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Hydrogen can be transported in pipelines similarly to natural gas. The tubes, with a typical diameter of 25-30 cm, are built using conventional pipe steel and operate at pressure of 10-20 bar. The volumetric energy density of hydrogen gas is 36% of the volumetric energy density of natural gas at the same pressure. In order to transport the same amount of energy, the hydrogen flux should be 2.8 times larger than the flux of natural gas. However, the viscosity of hydrogen (8.92x10^-6 Pa-s)is significantly smaller than that of natural gas (11.2x10^-6 Pa-s).
The minimum power P required to pump a gas through a pipe is given by
Where is the length of pipe, is the velocity and is the dynamic viscosity of gas. The transmission power per energy unit is therefore 2.2 times larger for hydrogen than natural gas.
Table. Operating Hydrogen Pipeline in Different Areas
Ruhr area in Germany
High Pressure Gas cylinders
The most common storage systems are high pressure gas cylinders with a maximum pressure of 20 MPa. New lightweight composite cylinders have been developed which are able to withstand pressure up to 80 MPa and so the hydrogen can reach a volumetric density of 36 kg/m^3, approximately half that in its liquid form at the normal boiling point. The gravimetric hydrogen density decreases with increasing pressure due to increasing thickness of the wall of the pressure cylinder. The wall thickness of a cylinder capped with two hemispheres is given by the following equation:
where is the wall thickness, is the outer diameter of the cylinder, the over pressure and the tensile strength of material varies from 50 MPa for aluminum to more than 1100 MPa for high quality steel. Future, developments of new composite materials have the potential to increase the tensile strength above that of steel with a material density less than half that of steel.
Figure. Volumetric density of compressed hydrogen gas as a function of gas pressure
including the ideal gasand liquid hydrogen. The ratio of the wall thickness to the outer
diameter of pressure cylinder is shown on the right-hand side for steel with a tensile
strength of 460MPa. A schematic drawing of the pressure cylinder is shown as an inset.
Figure. the six basic hydrogen storage methods and phenomena.
Liquid Hydrogen Tanks
The energy density of hydrogen can be improved by storing hydrogen in a liquid state. However, the issues with LH2 tanks are hydrogen boil-off, the energy required for hydrogen liquefaction, volume, weight, and tank cost. The energy requirement for hydrogen liquefaction is high; typically, 30% of the heating value of hydrogen is required for liquefaction. New approaches that can lower these energy requirements and thus the cost of liquefaction are needed. Hydrogen boil-off must be minimized or eliminated for cost, efficiency, and vehicle-range considerations, as well as for safety considerations when vehicles are parked in confined spaces. Insulation is required for LH2 tanks, and this reduces system gravimetric and volumetric capacity.
Liquid hydrogen (LH2) tanks can store more hydrogen in a given volume than compressed gas tanks. The volumetric capacity of liquid hydrogen is 0.070 kg/L, compared to 0.030 kg/L for 10,000-psi gas tanks.
Liquid tanks are being demonstrated in hydrogen-powered vehicles, and a hybrid tank concept combining both high-pressure gaseous and cryogenic storage is being studied. These hybrid (cryo-compressed tanks) insulated pressure vessels are lighter than hydrides and more compact than ambient-temperature, high-pressure vessels. Because the temperatures required are not as low as for liquid hydrogen, there is less of an energy penalty for liquefaction and less evaporative losses than for liquid hydrogen tanks.
Comparison of pressure storage and liquid storage
The gravimetric and volumetric hydrogen density depend strongly on the size of the storage vessel since the surface to volume ratio decreases with increasing size. Therefore, only the upper limit is defined. The large amount of energy necessary for the liquefaction, that is 40% of the upper heating value, makes liquid hydrogen not an energy efficient storage medium. Furthermore, the continuous boil-off of hydrogen limits the possible application for liquid hydrogen storage wywtems to cases where the hydrogen is consumed in a rather short time, for example air and space applications as Figure.
Figure. Hydrogen density for compressed hydrogen, liquid and solid hydrogen.
- Santen, Rutger, Kolb, Gunther, Barbaro, Pierluigi, Bianchini, Claudio, Garcia-Martinez, Javier, Moniz, Ernest, Hirose, Katsuhiko, Ozawa, Kazunori, Häring, Heinz-Wolfgang, Züttel, Andreas, Borgschulte, Andreas, & Schlapbach, Louis. (2008). Hydrogen as a future energy carrier. Vch Verlagsgesellschaft Mbh.