Discharge dynamics of coupled fuel cell and metal hydride hydrogen storage bed for small wind hybrid systems

In small hybrid wind systems, excess wind energy is stored for later use during the deficit power generation. Excess wind energy can be stored as hydrogen in a metal hydride storage bed and reused later to generate power using a fuel cell. This paper deals with the discharge dynamics of the coupled fuel cell and metal hydride storage bed during the power extraction. Thermal coupling of the fuel cell and metal hydride bed is also discussed. The waste heat generated in the fuel cell is removed using a water coolant. The exit fuel cell coolant stream is passed through the metal hydride storage bed to supply the necessary heat required for desorption of hydrogen from the bed. This will also lead to a reduction in the load on the radiator. The discharge dynamics and the thermal management of the coupled system are demonstrated through a system simulation model developed in Matlab/Simulink platform.     

Three-dimensional modeling of hydrogen sorption in metal hydride hydrogen storage beds

This paper conducts a three-dimensional (3D) modeling study to investigate the hydrogen absorption process and associated mass and heat transport in a metal hydride (LaNi₅) hydrogen storage tank. The 3D model is further implemented numerically for validation purpose and the detailed investigation on absorption process. Results indicate that at the very initial absorption stage the bed temperature evolves almost uniformly, while it varies greatly spatially at the latter stage. At the initial seconds, most hydrogen is absorbed in the region near the cooling wall due to the better heat removal. The absorption in the core is slow at the beginning, but becomes important at the very end stage. It also shows that the initial hydrogen flow in the bed is several-fold larger than the latter stage and the flow may provide extra cooling to the hydriding process. By analyzing the Peclet number, we find that the heat convection by the hydrogen flow may play an important role in local heat transfer. This work provides an important platform beneficial to the fundamental understanding of multi-physics coupling phenomena during hydrogen absorption and the development of on-board hydrogen storage technology.     

Experimental formula for estimating porosity in a metal hydride packed bed

An experimental formula for estimating porosity in a metal hydride packed bed is presented. The formula was developed by direct observation of the volume changes of a metal hydride packed bed under free expansion in a vessel. The experimental results showed that the cycles of expansion and contraction were repeated at large porosities above 60% after a rapid state change caused by early particle breakup. The formula for porosity was expressed as a function of the reacted fraction and as a function of the cycle number. The function formula of the reacted fraction can be used to compute different values of porosity for expansion by absorption and for contraction by desorption. The coefficients assuming 100% hydrogen storage based on the experiments with LaNi₅ were an expansion ratio of 16.7% and a contraction ratio of 8.4%, on average. This experimental porosity formula is useful for effective thermal conductivity calculations and for numerical simulations of metal hydride packed bed behavior.     

Effect of design parameters on enhancement of hydrogen charging in metal hydride reactors

The effects of heat transfer mechanisms on the charging process in metal hydride reactors are studied under various charging pressures. Three different cylindrical reactors with the same base dimensions are designed and manufactured. The first one is a closed cylinder cooled with natural convection, the fins are manufactured around the second reactor and the third reactor is cooled with water circulating around the reactor. The temperatures of the reactor at several locations are measured during charging with a range of pressure of 1–10 bar. The third reactor shows the lowest temperature increase with the fastest charging time under all charging pressures investigated. The effective heat transfer coefficients of the reactors are also calculated according to the experimental results and they are found to be 5.5 ± 1 W m⁻² K⁻¹, 35 ± 2 W m⁻² K⁻¹ and 113 ± 1 W m⁻² K⁻¹, respectively. The experimental results showed that the charging of hydride reactors is mainly heat transfer dependent and the reactor with better cooling exhibits the fastest charging characteristics.     

