Low-temperature Metal Hydrides Research Articles

Performance analysis of a metal hydride refrigeration system

The varying applications of metal hydride refrigeration systems, such as cold storage and space air conditioning, grant them important advantages over conventional ones. These advantages include being a low-grade heat driven, more environmentally friendly and renewable working fluid with greater compactness and fewer moving parts. However, a metal hydride refrigeration system always operates under unsteady conditions due to the cyclic hydriding and dehydriding processes involved. To analyse and optimise the metal hydride refrigeration system’s design and performance, in this paper, a comprehensive transient system model has been developed with a new and revised intrinsic kinetic correlation inclusive of the essential operating controls and applicable process conditions of regeneration, cooling and transitions in between. In addition, the correlative model on the characterisation process of pressure, concentration and temperature (PCT) profiles for the metal hydride alloys employed in the system has been developed and is introduced briefly in this paper. It is integrated in the system model and ensures the accurate prediction of maximum capacities for the metal hydride isothermal desorption and absorption processes. The developed transient system model has been validated through comparison with experimental results from literature on the medium-temperature cooling process of a metal hydride refrigeration system. The model simulation is conducted for a specially designed low-temperature metal hydride refrigeration system at different operating conditions and controls. In quantity, when the high-grade heat source temperature increases from 90 °C to 120 °C, the low-grade heat source temperature increases from −20 °C to 10 °C, the medium-grade heat sink temperature decreases from 30 °C to 15 °C, and the time period for regeneration or cooling process decreases from 10 min to 4 min, the cooling COP increases by 112.0%, 136.6%, 19.3% and 31.8% respectively. The optimisation strategies for the system operating conditions and controls are therefore recommended based on the detailed performance analyses of the system simulation results.

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.

Single-stage metal hydride-based heat storage system

The effective use of renewable energy sources (RES) becomes increasingly important, and energy storage systems are needed to ensure their stable operation. Metal hydrides (MH) can be used both for hydrogen storage and for thermal energy storage (TES). We present experimental results of heat, mass transfer and dynamic processes in a single-stage MH-based TES system based on a 5 kg La Ni4.8Mn0.3Fe0.1 metal hydride reactor. Stable cyclic operation in the temperature range 20–90 °C is demonstrated with volumetric energy storage density of 208 MJ/m3 and peak thermal power of 1.05 kW. The systems performance is analyzed regarding the energy and exergy efficiencies. The main bottlenecks include the connected thermal mass of the reactor and the necessity to use lightweight and efficient heat exchanger. In addition, a liquid flow rate in the heat exchanger should be kept low enough to decrease the difference between temperatures of the MH bed and coolant. Optimized low-temperature metal hydride TES systems can be used in climatic-oriented applications, such as MH starters for cold weather or “keep warm” systems.

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.

Numerical investigation of energy desorption from magnesium nickel hydride based thermal energy storage system

The use of dual metal hydride system for thermal energy storage consists of high and low-temperature metal hydrides. In this study, a 3D cylindrical Magnesium Nickel hydride bed is analyzed for thermal energy discharge. The energy discharge from metal hydride bed initially at temperature of 400 K, a heat transfer fluid at 500 K temperature is supplied to extract the heat generated due to exothermic chemical reaction. In this article, variation of the number of heat transfer fluid tubes and effect of variation of aspect ratio (ratio of diameter to height) on energy desorption and heat transfer from metal hydride bed is performed. The optimal number of heat transfer fluid tubes is determined for various aspect ratios. The temperature variation of the metal hydride bed with an increase in the number of heat transfer fluid tubes is analyzed. The study of aspect ratio variation on energy desorption and heat transfer characteristics is analyzed for three aspect ratios 0.5, 1, and 2. The variation of thermal energy desorbed, net heat transfer and temperature variation of metal hydride bed are analyzed. The adequate number of heat transfer fluid tubes for AR 0.5, 1, and 2 is identified as 32, 48, and 72, respectively. The cumulative heat released from MH bed with AR 0.5, 1, and 2 is 350.94 kJ, 330.56 kJ, and 310.42 kJ, respectively. The study will be useful in designing the optimized metal hydride bed reactor for thermal energy storage applications.

