and oxidizing the product into water.
This certainly seems a profligate use of hard-earned LH2, but BMW engineers have a logical alternative for future iterations: The boil-off can be directed to an onboard fuel cell, a small one dedicated to charging an auxiliary power unit. This APU would handle "hotel loads," the comfort and convenience features, steering assist and other electrical requirements.
Oh, the Humanity
As a fail-safe, should there be a breach of system integrity, the Hydrogen 7 has two redundant safety valves, one in the roof, the other, beneath the car, releasing the hydrogen into the atmosphere in a controlled and decentralized process. And, before you start thinking Hindenburg, be aware that hydrogen is inherently safer than gasoline in this regard: It disperses with alacrity; by contrast, spilled gasoline forms a puddle directly above which resides a perfectly stoichiometric - and explosive - mixture of gasoline vapor and air.
Driving the Dual-fuel Hydrogen 7
Driving the Hydrogen 7 was almost a non-event. A steering-wheel switch toggles between gasoline and hydrogen operation. With the latter, there's a slightly more metallic sound of combustion, traceable to hydrogen's quicker flame front. Also, engineers perform some tricks of fuel/air mixture to minimize NOx under light load, and this would occasionally - only occasionally, mind - manifest itself as a slight sag in torque. Otherwise, it took real sensitivity to recognize which fuel was being used.
Plenty of other drivers are going to share this experience during the year. BMW is putting 100 Hydrogen 7s into service in 2007, 50 in the U.S., the rest distributed among Tokyo, Beijing and six European cities. Obviously an important aspect of this is parallel development of infrastructure, with refueling stations cropping up here in Southern California, Washington, D.C., and elsewhere around the world.
I hope we get a Hydrogen 7 here in Newport Beach; I'm confident there's a place to refuel it.