Machine learning based prediction of metal hydrides for hydrogen storage,part I: Prediction of hydrogen weight percent

The openly available database provided by the US Department of Energy on hydrides for hydrogen storage were analyzed through supervised machine learning to rank features in terms of their importance for determining hydrogen storage capacity referred to as hydrogen weight percent. In this part: we employed four models, namely linear regression, neural network, Bayesian linear regression and boosted decision tree to predict the hydrogen weight percent. For each algorithm, the scored labels were compared to the actual values of hydrogen weight percent. Our investigation showed that boosted decision tree regression performed better than the other algorithms achieving a coefficient of determination of 0.83.     

Machine learning based prediction of metal hydrides for hydrogen storage,part II: Prediction of material class

The openly available dataset on hydrogen storage materials provided by the US Department of Energy was used to predict the optimal materials class of metal hydrides based on the desired properties, which included hydrogen-weight percent, heat of formation and operating temperature and pressure. We performed correlation and statistical analyses to investigate the statistical characteristics of each numeric features. We employed four classification algorithms: multiclass logistic regression, multiclass decision forest, multiclass decision jungle and multiclass neural network. Feature importance analysis was carried out to investigate how each classifier utilises the information available in the dataset. In overall, multiclass neural network classifier had better classification performance obtaining an accuracy of 0.80. The results suggest that the complex material class, followed by Mg is applicable for the most wide range of operating temperatures. Positive correlation was found between hydrogen weight percent, heat of formation and temperature, suggesting that the maximum hydrogen weight percent would be achieved in the complex material class operated at a high temperature.     

Effects of NH4+ doping on the hydrogen storage properties of metal hydrides

Doping can modify the properties of metal hydrogen storage materials significantly. Currently, the metal doping is a frequent strategy, while the non-metal cation doping has not been examined extensively so far. In this study, the effects of NH₄⁺ doping on the hydrogen storage properties of different metal hydrides, including TiH₂, Ti_0·25V_0·25Nb_0·25Zr_0·25H₂, Ti_0·5V_0·5H₂ and VH₂, are investigated by first-principles calculations. It is found that the NH₄⁺ presents a good affinity for metal hydrides and the NH₄⁺ incorporation leads to charge redistribution and formation of dihydrogen bond. Furthermore, the NH₄⁺ doping in metal hydrides is favorable for enhancing the hydrogen storage capacity and decreasing the thermal stability simultaneously. The possible reason for the NH₄⁺ doping induced destabilization in metal hydrides is the relatively weak interaction between NH₄⁺ and hydrogen atoms.     

Novel hydrogen generator/storage based on metal hydrides

A novel electrochemical system has been developed which integrates hydrogen production, storage and compression in only one device, at relatively low cost and higher efficiency than a classical electrolyser. The prototype comprises a six-electrode cell assembly using an AB₅ type metal hydride and Ni plates as counter electrodes, in a KOH solution. Metal hydride electrodes with chemical composition LaNi₄.₃Co_0.4Al₀.₃ have been prepared by high frequency vacuum melting followed by high temperature annealing. X-ray phase analysis showed typical hexagonal structure and no traces of other intermetallic compounds belonging to the La–Ni phase diagram. Thermodynamic study of the alloy has been performed in a Sievert-type apparatus produced by Labtech Ltd. In the present prototype during charging, hydrogen is absorbed in the metal hydride and corresponding oxygen is conveyed out of the system. Conversely, in the case of discharging the hydrogen stored in the metal hydride it is released to an external H₂ storage. Released hydrogen is delivered into the hydrogen storage up to a pressure of 15 bar. It is anticipated that the device will be integrated as a combined hydrogen generator in a stand-alone system associated to a 1 kW fuel cell.     

