Metal hydride hydrogen storage systems, where hydrogen atoms are absorbed into the interstitial spaces of a metal compound, were established in mobile applications early this century with the construction of the German Type 212 and Italian Todaro class diesel-electric submarines. Built by Thyssen Krupp Marine Systems, the submarines feature an air-independent propulsion system based on Siemens proton exchange membrane (PEM) compressed hydrogen fuel cells. Similar storage systems to those used for the submarines have also been used on autonomous underwater vehicles.
More recently, automotive use of hydrogen fuel cells has expanded with the introduction of vehicles such as the Toyota Mirai. Research has increasingly focused on the potential benefits of a metal hydride compressor capable of delivering pressures up to 1,000 bar for rapidly refuelling these vehicles, and it is technology that could also make rapid refuelling practical for vessels such as fuel-cell powered ferries.
Different metal hydrides are used for different applications. For example, Professor Craig Buckley of Curtin University in Australia is working on the usage of metal hydrides for thermal energy storage. The metal hydrides suitable for this application would have as high as possible reaction enthalpy (meaning they should release as much heat as possible when reacting with the hydrogen and should absorb as much heat as possible when releasing the hydrogen).
This is the opposite of what is desirable for hydrogen storage and compression. Nonetheless, heat is released during uptake of hydrogen into a metal hydride-based storage tank, and it has to be cooled if the hydrogen charging is rapid. Heat is required when the stored hydrogen is to be used and therefore released from the storage tank.
Some metal hydrides can release their stored hydrogen using the heat available in seawater (at temperatures ranging from 0-20°C). Others need heat at higher temperatures and can use the waste heat of a PEM fuel cell. Yet others need even higher temperatures which can be provided if the hydrogen is used in a high temperature fuel cell or burned in an internal combustion engine.
Hydrogen has the highest energy density per unit mass of any fuel, but its low volumetric density at ambient temperature and pressure means it has a low energy density per unit volume. Metal hydrides are a desirable storage medium, as they are the most compact and dense hydrogen storage possibility, says Dr. Martin Dornheim, Head of the Department of Nanotechnology, Materials Technology, at research institute Helmholtz-Zentrum Geesthacht in Germany. “Hydrogen is absorbed or sponged up by the solid material and thereby stored much denser than is the case for high pressure gas storage (700 bar) or liquified hydrogen at -252°C. It can store the hydrogen at low pressures (tens of bars) and ambient temperature.”
Metal hydride for onboard storage
Dornheim and his team are exploring applications for stationary hydrogen storage, the delivery of hydrogen shoreside as well as for onboard storage of hydrogen on ships, and he says the compact storage offered by metal hydrides means that a tank design can be fitted into any space available in a ship.
Storage tanks that use conventional metal hydrides have a key disadvantage, though, compared to 700 bar hydrogen gas storage or liquid hydrogen storage, says Dornheim. They weigh up to four times more than compressed gas storage tanks using type III and especially type IV tank shells (these are low or no metal containing cylinders which are stabilised for example by carbon nanofibers).
To overcome this issue, he is working on novel light metal hydrides and hydride composites with a storage capacity per weight for hydrogen which is more than five times higher than conventional metal hydrides. One light candidate is a sodium-aluminum-hydrogen compound. “However, this specific light candidate has only three times the weight capacity compared to classical conventional hydrides,” says Dornheim. “More promising are composites / mixtures of different light metal hydrides and borohydrides and amides.”
There are a large number of different metal hydrides currently being investigated, and they vary depending on the application, including the compression of hydrogen from low pressures to very high pressures for uses such as automotive refuelling. “This compression is possible just by using the physics of metal hydrides,” explains Dornheim. “They can be filled with hydrogen at rather low pressures and low temperatures. If the temperature is raised, however, the hydride wants to release the hydrogen again. The higher the temperature, the higher the hydrogen release pressure. This is a basic thermodynamic principle, since at high temperatures all systems want to go into the state where entropy is highest.”