Predictive calculation of the effective thermal conductivity in a metal hydride packed bed

The effective thermal conductivity of a metal hydride packed bed was calculated by considering the influence of expansion during hydrogen absorption and contraction during hydrogen desorption. The porosity was calculated using an experimental formula developed by direct observation, which was used in combination with other referenced methods. However, none of the methods could express the reported experimental value response to pressure change using only the experimental porosity formula. The area contact model was modified so that the porosity and the contact area changed with expansion and contraction. The contact area change was calculated by assuming a simple geometrical deformation caused the difference between the particle expansion and the packed bed expansion. The calculation results of the improved area contact model with the deformed factor and the shape factor were in good agreement with the reported experimental data. This calculation method of the effective thermal conductivity with the influence of expansion and contraction is expected to be useful for designing of heat transfer enhancement of a hydrogen storage tank.     

Demonstration of a micro-fabricated hydrogen storage module for micro-power systems

The objective of this work was to demonstrate a micro-fabricated hydrogen storage module for micro-power systems. Hydrogen storage materials were developed as thin-film inks to be compatible with an integrated manufacturing process. Performance and durability of storage modules were evaluated. Further, applications were demonstrated for a nickel-hydrogen battery and a micro-fabricated hydrogen-air PEM fuel cell. The ink making process, in which polymer binders and solvents were added to the palladium-treated alloys, slightly decreased the storage capacities, but had little effect on the activation properties of the treated alloys. After 5000 absorption/desorption cycles under hydrogen, the hydrogen storage capacities of the thin-film inks remained high. Absorption/desorption behavior of the ink was tested in the environment of a new type nickel-hydrogen battery, in which it would in contact with 26 wt% KOH solution, and the ink showed no apparent degradation. Storage modules were successfully used as the hydrogen source for PEM fuel cell.     

Impacts of external heat transfer enhancements on metal hydride storage tanks

The rate at which hydrogen can be drawn from a metal hydride tank is strongly influenced by the rate at which heat can be transferred to the reaction zone. In this work, the impacts of external convection resistance on thermodynamic behaviour inside the metal hydride tank are examined. A one-dimensional resistive analysis and two-dimensional transient model are used to determine the impact of external fins on the ability of a metal hydride tank to deliver hydrogen at a specified flow rate. For the particular metal hydride alloy (LaNi₅) and tank geometry studied, it was found that the fins have a large impact on the pressure of the hydrogen gas within the tank when a periodic hydrogen demand is imposed. Model results suggest that the metal hydride alloy at the centre of the tank can be removed to reduce weight and cost, without detrimental effects to the performance of the system.     

Improved hydrogen storage properties of Mg-based nanocomposite by addition of LaNi5 nanoparticles

Mg (200 nm) and LaNi₅ (25 nm) nanoparticles were produced by the hydrogen plasma-metal reaction (HPMR) method, respectively. Mg–5 wt.% LaNi₅ nanocomposite was prepared by mixing these nanoparticles ultrasonically. During the hydrogenation/dehydrogenation cycle, Mg–LaNi₅ transformed into Mg–Mg₂Ni–LaH₃ nanocomposite. Mg particles broke into smaller particles of about 80 nm due to the formation of Mg₂Ni. The nanocomposite showed superior hydrogen sorption kinetics. It could absorb 3.5 wt.% H₂ in less than 5 min at 473 K, and the storage capacity was as high as 6.7 wt.% at 673 K. The nanocomposite could release 5.8 wt.% H₂ in less than 10 min at 623 K and 3.0 wt.% H₂ in 16 min at 573 K. The apparent activation energy for hydrogenation was calculated to be 26.3 kJ mol⁻¹. The high sorption kinetics was explained by the nanostructure, catalysis of Mg₂Ni and LaH₃ nanoparticles, and the size reduction effect of Mg₂Ni formation.     

Performance simulation of metal hydride hydrogen storage device with embedded filters and heat exchanger tubes

A practical metal hydride based hydrogen storage device would consist of many filters to distribute hydrogen gas and heat exchanger tubes to cool or heat the hydride bed depending on whether hydrogen is being absorbed or desorbed. This paper presents the simulation of such a device with LaNi₅ as the hydriding alloy. A study of the geometric and operating parameters has been carried out to identify their influence in the hydriding performance of the storage device.     

