Hydrogen storage

The hydrogen storage is part of the hydrogen economy. Conventional methods of storage and storage of hydrogen are:

  • Pressure gas storage ( storage in pressure vessels by compacting with compressors )
  • LPG storage ( storage in liquid form by cooling and compressing )

Alternative forms of the hydrogen storage is based on solids or liquids:

  • Metal hydride (stored as chemical bond between hydrogen and a metal or an alloy )
  • Adsorptive ( adsorptive storage of hydrogen in highly porous materials )
  • Graphite nanofiber memory ( GNF) can theoretically save 75 % of its own weight in hydrogen. Convenient storage amounts of 10% to 15% by weight have been reached.
  • For storage and transport of hydrogen are also in process development that temporarily bind the hydrogen in organic substances, liquid, pressure storable compounds are formed ( " Chemically bonded hydrogen ").

Problem

Due to its chemical and physical properties of hydrogen handling of the previously used energy sources differs.

  • Hydrogen forming upon exiting a flammable mixture in a proportion of 4-75 %. An explosive mixture ( oxyhydrogen ) forms hydrogen only at a level of 18%. Because hydrogen is the lightest of all elements, it evaporates in an open environment before it can form an explosive mixture, or it will burn in hot environments already at the concentration of 4%.
  • Hydrogen has a low molar mass and resulting low volumetric energy density (about 1/ 3 of natural gas, but more than twice the mass energy density ). This requires to store an equivalent amount of energy three times as large a tank or three times as high pressure as for natural gas.
  • Due to the small molecular size and a low adsorption of hydrogen diffuses relatively well through a variety of materials, so that a high quality of the tank shell must be guaranteed. High temperatures and high internal pressures of this process is promoted. Due to hydrogen embrittlement metallic tank shells are under additional load. In cases of plastic, this effect does not occur.
  • When hydrogen liquefaction occurs due to unavoidable losses insulation outgassing. Can not be used this resulting hydrogen gas, result in substantial losses. For example, the half-full of liquid hydrogen tank of the BMW Hydrogen7 emptied when not in use in 9 days.
  • Not only for the production of hydrogen, but also to store large amounts of energy are required ( about 12% compression, liquefaction about 20%). Therefore, the hydrogen storage is often uneconomical despite many advantages at present ( 2012).

Types of hydrogen storage

Compressed hydrogen storage

The problems of storage in pressure vessels are now considered solved. Through the use of new materials, the effective loss is greatly reduced by diffusion. Were still pressure tanks with 200 to 350 bar are used for the automotive sector by 2000, it is 2011 already 700 - and 800 -bar tanks with higher capacity. The complete hydrogen fueling system for a car weighs only 125 kg. The energy required for compression to 700 bar is about 12 % of the energy content of the hydrogen. The currently available on the commercial use of pressure tanks meet all safety requirements of vehicle manufacturers and are approved by TÜV. Pressure tanks up to 1200 bar are technically possible.

A special case of compressed hydrogen storage with very high storage is the storage in underground gas storage facilities similar to the memories in the natural gas network → See: hydrogen in pipelines

Liquid hydrogen storage

For large quantities of LPG storage can be used. To the hydrogen is liquefied ( LH2 ), and under ambient pressure at low temperature (boiling point -252.8 ° C, 20.4 K) stored. The energy required for the liquefaction of about 20 % of the energy content of the hydrogen. The pressure is then used for the design of the tank no problem, a great effort has to be in the thermal insulation of the tank and lines. Advantageous is the lower reactivity at low temperatures and the higher density by a factor of 800 of liquid hydrogen in comparison with gaseous hydrogen at atmospheric pressure. Nevertheless, requires liquid hydrogen per unit weight much space. He has 71 kg / m³ only a slightly higher density than kleinporig polystyrene foam ( in a 20- liter bucket fit only 1.42 kg of liquid hydrogen ). Disadvantages are fundamentally dictated evaporative losses in non-acceptance of the resulting hydrogen gas. Other measures ( boil off management) can be further minimized the losses due to evaporation, in stationary applications, for example, by coupling with a combined heat and power (CHP).

For use in automobiles tank robots have been developed, which assume the coupling and refueling. The energy required for liquefaction is about 20 % of the energy content of hydrogen (TU Dresden ), but this falls to only once, later transferring consumes relatively little energy, such as in the automotive sector from the factory to tanker trucks to gas stations and vehicles with liquid hydrogen.

Metal hydride

Another possibility for the pressure reduction of the molecular hydrogen is the solution in the other memory means. Because of its largely electrically and magnetically neutral properties it does not use a liquid solvent, but solid storage materials such as metal hydrides. The hydrogen is stored in the gaps of the ( cold ) the metal grid and is discharged by heating of the memory again. One cubic meter of metal contains more hydrogen atoms than a cubic meter of liquefied hydrogen. In a metal hydride can be five times more electrical energy can be stored as in a lead-acid battery of the same weight. However, they proved to be so expensive and difficult that they are only used in submarines, where both factors play no role. Critical to the selection of the materials are the absorption and desorption temperature and pressure at which hydrogen is stored and released again, and the high weight of the tank.

Researchers from the Université Catholique de Louvain in Belgium and the University of Aarhus in Denmark in 2011 presented a new highly porous form of magnesium borohydride before, the hydrogen can be chemically bound and physically adsorbed store. Magnesium borohydride (Mg (BH4 ) 2) are hydrogen at relatively low temperatures and stores a high proportion by weight of hydrogen (about 15 %).

