Net energy analysis of hydrogen storage options

Hydrogen storage is critical for developing viable hydrogen vehicles. This paper compares compressed hydrogen, cryogenic hydrogen and metal hydride (Mg and FeTi) options using net energy analysis. A simulation of an Indian vehicle with an urban drive cycle using a fuel cell stack is carried out to determine the total hydrogen required per km of travel.Net energy analysis is carried out considering the energy requirements of the storage device and the energy required to produce and store the hydrogen. From net energy analysis compressed hydrogen is the preferred option. The direct energy requirement is more than 55% for magnesium hydride as compared to compressed hydrogen due to the combined effect of increase in weight and higher heat of desorption.In addition to volumetric and gravimetric storage density, it is felt that net energy analysis should be also included as an additional criteria for evaluating any storage option. For metal hydride storage the net energy required to produce the tank should be minimum. This could be used as a selection criterion to design an optimum metal hydride storage. The performance of other materials like porous carbon, carbon nanotubes and hybrids can be evaluated using net energy analysis.     

A metal hydride system for a forklift: Feasibility study on on-board chemical storage of hydrogen using numerical simulation

This paper describes a technical feasibility study of on-board metal hydride storage systems. The main advantages of these systems would be that of being able to replace counterweights with the weight of the storage system and using the heat emissions of fuel cells for energy, making forklifts a perfect use case. The main challenge is designing a system that supplies the required energy for a sufficiently long period. A first draft was set up and analyzed to provide a forklift based on a fuel cell with hydrogen from HydralloyC5 or FeTiMn. The primary design parameter was the required amount of stored hydrogen, which should provide energy equal to the energy capacity of a battery in an electric vehicle. To account for highly dynamic system requirements, the reactor design was optimized such that the storage was charged in a short time. Additionally, we investigated a system in which a fixed amount of hydrogen energy was required. For this purpose, we used a validated simulation model for the design concepts of metal hydride storage systems. The model includes all relevant terms and parameters to describe processes inside the system's particular reactions and the thermal conduction due to heat exchangers. We introduce an embedded fuel cell model to calculate the demand for hydrogen for a given power level. The resulting calculations provide the required time for charging or a full charge depending on the tank's diameter and, therefore, the necessary number of tanks. We conclude that the desired hydrogen supply times are given for some of the use cases. Accordingly, the simulated results suggest that using a metal hydride system could be highly practical in forklifts.     

Hydride bed control: Understanding of hydride bed using tank efflux

Control of dehydriding (desorption of hydrogen or hydrogen isotope) rate from a hydride bed in fusion fuel cycle is one important design point to estimate a real supplying amount of hydrogen from the hydride bed. In a real system tens of batch-type hydride beds are to be utilized for supplying a certain amount of hydrogen isotope at the same time. A study on efflux time from a hydraulic water tank was applied as a fundamental similarity test of the gas fueling system. As a result, liquid efflux from a tank shows a similar behavior with the desorption pattern of the depleted uranium hydride bed system. As much important as keeping a hydride buffer vessel pressure in a hydride bed system, similar tendency was studied in the tank efflux system; i.e., to keep the secondary vessel height there needs a certain amount of liquid flow from the upper tank and the tank height difference. From one tank with connected another tank flow with understanding of tank efflux model a complicated multi-tanks behavior could be understood by simulating its complex efflux characteristics, and it is likely to be applied to the multi-hydride beds system.     

Modeling and optimization of multi-tubular metal hydride beds for efficient hydrogen storage

This work presents a novel systematic approach for the optimal design of a multi-tubular metal hydride tank, containing up to nine tubular metal hydride reactors, used for hydrogen storage. The tank is designed to store enough amount of hydrogen for 25 km range¹, for a fuel cell vehicle. A detailed 3D Cartesian, mathematical model is developed and validated against a 2D cylindrical developed by Kikkinides et al. [1]. The objective is to find the optimal process design so as to increase the overall thermal efficiency, and thus minimize the storage time. Optimization results indicate that almost 90% improvement of the storage time can be achieved, over the case where the tank is not optimized and for a minimum storage capacity of 99% of the maximum value.     

