the tank volume is 107 gallons, and its weight is 225 lbs. Increasing tank pressure leads to greater
safety problems and increased energy loss for compressing the hydrogen; at 6,000 psi, the energy
cost of compression is approximately 10 to 15 percent of the fuel energy. Realistically, pressures
over 6,000 psi are not considered safe,^138 and tank capacity over 30 or 40 gallons would seriously
compromise the room available in a car. Hence, compressed hydrogen gas storage in a car would
have the energy equivalent of only about 3.0 gallons of gasoline for a 6,000 psi tank of a size that
could be accommodated without seriously impairing trunk room.
Liquid storage is possible because hydrogen liquefies at -253oC, but a highly insulated--and,
thus, heavy and expensive--cryogenic storage tank is required. A state-of-the-art tank designed by
BMW accommodates 25 gallons of liquid hydrogen.^139 It is insulated by 70 layers of aluminum
foil with interlayered fiberglass matting. The weight of the tanks when fill is about 130 lbs, and
hydrogen is held at an overpressure of up to 75 psi. The total system volume is about five times
that of an energy equivalent gasoline tank (gasoline has 3.8 times the energy content of liquid
hydrogen per unit volume), and the weight is twice that of the gasoline tank. Heat leakage results
in an evaporation loss of 1 to 2 percent of the tank volume per day. Although the container size
for a 120-liter tank would fit into the trunk of most cars, there are safety concerns regarding the
venting of hydrogen lost to evaporation, and crash-safety-related concerns.^140 There is also an
important sacrifice in overall energy efficiency, because the energy required to liquefy hydrogen is
equal to about one-third the energy content of hydrogen.
Metal hydride storage utilizes a process by which metals such as titanium and vanadium react
exothermally (that is, the reaction generates heat) with hydrogen to form a hydride. During
refueling, heat must be removed when hydrogen is reacting with the metals in the tank; when the
vehicle powerplant requires fuel heat must be supplied to release the hydrogen from the tank. For
these reasons, the entire tank must be designed as a heat exchanger, with cooling and heating
water flow ducts. The hydrogen used must also be very pure, as gaseous impurities impair the
chemical reactions in the metal hydride tank Moreover, the weight of metal required to store
hydrogen is very high: to store the energy equivalent of 10 gallons of gasoline, the tank would
weigh more than 500 lbs.^141 The main advantages of the system are safety and low hydrogen
pressure. The overall process is so cumbersome, however, that it seems an unlikely prospect for
light duty vehicles, although such systems can be used in buses and trucks.
Adsorption in carbon sieves was thought to be a promising idea to increase the capacity of
compressed gas cylinders, although there is a weight penalty. However, most recent work on
carbon sieves have concluded that the capacity increase is significant only at pressures in the
1,000 to 1,500 psi range; at 3,000 psi or higher pressure, carbon sieves appear to offer no benefit
over compressed gas cylinders. 142 Because a pressure of 5,000 psi or more is desirable, it does not
appear that this technology is of use for on-board storage.(^138) J. Zieger, "Hypasse - Hydrogen Powered Automobiles,” paper presented at the 10th World Hydrogen Conference, June 1994.
(^139) D. Riester and W. Strobl, “Current Development and Outlook for a Hydrogen Fueled Car,” paper presented at the 8th World Hydrogen Energy
Conference, June 1992. 140
141 In the event of a spill, contact with the liquid hydrogen (during the brief period before it would evaporate) would be extremely dangerous.
Daimler-Benz, see footnote 132.
(^142) J. Bentley et al., “Development of Advanced Hydrogen Storage Systems for Transportation Application,” paper presented at the Automotive
Technology Development Contractors Coordination Meeting U.S. Department of Energy, October 1994.