Hydrogen storage
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Hydrogen storage is the main technological problem of a viable hydrogen economy. Some attention has been given to the role of hydrogen to provide grid energy storage for unpredictable energy sources, like wind power, but most research into hydrogen storage is focused on storing hydrogen in a lightweight, compact manner for mobile applications.
Hydrocarbons are stored extensively at the point of use, be it in the gasoline tanks of automobiles or propane tanks hung on the side of barbecue grills. Hydrogen, in comparison, is quite difficult to store or transport with current technology. Hydrogen gas has good energy density per weight, but poor energy density per volume versus hydrocarbons, hence it requires a larger tank to store. A large hydrogen tank will be heavier than the small hydrocarbon tank used to store the same amount of energy, all other factors remaining equal. Increasing gas pressure would improve the energy density per volume, making for smaller, but not lighter container tanks (see pressure vessel). Compressing a gas will require energy to power the compressor. Higher compression will mean more energy lost to the compression step. Alternatively, higher volumetric energy density liquid hydrogen may be used (like the Space Shuttle). However liquid hydrogen is cryogenic and boils around 20.268 K (–252.882 °C or -423.188 °F). Hence, its liquefaction imposes a large energy loss, used to cool it down to that temperature. The tanks must also be well insulated to prevent boil off. Ice may form around the tank and help corrode it further if the insulation fails. Insulation for liquid hydrogen tanks is usually expensive and delicate. Assuming all of that is solvable, the density problem remains. Even liquid hydrogen has worse energy density per volume than hydrocarbon fuels such as gasoline by approximately a factor of four.
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[edit] Mobile storage targets
Targets are set by the FreedomCAR Partnership in January of 2002 between the United States Council for Automotive Research (USCAR) and DOE (Targets assume a 5-kg H2 storage system).
[edit] Ammonia storage
Ammonia (NH3) can be used to store hydrogen chemically and then release it in a catalytic reformer. Ammonia provides exceptionally high hydrogen storage densities as a liquid with mild pressurization and cryogenic constraints. It can also be stored as a liquid at room temperature and pressure when mixed with water. Ammonia is the second most commonly produced chemical in the world and a large infrastructure for making, transporting and distributing ammonia already exists. Ammonia can be reformed to produce hydrogen with no harmful waste, or can mix with existing fuels and burn efficiently. Pure ammonia burns poorly at the atmospheric pressures found in Natural Gas fired water heaters and stoves. Under compression in an automobile engine it is a suitable fuel for slightly modified gasoline engines. Ammonia is very energy expensive to make and the existing infrastructure would have to be greatly enlarged to handle replacing transportation energy needs. Ammonia is a toxic gas at normal temperature and pressure and has a potent odor. See: Ammonia production.
Technical University of Denmark scientists announced in September 2005 a method of storing hydrogen in the form of ammonia saturated into a salt tablet. They claim it will be an inexpensive and safe storage method. [1] New Scientist [2] state that Arizona State University is investigating using a Borohydride solution to store hydrogen, which is released when the solution flows over a catalyst made of ruthenium.
[edit] Metal hydrides
There are proposals to use metal hydrides as the carrier for hydrogen instead of pure hydrogen. Hydrides can be coerced, with varying degrees of ease, into releasing and absorbing hydrogen. Some are easy-to-fuel liquids at ambient temperature and pressure, others are solids which could be turned into pellets. Proposed hydrides for use in a hydrogen economy include simple hydrides of magnesium or transition metals and complex metal hydrides, typically containing sodium, lithium or calcium and aluminium or boron. These have good energy density per volume, although their energy density per weight is often worse than the leading hydrocarbon fuels and often require high temperatures to release their hydrogen content.
Solid hydride storage is a leading contender for automotive storage. A hydride tank is about three times larger and four times heavier than a gasoline tank holding the same energy. For a standard car, that's about 45 US gallons (0.17 m3) of space and 600 pounds (270 kg) versus 15 US gallons (0.057 m3) and 150 pounds (70 kg). A standard gasoline tank weighs a few dozen pounds (tens of kilograms) and is made of steel costing less than a dollar a pound ($2.20/kg). Lithium, the primary constituent by weight of a hydride storage vessel, currently costs over $40 a pound ($90/kg). Any hydride will need to be recycled or recharged with hydrogen, either on board the automobile or at a recycling plant. A metal-oxide fuel cell, (i.e. zinc-air fuel cell), may provide a better use for the added weight, than a hydrogen fuel cell with a metal hydride storage tank.