Metal Organic Framework

Metal-organic frameworks (English metal- organic frameworks, MOFs ) are porous materials with well- ordered crystalline structure. They consist of complexes with transition metals (usually Cu, Zn, Ni or Co ) as a " node ", and organic molecules (ligands) as the compound ( "linker" ) between the nodes. By using appropriate node and linker as well as by impregnation with different host species, the MOF can be optimized for the hydrogen storage. The MOF form an active field of research and are considered one of the most promising technologies for hydrogen storage.

Chemically bound hydrogen

In addition to the possibilities of storing molecular hydrogen there is a whole range of options of transport and storage in chemically bound form. These opportunities do not count for hydrogen storage in the narrow sense, which refers to the technical process of storage of molecular hydrogen. In the context of a hydrogen economy, however, this possibility is added in, as there is storage and removal of hydrogen from the subject of the productive process.

Since most are organic substances in the hydrogen carriers, they are also called "Liquid Organic Hydrogen Carriers " ( LOHC, liquid organic hydrogen carriers ).

Methanol

Suitable as hydrogen carriers are especially alcohols, such as methanol. By means of reforming it can produce a hydrogen-rich gas mixture.

Liquid Organic Hydrogen Carriers ( LOHC )

In Liquid organic hydrogen carriers ( LOHC ), hydrogen is chemically bound to these by chemical reaction with an unsaturated compound ( hydrogenation). To release the substances resulting from storing saturated compound is dehydrated again, wherein the unsaturated compound is re-formed and hydrogen gas is produced. A large number of materials for this purpose is in principle in question. However, only aromatic compounds are suitable for industrial application.

Toluene

The oldest explored LOHC system is based on the hydrogenation of toluene to methylcyclohexane ( or the corresponding reverse reaction ). This system has been shown in a demonstration plant. Due partly to unfavorable properties but increasingly other substances have been studied for several years.

N-ethylcarbazole

N- ethylcarbazole is regarded as the most promising candidate among the hydratable organic substances. For the recovery of hydrogen for operating a hydrogen combustion engine or a fuel cell is the relatively low temperature required for the release of advantage. The " discharged " carbazole can be exchanged with hydrogen " charged " Perhydro -N -ethylcarbazole (also Perhydro - carbazole ) at a gas station again; the current refueling infrastructure could be maintained with minor changes. However, the method is currently (2011) still in development stage.

Dibenzyltoluene

In the current research, the reversible hydrogenation of dibenzyltoluene is examined as a potential LOHC strengthened.

Use

In the methods for technical storage of hydrogen in elemental form pressure vessels are required, what is often a metallic outer shell is used. This also applies to LPG storage, and the metal hydride having a temperature-dependent pressure. For the high-pressure storage at 700 bar and carbon fiber reinforced plastics are used to hold the weight of the tank low.

For large quantities in stationary systems are currently in use LPG storage. For small amounts of compressed memory can be used up to 700 bar. Metal hydride are used there where the memory weight does not matter, as on ships. For vehicles and aircraft pressure tanks are due to the low weight today exclusively used:

Toyota uses it in its fuel cell vehicle FCHV -adv and thus a range of 830 km. The vehicle is already in commercial use and can be leased.

Volkswagen is building a a 700 - bar hydrogen tank in the Tiguan HyMotions, Mercedes in the A-Class F -Cell " plus" and Opel in HydroGen4.

For buses now also pressure tanks are used, such as in the Citaro Fuel Cell Hybrid from Mercedes.

Companies that are involved in the research and manufacture of hydrogen storage tanks, the Linde AG, for example, in Germany, in Norway and Iceland StatoilHydro and the United States Quantum Fuel Technologies Worldwide.

Risk of accident

The technique used industrially today considered the Hochentzündlichkeit of hydrogen as well as its ability to form explosive gas. Lines and tanks are designed accordingly, so that in daily use are no greater risk than, for example, by the use of gasoline.

However, the risks are still partially unknown due to the currently only limited application. Thus, oxygen / hydrogen mixtures may be formed with a share of less than 10.5 percent by volume hydrogen, which are heavier than air and will sink to the bottom. The segregation is not instantaneous, so that up to below the 4- volume percent limit, the ignition remains. When dealing with hydrogen safety and ventilation systems must consider this abnormal behavior.

Hydrogen vehicles with pressure tanks can be easily parked in car parks and underground garages. There is no statutory provision which restricts the. Vehicles with liquid hydrogen storage tanks must not be parked in an enclosed space because of the inevitable outgassing.

Energy densities compared

Based on the mass ( in kWh / kg):

  • Hydrogen: 33.3
  • Hydrogen storage with Perhydro -N -ethylcarbazole: 1.9
  • Natural gas: 13.9
  • Gasoline: 11.1 to 11.6 ( 40.1 to 41.8 MJ / kg) ( Note 1 )
  • Diesel: 11.8 to 11.9 ( 42.8 - 43.1 MJ / kg) ( Note 1 )
  • Methanol: 6.2
  • LOHC (N- ethylcarbazole ): 1.93
  • Li -ion Battery: 0.2 ( approximately, depending on the type )

In the volume -related ( in kWh / l):

  • Hydrogen gas (atmospheric pressure): 0.003
  • Hydrogen gas ( 20 MPa / 200 bar): 0.53
  • Hydrogen gas ( 70 MPa / 700 bar): 1,855
  • Hydrogen storage with Perhydro -N -ethylcarbazole: 2.0
  • Hydrogen (liquid, -253 ° C): 2.36
  • Natural gas ( 20 MPa): 2.58
  • Gasoline: 8.2 to 8.6 ( Note 2 )
  • Diesel: 9.7 ( Note 2 )
  • LOHC (N- ethylcarbazole ): 1.89
  • Li -ion battery: 0.25 to 0.675
The Linde Group Digital Object Identifier
814392
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