On-board and Off-board performance of hydrogen storage options for light-duty vehicles

Leading physical and materials-based hydrogen storage options are evaluated for their potential to meet the vehicular targets for gravimetric and volumetric capacity, cost, efficiency, durability and operability, fuel purity, and environmental health and safety. Our analyses show that hydrogen stored as a compressed gas at 350–700 bar in Type III or Type IV tanks cannot meet the near-term volumetric target of 28 g/L. The problems of dormancy and hydrogen loss with conventional liquid H₂ storage can be mitigated by deploying pressure-bearing insulated tanks. Alane (AlH₃) is an attractive hydrogen carrier if it can be prepared and used as a slurry with >50% solids loading and an appropriate volume-exchange tank is developed. Regenerating AlH₃ is a major problem, however, since it is metastable and it cannot be directly formed by reacting the spent Al with H₂. We have evaluated two sorption-based hydrogen storage systems, one using AX-21, a high surface-area superactivated carbon, and the other using MOF-177, a metal-organic framework material. Releasing hydrogen by hydrolysis of sodium borohydride presents difficult chemical, thermal and water management issues, and regenerating NaBH₄ by converting B–O bonds is energy intensive. We have evaluated the option of using organic liquid carriers, such as n-ethylcarbazole, which can be dehydrogenated thermolytically on-board a vehicle and rehydrogenated efficiently in a central plant by established methods and processes. While ammonia borane has a high hydrogen content, a solvent that keeps it in a liquid state needs to be found, and developing an AB regeneration scheme that is practical, economical and efficient remains a major challenge.     

Experimental results of a compact thermally driven cooling system based on metal hydrides

In this paper a detailed experimental analysis of a metal hydride based cooling system is presented. For the high temperature side an AB₅ type alloy (LmNi_4.91Sn_0.15) was chosen, whereas an AB₂ type alloy (Ti_0.99Zr_0.01V_0.43Fe_0.09Cr_0.05Mn_1.5) is used for the low temperature side. Due to very good heat and also mass transfer characteristics (among others, large heat transfer surface area) of the utilized capillary tube bundle reaction bed, very short half-cycle times in the order of 100 s have been reached. Consequently, the specific cooling power of the system is up to 780 W per kg desorbing metal hydride – depending on the temperature boundary conditions. The system was experimentally analyzed for different cooling and ambient temperatures, whereas the heating temperature was fixed to 130 °C.     

Compressed hydrogen production via reaction between liquid ammonia and alkali metal hydride

Ammonia NH₃ is recognized as one of the attractive hydrogen H₂ carriers because it has a high hydrogen content of 18 mass% and it is easily liquefied under about 1 MPa of pressure at a room temperature. NH₃ can react with alkali metal hydrides and generate H₂ even at room temperature, resulting that metal amides are formed as reaction products. The H₂ generation is exothermic reaction, and it is not effectively prevented by H₂ partial pressure in a closed system as thermodynamic properties. In this work, we demonstrated the production of compressed H₂ by the reaction between liquid NH₃ and lithium hydride LiH in a closed pressure vessel, where liquid NH₃ would realize better kinetic properties for the reaction with metal hydride than gaseous NH₃. Actually, more than 12 MPa H₂ was obtained within several hours.     

Thermal management and power saving operations for improved energy efficiency within a renewable hydrogen energy system utilizing metal hydride hydrogen storage

To ensure the energy efficiency of renewable hydrogen energy systems, power conservation and thermal management are necessary. This study applies these principals to the operation of metal hydride tanks (MHTs) in a bench-scale hydrogen system, named Hydro Q-BiC?, comprising photovoltaic panels (20 kW), an electrolyzer (5 Nm³/h), MHTs containing a TiFe-based MH (40 Nm³), fuel cells (FC; 3.5 kW(power)/2.5 kW(heat)), and Li-ion batteries (20 kW/20 kWh). Here, we show that in a modified hydrogen production operation, with limited use of auxiliaries for cooling the MHTs, the power consumption of the MHTs was reduced by more than 99% compared to a typical operation. The thermal requirements for the MHTs were reduced by ceasing production in a pressurized state. During the hydrogen use operation, the power consumption was reduced to 1/4 and the FC heat output could be fully used; hence, the overall energy efficiency (power-to-hydrogen-to-power/heat) was as high as ~ 60% (43% for the typical operation).     

Steady-state investigation of water vapor adsorption for thermally driven adsorption based greenhouse air-conditioning system

In the present study, water vapor adsorption onto silica-gel, activated carbon powder (ACP) and activated carbon fiber (ACF) has been experimentally measured at 20, 30 and 50 °C using a volumetric method based adsorption measurement apparatus for greenhouse air-conditioning (AC). The Guggenheim–Anderson–De Boer and Dubinin–Astakhov adsorption models are used to fit the adsorption data of silica-gel and ACP/ACF, respectively. The isosteric heat of adsorption is determined by Clausius–Clapeyron relationship. The adsorbents are evaluated for low-temperature regeneration with aim to develop solar operated AC system for greenhouses. Ideal growth zone for agricultural products is determined by which the steady-state desiccant AC cycle is evaluated on the psychometric chart and adsorption isobars.Steady-state moisture cycled (MC_SS) by each adsorbent is determined for demand category-I, II and III which are based on 60%, 40% and 20% relative humidity of dehumidified air, respectively. In case of demand category-I, the ACP enables maximum MC_SS at all regeneration temperatures (T_reg), ideally sitting at 47 °C. The ACF enables double MC_SS as compared to silica-gel during demand category-II at T_reg ≥59 °C. However, the silica-gel is found the only applicable adsorbent for the demand category-III.     