Often hydrides react by combusting rather violently upon exposure to moist air, and are quite toxic to humans in contact with the skin or eyes, hence cumbersome to handle (see borane, lithium aluminum hydride). This is why such fuels, despite being proposed and vigorously researched by the space launch industry, have never been used in any actual launch vehicle.
Few hydrides provide low reactivity (high safety) and high hydrogen storage densities (above 10% per weight). Leading candidates are sodium borohydride, lithium aluminum hydride and ammonia borane. Sodium borohydride and ammonia borane can be stored as a liquid when mixed with water, but must be stored at very high concentrations to produce desirable hydrogen densities, thus requiring complicated water recycling systems in a fuel cell. As a liquid, sodium borohydride provides the advantage of being able to react directly in a fuel cell, allowing the production of cheaper, more efficient and more powerful fuels cells that do not need platinum catalysts. Recycling sodium borohydride is energy expensive and would require recycling plants. More energy efficient means of recycling sodium borohydride are still experimental. Recycling ammonia borane by any means is still experimental.
Hydrogen produced for MH storage has to be of a high purity, contaminants alter the MH surface and prevent absorption, oxygen 10 ppm O2 maximum in H2, carbon monoxide hydro-carbons and water at very low levels.
[edit] Synthesized hydrocarbons
An alternative to hydrides is to use regular hydrocarbon fuels as the hydrogen carrier. Then a small hydrogen reformer would extract the hydrogen as needed by the fuel cell. However, these reformers are slow to react to changes in demand and add a large incremental cost to the vehicle powertrain.
Direct methanol fuel cells do not require a reformer, but provide a lower energy density compared to conventional fuel cells, although this could be counter balanced with the much better energy densities of ethanol and methanol over hydrogen. Alcohol fuel is a renewable resource.
Solid-oxide fuel cells can run on light hydrocarbons such as propane and methane without a reformer, or can run on higher hydrocarbons with only partial reforming, but the high temperature and slow startup time of these fuel cells makes them prohibitive for automobiles.
[edit] Research
[edit] Carbon nanotubes
More exotic hydrogen carriers based on nanotechnology are examened, such as carbon buckyballs and nanotubes, carbon nanotubes are able to store up to 8% weight hydrogen in small samples, research is at an early stage.
[edit] Phosphonium borate
In 2006 researchers of University of Windsor reported on reversible hydrogen storage in a non-metal phosphonium borate [3] [4] [5]:
The phosphino-borane on the left accepts one equivalent of hydrogen at one atmosphere and 25°C and expels it again by heating to 100°C. The storage efficiency is 0.25% still rather below the 6 to 9% required for practical use.
[edit] Polymer
Aug 4 2006 - A team of Korean researchers led bij Professor Lim Ji-sun of Seoul National University’s School of Physics found a new material with the hydrogen storage efficiency at 7.6 percent, the hydrogen can be stored in solid matter in normal temperatures and pressures by attaching a titanium atom to a polyacetylene. [[1]] [[2]]
[edit] Glass microspheres
Hollow glass microspheres can be utilized for controlled storage and release of hydrogen.
[edit] See also
[edit] External links
- United States Department of Energy Planned program activities for 2003-2010
- Hyweb (1996)
- Research into metal-organic framework or Nano Cages [[3]][[4]]
[edit] References
- ^ http://www.netpublikationer.dk/um/6567/html/chapter12.htm
- ^ New type of hydrogen fuel cell powers up. newscientist. Retrieved on 2006-09-16.
- ^ Reversible, Metal-Free Hydrogen Activation Gregory C. Welch, Ronan R. San Juan, Jason D. Masuda, Douglas W. Stephan Science (journal) 17 November 2006: Vol. 314. no. 5802, pp. 1124 - 1126 DOI:10.1126/science.1134230
- ^ H2 Activation, Reversibly Metal-free compound readily breaks and makes hydrogen Elizabeth Wilson Chemical & Engineering News November 20, 2006 Link
- ^ Mes stands for a mesityl substituent and C6F5 for a pentafluorophenyl group, see also tris(pentafluorophenyl)boron
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