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.

Experimental research of metal hydride heat storage reactor processes

The results of the experimental research of thermal, mass exchange and dynamical characteristics of processes inside the low temperature metal hydride (MH) thermal energy storage system are presented. Single stage pressure driven MH heat storage system of closed cycle concept was studied and tested. Intermetallic compound (IMC) LaFe0.1Mn0.3Ni4.8 in the quantity of 5 kg was used as main hydrogen storage/heat emitter element in the reactor. Nominal maximum hydrogen capacity of the reactor is 850 st.l. with though resulting effective volume of cycled hydrogen ended up to be around 240-250 st.l. The reactor type and intermetallic alloy, which were used in the series of experiments, proved to be somewhat suitable for the task, but more advanced heat exchange design along with selection of different type of IMC promise to increase the cycled effective volume along with the system dynamics, resulting in greater thermal energy power output.

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.

La–Ni Based Alloy Modification by Ce and Fe for the Next Hydrogen Storage in Low-Temperature Metal Hydrides

La–Ni based alloys have been established as suitable materials for reversible hydrogen storage. Low-temperature metal hydrides (LT MH) facilitate hydrogen storage at ambient temperatures and pressure of 1–1.5 MPa. Nevertheless, further research and modifications of these alloys are needed. In this paper, the parent LaNi5 alloy was modified by partially replacing La with Ce and Ni with Fe. The alloys were prepared in two different ways: conventional melting of pure powder metals, and thermochemical reaction using corresponding metal chlorides. Steps were taken to optimize the time-temperature parameters of alloy synthesis. A comparative analysis of the obtained alloy samples followed. Hydrogen sorption isotherms were obtained for the unmodified LaNi5 and modified (La0.5Ce0.5)Ni5 samples made from pure powder metals. Due to strong oxidation of the alloys prepared from metal chlorides, sorption isotherms for these alloys were not obtained.

Hydrogen KW-Scale Energy Storage Systems on the Base of Metal Hydrides and Fuel Cells

The results of demonstration projects on development and testing of hydrogen fuel cell power units utilizing low-temperature metal hydrides of AB5 type as hydrogen storage and purification tool are presented. In the effort to enable the technology for autonomous applications, the novel concept of using fuel cell exhaust air for hydrogen desorption process replacing an external heating agent was successfully proved. The technology of through-flow metal-hydride based hydrogen purification was also successfully utilized for biohydrogen. The results of two proofof- concept projects show a good perspective for autonomous applications and open the opportunity for further research in the field of power system control.

Hydrogen storage systems for fuel cells: Comparison between high and low-temperature metal hydrides

The need for the enhancement of alternative energy sources is increasingly recognised and, in this perspective, the achievement of hydrogen economy seems to be fundamental. In this regard, fuel cells represent an interesting option for small and medium scale distributed renewable generation; however, these systems are inextricably linked with the concept of hydrogen storage. Research on metal hydrides revealed the opportunity to use these materials as basic elements in hydrogen storage devices, called MH systems. This means that interest exists in investigating the behaviour of metal hydrides: in fact, MH system operation is based on the hydriding/dehydriding reactions hydrides undergo, and, with the aim of evaluating the performance of such devices, these processes must be discussed and modelled.In the light of this, a simple numerical model to study hydride-based storage systems and their integration with fuel cells was developed: two low-temperature hydrides (LaNi5, LaNi4·8Al0.2) and two high-temperature hydrides (Mg, Mg2Ni) were selected and their behaviours in a MH system were simulated and compared with the help of such a model. This is an essential step in identifying the hydrides more suited to the application in question. Results showed that the choice is the trade off between encumbrance and reaction times; this implies that low-temperature hydrides are preferable because their encumbrance is limited and their reaction temperature range grants a greater versatility in small scale generation.

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.

