Five kg of liquid hydrogen would give a high-efficiency hydrogen fuel cell or internal combustion engine vehicle a 600-km driving range, according to Salvador Aceves, an ASME member and associate program leader for transportation at Lawrence Livermore National Laboratory in Livermore, Calif. Aceves is a past chairman of ASME's Advanced Energy Systems division.
However, drawbacks of cryogenic storage are the high electrical consumption required to liquefy the hydrogen, and the evaporative losses that can occur when the low-pressure tanks are filled and when the car is parked."It would take about 65 kilowatt-hours of electricity to liquefy the 5-kg target for vehicular hydrogen," said Aceves
The loss of cryogenic hydrogen by evaporation is caused by heat transfer between the tank and environment.
"If the car was driven 5 km or more per day, there was virtually no evaporative loss," said Aceves
An insulated cylinder, designed to contain hydrogen chilled to 20 K, has to hold up in a bonfire at Authorized Testing Inc.
before the tank is deemed worthy of testing on automobiles.
"These tanks would require 16 percent less energy when running on compressed fuel than when using cryogenic hydrogen, and would experience less evaporative losses when charged with subzero (20 K) hydrogen by virtue of their insulation," noted Aceves
colleagues built openings in the outer jackets for thermocouples, strain gauges, and a capacitive level sensor to measure pressure, temperature, and level within the tanks.They also equipped the tanks with safety devices to prevent catastrophic failure in case hydrogen leaked into the vacuum.These were relief valves that would open if the pressure limits were exceeded, and rupture discs if the relief valves failed.
The researchers then subjected the tanks to a variety of Department of Transportation
and Society of Automotive Engineers tests at Lawrence Livermore's High Pressure Laboratory.
For example, the outer jacket did not leak after being subjected to the stresses of 1,000 vacuum cycles.
"We stayed in a safety bunker while the tanks were tested in the open," said Aceves
The laboratory subjected the tanks to a variety of certification tests to meet the standards of the U.S. Department of Transportation
.These included cycling the tanks, at ambient temperatures, 10,000 times from less than 10 percent of the service pressure to full service pressure.
In a series of environmental cycling tests, the tanks were filled with gaseous hydrogen to determine their performance in extreme climates.First, the tanks were cycled 5,000 times from zero to full service pressure, with an internal tank temperature of 140°F and external ambient air temperatures in 95 percent humidity.Then, the tanks were cycled 5,000 times from zero to service pressure at -60°F tank temperature.
"After we pass the SAE tests, we want to install the insulated cryogenic tanks on vehicles for field testing," Aceves
"One, a Ford F 250, will carry liquefied natural gas, and the other, a Ford Ranger, will hold liquid hydrogen," Aceves
The project is funded by the Department of Energy's
hydrogen program and the South Coast Air Quality Management District.
Hydrides on a Diet
Compressed gas tanks store hydrogen as a gas, and their cryogenic counterparts store it as a liquid.A less familiar method of storing hydrogen is as a solid in metal hydrides, alloys of rare earth, transition metal, and magnesium.These granulated materials absorb hydrogen.Because the hydrogen is chemically bonded to the alloys, heat is required to release it.