Storage Tank

Liquid hydrogen has its advantages, however. It is relatively easy and safe to store and transport in compact, lightweight, low-pressure containers. According to project engineer Gene Berry, a hydrogen car designed to get the equivalent of 60 miles per gallon of gasoline uses 100 liters of liquid hydrogen to travel 420 miles, compared to 240 miles on high-pressure (70 megapascals) compressed hydrogen and 140 miles on moderate-pressure (35 megapascals) compressed hydrogen. Liquid hydrogen is thus ideal for long highway trips. Its advantages help explain why it has become the favorite of BMW’s 20-year hydrogen vehicle development program.

The rub is that liquid hydrogen is extremely sensitive to heat, expanding significantly when warmed only a few degrees. When a hydrogen-fueled car is not being driven, heat exchange between the storage tank and the outside environment warms the fuel, causing it to evaporate. As the hydrogen fuel expands, the pressure in the tank increases. The hydrogen must therefore be vented to the atmosphere to prevent the dangerous buildup of excessive pressure.

Cryogenic hydrogen fuel in a standard liquid hydrogen tank typically warms up enough in 3 to 4 days after refueling to require venting. The tank team used computer analysis to determine that when liquid hydrogen is stored in a conventional low-pressure (0.5-megapascal) tank in a car that gets the equivalent of 80 miles per gallon, it would begin to vent and lose fuel if driven (on average) less than about 15 miles daily. Evaporation losses increase sharply if the car is driven less. In a parked car, an entire tank of liquid hydrogen fuel will completely evaporate in just 3 weeks. Eliminating this evaporation is a primary goal of Livermore’s tank design.

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