Simulation of transient heat and mass transfer during hydrogen sorption in cylindrical metal hydride beds

A CFD analysis of heat and mass transfer in cylindrical metal hydride beds is carried out using the commercial code Fluent 6.2. The effect of bulk diffusion is considered for mass transfer in the solid phase. Temporal and spatial variations of temperature and concentration in hydride bed are plotted. Emphasis is given to monitor the motion of hydrogen within the bed and to the influence of the L/D$L/D$ ratio and porosity. It is observed that a concentration variation in the bed is the driving force for hydrogen flow in hydride beds. The gas movement is observed to be from saturated cooler peripheral region towards the unsaturated hotter core region of the bed.     

Exergy analysis of self-ignition combustion synthesis for producing rare-earth-based hydrogen storage alloy

A new production system for rare-earth-based hydrogen storage alloys is proposed. We applied self-ignition combustion synthesis (SICS) utilizing hydrogenation heat of metallic calcium. The required primary energy and total exergy loss (EXL) for the production of 1 kg of LaNi₅ alloy with the proposed and conventional systems were evaluated. The results revealed that the production of raw materials accounted for more than 90% of the total EXL in both systems. Specifically, the use of calcium had decisive effects on the total EXL of the system for producing LaNi₅ alloy. The proposed system reduced the total EXL by 14.6 MJ/kg-LaNi₅ as compared with the conventional system. The SICS was remarkably exergy-saving because the heating temperature was decreased by utilizing the hydrogenation heat of calcium and the product absorbed hydrogen without an activation treatment.     

Development of a hydrogen catalytic heater for heating metal hydride hydrogen storage systems

This paper describes the design, fabrication and performance evaluation of a high efficiency, compact heater that uses the catalytic oxidation of hydrogen to provide heat to a hydrogen storage system. The heater was designed to transfer up to 30 kW of heat from the catalytic reaction to the hydrogen storage system via a recirculating heat transfer fluid.     

System simulation model for high-pressure metal hydride hydrogen storage systems

On-board hydrogen storage systems employing high-pressure metal hydrides promise advantages including high volumetric capacities and cold start capability. In this paper, we discuss the development of a system simulation model in Matlab/Simulink platform. Transient equations for mass balance and energy balance are presented. Appropriate kinetic expressions are used for the absorption/desorption reactions for the Ti_1.1CrMn metal hydride. During refueling, the bed is cooled by passing a coolant through tubes embedded within the bed while during driving, the bed is heated by pumping the radiator fluid through same set of tubes. The feasibility of using a high-pressure metal hydride storage system for automotive applications is discussed. Drive cycle simulations for a fuel cell vehicle are performed and detailed results are presented.     

Metal hydride hydrogen storage and compression systems for energy storage technologies

Along with a brief overview of literature data on energy storage technologies utilising hydrogen and metal hydrides, this article presents results of the related R&D activities carried out by the authors. The focus is put on proper selection of metal hydride materials on the basis of AB₅- and AB₂-type intermetallic compounds for hydrogen storage and compression applications, based on the analysis of PCT properties of the materials in systems with H₂ gas. The article also presents features of integrated energy storage systems utilising metal hydride hydrogen storage and compression, as well as their metal hydride based components developed at IPCP and HySA Systems.     

Hydrogen energy storage systems to improve wind power plant efficiency considering electricity tariff dynamics

One of the limitations of the efficiency of renewable energy sources is the stochastic nature of generation; consequently, it is necessary to use high-capacity energy storage systems such as hydrogen storage for its integration into existing power networks. At the same time, electricity market tariffs for large enterprises change during the day. Therefore, it can be assumed that storing energy during cheaper hours and usage in more expensive hours allows increasing the efficiency of renewable energy sources. Evaluation of the economic efficiency of an energy storage system requires simulation with a step of at least 1 h for several years since the use of averaged production volume and averaged electricity tariffs will not allow obtaining an adequate to the task accuracy. A simulation model and software have been implemented to perform simulations and calculate the economic efficiency of a wind turbine with and without a hydrogen storage device. The methodology has been approved on three-year real data of wind speeds and electricity tariffs in the Novosibirsk region and Krasnodar Territory (Russian Federation).     

