Introduction
Conclusions
Storage
Every day, 20 million barrels of oil are consumed in the United States. Of these, 12 million go towards mobile power plants for transportation in trucks, buses, cars, trains, planes and ships.
For the hydrogen economy to take hold, there must be significant technological breakthroughs that allows hydrogen energy to be harnessed on a small scale by vehicles and appliances. As part of the FreedomCAR initiative, the federal government has established targets for hydrogen storage that will allow for its use in a fuel cell vehicle. [1] They include:
Current technologies may need to improve by a factor of two or more in order to meet some of these needs. The figure below compares the size of storage tanks necessary to power a car approximately 250 miles under current storage capabilities.
Relative size of a 4kg hydrogen storage tank using four methods of storage: magnesium hydrides, lanthanum hydrides, liquid cryogenic hydrogen, and high pressure storage.
Source: Schlapbach and Zuttel (2001).
The sections below detail the different approaches for storing hydrogen in a mobile setting.
High Pressure Storage
The most traditional method of hydrogen storage, high pressure tanks remain the most studied, and until revolutionary advances are made in some of the newer methods of hydrogen storage, high pressure storage also remains the most practical option for storage.
A key drawback of hydrogen gas is that it has an extremely low density. At ambient temperatures and pressures, hydrogen has an energy density of only 10.7kJ/L, compared to 31.6MJ/L for gasoline. Thus, in order to improve the energy density of hydrogen to useful levels, pressurization to as high as 5,000 or 10,000psi is necessary. In contrast, natural gas is generally not stored at pressures above 3,600psi. Thus, traditional tanks for fossil fuel storage will not be sufficient for hydrogen storage.
Particularly when the hydrogen needs to be stored in a vehicle, the material needed for the tank needs to be light and strong. Carbon composite such as wound carbon fibers, with a yield strength of 1,900MPa, offer the lightest and strongest material properties, but they are often quite expensive. Steel, which offers fairly high yield strength (690MPa) become subject to hydrogen embrittlement, which significantly increases the risk of burst failure. Titanium and aluminum are much lighter but cannot offer the strength of carbon fiber tanks. One precaution that must be taken is the installation of a liner to prevent hydrogen diffusion through the carbon tank.
Even at 10,000psi, however, the energy density of hydrogen gas is only 4.4MJ/L, which means that a hydrogen fuel tank will need to be several times larger than a conventional gas tank, once greater efficiencies of hydrogen fuel cells are factored in. At 4% hydrogen by mass, these tanks represent some of the most effective currently available techniques, but still fall short of the FreedomCAR standards.
Thus there is a great deal of research into materials that can provide hydrogen energy densities higher than 4.4MJ/L. These methods include cryogenic storage, storage in hydrides, and storage in nanostructures.
Cryogenic Storage
Source: http://www.llnl.gov/str/June03/Aceves.html
At a temperature of about -260ºC, hydrogen becomes a condensed liquid with a density of 70.8kg/m 3. The energy density increases correspondingly to 8.4MJ/L. An even greater increase in energy density can be obtained by creating “slush” hydrogen where the solid and liquid forms are in equilibrium.
There are two major drawbacks to using cryogenic hydrogen. One is the energy (approximately 1/3 of the energy available in the hydrogen itself) necessary to bring the hydrogen to such a cold temperature. The other is insulation. Any outside heat reaching the tank will cause boil-off of the hydrogen. Therefore extremely good insulation is necessary to prevent dangerous boil-off, which can cause explosions if there is no method for alleviating the pressure.
Some insulation materials that are used in cryogenic hydrogen storage are polyurethane and polyvinylchloride foams, glass fibers, aluminum foil, glass paper laminate, and silica powder. The best forms of insulation are also evacuated, meaning that there is no air present in the insulation layer to transmit heat by conduction. The reflectivity of the glass and aluminum foil also prevents heat absorption.