Metal hydride compressors can deliver very pure hydrogen. This can be difficult to achieve with mechanical compressors which can suffer hydrogen purity issues as a result of the lubrication and abrasion of the piston. Ionic compressors are another option. They have fewer moving parts than mechanical compressors and have been used to deliver hydrogen at pressures up to 700 bar. However, metal hydride compressors offer the advantage of having no moving parts, because they use thermal energy rather than mechanical energy. This means they operate silently and require no maintenance.
Hydrogen compressors are usually stationary systems, so their weight is not as important as their compression efficiency: with as small a temperature difference as possible, the hydrogen should be compressed as much as possible. Furthermore, hydrogen compressors should ideally have an ambient operating temperature for hydrogen uptake and temperatures below 100°C for the release of the hydrogen, says Dornheim. If the hydrogen is to be compressed to very high pressures like 800 bar, metal hydrides with a rather low stability (low bonding or reaction heat) are required.
Burckhardt Compression and GRZ Technologies in Switzerland are developing hydrogen compression technology that uses thermally active metal hydrides. The Static Hydrogen Compressor from Burckhardt Compression is based on GRZ Technologies’ HYCO laboratory compressor that is already in use for small amounts of hydrogen. Without moving parts, it is noise and vibration free and is hermetically sealed from the environment. This means it can operate without any gas leakage and can be used in sensitive areas.
Scaled-up, the technology will be designed for high-pressure applications of 200, 350 and 700 bar. As well as its refuelling potential, the technology can store renewably produced electricity, supply peak power for an extended period of several months and be used as a backup power supply.
Dornheim is also collaborating with GRZ Technologies along with the Ecole polytechnique fédérale de Lausanne (EPFL) research institute in Switzerland as part of the International Energy Agency Hydrogen Technology Collaboration Program – Task 40 “Energy Storage and Conversion based on Hydrogen.” Over the next few years, the researchers aim to develop reversible or regenerative hydrogen storage materials and systems suitable for mobile, stationary and portable applications, electrochemical storage and solar thermal heat storage. This will involve furthering the fundamental understanding of hydrogen storage chemistry. For example, the effect of catalysts is not yet well understood in complex or liquid hydrides even though catalysts are important for hydrogenation/dehydrogenation processes.
Dornheim’s team is also collaborating on a number of other projects including the EU project “Hydride4Mobility” which aims to improve metal hydride-based hydrogen storage tanks for mobile applications and for metal hydride-based compressors. The project partners are addressing critical commercialisation issues for hydrogen powered utility vehicles using metal hydride hydrogen storage and PEM fuel cells, together with the systems needed for their refuelling at industrial facilities. A first test case will be a forklift.
For this type of application, metal hydrides with a high specific weight are an advantage, as the unit can then function as a vehicle counterbalance without any extra cost. However, the slow hydrogen charge and discharge of these metal hydride systems, the complexity of their design and the efficiency of system integration remain challenges to overcome.
Dornheim is also collaborating with Volkswagen, plant apparatus developer Panco and tank experts Stühff, on the design and construction of a hydrogen storage system for a fuel cell car as part of the German funded “H2HybridTank” project. Under the framework of the European HyCARE project, he is also developing a stationary hydrogen storage system for the storage of 50kg of hydrogen, around 10 times more than that stored in a fuel cell car.
A key challenge for mobile applications in the future is the reduction of hydrogen refuelling pressures and shortening of refuelling time. Research suggests that the properties of metal hydrides in higher pressure applications may not behave the same as they do in low pressure systems, and at high pressures, degradation over multiple cycles could occur. Apart from optimising the composition of the hydrides, another challenge will be to reduce the cost of manufacturing them.
For hydrogen compressors, the challenges include minimising void space to reduce losses of productivity at high pressure, effective heat exchange between the metal hydride and the heating/cooling fluid, minimising the heat lost during periodic heating/cooling of the metal hydride and hydrogen gas containment at operating pressures over 500 bar.
The metal hydrides being developed for hydrogen compression are expected to be similar to the ones for hydrogen storage. Ideally, they should have well-matched operating pressure and temperature ranges, high reversible hydrogen sorption capacities, fast kinetics, minimal volume decrease upon dehydrogenation and high cycle stability.