Hydrogen storage

DESCRIPTION

Metal hydride tank is a container loading with hydrogen storage alloy powder, heat exchange parts, and gas transport components. The container body materials are generally aluminum alloy or stainless steel. Our hydrogen storage vessels are based on AB5 metal hydride alloys. Hydrogen being stored at low pressure in the vessel, they provide a safe and reliable energy storage, particularly for portable applications, in-house and in-board storage.

Individual vessels can be combined in arrays to reach specific H₂ volumes and discharge rates. Contact us for custom-built hydrogen storage.

Chose your storage depending on the quantity of hydrogen you wish to store. The number in the product name refers to the quantity of normal liters of gas kept in the vessel (for instance, the MH150 tank can store up to 150NL of hydrogen). Typical uses of the tanks include fuel cell power, gas chromatography and lab experiments/chemical reactions.

The performance of a metal hydride tank mainly includes the following aspects:
. Capacity — how much hydrogen can be stored, generally indicated as in standard liters NL;
. Releasing rate — the rate that hydrogen is released from the tank. A sufficient heat supply is required for hydrogen releasing. The rate is determined by the type of metal hydride, efficiency of heat exchange as well as gas transport resistance;
. Recharging time — time that used to refueling the tank;
. Cycle life — the cycle number that the tank can be refueled without a significant degradation in capacity.

The main features of the metal hydride tank:
. High storage density；
. Capability of low temperature releasing
. Quick locking;
. Flexibility in tank size;
. Body materials: stainless steel or aluminum alloy

SAFETY

Metal hydride tanks are ideal for indoor use thanks to their low storage pressure.
However the user should consider the volume of the room where the vessel is stored compared to the volume of hydrogen stored. Hydrogen has a Lower Flammable Limit (LFL) of 4% diluted in air. We recommend that the uncompressed total volume of hydrogen stored does not exceed 2% (half of the LFL) of the total volume of the room.

Our tanks are delivered filled with argon to comply with air transportation security rules. Therefore, they need to be charged before being usable.

HOW DOES METAL HYDRIDE TANKS WORK

Storage Mechanism
Hydrogen is stored in the form of so-called “metal hydride”.
Most metals or alloys can react with hydrogen to form new compounds, which are named as metal hydrides. The formation of metal hydride is an exothermic process associated with heat releasing. With sufficient heat supply, hydrogen can be released from the as-formed metal hydride. Such a reversible reaction process can be expressed as follows:

2/nM + H₂ = 2/nMH_n +ΔH

where M-metal or alloy; MH_n-metal hydride; ΔH-thermal effect associated with the reaction. Pressure and temperature changed, the reaction will take place alternatively and hydrogen will be absorbed or desorbed.

Atoms of hydrogen, under low temperature or high pressure, can enter the holes of crystalline of the parent metal or alloy, forming solid solution — α phase; then reaction between hydrogen and metal will continue to form the new phase — β phase. The above figure is the simple model of hydrogen atoms locating on the grid of metals.

Thermodynamic
From the thermodynamics point of view, the enthalpy associated with the formation reaction of metal hydride reflects the strength of the metal-hydrogen bond in the metal hydride phase. The relationship among the pressure, temperature and composition is generally described by a Pressure—Composition—Isotherm plot, namely PCI curve.

In this figure, the sloping part on the left side is for α—phase region. Both α and β phases coexist in the middle region and only β—phase is present on the right side. The pressure relative to the middle flat region is called the equilibrium plateau pressure.
The relationship between pressure and temperature is described by the Van’t Hoff equation:

LnPH₂ = ΔH₀/RT – ΔS₀/R

Where ΔH₀ and ΔS₀ are the enthalpy and entropy of the hydrogen absorption reaction, “R” the gas constant and “T” the absolute temperature. This relationship can be verified experimentally by carrying out PCI measurements on a material at several temperatures. Adopting the Van’t Hoff relationship and PCI curve, it is possible to determine the enthalpy and entropy of the metal and hydrogen equilibrium.

Type of metal hydrides
From the engineering point of view, three types of metal hydrides mostly used, which are listed as follows.
– AB type: Ti-Fe-C, Ti-Fe-Ca, Ti-Fe, Ta, Ti-Fe-Mm,…
– AB2 type: Ti-Cr-Fe, Ti-Zr-Cr-Fe, Ti-Zr-Cr-Fe-Mn-Cu, Ti-Mn-V, Ti-Zr-Mn-V-Fe,…
– AB5 type: Ca-Mm-Ni-Al, Mm-Ni-Mn-Co,…