Optimized heat transfer fin design for a metal-hydride hydrogen storage container

This paper presents a heat transfer fin optimization for a LaNi₅ hydrogen storage container. In this simplified approach, a one-dimension fin model is proposed in order to avoid geometrical restrictions and constrains associated to a particular technological solution. Therefore, the presented model can be utilized as a general framework for the development of containers with inner fins.     

Structural analysis of metal hydride-based hybrid hydrogen storage systems

Hybrid hydrogen storage systems, which see the adoption of metal hydride materials charged at high pressure, can be a viable method to reach good gravimetric and volumetric capacities under selected conditions, since hydrogen is stored both as element bound to the hydride and as high pressure gas. A general structural model, which can simulate high pressure hybrid storage tanks, has been developed, with the aim of describing the performance of the system under various operating conditions. A baseline case has been simulated, comparing tanks composed of SS316 and IM6 graphite fiber reinforced epoxy composite that contain metal hydride materials that can store weight fractions of bound hydrogen ranging from 2% to 8%. Sensitivity analyses were performed for the baseline studies with the aim of determining the operating conditions that maximize gravimetric and volumetric capacities. Results show that high pressure systems are optimal (in terms of gravimetric and volumetric capacity) for tank materials having low density and a high allowable stress, while a low operating pressure is preferable for high density tank materials, especially when coupled with metal hydrides capable of storing a high weight fraction of bound hydrogen.     

Improvement of thermal performance of spiral heat exchanger on hydrogen storage by adding copper fins

The reaction time of hydrogen in metal-hydride vessels (MHVs for short) is strongly influenced by the heat transfer from/to the hydride bed. In the present work an experimental study of the geometric and the operating parameters of a finned spiral heat exchanger has been carried out to identify their influence on the performance of the charging process of the MHV. The experimental results show that the charge time of the reactor is considerably reduced, when finned spiral heat exchanger is used. In addition, the effect of different parameters (flow mass and temperature of the cooling fluid, applied pressure of hydrogen in the case of absorption and desorption) has been discussed and obtained results show that a good choice of these parameters is important.     

Experimental study of a metal hydride vessel based on a finned spiral heat exchanger

The reaction time of hydrogen in metal hydride vessels (MHVs for short) is strongly influenced by the heat transfer from/to the hydride bed. In the present work an experimental study of the geometric and the operating parameters of a finned spiral heat exchanger has been carried out to identify their influence on the performance of the charging process of the MHV. The experimental results show that the charge time of the reactor is considerably reduced, when finned spiral heat exchanger is used. In addition, the effect of different parameters (flow mass and temperature of the cooling fluid, applied pressure, and hydrogen tank volume) has been discussed and obtained results show that a good choice of these parameters is important.     

The possibilities of cooperation between a hydrogen generator and a wind farm

In this paper, a hydrogen generator and a wind farm were taken as the research objects. The H₂ generator consisted characteristics of laboratory-tested electrolyzers were determined as a function of the hydrogen mass flow. Determining the auxiliary power index of the device allowed the efficiency of the hydrogen generator to be determined as a function of hydrogen mass flow as well as the hydrogen generator relative power. The dynamic characteristics of a generator were also presented. The possibility of a given wind farm cooperating with hydrogen generators that are characterized by different powers and various efficiencies was simulated. Algorithm enables determination of hydrogen generators efficiency for devices with various performance in nominal operation point is shown. It has been shown that proper selection of the power of the hydrogen generator in relation to the power of the wind farm can ensure a high efficiency for the device.