Low Liquid Hydrogen Energy Density Presents Gas Tank Challenges

Lawrence Livermore National Laboratory has claimed a world record for hydrogen-fueled car endurance. Using a modified Toyota Pruis, researchers drove 1,050 kilometers (about 653 miles) on 150 liters (about 40 gallons) of liquid hydrogen. That’s 105 km/kilogram on a per-mass basis or 7 km/liter (16.3 miles/gallon) on a per-volume basis. This feat is reported in “H2 Going the Distance” from ASME’s Mechanical Engineering Magazine and is detailed in “Setting a World Driving Record With Hydrogen” from Science and Technology Review (S&TR).

Not surprisingly, on a per-volume basis, the Livermore numbers come out low compared to gasoline-fired automobiles. According to fueleconomy.gov, the 2008 Prius gets over 45 miles/gallon of gasoline on the highway. The number disparity becomes clearer when comparing the energy density of gasoline (922 BTU/ft³) to liquid hydrogen (270 BTU/ft³). To put the Lawrence Livermore accomplishment in perspective, if their Prius were running on a fuel with the same energy volume density as gasoline, it would have gone over 3.4 times farther, topping 55.6 miles per gallon. So, compared against the 45 mpg rating of an assembly line Prius, the Livermore hybrid is an overachiever.

Despite hydrogen being among the most energetic fuels on a per-mass basis, liquid hydrogen contains only about 29% of the energy on a per-volume basis as gasoline. So, while hydrogen fuel weighs less than tank gasoline with the same total energy, it takes a much larger volume to store. Unfortunately volume is at a premium in today’s small, efficiency-focused automobiles.

Another major liquid hydrogen drawback is the energy required to liquefy it (~ 30 percent of the energy content of the hydrogen molecule, according to S&TR). Moreover, to maintain its liquid state, hydrogen must be vented to remove latent heat and keep the tank from an overpressure explosion. So, liquid hydrogen vehicles “burn” fuel even when sitting parked and off in a driveway. While insulation can be added surrounding the fuel tank to reduce fuel bleed off, insulation eats up precious volume.

Despite these challenges, the Livermore team has made some impressive advances in liquid hydrogen fuel tank technology. Their pressure vessel is 47 inches (120 cm) long and 23 inches (58 cm) in diameter. It is smaller than previous tanks and yet stores more hydrogen due to its design: a carbon-fiber-coated aluminum vessel holds pressure and is surrounded by a vacuum space filled with reflective plastic.

Hydrogen storage technology still requires a long evolution to become commercially viable for automobiles, but the Lawrence Livermore team has provided a glimpse at a possible hydrogen-fired transportation future.