|BMW’s liquid-hydrogen fueled V12 from the H2R. Click to enlarge.|
Of the major automakers working with hydrogen as a fuel, BMW holds a unique position—not only has it settled on hydrogen-fueled combustion engines as the interim step to a longer-term fuel-cell future (a stance Ford also takes), the German automaker focuses on liquid hydrogen fuel, rather than gaseous.
Liquid hydrogen offers a higher volumetric density (0.070 kg/liter) than gaseous (0.030 kg/liter @ 10,000 psi)—an important consideration when faced with a limited storage volume in a vehicle.
There are, however, several critical practical issues with using liquid hydrogen in a vehicle, including:
Hydrogen boil-off. Boil-off impacts cost, efficiency and vehicle range, not to mention safety consideration for vehicles parked in confined spaces. Insulation is needed to keep the temperature ultra-low, and this reduces gravimetric and volumetric capacity.
A high energy requirement for liquefaction of the gas—around 30% of the heating value of the hydrogen.
|A liquid hydrogen storage tank from Linde. Click to enlarge.|
BMW is bringing to NHA a liquid hydrogen storage system that is double-walled. Between the two walls, there are approximately 70 layers of aluminum-coated synthetic foil under high vacuum. According to BMW, a 17-meter (50-foot) layer of polystyrene would be necessary to achieve the same insulation effect.
In September 2004, BMW demonstrated the power potential of its approach by setting nine international speed records for hydrogen-driven vehicles with the BMW H2R research vehicle. This unique prototype is powered by a 6.0-liter V-2 engine that develops approximately 285 hp; among the records it set was 186.11 mph for the flying-start kilometer. (Earlier post.)
On a slightly more practical level, the company is developing a limited-production 7 series bi-fuel model capable of burning either gasoline or liquid hydrogen. At the National Hydrogen Association’s annual conference in Long Beach this week, BMW will display its current work on a bi-fuel hydrogen-gasoline engine as well as the in-vehicle liquid hydrogen storage system.
During the development of these bi-fuel engines, BMW has hit output levels of more than 170kW (230hp)—with more potentially available, according to the company.