High-temperature Metal Hydride Research Articles

Heat discharge performance of metal hydride thermal battery under different heat transfer conditions: Experimental findings

Thermal batteries utilizing metal hydride pairs are gaining tremendous research appeal for their applications in thermochemical energy storage. The pair consisting of a high-temperature metal hydride (HTMH: Mg-based hydride) and a low-temperature metal hydride (LTMH: AB2 type hydride) is attractive due to its relatively high energy storage density and medium energy storage efficiency. The energy storage efficiency can be significantly increased by improving the heat discharging performance of the thermal battery. In this study, we experimentally explore the heat releasing performance of an MgH2/(TiZr)(MnFeCr)2-based thermal battery. More specifically, the effects of LTMH bed heat transfer conditions on the heat discharging performance were briefly discussed. These heat transfer conditions include natural convection, forced convection, and resistive heating. The results showed that when operating the LTMH bed under active heat transfer conditions (forced convection or resistive heating), the thermochemical energy storage density varies between 1500 and 1820 kJ/kg-Mg with relatively high-temperature lift (heat upgrade) between 47 and 55 °C. On the other side, the thermal battery discharges heat at a relatively high specific power, ca. 100–225 W/kg-Mg, which can be a benefit for heat-to-work conversion applications using organic or steam Rankine cycles.

Parametric assessment of a hybrid heat storage unit based on paired metal hydrides and phase change materials

In order to generate electricity at night and on cloudy days, concentrated solar power (CSP) plants need a sufficient thermal energy storage (TES) unit. The most effective TES unit that combines thermochemical and latent heat storage techniques. In the present paper, for CSP plants, a new MH/PCM-TES unit design is proposed. This unit consist of paired metal hydride (MH) tanks that are connected. The high-temperature metal hydride (HTMH) tank filled with Mg2Fe alloy is used to store solar heat, while the low-temperature metal hydride (LTMH) tank filled with Na3Al alloy is used to store hydrogen that desorbed by the HTMH tank during heat charging process. The thermochemical heat released by the LTMH tank is stored as latent heat in a PCM truncated heat exchanger for reuse during desorption pocess. A two-dimensional mathematical model is established and a numerical code written in Fortran-90 is developed to study the performance of the proposed hybrid heat storage unit. The volumetric storage capacity, the specific power, and the energy efficiency are used as key performance indicators of the new MH/PCM-TES unit. The effects of the PCM's thermo-physical proprieties and the operating temperatures are investigated and discussed. The obtained numerical results demonstrated that the heat charging/discharging temperature and thermo-physical properties of PCM (melting temperture, density, thermal conductivity, specific heat) had a substantial effect on the used key performance indicators. In addition, it was found that the use of PCM with the LTMH tank enhances the energy recovery efficiency of the hybrid heat storage unit by about 30%.

Thermochemical energy storage using coupled metal hydride beds of Mg-LaNi5 composites and LaNi5 based hydrides for concentrated solar power plants