Techno-economic analysis of screening metal hydride pairs for a 910 MWhth thermal energy storage system

Matching of metal hydride pairs has a significant influence on performance of thermal energy storage (TES) system. This article conducts a complete techno-economic analysis of screening metal hydride pairs (MgH2&LaNiAl and MgH2&TiFeMn). A mathematical model is developed to calculate the energy consumption, which is solved by COMSOL Multiphysics v5.1. Firstly, thermodynamic matching is analyzed to judge the energy consumption qualitatively. Further, a cost model of thermal energy is established to estimate the energy consumption cost. It is found that the charging energy consumption cost of MgH2&LaNiAl system is reduced to be zero due to a good thermodynamic matching, whereas that of MgH2&TiFeMn system accounts for as high as 63.8% of the cycle energy consumption cost. Based on the life cycle economic analysis, matching of MgH2&TiFeMn is considered to be a better selection due to a smaller levelized thermal storage cost (28 USD/kWhth), where two major expenses are the capital cost and energy consumption cost, 74.3% and 19.3% respectively. Therefore, a matching principle is concluded that screening metal hydride pairs for TES should be considered in two ways: firstly, the hydrogen storage cost due to the expensive price of low temperature metal hydride; secondly, the thermodynamic matching, which determines the energy consumption cost.

High temperature metal hydrides for energy systems Part B: Comparison between high and low temperature metal hydride reservoirs

A dynamic analysis model aimed at describing the hydrogen absorption and desorption phases of a metal hydride has been calibrated for magnesium hydride in Part A of the present work. We can make use of the estimate of the main kinetic parameters associated to this kind of hydride in order to study the behaviour of a metal hydride-based energy system.A metal hydride becomes the basis of an energy system when the enthalpy related to its hydriding/dehydriding reactions is used by an applicator. Therefore, magnesium hydride, which is a high-temperature hydride, can be virtually placed in an energy system thanks to the model and its main energy-related characteristics can be calculated. This allows us to get a glimpse of the performances of magnesium hydride in the field of energy production and to compare them to those of well-established low-temperature hydrides, such as LaNi5 hydride.

The use of air as heating agent in hydrogen metal hydride storage coupled with PEM fuel cell

The possibility to refuel air-cooled PEMFC by hydrogen desorbed from low temperature metal hydride storage using the FC exhaust air was successfully demonstrated. The volumetric flow of hydrogen exceeded the values needed to ensure 1.1 kW (e) FC capacity level. Experimental setup, the results of experimental investigations are presented and discussed, as well as the reserves for the technology application for kW scale power production units based on air-cooled fuel cells.

High performance metal hydride based thermal energy storage systems for concentrating solar power applications

Thermal energy storage systems based on metal hydride pairs using high efficiency materials are evaluated. The low temperature metal hydrides NaAlH4 and Na3AlH6 were cycled to determine stability of hydrogen capacity over extended cycling. Addition of aluminum and expanded natural graphite were found to enhance the cycling stability of NaAlH4. Potential high temperature metal hydrides were investigated based on NaMg materials. A techno-economic analysis was performed to evaluate the performance a thermal energy storage system based on two metal hydride pairs: NaMgH3:NaAlH4 and NaMgH2F:Na3AlH6. The resulting analysis suggests that the two systems have the potential to reach low cost and high efficiency performance targets.

Complex Rare-Earth Aluminum Hydrides: Mechanochemical Preparation, Crystal Structure and Potential for Hydrogen Storage

A novel type of complex rare-earth aluminum hydride was prepared by mechanochemical preparation. The crystal structure of the REAlH(6) (with RE = La, Ce, Pr, Nd) compounds was calculated by DFT methods and confirmed by preliminary structure refinements. The trigonal crystal structure consists of isolated [AlH(6)](3-) octahedra bridged via [12] coordinated RE cations. The investigation of the rare-earth aluminum hydrides during thermolysis shows a decrease of thermal stability with increasing atomic number of the RE element. Rare-earth hydrides (REH(x)) are formed as primary dehydrogenation products; the final products are RE-aluminum alloys. The calculated decomposition enthalpies of the rare-earth aluminum hydrides are at the lower end for reversible hydrogenation under moderate conditions. Even though these materials may require somewhat higher pressures and/or lower temperatures for rehydrogenation, they are interesting examples of low-temperature metal hydrides for which reversibility might be reached.