The use of liquid hydrogen to power fuel cells has recently been demonstrated by Ballard and others. In this case, the cold hydrogen is vaporized before entering the fuel cell. The use of liquid hydrogen in heavy vehicles is not without challenges, however.
In terms of energy density, liquid hydrogen enables a higher volumetric energy density of 8.5 MJ/L, almost four times of that of hydrogen gas at 350 bar (2.2 MJ/L). Therefore, with the same travel distance, the required liquid hydrogen occupies much less space, (omitting all the additional insulation around the storage vessel) than the pressurized gas. Liquid hydrogen can also be stored under near atmosphere pressure, which is safer to some respect than high pressure hydrogen.
However, hydrogen liquefaction is very energy consuming. About 40% of the energy is wasted through the liquefaction process. Typically, 12 kWh of energy is required to produce one kilogram of liquid hydrogen. Another problem associated with the use of liquid hydrogen is its boil-off. The later must be allowed to occur for the hydrogen “vapor” to escape, so the pressure inside the cryogenic storage vessel does not exceed the design pressure.
Very well engineered liquid hydrogen vessels can have a boil off of 0.1% per day, but this incurs additional cost. Typical vessels without advanced insulation will have a boil-off of 1-5% per day. To avoid boil-off hydrogen accumulation, a good ventilation is thus required around the storage space and as such liquid hydrogen is not the best long term storage solution for hydrogen.
An additional challenge is around the refilling rate of liquid hydrogen. Based on its long-standing experience of handling liquid hydrogen for its space program, NASA has developed a range of solutions. With NASA’s technology, 340,000 gallons (1,287m3) of liquid hydrogen can be transferred in 90 minutes, but three hours are needed for pre-cooling. Compared with gas compression, this is less of an attractive option. Highly efficient pumps for liquid hydrogen transfer for refueling stations, trucks and train are yet to be developed.
Hyundai Rotem, in collaboration with Korea Railroad Research Institute, is currently developing a liquid hydrogen-based locomotive. This project is aiming for a 1,000km distance or more at a top speed of 150 km/hour on a single charge, with the target of enabling 1.6 times the range of a 700-bar hydrogen train.
Today all the hydrogen trains developed are DMU replacements. DMUs typically have a diesel engine under the floor of each carriage which is about the same size as a truck engine – around 300 kW. With this power, compressed hydrogen can give a range of up to 800 km, but these trains are not for high-speed service. A higher speed train might need four times the power, and much more fuel. This is beyond what can be achieved with compressed hydrogen and better solutions are needed.
An alternative is to store hydrogen as a solid and we are developing solutions for heavy vehicles. In this case, the hydrogen atoms are held tightly within a metallic alloy. This enables hydrogen volumetric storage densities that are twice that of liquid hydrogen. An additional major advantage of solid hydrogen is the unparalleled levels of safety as compared to compressed or liquid hydrogen. With hydrogen stored as a solid, catastrophic release of hydrogen is unlikely and refiling is an easy operation.
About the authors
Francois Aguey-Zinsou is a professor of chemistry at The University of Sydney and president and CTO of H2potential – Australia’s first scientist-led company completely focused on Hydrogen; and Frank Szanto is design authority at rolling stock manufacturer, Downer Rail and Transit Systems