The potential of metal hydrides paired with compressed hydrogen as thermal energy storage for concentrating solar power plants

Concentrating solar power (CSP) plants require thermal energy storage (TES) systems to produce electricity during the night and periods of cloud cover. The high energy density of high-temperature metal hydrides (HTMHs) compared to state-of-the-art two-tank molten salt systems has recently promoted their investigation as TES systems. A common challenge associated with high-temperature metal hydride thermal energy storage systems (HTMH TES systems) is storing the hydrogen gas until it is required by the HTMH to generate heat. Low-temperature metal hydrides can be used to store the hydrogen but can comprise a significant proportion of the overall system cost and they also require thermal management, which increases the engineering complexity. In this work, the potential of using a hydrogen compressor and large-scale underground hydrogen gas storage using either salt caverns or lined rock caverns has been assessed for a number of magnesium- and sodium-based hydrides: MgH₂, Mg₂FeH₆, NaMgH₃, NaMgH₂F and NaH. Previous work has assumed that the sensible heat of the hydrogen released from the HTMH would be stored in a small, inexpensive regenerative material system. However, we show that storing the sensible heat of the hydrogen released would add between US$3.6 and US$7.5/kWh_th to the total system cost for HTMHs operating at 565 °C. If the sensible heat of released hydrogen is instead exploited to perform work then there is a flow-on cost reduction for each component of the system. The HTMHs combined with underground hydrogen storage all have specific installed costs that range between US$13.7 and US$26.7/kWh_th which is less than that for current state-of-the-art molten salt heat storage. Systems based on the HTMHs Mg₂FeH₆ or NaH have the most near term and long term potential to meet SunShot cost targets for CSP thermal energy storage. Increasing the operating temperature and hydrogen equilibrium pressure of the HTMH is the most effective means to reduce costs further.     

Monitoring and control of a hydrogen production and storage system consisting of water electrolysis and metal hydrides

Renewable energy sources such as wind turbines and solar photovoltaic are energy sources that cannot generate continuous electric power. The seasonal storage of solar or wind energy in the form of hydrogen can provide the basis for a completely renewable energy system. In this way, water electrolysis is a convenient method for converting electrical energy into a chemical form. The power required for hydrogen generation can be supplied through a photovoltaic array. Hydrogen can be stored as metal hydrides and can be converted back into electricity using a fuel cell. The elements of these systems, i.e. the photovoltaic array, electrolyzer, fuel cell and hydrogen storage system in the form of metal hydrides, need a control and monitoring system for optimal operation. This work has been performed within a Research and Development contract on Hydrogen Production granted by Solar Iniciativas Tecnológicas, S.L. (SITEC), to the Politechnic University of Valencia and to the AIJU, and deals with the development of a system to control and monitor the operation parameters of an electrolyzer and a metal hydride storage system that allow to get a continuous production of hydrogen.     

Application of hydrides in hydrogen storage and compression: Achievements,outlook and perspectives

Metal hydrides are known as a potential efficient, low-risk option for high-density hydrogen storage since the late 1970s. In this paper, the present status and the future perspectives of the use of metal hydrides for hydrogen storage are discussed. Since the early 1990s, interstitial metal hydrides are known as base materials for Ni – metal hydride rechargeable batteries. For hydrogen storage, metal hydride systems have been developed in the 2010s [1] for use in emergency or backup power units, i. e. for stationary applications.With the development and completion of the first submarines of the U212 A series by HDW (now Thyssen Krupp Marine Systems) in 2003 and its export class U214 in 2004, the use of metal hydrides for hydrogen storage in mobile applications has been established, with new application fields coming into focus.In the last decades, a huge number of new intermetallic and partially covalent hydrogen absorbing compounds has been identified and partly more, partly less extensively characterized.In addition, based on the thermodynamic properties of metal hydrides, this class of materials gives the opportunity to develop a new hydrogen compression technology. They allow the direct conversion from thermal energy into the compression of hydrogen gas without the need of any moving parts. Such compressors have been developed and are nowadays commercially available for pressures up to 200 bar. Metal hydride based compressors for higher pressures are under development. Moreover, storage systems consisting of the combination of metal hydrides and high-pressure vessels have been proposed as a realistic solution for on-board hydrogen storage on fuel cell vehicles.In the frame of the “Hydrogen Storage Systems for Mobile and Stationary Applications” Group in the International Energy Agency (IEA) Hydrogen Task 32 “Hydrogen-based energy storage”, different compounds have been and will be scaled-up in the near future and tested in the range of 500 g to several hundred kg for use in hydrogen storage applications.     