• A MHTES system is proposed for CSP plant application. • Energy storage and recovery temperatures are estimated. • PCIs of Mg based composites and LaNi 5 based alloys are measured. • Reaction enthalpy and entropy are evaluated using van’t Hoff equation. • The performance of the MHTES system is analyzed thermodynamically. • The effect of operating temperatures on the performance of the MHTES is studied. Thermochemical energy storage based on coupled hydride beds is an attractive option for Concentrated Solar Power plant applications due to its efficient long term and high energy storage density. In the coupled system, a high temperature metal hydride (HTMH) bed is coupled with a low temperature metal hydride (LTMH) bed. In the present study, Mg-LaNi 5 composite hydrides are taken as HTMHs due to their high absorption capacity and high reaction enthalpy, and LaNi 5 based alloys are taken as LTMHs as these hydrides are ideal for hydrogen storage at ambient conditions. The thermodynamic properties such as reaction enthalpy and entropy are evaluated by measuring Pressure Concentration Isotherms of Mg- x wt% LaNi 5 ( x =20 and 30) composites, LaNi 4.7 Al 0.3 and LaNi 4.6 Al 0.4 . A new methodology for storing and recovering thermal energy together with upgradation of the quality of energy is established. Methodology to determine the minimum energy storage temperature and maximum energy recovery temperature is proposed. The quality of the thermal energy will be improved by increase in delivery temperature. Thermodynamic analysis is performed using measured properties to evaluate the performance parameters such as heat upgradation, coefficient of performance (COP), and energy storage density (ESD). Maximum COP and ESD of 0.705 and 418.841 kJ/kg, respectively is obtained for hydride pair Mg-5wt% LaNi 5 -La 0.8 Ce 0.2 Ni 5 . Maximum degree of upgradation of 34°C is obtained using the hydride pair Mg-20wt% LaNi 5 -LaNi 4.7 Al 0.3 . The effects of variation of heat source temperature, regeneration temperature and ambient temperature on the performance of the MHTES is studied. It is observed that the regeneration temperature is a significant parameter that influences the performance of the MHTES.

Charging and discharging analysis of thermal energy using magnesium nickel hydride based thermochemical energy storage system

A concentrated solar power plant integrated with thermal energy storage system helps to operate the plant during non-sunshine hours. This helps to increase the power generation capacity and improve the dispatchability of the power plant. Thermal energy systems are classified into sensible, latent, and thermochemical energy storage systems. Thermochemical energy storage with high energy storage density is one of the potential energy storage techniques available. The metal hydride-based thermal energy storage system has high energy storage density, better chemical reversibility, long cyclic, and high temperature stability. A dual bed metal hydride system uses high and low-temperature metal hydrides. High temperature metal hydride is used as energy storage media, while low-temperature metal hydride is used as hydrogen storage media. In this article, a detailed discussion on the energy sorption (absorption and desorption) analysis of a metal hydride suitable for high temperature in a dual bed metal hydride-based thermal energy storage system is performed. A 3-D model of Magnesium Nickel alloy in cylindrical geometry with radially distributed axial tubes is developed in COMSOL Multiphysics 5.5. The energy absorption and desorption analysis of metal hydride with variation in the number of heat transfer fluid tubes and different aspect ratio geometry are performed in detail. Study on variation of the number of heat transfer fluid tubes includes identifying the requirement of the adequate number of heat transfer fluid tubes to supply/remove heat to/from metal hydride during energy absorption/desorption. Variation of aspect ratios on the heat energy sorption characteristics and heat transfer phenomenon is studied in detail for both energy absorption and desorption cycle. The adequate number of heat transfer fluid tubes for the aspect ratio 0.5, 1, and 2 are found as is 32, 48, and 72, respectively. The stored and released energy for absorption and desorption study for the aspect ratio 0.5, 1, and 2 are 274.83 kJ, 255.44 kJ, and 240.27 kJ and 250.11 kJ, 234.43 kJ, and 224.10 kJ, respectively. The energy storage efficiency of the metal hydride bed for aspect ratios 0.5, 1, and 2 is 91%, 91.77%, and 93.27% respectively, while the overall system efficiency of the metal hydride bed for aspect ratios 0.5, 1, and 2 is 70.46%, 71.23%, and 72.39% respectively. The outcomes of this study helps to design the metal hydride based thermochemical energy storage system for process heating and power generation.

Operating Characteristics of Metal Hydride-Based Solar Energy Storage Systems

Thermochemical energy storage systems, based on a high-temperature metal hydride coupled with a low-temperature metal hydride, represent a valid option to store thermal energy for concentrating solar power plant applications. The operating characteristics are investigated for a tandem hydride bed energy storage system, using a transient lumped parameter model developed to identify the technical performance of the proposed system. The results show that, without operational control, the system undergoes a thermal ratcheting process, causing the metal hydride concentrations to accumulate hydrogen in the high-temperature bed over time, and deplete hydrogen in the low temperature. This unbalanced system is compared with a ’thermally balanced’ system, where the thermal ratcheting is mitigated by thermally balancing the overall system. The analysis indicates that thermally balanced systems stabilize after the first few cycles and remain so for long-term operation, demonstrating their potential for practical thermal energy storage system applications.