Finite element model for charge and discharge cycle of activated carbon hydrogen storage

One of the main challenges to introduce hydrogen on the energy market is to improve on-board hydrogen storage and develop more efficient distribution technologies to increase the amount of stored gas while lowering the storage pressure. The physisorption of hydrogen on activated carbons (AC) is being investigated as a possible route for hydrogen storage. The objective of this work is to study the performance of adsorption-based hydrogen storage units from a "systems" point of view. A realistic two-dimensional axisymmetric geometric model which couples mass, momentum and energy balances is established based on the thermodynamic conservation laws using finite element method as implemented in COMSOL Multiphysics™. We consider the charging and discharging of the storage unit at a rated pressure of 9 MPa, and at an initial temperature of 302 K. The results are compared with experimental data obtained at the Hydrogen Research Institute of the University of Quebec at Trois-Rivieres. The storage tank is cooled by ice water. Research results show that both the simulated variations of pressure and temperatures during charge and discharge processes are in good agreement with the experimental data. The temperatures in the central region of tank are higher than those at the entrance and near the wall at the end of charge time while they are lower than those at the entrance and near the wall at the end of discharge time. The velocities are largest at the entrance, and decrease gradually along the axis of the tank. Owing to thermal effects, the larger flow rates result in less amount of adsorption in the condition of the same charging pressure. Hence measures of increasing heat transfer should be adopted, such as increasing the thermal conductivity of the storage bed. From the point of view of storage capacity, it is therefore possible to realize rapid hydrogenation, which is conducive to the use of such systems for on-board hydrogen storage based on activated carbon adsorption.     

Development of a liquid hydrogen car

Due to stricter emission control standards and the higher cost of fossil fuels, hydrogen is considered to be of great promise as a future automotive fuel. However, the problem of on-board storage has yet to be solved. At present, storage of the hydrogen as a liquid is the only method in practice. In order to participate in the SEED Rally held in the U.S.A. in 1975, a Datsun B-210 passenger car with a 1.4 liter engine was converted to a hydrogen fueled car using the liquid storage method. This was accomplished within the short space of 4 months. Since the car had to travel at least 650 km (400 miles) between fuelings, it was equipped with a tank which could contain 230 l (61 gal) of liquid hydrogen at 5 atm. gage (71 psig). Because cooler hydrogen gas induces better engine performance, the fuel line from the rear tank to the front engine was a vacuum insulated pipe. Fuel was injected intermittently into the intake port by a mechanical valve. This car successfully completed the Rally, 2800 km (1730 miles), and proved that the liquid hydrogen car has a bright future with regard to energy economy, performance, emissions, and safety.     

Numerical analysis of an energy storage system based on a metal hydride hydrogen tank and a lithium-ion battery pack for a plug-in fuel cell electric scooter

This work investigates on the performance of a hybrid energy storage system made of a metal hydride tank for hydrogen storage and a lithium-ion battery pack, specifically conceived to replace the conventional battery pack in a plug-in fuel cell electric scooter. The concept behind this solution is to take advantage of the endothermic hydrogen desorption in metal hydrides to provide cooling to the battery pack during operation.The analysis is conducted numerically by means of a finite element model developed in order to assess the thermal management capabilities of the proposed solution under realistic operating conditions.The results show that the hybrid energy storage system is effectively capable of passively controlling the temperature of the battery pack, while enhancing at the same time the on-board storage energy density. The maximum temperature rise experienced by the battery pack is around 12 °C when the thermal management is provided by the hydrogen desorption in metal hydrides, against a value above 30 °C obtained for the same case without thermal management. Moreover, the hybrid energy storage system provides the 16% of the total mass of hydrogen requested by the fuel cell stack during operation, which corresponds to a significant enhancement of the hydrogen storage capability on-board of the vehicle.     

Recent challenges of hydrogen storage technologies for fuel cell vehicles

Fuel cell vehicles have a high potential to reduce both energy consumption and carbon dioxide emissions. However, due to the low density, hydrogen gas limits the amount of hydrogen stored on board. This restriction also prevents wide penetration of fuel cells. Hydrogen storage is the key technology towards the hydrogen society. Currently high-pressure tanks and liquid hydrogen tanks are used for road tests, but both technologies do not meet all the requirements of future fuel cell vehicles. This paper briefly explains the current status of conventional technologies (simple containment) such as high-pressure tank systems and cryogenic storage. Another method, hydrogen-absorbing alloy has been long investigated but it has several difficulties for the vehicle applications such as low temperature discharge characteristics and quick charge capability due to its reaction heat. We tested a new idea of combining metal hydride and high pressure. It will solve some difficulties and improve performance such as gravimetric density. This paper describes the latest material and system development.     