However, even with the best insulation, there will always be heat absorption into the tank and boil-off of hydrogen. Therefore, a system using cryogenic hydrogen must have a constant low-level usage of fuel, otherwise material is lost or dangerous pressure build-ups occur. This makes cryogenic storage impractical for applications where the tank must last more than a day or two without losses.
Hydride Storage
Certain light metals and transition metals form hydride compounds under high temperatures and pressures when exposed to hydrogen gas. Researchers hope to use these compounds as a basis for storing hydrogen at higher densities than in pressurized tanks. Some example compounds include LaNi5H6 and Mg2NiH4 which have hydrogen densities currently reported at 1-3.5% [2].
Temperature-Hydrogen content graph of lanthanum-nickel hydride material.
Source: Schlapbach and Zuttel (2001)
The graph on the left represents the hydrogen content (horizontal axis) at different pressures (vertical axis). The horizontal section of the graph represents the equilibrium temperature and pressure where the uptake and release of hydrogen occurs.
Of particular interest are a class of compounds called alanides, which contain aluminum and hydrogen. The decomposition of sodium alanate at high temperatures is given by
3NaAlH4 → Na3AlH6 + 2Al + H2
Na3AlH6 → 3NaH + Al + 3/2H2
These reactions can yield some of the highest weight percentages of hydrogen for any solid-state storage, demonstrated up to 5.5%. However, the temperatures required for decomposition are high for an on-board setting and manufacture of the materials is difficult. [3]
Nanosurface Storage
The possibility of storing hydrogen on the surface of nano-surfaces has raised a great deal of excitement although practical results thus far have been very limited. Rolled sheets of carbon, which form "nanotubes" of a few nanometers in width, can attract molecular hydrogen through van der Waals forces. While it is unclear whether the tubes can hold a second layer of hydrogen molecules, or the extent of the reversibility of the physisorption, some researchers have demonstrated up to 6% hydrogen storage by weight in carbon nanotubes. More common and duplicable results have been shown to be in the range of 1-3% by weight.
Source: Weidenkaff et al (2002) [4]
Methanol Storage
Unlike any of the above methods, methanol storage contains protons in the form of an alcohol molecule, which must be "reformed" to yield H2 for the fuel cell or for direct combustion. This reforming process, through steam reforming or a similar process, has the tremendous advantage of being able to store the fuel in an uncompressed, liquid form. However, there are significant drawbacks to reforming:
It was found that the fuel efficiencies of a simulated fuel cell vehicle running on a methane reformer had a fuel efficiency of only half that running on direct hydrogen. [5]
Sources:
[1]“Basic Research Challenges for Hydrogen Storage: Report of the Basic Energy Sciences Working Group on Hydrogen Production, Storage and Use.” U.S. Department of Energy, Office of Science. 15 May 2003. http://www.sc.doe.gov/bes/hydrogen.pdf
[2]Schlapbach, L. and Andreas Zuttel. "Hydrogen-Storage Materials for Mobile Applications." Nature 414: 353-358. 2001.
[3] Seayad, Abdul M. and David Antonelli. "Recent Advances in Hydrogen Storage in Metal-Containing Inorganic Nanostructures and Related Materials." Advanced Materials, 16, No. 9-10, 17 May 2004.
[4] Weidenkaff, A, S.G. Ebbinghaus, Ph. Mauron, A. Reller, Y. Zhang and A. Zuttel, "Metal nanoparticles for the production of carbon nanotube composite materials by decomposition of different carbon sources." Materials Science and Engineering C, 19, 119-123 (2002).
[5 Boettner, Daisie D., Gino Paganelli, Yann Gueznec, Giorgio Rizzoni and Michael J. Moran, "On-Board Reforming Effects on the Performance of Proton Exchange Membrane (PEM) Fuel Cell Vehicles." Journal of Energy Resources Technology, Vol. 124, No. 3, pp. 191–196, September 2002