An innovative multi-zone configuration to enhance the charging process of magnesium based metal hydride hydrogen storage tank

High-temperature metal hydride (MH), such as magnesium hydride, is considered as one of the most promising technology to store hydrogen. However, there are two main bottlenecks, including the low rate of hydrogen absorption and low capacity of the MH reactor. In this regard, heat removal from the MH tank plays a crucial role in the hydrogen storage process. In the present study, to increase the hydrogen absorption performance, a novel configuration of the MH reactor is proposed and simulated using computational fluid dynamics (CFD). The study aims to assess the geometrical parameters of the proposed heat exchanger. Besides, a sensitivity analysis of the operating parameters of the reactor, including the Reynolds number and temperature of the air as well as hydrogen supply pressure is performed. The results indicate that the charging time drops significantly by increasing the number of air passages since the heat transfer rate improves dramatically. By raising the heat transfer fluid initial temperature, the charging time increases; however, as the heat transfer fluid Reynolds number and the inlet pressure of hydrogen increase, the absorption process accelerates. The recommended configuration is introduced by considering both charging time and manufacturing limitations. It is shown that the loading is approximately 30 minutes for the new multi-zone hydrogen energy storage using four passages for the air which provides a more applicable hydrogen fuel system.

Alloy Selections in High-Temperature Metal Hydride Heat Pump Systems for Industrial Waste Heat Recovery

Alloy Selections in High-Temperature Metal Hydride Heat Pump Systems for Industrial Waste Heat Recovery

Numerical modeling and performance comparison of high-temperature metal hydride reactor equipped with bakery system for solar thermal energy storage

The current study provides an optimal design strategy for the metal hydride reactor for the thermal energy storage system for solar bakery applications. The sensitivity analysis of the annular truncated hollow conical fins type reactor was conducted by using COMSOL Multiphysics 5.3a software. The gravimetric exergy output rate and exergy output of different designs were compared. It was found that the optimal geometric parameters and operating conditions for the annular truncated hollow conical fins type reactor are fin angle (θ)=20°, fin radius (Rf) = 0.07 m, number of fins (Nf)= 32 and fin spacing (Sf) = 0.02 m, fin thickness (thf) = 0.008 m, hydrogen supply pressure (Pin, H2) = 5 MPa and inlet heat transfer fluid velocity (VHTF,in) = 20 m/s. Finally, it was concluded that the Deign IV is the most favorable design, which has a uniform reaction, faster reaction time (15,000 s), and higher values of gravimetric exergy output rate and exergy output (1.23 W/kg, 0.028 kW).

Performance improvement of solar bakery unit by integrating with metal hydride based solar thermal energy storage reactor

Storage of solar thermal energy is an effective way to overcome the issue of incompatibility between the solar system and its users. This paper proposed a novel perception by integrating a metal hydride based thermochemical energy storage (MH-TES) system with a solar bakery unit (SBU). A numerical simulation was performed by using COMSOL MULTIPHYSIS 5.3a to assess the dynamic behavior of the reactor. The results showed that the high-temperature metal hydride (HTMH) reactor during the discharging process has a high improvement potential rate (0.195 kW), a high exergetic factor (59.97%) and low exergy efficiency (30.25%) as compared to HTMH reactor during the heat charging process. The overall exergy efficiency of the HTMH reactor was estimated as 34.03%. The maximum value of energy output (Qout), exergy output (Exout), gravimetric exergy output rate (GEOR) and gravimetric efficient heat output rate (GEHOR) during discharging period was calculated as 1560 W, 174 W, 7.4 W/kg and 66 W/kg, respectively. The energy efficiency of SBU is improved from 6% to 42.89% by integrating with the MH-TES system. This paper presents a comprehensive approach for thermal analysis of HTMH for the solar bakery system, which contributes to system optimization for efficient solar energy storage.