A thermally coupled metal hydride hydrogen storage and fuel cell system

This paper examines the ability of metal hydride storage systems to supply hydrogen to a fuel cell with a time varying demand, when the metal hydride tanks are thermally coupled to the fuel cell. A two-dimensional mathematical model is utilized to compare different heat transfer enhancements and storage tank configurations. The scenario investigated involves two metal hydride tanks containing the alloy Ti_0.98Zr_0.02V_0.43Fe_0.09Cr_0.05Mn_1.5, located in the air exhaust stream of a fuel cell. Three cases are simulated: a base case with no heat transfer enhancements, a case with external fins attached to the outside of the tank, and a case where an annular tank design is used. For the imposed duty cycle, the base case is insufficient to provide the hydrogen demands of the system, while both the finned and annular cases are able to meet the demands. The finned case yields higher pressures and occupies more space, while the annular case yields acceptable pressures and requires less space. Furthermore, the annular metal hydride tank meets the requirements of the fuel cell while providing a more robust and compact hydrogen storage system.     

Comparing the cost of electricity sourced from a fuel cell-based renewable energy system and the national grid to electrify a rural health centre in India: A case study

The provision of electricity is a key component in the development of a country’s health care facilities. This study was performed to estimate the cost of powering a rural primary health centre, in India with a decentralised renewable energy system. The costs were also compared between a decentralised renewable energy system and providing electricity from a grid source. The critical or break-even distance that makes electricity from a decentralised renewable energy system cost effective over that from a grid source was determined. The decentralised renewable energy system considered was a hydrogen-based fuel cell for the generation of electricity with hydrogen extracted from biogas obtained from biomass. The software program HOMER was used for the simulation analysis. The cost of a decentralised renewable energy system was found to be between seven times and less than half that of conventional energy, and the break-even distance was between 43.8 km to a negative distance for varying ranges of input component costs. The results of this study indicated that the use of a decentralised renewable energy system to power a rural primary health centre is both feasible and cost effective, and may even be cheaper than using electricity from a grid source.     

Neural network based optimization for cascade filling process of on-board hydrogen tank

Compressed hydrogen storage is widely used in hydrogen fuel cell vehicles (HFCVs). Cascade filling systems can provide different pressure levels associated with various source tanks allowing for a variable mass flow rate. To meet refueling performance objectives, safe and fast filling processes must be available to HFCVs. The main objective of this paper is to establish an optimization methodology to determine the initial thermodynamic conditions of the filling system that leads to the lowest final temperature of hydrogen in the on-board storage tank with minimal energy consumption. First, a zero-dimensional lumped parameter model is established. This simplified model, implemented in Matlab/Simulink, is then used to simulate the flow of hydrogen from cascade pressure tanks to an on-board hydrogen storage tank. A neural network is then trained with model calculation results and experimental data for multi-objective optimization. It is found to have good prediction, allowing the determination of optimal filling parameters. The study shows that a cascade filling system can well refuel the on-board storage tank with constant average pressure ramp rate (APRR). Furthermore, a strong pre-cooling system can effectively lower the final temperature at a cost of larger energy consumption. By using the proposed neural network, for charging times less than 183s, the optimization procedure predicts that the inlet temperature is 259.99–266.58 K, which can effectively reduce energy consumption by about 2.5%.     

Hydride tank storage system dimensioning on the base of their dynamic behavior

The conditions in which the metal hydride adsorbs and release hydrogen are crucial aspects for integrating the fuel tank in a working system. In this paper the characterization of a metal hydride alloy tank has been analyzed and reported. In particular the dynamic behavior has been evaluated to propose a method to design a tank system. Indeed, to calculate the number of tanks, the evaluation of the energy requirement could be not enough because the available hydrogen depends not only on the quantity contained in the tank, but also on other dynamic factors which influence the kinetics and hydrogen flow such as the temperature and, therefore, the heat exchange system. By experimental data it has been individuated a procedure to build a curve, realizing a relationship between flow and maintaining time, in order to dimensioning a hydrogen system storage.     

