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A systematic study of different ratios of CO, CO2, N2 gas components on the hydrogen storage properties of the Na3AlH6 complex hydride with 4 mol% TiCl3, 8 mol% aluminum and 8 mol% activated carbon is presented in this paper. The different concentrations of CO and CO2 in H2 and CO, CO2, N2 in H2 mixture were investigated. Both CO and CO2 gas react with the complex hydride forming Al oxy-compounds, NaOH and Na2CO3 that consequently cause serious decline in hydrogen storage capacity. These reactions lead to irreversible damage of complex hydride under the current experimental condition. Thus, after 10 cycles with 0.1 vol % CO + 99.9 vol %H2 and 1 vol % CO + 99 vol %H2, the dehydrogenation storage capacity of the composite material decreased by 17.2% and 57.3%, respectively. In the case of investigation of 10 cycles with 1 vol % CO2 + 99 vol % H2 gas mixture, the capacity degradation was 53.5%. After 2 cycles with 10 vol % CO +90 vol % H2, full degradation was observed, whereas after 6 cycles with 10 vol % CO2 + 90 vol % H2, degradation of 86.8% was measured. While testing with the gas mixture of 1.5 vol % CO + 10 vol % CO2 + 27 vol % H2 + 61.5 vol % N2, the degradation of 94% after 6 cycles was shown. According to these results, it must be concluded that complex aluminum hydrides cannot be used for the absorption of hydrogen from syngas mixtures without thorough purification.

The potential and research surrounding metal hydride (MH) based thermal energy storage is discussed, focusing on next generation thermo-chemical energy storage (TCES) for concentrated solar power. The site availability model to represent the reaction mechanisms of both the forward and backward MH reaction is presented, where this model is extrapolated to a small pilot scale reactor, detailing how a TCES could function/operate in a real-world setting using a conventional shell & tube reactor approach. Further, the important parameter of effective thermal conductivity is explored using an innovative multi-scale model, to providing extensive and relevant experimental data useful for reactor and system design. Promising high temperature MH material configurations may be tuned by either destabilisation, such as using additions to Ca and Sr based hydrides, or by stabilisation, such as fluorine addition to NaH, MgH2, or NaMgH3. This versatile thermodynamic tuning is discussed, including the challenges in accurately measuring the material characteristics at elevated temperatures (500 –700 °C). Attention to scale up is explored, including generic design and prototype considerations, and an example of a novel pilot-scale pillow-plate reactor currently in development; where materials used are discussed, overall tank design scope and system integration.

Thermochemical energy storage materials have advantage of much higher energy densities compared to latent or sensible heat storage materials. Metal hydrides show good reversibility and cycling stability combined with high enthalpies. They can be used for short and long-term heat storage applications and can increase the overall flexibility and efficiency of solar thermal energy production. Metal hydrides with working temperatures less than 500 °C were in the focus of research and development over the last years. For the new generation of solar thermal energy plants new hydrides materials with working temperatures above 600 °C must be developed and characterized. In addition to thorough research on new metal hydrides, the construction and engineering of heat storage systems at these high temperatures are challenging. Corrosion problems, hydrogen embrittlement and selection of heat transfer fluids are significant topics for future research activities.

Sodium aluminum hydride has been extensively investigated for hydrogen storage applications whereas its intermediate decomposition compound Na3AlH6 received much less attention, despite having a lower dissociation pressure and a reasonable hydrogen storage capacity of 3.0 wt%. In this work, Na3AlH6 is synthesized through ball milling, starting from NaAlH4 and 2 NaH in the presence of TiCl3 catalyst precursor, and evaluated on its hydrogen sorption properties and cycle stability. Further addition of 8 mol% Al and 8 mol% activated carbon (AC) and their effect on both the hydrogen sorption properties and cycle stability have been investigated. In order to explore whether the introduction of the Al and AC additives would be more beneficial (in terms of hydrogen sorption behavior and cycle stability) after the Na3AlH6 synthesis or during its synthesis, pre-synthesized Na3AlH6-based measurements were also included in this work. TiCl3-catalyzed NaAlH4 + 2 NaH sample showed a stable reversible hydrogen storage capacity of 1.7 wt%, which was further increased to 2.1 wt% with the addition of Al-powder and activated carbon AC.