Integrating hydrogen generation and storage in a novel compact electrochemical system based on metal hydrides

The development of efficient and reliable energy storage systems based on hydrogen technology represents a challenge to seasonal storage based on renewable hydrogen. State of the art renewable energy generation systems include separate units such as electrolyzer, hydrogen storage vessel and a fuel cell system for the conversion of H₂ back into electricity, when required. In this work, a novel electrochemical system has been developed which integrates hydrogen production, storage and compression in only one device, at relatively low cost and high efficiency. The developed prototype comprises a six-electrode cell assembly using an AB₅-type metal hydride and Ni plates as counter electrodes, in a 35-wt% KOH solution. Metal hydride electrodes with chemical composition LaNi_4.3Co_0.4Al_0.3 were prepared by high frequency vacuum melting followed by high temperature annealing. X-ray phase analysis showed typical hexagonal structure and no traces of other intermetallic compounds belonging to the La–Ni phase diagram. Thermodynamic study has been performed in a Sieverts type of apparatus produced by Labtech Int. During cycling, the charging/discharging process was studied in situ using a gas chromatograph from Agilent. It is anticipated that the device will be integrated as a combined hydrogen generator and storage unit in a stand-alone system associated to a 1-kW fuel cell.     

Experimental analysis of a metal hydride hydrogen storage system with hexagonal honeycomb-based heat transfer enhancements-part B

This study is a continuation of the computational analysis of the reactor equipped with hexagonal honeycomb based heat transfer enhancements, performed in Part A of the study. In the present study, the performance of the metal alloy and the reactor is investigated experimentally. The gravimetric capacity and reaction kinetics of the alloy La_0.9Ce_0.1Ni₅ are determined. The performance of the reactor under different external environments is noted. The influence of operating conditions such as supply pressure, heat transfer fluid, heat transfer fluid temperature on the reactor performance is investigated. Evaporative cooling as a heat removal technique for metal hydride based hydrogen storage reactors is tested for the first time and compared to conventional heat removal methods. It is found to improve the heat transfer from the alloy bed significantly.     

Bounding material properties for automotive storage of hydrogen in metal hydrides for low-temperature fuel cells

Metal hydride material properties required for on-board hydrogen storage for use with automotive polymer electrolyte fuel cell systems are discussed. Thermodynamic relationships between enthalpy and entropy of sorption are determined such that the storage system can be thermally integrated with the fuel cell system and be refueled at reasonable H₂ supply pressures of 50–200 atm. Simple criteria are developed for specifying minimum discharge kinetic rates needed to satisfy hydrogen demand on automotive duty cycles. Simple criteria are also developed for specifying minimum charge kinetic rates needed to refuel metal hydride tanks in reasonable time. Accessible intrinsic capacity and bulk density of the metal hydride are determined for the storage system to achieve system level targets for gravimetric and volumetric capacities. Based on these analyses, it is recommended that the storage media properties be measured on samples prepared by mixing the metal hydride with a high thermal conductivity material, and compacted to 600 kg m⁻³ bulk density. The compact should have a minimum effective thermal conductivity of 8.5 W m⁻¹ K⁻¹.     

HYDRIDE4MOBILITY: An EU HORIZON 2020 project on hydrogen powered fuel cell utility vehicles using metal hydrides in hydrogen storage and refuelling systems

The goal of the EU Horizon 2020 RISE project 778307 “Hydrogen fuelled utility vehicles and their support systems utilising metal hydrides” (HYDRIDE4MOBILITY), is in addressing critical issues towards a commercial implementation of hydrogen powered forklifts using metal hydride (MH) based hydrogen storage and PEM fuel cells, together with the systems for their refuelling at industrial customers facilities. For these applications, high specific weight of the metallic hydrides has an added value, as it allows counterbalancing of a vehicle with no extra cost. Improving the rates of H₂ charge/discharge in MH on the materials and system level, simplification of the design and reducing the system cost, together with improvement of the efficiency of system “MH store-FC”, is in the focus of this work as a joint effort of consortium uniting academic teams and industrial partners from two EU and associated countries Member States (Norway, Germany, Croatia), and two partner countries (South Africa and Indonesia).The work within the project is focused on the validation of various efficient and cost-competitive solutions including (i) advanced MH materials for hydrogen storage and compression, (ii) advanced MH containers characterised by improved charge-discharge dynamic performance and ability to be mass produced, (iii) integrated hydrogen storage and compression/refuelling systems which are developed and tested together with PEM fuel cells during the collaborative efforts of the consortium.This article gives an overview of HYDRIDE4MOBILITY project focused on the results generated during its first phase (2017–2019).     