Hydrogen storage systems based on hydride materials with enhanced thermal conductivity

The reaction of hydrogen gas with a metal to form a metal hydride is exothermic. If the heat released is not removed from the system, the resulting temperature rise of the hydride will reduce the hydrogen absorption rate. Hence, hydrogen storage systems based on hydride materials must include a way to remove the heat generated during the absorption process. The heat removal rate can be increased by (i) increasing the effective thermal conductivity of the metal hydride by mixing it with high-conductivity materials such as aluminum foam or graphite, (ii) optimizing the shape of the tank, and (iii) introducing an active cooling environment instead of relying on natural convection. This paper presents a parametric study of hydrogen storage efficiency that explores quantitatively the influence of these parameters. An axisymmetric mathematical model was formulated in Ansys Fluent 12.1 to evaluate the transient heat and mass transfer in a cylindrical metal hydride tank, and to predict the transient temperatures and mass of hydrogen stored as a function of the thermal conductivity of the enhanced hydride material, aspect ratio of the cylindrical tank, and thermal boundary conditions. The model was validated by comparing the transient temperature at selected locations within the storage tank with concurrent experiments conducted with LaNi₅ material. The parametric study revealed that the aspect ratio of the tank has a stronger influence when the effective thermal conductivity of the metal hydride bed is low or when the heat removal rate from the tank surface is high (active cooling). It was also found that for a hydrogen filling time of 3 min, adding 30% aluminum foam to the metal hydride maximizes hydrogen absorption under natural convection, whereas the addition of only 10% aluminum foam maximizes the hydrogen content under active cooling. For filling times beyond 3 min, the amount of aluminum foam required to maximize hydrogen content can be reduced for both natural convection and active cooling. This study should prove useful in the design of practical metal hydride-based hydrogen storage systems.     

Numerical investigation of hydrogen absorption in a stackable metal hydride reactor utilizing compartmentalization

In this paper, a three-dimensional model for hydrogen absorption in a metal alloy has been developed, validated against the experimental data in the literature, and then applied to a novel design for a hydrogen storage unit. The proposed design is similar to the fuel cell stack, but here the Membrane Electrode Assembly (MEA) has been replaced by a metal hydride (MH) reactor placed between the flow-field plates. These are stacked together to achieve the required amount of hydrogen storage. The flow-field plates have channels engraved on one side for hydrogen supply and on the other, for coolant/heating medium. It is known that the effectiveness of a hydrogen storage unit is directly related to its heat transfer area, and therefore, the choice of its geometry is very important. The larger the size, the more the resistance to heat transfer. Although, the internal tubular heat exchangers have proven to be effective in heat transfer, they pose severe challenges such as cooling/heating medium leakage due to tube erosion, stresses generated, etc. and they displace the active metal hydride from the tank. The present stacked MH reactor configuration helps to overcome these challenges by stacking small MH reactors together and there is no chance of the cooling/heating medium leaking into the metal hydride. Numerical simulations were performed to investigate the effect of coolant flow rate and percentage of flow-field plate rib area exposed to the MH reactor on temperature evolution and the amount of hydrogen stored. Further, a detailed study was carried out to understand the effect of compartmentalization of the MH reactor on temperature distribution. The results revealed that compartmentalization substantially helps to uniformly distribute the temperature in the metal bed, which is very important to maintain uniform utilization of the metal powder. Consequently, the uniform metal powder density for repeated absorption-desorption cycles without significant loss of its hydrogen storage capabilities.     

Experimental study of hydrogen storage with reaction heat recovery using metal hydride in a totalized hydrogen energy utilization system

Experimental results for hydrogen storage tanks with metal hydrides used for load leveling of electricity in commercial buildings are described. Variability in electricity demand due to air conditioning of commercial buildings necessitates installation of on-site energy storage. Here, we propose a totalized hydrogen energy utilization system (THEUS) as an on-site energy storage system, present feasibility test results for this system with a metal hydride tank, and discuss the energy efficiency of the system. This system uses a water electrolyzer to store electricity energy via hydrogen at night and uses fuel cells to generate power during the day. The system also utilizes the cold heat of reaction heat during the hydrogen desorption process for air conditioning. The storage tank has a shell-like structure and tube heat exchangers and contains 50 kg of metal hydride. Experimental conditions were specifically designed to regulate the pressure and temperature range. Absorption and desorption of 5,400 NL of hydrogen was successfully attained when the absorption rate was 10 NL/min and desorption rate was 6.9 NL/min. A 24-h cycle experiment emulating hydrogen generation at night and power generation during the day revealed that the system achieved a ratio of recovered thermal energy to the entire reaction heat of the hydrogen storage system of 43.2% without heat loss.     

