Booster for metal hydrides
When metals combine with hydrogen to form metal hydrides, large amounts of hydrogen are stored in a confined space in the metal lattice. This reversible reaction takes place under slight pressure and releases heat. Metal hydrides are therefore suitable for storing both hydrogen and thermochemical heat. Previous disadvantage: The poor heat conductivity of the hydrides limits the charging and discharging speed considerably. To improve this, researchers are processing the metal hydrides together with graphite, which is highly thermally conductive, to form metal hydride composites in pellet form. With different groups of substances, they can cover the temperature range from room temperature to 400 degrees Celsius.
Metal hydrides are usually used as a powder in bulk. In loose structures, the crystalline particles only have small contact surfaces. If these particles are compacted, the air spaces between them are reduced. The contact areas, on the other hand, increase. That is why their compression into pellets improves the thermal conductivity. But that alone is not enough, since although the starting metals generally conduct heat very well, the thermal conductivity in the hydrated state deteriorates drastically. "In a pure magnesium hydride pellet, for example, the thermal conductivity is less than one watt per metre and Kelvin," explains project manager Dr Lars Röntzsch, and adds: "If we mix the material with 10 per cent graphite by weight, we can increase the value to more than 15."Depending on the starting material, graphite improves the heat transfer by up to 50 times.
"Graphite is very cost effective and its thermal conductivity is very high," says Röntzsch in explaining the choice of material. For example, although copper as a metallic alternative has a greater conductivity than graphite, the non-ferrous metal would be too expensive for large storage systems. During compaction, the hydride particles and the graphite align themselves perpendicular to the pressing direction. This anisotropic orientation within the composite material improves the permeability of the pellet for hydrogen in a defined direction. For all the investigated metal composites, the researchers were able to demonstrate that the hydrogen permeability is not a limiting factor for the storage process.
A composite for all temperatures
The researchers wanted to cover a wide temperature range with various metal hydrides. They achieved this with metal hydride composites from three hydride classes:
- Metal hydrides based on magnesium for the high temperature range between 250 and 400 degrees Celsius,
- Sodium aluminium hydride for average temperatures between 120 and 200 degrees Celsius and
- Hydrides based on titanium-manganese for the low temperature range up to 100 degrees Celsius.
They formed the hydride-graphite mixtures into cylindrical pellets and then investigated the reaction dynamics and cycle stability. The hydrides for the low temperature range showed particularly promising results. The researchers succeeded in developing reliable and cost-effective production methods under ambient conditions. The composite material proved its long-term stability and robustness in tests with over 1000 charging and discharging cycles. It is particularly suitable for hydrogen storage systems in conjunction with fuel cells or hydrogen combustion engines.
A demonstrator in the electrical enclosure
In addition to the reactors with which the scientists tested the hydrides in the lab, they also developed a highly dynamic hydrogen storage system as a practical demonstrator. The storage unit is integrated in a standard module that fits into a 19-inch electrical enclosure. Fuel cell manufacturers offer some of their equipment in this format. Electrolysers are also available in rack structures. The performance data and means of operation were adapted to specific devices that are available.
The demonstrator can be charged and discharged at a flow rate of 3.5 standard litres per minute within about two hours. Tests have shown that the storage system is also quite capable of providing higher flow rates.
Further details on the HD-HGV research project are available on the project summary.