A novel electrical energy storage system based on a reversible solid oxide fuel cell coupled with metal hydrides and waste steam

Reversible solid oxide fuel cells (RSOFCs), with high energy densities, long operating times, and intermediate power ratings, have become promising devices for renewable energy storage. A metal hydride (MH) tank is a prospective thermochemical heat and hydrogen storage unit. External heat source such as waste steam is a well-known efficiency booster for high temperature electrolysis system. Here, we propose a novel RSOFC system coupled with MH and waste steam. The MH materials of MgH2-5 at.% V and LaNi5 were used for high-temperature MH (HTMH) case and low-temperature MH (LTMH) case calculations, respectively. We found that, In HTMH case, the H2 compression power was low, but the MH tank produced steam only during the last 29% of the total absorption time. When the MH tank produced steam, the SOEC mode efficiency increased by 19.3% points. In the SOFC mode, the MH tank stored 76% of heat released from stack, and the system efficiency was lower than stack efficiency by 6% points. The system round trip efficiencies of HTMH system and LTMH system were 45.6% and 48.1%, respectively. For a specific HTMH material, there is an optimal current density in the SOEC mode where heat from MH tank can be used completely and the external heat source is minimal. By choosing appropriate operating strategy or MH material, the high temperature MH can result in a system round-trip efficiency comparable to that of a low temperature MH combined with an external heat utilization system.

Metal Hydride Beds-Phase Change Materials: Dual Mode Thermal Energy Storage for Medium-High Temperature Industrial Waste Heat Recovery

Heat storage systems based on two-tank thermochemical heat storage are gaining momentum for their utilization in solar power plants or industrial waste heat recovery since they can efficiently store heat for future usage. However, their performance is generally limited by reactor configuration, design, and optimization on the one hand and most importantly on the selection of appropriate thermochemical materials. Metal hydrides, although at the early stage of research and development (in heat storage applications), can offer several advantages over other thermochemical materials (salt hydrates, metal hydroxides, oxide, and carbonates) such as high energy storage density and power density. This study presents a system that combines latent heat and thermochemical heat storage based on two-tank metal hydrides. The systems consist of two metal hydrides tanks coupled and equipped with a phase change material (PCM) jacket. During the heat charging process, the high-temperature metal hydride (HTMH) desorbs hydrogen, which is stored in the low-temperature metal hydride (LTMH). In the meantime, the heat generated from hydrogen absorption in the LTMH tank is stored as latent heat in a phase change material (PCM) jacket surrounding the LTMH tank, to be reused during the heat discharging. A 2D axis-symmetric mathematical model was developed to investigate the heat and mass transfer phenomena inside the beds and the PCM jacket. The effects of the thermo-physical properties of the PCM and the PCM jacket size on the performance indicators (energy density, power output, and energy recovery efficiency) of the heat storage system are analyzed and discussed. The results showed that the PCM melting point, the latent heat of fusion, the density and the thermal conductivity had significant impacts on these performance indicators.

Defecton Contribution to the High-Temperature Superconductivity of Hydrides

We introduce below the concept of defecton, and describe briefly the electron polaron effect, the clustering of defectons, and also the defecton mechanism of superconductivity. It is shown that in the case of high-temperature superconducting metal hydrides, which acquire superconducting properties under pressure of hundreds of gigapascals, this mechanism can make a significant contribution to the Cooper pairing.