Characterization of metal hydrides by in-situ XRD

In-situ synchrotron radiation powder X-ray diffraction (SR-PXD) technique is a powerful tool to gain a deeper understanding of reaction mechanisms in crystalline materials. In this paper, the implementation of a new in-situ SR-PXD cell for solid–gas reactions is described in detail. The cell allows performing measurements in a range of pressure which goes from light vacuum (10⁻² bar) up to 200 bar and temperatures from room temperature up to 550 °C. The high precision, with which pressure and temperature are measured, enables to estimate the thermodynamic properties of the observed changes in the crystal structure and phase transformations.     

Metal (boro-) hydrides for high energy density storage and relevant emerging technologies

The current energy transition imposes a rapid implementation of energy storage systems with high energy density and eminent regeneration and cycling efficiency. Metal hydrides are potential candidates for generalized energy storage, when coupled with fuel cell units and/or batteries. An overview of ongoing research is reported and discussed in this review work on the light of application as hydrogen and heat storage matrices, as well as thin films for hydrogen optical sensors. These include a selection of single-metal hydrides, Ti–V(Fe) based intermetallics, multi-principal element alloys (high-entropy alloys), and a series of novel synthetically accessible metal borohydrides. Metal hydride materials can be as well of important usefulness for MH-based electrodes with high capacity (e.g. MgH₂ ~ 2000 mA h g⁻¹) and solid-state electrolytes displaying high ionic conductivity suitable, respectively, for Li-ion and Li/Mg battery technologies. To boost further research and development directions some characterization techniques dedicated to the study of M-H interactions, their equilibrium reactions, and additional quantification of hydrogen concentration in thin film and bulk hydrides are briefly discussed.     

Multi-criteria analysis for screening of reversible metal hydrides in hydrogen gas storage and high pressure delivery applications

Metals and alloys forming reversible hydrides with hydrogen gas are potential building blocks for compact, solid state hydrogen storage systems. Based on the materials’ thermodynamic characteristics, their use as temperature-swing gas compression and delivery systems in the hydrogen economy is also possible. Given the wide variety of materials developed and tested at laboratory and pilot scales, a harmonized method of selecting the feasible material(s) for a particular real-life application is required. This study proposes a system selection framework based on a normalized, multi-criteria metric. Using calculated values of multi-criteria metric, multi-criteria screening and ranking of potential materials has been demonstrated for a particular use case. It is found that the alloy TiMn_1.52 having value of additive metric between 0.25 and 0.35 represents the best material for a single stage system. The alloy pair CaNi₅–Ti_1.5CrMn represents the best alternative for a two-stage system with additive metric values between 0.63 and 0.82. Energy and economic characteristics of the metal hydride gas compression and delivery systems are evaluated and compared with an equivalent mechanical compression system producing the same final effect (i.e., delivery of a given quantity of gas at a defined pressure).     

The problem of solid state hydrogen storage

A short review of the materials under investigation suitable for solid state hydrogen storage is presented, with particular reference to the experimental activity carried out at the laboratory of Hydrogen Group of Padova University.     

Large-scale storage of hydrogen

The large-scale storage of hydrogen plays a fundamental role in a potential future hydrogen economy. Although the storage of gaseous hydrogen in salt caverns already is used on a full industrial scale, the approach is not applicable in all regions due to varying geological conditions. Therefore, other storage methods are necessary. In this article, options for the large-scale storage of hydrogen are reviewed and compared based on fundamental thermodynamic and engineering aspects. The application of certain storage technologies, such as liquid hydrogen, methanol, ammonia, and dibenzyltoluene, is found to be advantageous in terms of storage density, cost of storage, and safety. The variable costs for these high-density storage technologies are largely associated with a high electricity demand for the storage process or with a high heat demand for the hydrogen release process. If hydrogen is produced via electrolysis and stored during times of low electricity prices in an industrial setting, these variable costs may be tolerable.     