The development and preliminary tests of a 5 kg Mg2FeH6 heat storage system which is useable for short and long-term storage applications at temperatures around 500 °C are described. The heat transfer for the heat storage process (dehydrogenation of the hydride Mg2FeH6) and heat release (hydrogenation of the hydride precursor 2Mg-Fe) is done by the flow of molten salt in appropriate heat exchangers serving as heat source or heat sink. The construction of the tube bundle reactor as a heat storage tank is presented. 1.6 kWh of heat could be released and 1.5 kWh of heat could be stored during the first experimental tests. Difficulties, which occured during the preliminary tests, are described.

This review describes recent research in the development of tank systems based on complex metal hydrides for thermolysis and hydrolysis. Commercial applications using complex metal hydrides are limited, especially for thermolysis-based systems where so far only demonstration projects have been performed. Hydrolysis-based systems find their way in space, naval, military and defense applications due to their compatibility with proton exchange membrane (PEM) fuel cells. Tank design, modeling, and development for thermolysis and hydrolysis systems as well as commercial applications of hydrolysis systems are described in more detail in this review. For thermolysis, mostly sodium aluminum hydride containing tanks were developed, and only a few examples with nitrides, ammonia borane and alane. For hydrolysis, sodium borohydride was the preferred material whereas ammonia borane found less popularity. Recycling of the sodium borohydride spent fuel remains an important part for their commercial viability.

Here we present the development of an aluminium alloy based hydrogen storage tank, charged with Ti-doped sodium aluminium hexahydride Na3AlH6. This hydride has a theoretical hydrogen storage capacity of 3 mass-% and can be operated at lower pressure compared to sodium alanate NaAlH4. The tank was made of aluminium alloy EN AW 6082 T6. The heat transfer was realised through an oil flow in a bayonet heat exchanger, manufactured by extrusion moulding from aluminium alloy EN AW 6060 T6. Na3AlH6 is prepared from 4 mol-% TiCl3 doped sodium aluminium tetrahydride NaAlH4 by addition of two moles of sodium hydride NaH in ball milling process. The hydrogen storage tank was filled with 213 g of doped Na3AlH6 in dehydrogenated state. Maximum of 3.6 g (1.7 mass-% of the hydride mass) of hydrogen was released from the hydride at approximately 450 K and the same hydrogen mass was consumed at 2.5 MPa hydrogenation pressure. 45 cycle tests (rehydrogenation and dehydrogenation) were carried out without any failure of the tank or its components. Operation of the tank under real conditions indicated the possibility for applications with stationary HT-PEM fuel cell systems.

The storage of heat at high temperatures, which can be used to generate electricity after sunset in concentrating solar power plants, is one of the most challenging technologies. The use of metal hydride could be one possibility to solve the problem. During the endothermic heat storage process, the metal hydride is decomposed releasing hydrogen, which then can be stored. During the exothermic reaction of the metal with the hydrogen gas, the stored heat is then released. Previous research had shown that Mg and Fe powders can be used at temperatures up to 550 °C for heat storage and shows excellent cycle stability over hundreds of cycles without any degradation. Here, we describe the results of testing of a tube storage tank that contained 211 g of Mg and Fe powders in 2:1 ratio. Twenty-three dehydrogenations (storage) and 23 hydrogenations (heat release) in the temperature range between of 395 and 515 °C and pressure range between 1.5 and 8.6 MPa were done. During the dehydrogenation, 0.41–0.42 kWhth kg−1 of heat based on material 2 Mg/Fe can be stored in the tank. After testing, mainly Mg2FeH6 was observed and small amounts of MgH2 and Fe metal can be detected in the hydride samples. This means that the heat storage capacity of the system could be further increased if only Mg2FeH6 is produced during subsequent cycles.

In this publication the authors present an aluminium alloy tank for hydrogen storage using 1921 g of Na 3 AlH 6 doped with 4 mol% of TiCl 3 and 8 mol% of activated carbon. The tank and the heat exchangers are manufactured by extrusion moulding of Al-Mg-Si based alloys. EN AW 6082 T6 alloy is used for the tank and a specifically developed alloy with a composition similar to EN AW 6060 T6 is used for the heat exchangers. The three heat exchangers have a corrugated profile to enhance the surface area for heat transfer. The doped complex hydride Na 3 AlH 6 is densified to a powder density of 0.62 g cm −3 . The hydrogenation experiments are carried out at 2.5 MPa. During one of the dehydrogenation experiments approximately 38 g of hydrogen is released, accounting for gravimetric hydrogen density of 2.0 mass-%. With this tank 15 hydrogenation and 16 dehydrogenation tests are carried out.