Numerical investigation of high temperature metal hydride water pumping system

I n the present work, a high temperature Metal Hydride Water Pumping System (MHWPS) equipped with a latent heat exchanger was investigated numerically. The operating concept of the pump was presented and the mathematical model of heat and mass transfer within the pump was established. We simulated the pump under different operating conditions using the alloy Mg2Ni as a metal hydride and the KNO3 as a Phase Change Material (PCM). The obtained results have shown that i) the developed numerical model is flexible and accurate in predicting the dynamic behavior of the pump ii) the numerical model without radiative heat transfer gives results with errors that can reach 38%, particularly for pumping time iii) the integration of the PCM provides a reduction in the pumping time of about 90% and an increase of the efficiency of the pump of about 7.6 times compared to the case without PCM which represents an improvement of 86% and iv) the Mg2Ni alloy requires high temperature and a mass of PCM about 9 times larger than the case of LaNi5 alloy to pump a volume 6.5 times greater than that pumped with LaNi5.

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/kWhth 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/kWhth 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.

Thermal conductivity measurements of magnesium hydride powder beds under operating conditions for heat storage applications

One of the major issues of the change in energy politics is the storage of renewable energy in order to facilitate a continuous energy supply to the grid. An efficient way to store energy (heat) is provided by the usage of Thermochemical Energy Storage (TES) in metal hydrides. Energy is stored in dehydrogenated metal hydrides and can be released by hydrogenation for consumption. One prominent candidate for high temperature (400 °C) heat storage is magnesium hydride. It is a well-known and investigated material which shows high cycling stability over hundreds of cycles. It is an abundant material, non-toxic and easy to prepare in bigger scales. One of the major drawbacks for heat storage applications is the low heat transfer capability of packed beds of magnesium hydrides. In this work we present results of effective thermal conductivity (ETC) which were measured under hydrogen pressure up to 25 bar and temperatures up to 410 °C in order to meet the operating conditions of magnesium hydride as a thermochemical heat storage material. We could show that the effective thermal conductivity of a magnesium hydride – hydrogen system at 410 °C and 25 bar hydrogen increases by 10% from 1.0 W m−1 K−1 to 1.1 W m−1 K−1 after 18 discharging and charging cycles. In dehydrogenated magnesium hydride this increase of the thermal conductivity was found to be at 50% from 1.20 W m−1 K−1 to 1.80 W m−1 K−1 at 21 bar hydrogen. These data are very important for the design and construction of heat storage tanks based on high temperature metal hydrides in the future.

Methods for accurate high-temperature Sieverts-type hydrogen measurements of metal hydrides

The potential of high-temperature metal hydrides (HTMHs) as thermal energy storage materials means that accurate assessment of their properties is required on the laboratory scale. Above ≃ 450 °C, the hydrogen permeability through sample cell reactors (SCRs) used in experiments to characterise HTMHs can have an appreciable impact on the data. In this work, the thermodynamic and kinetic properties of TiHx have been measured at temperatures up to 725 °C as a test of different practical methods for limiting the effects of hydrogen permeability and solubility in SCRs during high-temperature characterisation of metal hydrides. Aluminium-coated stainless steel and α-SiC were used to construct SCRs with reduced hydrogen permeability using commercially available techniques. When steel SCRs are the only practical option, guidelines have been developed to choose experimental conditions that minimise the effect on the data collected due to their hydrogen permeability and solubility. A method has also been developed to correct collected data for the hydrogen permeability and solubility of SCRs.

Adiabatic magnesium hydride system for hydrogen storage based on thermochemical heat storage: Numerical analysis of the dehydrogenation

With hydrogen becoming more and more important as storage and carrier for renewable energy, there is an increasing need for flexible and efficient storage technologies. However, existing technologies, such as liquefaction or compression, often require a significant share of the hydrogens lower heating value. High-temperature metal hydrides (HT-MHs), such as magnesium hydride, are a promising alternative. Due to high operation temperatures, their application is challenging. A novel adiabatic hydrogen storage reactor based on the combination of a HT-MH with a thermochemical energy storage system (TCSS), such as Mg(OH)2/MgO + H2O, can be a solution. In this work, the previously published numerical simulations for hydrogen absorption are extended to the desorption process. A two-dimensional model for the hydrogen release was set up. The performance of the storage reactor is strongly dependent on the thermodynamic equilibrium of the reactions involved and less dependent on the reaction kinetics. Dehydrogenation is possible within 132 min, which is in the vicinity of the hydrogenation time. To enhance the dehydrogenation process, the water vapor pressure can be adjusted aiming for higher temperatures during the MgO hydration. Hydrogen can either be provided at constant pressure or constant mass flow rate.