Design of a large-scale metal hydride based hydrogen storage reactor: Simulation and heat transfer optimization

An optimized design for a 210 kg alloy, TiMn alloy based hydrogen storage system for stationary application is presented. A majority of the studies on metal hydride hydrogen systems reported in literature are based on system scale less than 10 kg, leaving questions on the design and performance of large-scale systems unanswered. On the basis of sensitivity to various design and operating parameters such as thermal conductivity, porosity, heat transfer coefficient etc., a comprehensive design methodology is suggested. Following a series of performance analyses, a multi-tubular shell and tube type storage system is selected for the present application which completes the absorption process in 900 s and the desorption process in 2000 s at a system gravimetric capacity of 0.7% which is a vast improvement over similar studies. The study also indicates that after fifty percent reaction completion, heat transfer ceases to be the major controlling factor in the reaction. This could help prevent over-designing systems on the basis of heat transfer, and ensure optimum system weight.     

Simulation of high temperature thermal energy storage system based on coupled metal hydrides for solar driven steam power plants

Concentrating solar power plants can achieve low cost and efficient renewable electricity production if equipped with adequate thermal energy storage systems. Metal hydride based thermal energy storage systems are appealing candidates due to their demonstrated potential for very high volumetric energy densities, high exergetic efficiencies, and low costs. The feasibility and performance of a thermal energy storage system based on NaMgH₂F hydride paired with TiCr_1.6Mn_0.2 is examined, discussing its integration with a solar-driven ultra-supercritical steam power plant. The simulated storage system is based on a laboratory-scale experimental apparatus. It is analyzed using a detailed transport model accounting for the thermochemical hydrogen absorption and desorption reactions, including kinetics expressions adequate for the current metal hydride system. The results show that the proposed metal hydride pair can suitably be integrated with a high temperature steam power plant. The thermal energy storage system achieves output energy densities of 226 kWh/m³, 9 times the DOE SunShot target, with moderate temperature and pressure swings. In addition, simulations indicate that there is significant scope for performance improvement via heat-transfer enhancement strategies.     

Thermodynamic and kinetic factors effecting hydrogen absorption on metal hydrides

The work of Guvendiren et al. on the effects of additives on mechanical milling and hydrogenation of Magnesium Powder which is published in this journal [Guvendiren M, Bayboru E, Ozturk T. International Journal of Hydrogen Energy 2004; 29: 491–496] shows excellent experimental work which agrees with previous published work. However they did not explain the right phenomenon which is undergoing during hydrogen absorption on magnesium hydride's system. In this communication it is the objective to distinguish between thermodynamic and kinetics factor effecting hydrogen absorption. It will be shown that the phase rule of thermodynamics will determine the variation of Pressure–composition isotherms at constant temperature during hydrogen absorption in Magnesium or Titanium. This is because the Pressure–composition isotherms at constant temperature is different for a single-phase (e.g. beta-Titanium) than two phases (e.g. delta and epsilon phases). Thus the data of Guvendiren et al. published in this journal (Fig. 5) can be explained by this effect.     

Design of a AB5-metal hydride cylindrical tank for hydrogen storage

Hydrogen storage in metal hydrides presents distinct challenges which encourage the study of effective heat management strategies. Hydrogen absorption in metal hydrides is an exothermic reaction, consequently the generated heat must be removed effectively to achieve the desired performance. This work presents a mathematical model describing the adsorption of hydrogen in La Ni_4.7Co_0.3 metal hydride as a storage material. Heat and mass transfer effects are modeled in detail. The effect of heat transfer coefficient is also estimated. Besides, a heat transfer fluid for cooling is incorporated to the model. The problem is mathematically formulated presenting a numerical simulation of a design of a cylindrical tank for hydrogen storage. The alloy is studied by using pressure-composition-temperature curves which are carried out at different temperatures. Thermodynamic parameters and hydrogen storage capacity are determined. For isotherm's kinetics, the Jonhson-Mehl-Avrami-Kolomogorov model is used, from which the kinetic constant of the hydriding process is determined.