H₂ Storage

M. RUDLOFF, AMBARtec AG, Dresden, Germany

The author's company's hydrogen (H₂) storing process^a is based on the cyclic reduction of iron oxide as a storage medium by H₂ and the oxidation of the iron by water vapor, which again releases H₂ from the storage component (FIG. 1). The proprietary storage material consists of a specially designed iron oxide (FeOx) and possesses the unique properties of being reliably and easily reducible and oxidizable, while also exhibiting high thermal stability necessary for long-term usage. The released H₂ can then be used directly or converted into electrical energy in a fuel cell, internal combustion engine (ICE) or gas turbine.

 

 

During the H₂ storage process, H₂ can be stored at low- to high-pressure levels (0.5 bar–100 bar) and is fully compatible with any H₂ production unit. Fluctuating production rates (depending on wind speed or solar irradiation) are of no concern as the storage process^a can adapt readily to changing H₂ flows within wide ranges. Additionally, the storage process^a is indifferent to residual water contents or gas impurities from H₂ production, eliminating the need for any upstream gas treatment stages, particularly alkali metal residues from electrolysis or carbon monoxide (CO)/carbon dioxide (CO₂) remaining from gray H₂ production, which is critical for other H₂ storage technologies (e.g., pressurized or liquified storage). 

During the storage process, the H₂ reacts with oxygen in an endothermic reduction of the FeOx and is released as water vapor. The endothermic H₂ storage reaction requires thermal energy amounting to 3.16 kilowatt hours (kWhr)/kilogram (kg) of H₂, which is stored in the storage unit. The storage reaction takes place above 600°C, making it a perfect partner for high-temperature solid oxide electrolyzer cells (SOEC) and raising the efficiency of the SOEC by 10%–15%. If a conventional electrolyzer is used, the water exiting the storage unit is condensed, the energy is extracted and used for H₂ heating to reaction temperature, and the water is made available to the electrolysis for renewable H₂ production. This leads to significantly reduced operating expenditure (OPEX), especially in arid regions, which are excellent areas for photovoltaic power stations. 

The described chemical reaction makes the storage unit^a unique because the H₂ is not stored elementally. Only the potential for facile H₂ release is preserved. Consequently, obtaining the required permits and safety licenses can be significantly accelerated. The H₂ in the piping and auxiliary equipment during loading and unloading must be considered in any safety review, making the storage solution^a an interesting choice for all safety-sensitive applications (e.g., shipping or on-road mobility). In contrast to cryogenic liquid H₂, which exhibits high losses during storage, the author’s company’s technology^a provides loss-free H₂ storage potential over long periods. Possible benefiting applications are long-range transportation or seasonal H₂ storage. 

The H₂ release can be spatially and temporally separated from the H₂ loading process by inserting steam into the storage unit. At > 450°C, the reduced iron reacts with the water molecules, oxidizes to FeOx in an exothermic reaction and simultaneously releases H₂. The thermal energy needed during H₂ storage is released, amounting to 3.16 kWhr/kg of H₂. The remaining steam leaves the storage unit with the H₂ in a mixture with more than 90 vol% H₂ in the gas stream. The H₂ is cooled, the thermal energy is recirculated, and the water vapor is condensed. A H₂ quality of 99.9% is achievable using a drying unit with only water as the main impurity. A commercially available water separation system can increase the H₂ quality to 99.999% if higher qualities are required. Depending on the intended use of the H₂, it can be made available at a pressure of up to 100 bar, independent of the original H₂ pressure. For higher pressures (e.g., gas stations), a H₂ compressor is required. 

Since loading and unloading are high-temperature processes, strict temperature management inside the storage unit is key for successful storage operation. To increase the reaction rate and, thus, the H₂ storage and release process speed, a temperature level of > 450°C is targeted. A high-temperature consistency over the entire storage and release process is crucial for continuous process control. Theoretically, there is a risk that the exothermic reaction during H₂ release could lead to a local temperature increase at the storage material. This can prevent H₂ release or damage the storage material (Eq. 1). 

3 Fe + 4 H₂O → Fe₃O₄ + 4 H₂ ΔH° = -153 kJ/mol (Eq. 1) 

Similarly, during the storage process, a reduction in the temperature of the storage material can be expected due to reaction endothermy, which must be compensated for by external heating or temperature control of the input gas stream (Eq. 2). 

Fe₃O₄ + 4 H₂ → 3 Fe + 4 H₂O ΔH° = 153 kJ/mol (Eq. 2) 

However, the latest experimental results in the pilot facility suggest that the temperature of the H₂ storage unit is only slightly influenced by the heat release during the oxidation reaction (H₂ release) or the reduction reaction (H₂ storage) (FIG. 2).

 

 

The multi-zone furnace used was set to constant temperatures for this purpose. FIG. 3 shows the temperature profile in the front, middle and rear reaction zones qualitatively plotted against the progress of the H₂ release (left) and H₂ storage (right).

 

 

It is clear that with the operation mode applied and near constant heating power of the external heating, there is a high-temperature stability in the storage material. Local hot spots during H₂ release or respective cold spots during H₂ storage cannot be detected. This means the storage unit^a can be uniformly loaded or unloaded with H₂ without active thermal management, significantly reducing equipment and control requirements. 

The author’s company is working on commissioning a scaled-up storage^a facility with a storage capacity of 7.5 kg of H₂. Delivery of the first storage modules (250 kWhr) is possible in 2023. Storage facilities with a 600 kg of H₂/20,000 kWhr capacity will be available from 2024 and planning for individual projects has already begun.H₂T

Notes 

^a HyCS storage process