Refuelling scenarios of a light urban fuel cell vehicle with metal hydride hydrogen storage. Comparison with compressed hydrogen storage counterpart

The high price of hydrogen fuel in the fuel cell vehicle refuelling market is highly dependent on the one hand from the production costs of hydrogen and on the other from the capital cost of a hydrogen refuelling station's components to support a safe and adequate refuelling process of contemporary fuel cell vehicles. The hydrogen storage technology dominated in the vehicle sector is currently based on high-pressure compressed hydrogen tanks to extend as much as possible the driving range of the vehicles. However, this technology mandates the use of large hydrogen compression and cooling systems as part of the refuelling infrastructure that consequently increase the final cost of the fuel. This study investigated the prospects of lowering the refuelling cost of small urban hydrogen vehicles through the utilisation of metal hydride hydrogen storage. The results showed that for low compression hydrogen storage, metal hydride storage is in favour in terms of the dispensed hydrogen fuel price, while its weight is highly comparable to the one of a compressed hydrogen tank. The final refuelling cost from the consumer's perspective however was found to be higher than the compressed gas due to the increased hydrogen quantity required to be stored in fully empty metal hydride tanks to meet the same demand.     

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.     

Optimization of heat exchanger designs in metal hydride based hydrogen storage systems

Design of the heat exchanger in a metal hydride based hydrogen storage system influences the storage capacity, gravimetric hydrogen storage density, and refueling time for automotive on-board hydrogen storage systems. The choice of a storage bed design incorporating the heat exchanger and the corresponding geometrical design parameters is not obvious. A systematic study is presented to optimize the heat exchanger design using computational fluid dynamics (CFD) modeling. Three different shell and tube heat exchanger designs are chosen. In the first design, metal hydride is present in the shell and heat transfer fluid flows through straight parallel cooling tubes placed inside the bed. The cooling tubes are interconnected by conducting fins. In the second design, heat transfer fluid flows through helical tubes in the bed. The helical tube design permits use of a specific maximum distance between the metal hydride and the coolant for removing heat during refueling. In the third design, the metal hydride is present in the tubes and the fluid flows through the shell. An automated tool is generated using COMSOL-MATLAB integration to arrive at the optimal geometric parameters for each design type. Using sodium alanate as the reference storage material, the relative merits of each design are analyzed and a comparison of the gravimetric and volumetric hydrogen storage densities for the three designs is presented.     

Boron Compounds as Hydrogen Storage Materials

Abstract

Hydrogen exhibits the highest heating value per mass of all chemical fuels. Furthermore, hydrogen is regenerative and environmentally friendly. Hence, hydrogen storage is very important for humans. Hydrogen storage in metal hydrides is considered as one of the most attractive methods. In the present work, the hydrogen absorption-desorption behavior of the boron compounds has been compared. We present recent developments in the search for hydrogen-storage capacity of boron. Boron compounds have a very high energy density, much better than that of liquid hydrogen and also a lot safer. LiBH₄ is a complex hydride that consists of 18 mass% of hydrogen. It has stability compared with other chemical hydrides and an easy conversion to H₂. Thus, there are a good many reasons that hydrogen-storage materials for LiBH₄ will be used in the future at many ranges for power sources. The future warrants further investigations of the B-H system from the viewpoint of hydrogen energy storage.     

Making markets for hydrogen vehicles: Lessons from LPG

The adoption of liquefied petroleum gas vehicles is strongly linked to the break-even distance at which they have the same costs as conventional cars, with very limited market penetration at break-even distances above 40,000 km. Hydrogen vehicles are predicted to have costs by 2030 that should give them a break-even distance of less than this critical level. It will be necessary to ensure that there are sufficient refuelling stations for hydrogen to be a convenient choice for drivers. While additional LPG stations have led to increases in vehicle numbers, and increases in vehicles have been followed by greater numbers of refuelling stations, these effects are too small to give self-sustaining growth. Supportive policies for both vehicles and refuelling stations will be required.