STUDY OF SORPTION KINETICS IN METAL HYDRIDE PAIRS

We present the results of studying sorption processes in metal hydride pairs. Technique and laboratory setup are developed for experimental study of kinetics and determination of the duration of interaction between hydrogen and hydrides formed upon saturation of two intermetallic compounds (IMC) of different compositions. The design of the setup and principle of operation are presented. The setup includes two steel retorts which contain the alloys capable of multicycle reversible sorption of hydrogen with formation of the high- and low-temperature hydrides. The system provides measurements of pressure in the hydrogen pipeline between the retorts and temperature of circulating or stirred heat carriers which cool the retorts during sorption and heat them during desorption. The dynamics of hydrogen pressure change in the pipeline and temperature of the heat carrier under simultaneous operation of two hydrides of alloys Mm 1– y La y Ni 4 Co and LaNi 5– x Al x are studied. The duration of hydrogen desorption from the first hydride with simultaneous sorption by the second one is determined. The thermal balance for the low-temperature module of the setup was calculated to prove the obtained kinetic characteristics. Similar results were obtained for desorption of hydrogen from high-temperature metal hydride. The technique and results can be used for simulation of sorption processes in metal hydride pair and determination of the parameters of optimal design for efficient operation of sorption heat pumps.

Numerical study of high temperature metal-hydrogen reactor (Mg2Ni-H2) with heat reaction recovery using phase-change material during desorption

High temperature metal hydrides used for storing hydrogen have promising future prospect due to the interesting density that characterizes it. This type of hydride offers high hydrogen storage capacities and its formation is highly exothermic. Therefore, the formation enthalpy represents 30% of the lower heating value (LHV) of the absorbed hydrogen, which represents a significant energy cost.This article introduces a numerical study of a new cylindrical conception of a high temperature metal-hydrogen reactor (MHR) integrating a phase change material (PCM). This system was developed to store efficiently the realized heat during hydrogen absorption process in order to recover it during desorption. A mathematical model was developed in two-dimensional (2D) and solved using the unstructured control volume finite element method (CVFEM). For this model, the proposed expressions of the reaction kinetic and the equilibrium pressure of the hydride system (Mg2NiH4) [1] and the analytic approximation based on the Heaviside step function [2] to describe the liquid fraction were used to simulate the dynamic behavior of the reactor during desorption process. Numerical results showed that the new composition Mg69Zn28Al3, which investigated by Garrier et al. [3] is the appropriate PCM that met the requirements to store the exothermic released heat for our hydride system. Additionally, the results have shown that the time required for the reactor to desorb all the hydrogen do not exceed 2.5 h.

High-temperature high-pressure calorimeter for studying gram-scale heterogeneous chemical reactions.

We present an instrument for measuring pressure changes and heat flows of physical and chemical processes occurring in gram-scale solid samples under high pressures of reactive gases. Operation is demonstrated at 1232 °C under 33 bars of pure hydrogen. Calorimetric heat flow is inferred using a grey-box non-linear lumped-element heat transfer model of the instrument. Using an electrical calibration heater to deliver 900 J/1 W pulses at the sample position, we demonstrate a dynamic calorimetric power resolution of 50 mW when an 80-s moving average is applied to the signal. Integration of the power signal showed that the 900 J pulse energy could be measured with an average accuracy of 6.35% or better over the temperature range 150-1100 °C. This instrument is appropriate for the study of high-temperature metal hydride materials for thermochemical energy storage.