Novel reactor design for metal hydride cooling systems

In this paper a plate reactor for metal hydride applications is presented and experimentally characterized. Through an optimized heat transfer characteristic the concept is suitable for all metal hydride high performance systems, where short reaction times are required. The experimental characterization using metal hydride Hydralloy^® C5 reveals that very short half-cycle times (t < 60 s) are feasible enabling small and compact systems. With regard to the application of an open metal hydride cooling system for fuel cell vehicles and a cooling temperature of 10 °C, single reactor experiments lead to a high specific cooling power of 1.31 kW kg_MeH^?1. In a continuous working system where thermal losses are considered, still 690 W kg_MeH^?1 can be reached.     

Experimental study of metal hydride-based hydrogen storage tank at constant supply pressure

Metal hydride-based hydrogen storage tank is tested using 1 kg of AB₅ alloy, namely LaNi₅. The hydrogen tank is of annular cylindrical with inner and outer heat exchangers. The inner one is a finned spiral heat exchanger and the outer one is a conventional jacket. Performance (storage capacity and storage time) studies are carried out by varying the supply pressure and the cooling temperature of the hydride bed. At any given cooling temperature, hydrogen storage rate is found to increase with supply pressure. Cooling temperature is found to have a significant effect on hydrogen storage capacity at lower supply pressures.     

Hybrid fuel cell-based energy system with metal hydride hydrogen storage for small mobile applications

This paper describes the general architecture of a hybrid energy system, whose main components are a proton exchange membrane fuel cell, a battery pack and an ultracapacitor pack as power sources, and metal hydride canisters as energy storage devices, suitable for supplying power to small mobile non-automotive devices in a flexible and variable way. The first experimental results carried out on a system prototype are described, showing that the extra components, required in order to manage the hybrid system, do not remarkably affect the overall system efficiency, which is always higher than 36% in all the test configurations examined. In fact, the system allows the fuel cell to work most often at quasi-optimal conditions, near its maximum efficiency (i.e. at low/medium loads), because high external loads are met by the combined effort of the fuel cell and the ultracapacitors. For the same reason, the metal hydride storage system can be used also under highly dynamic operating conditions, notwithstanding its usually poor kinetic performance.     

New sorbent materials for the hydrogen storage and transportation

Activated carbon fiber was chosen as an efficient gas (ammonia, methane, hydrogen) sorption material to design a gas storage system. To increase gas sorption capacity a complex compound (activated carbon fiber+chemicals$\text{fiber}+\text{chemicals}$) was applied. The application of a heat pipe in gas accumulator enables one to control the temperature of sorbent bed and provide optimum operational conditions.     

Study on absorption/desorption characteristics of a metal hydride tank for boil-off gas from liquid hydrogen

We have been performing research on the Totalized Hydrogen Energy Utilization System (THEUS) which has applications to commercial buildings and a planned added function of supplying energy to stations for hydrogen and electric vehicles. In that case we will utilize liquid hydrogen transported from a hydrogen station and all Boil-Off Gas (BOG) will be recovered in THEUS’s metal hydride tanks. It is known that BOG is chiefly composed of para-hydrogen, which has different thermo-physical properties from normal hydrogen. It has been reported that some metal hydride alloys work as a catalyst to accelerate the para-ortho conversion and the conversion proceeds relatively fast in the case of La–Ni₅. The conversion is considered to be an endothermic reaction. A misch metal (Mm)-Ni₅ metal hydride alloy, which contained La and Ni, was used in our THEUS metal hydride tank. To examine the effect of the para-ortho conversion on the THEUS operation, we investigated the absorption/desorption characteristics of the metal hydride tank with BOG. We confirmed that the effect of the heat of conversion was very small and BOG could be treated as normal hydrogen for practical application.     

Application of metal hydride sheet to thermally driven cooling system

This paper describes an application of a metal hydride (MH) sheet, which consists of MH powder, carbon fiber, and aramid pulp, in a metal hydride heat pump (MHHP) system with a TiFe_0.9Ni_0.1/La_0.6Y_0.4Ni_4.9Al_0.1 working pair (MH1/MH2). In the experiments, the effect of the use of MH sheet on the system performances was investigated, in which the MH sheets were used to replace part of the MH powder to improve the heat exchange performance. The sheets and powder were packed alternately into the MH beds in layers with an aspect ratio less than one. The MH sheet significantly accelerated the heat exchange ratio of both MH packed beds. Using the MH sheet in both reactors, the specific cooling power increased by 1.2 times. The results also indicated that the role of heat exchange in an MH2 reactor as a cooling output side was more important in the enhancement of system performance than that in an MH1 reactor as a heat source side. In addition, the proposed MH sheet was effective not only for improving the system performance but also for decreasing the stress on the reactor vessel due to the expansion of MH during the hydrogen absorption/desorption.     

A hydrogen purification and storage system using metal hydride

This work is to develop a new hydrogen purification and storage system for daily start and stop (DSS) operations. The new system enables us to minimize emissions of carbon dioxide by using compact and highly efficient fuel cells. The new system first removes carbon monoxide, which is poisonous to metal hydride, from reformed gas by using a special carbon monoxide adsorbent. After removing carbon monoxide, the reformed gas is introduced to a metal hydride bed to purify and store hydrogen. Some 100 NL/h Laboratory scale apparatus was operated in daily start and stop operations for 100 cycles for a total of 150 h with quite good efficiency. The new process has achieved an 83% hydrogen recovery ratio in one-month DSS operations.     

Dynamic model and experimental results of a thermally driven metal hydride cooling system

This paper presents a dynamic model and experimental results of a metal hydride cooling system based on a coupled pair of reactors: a hot reaction bed (alloy LmNi_4.91Sn_0.15) and a cold reaction bed (Ti_0.99Zr_0.01V_0.43Fe_0.09Cr_0.05Mn_1.5). The driving power is waste heat removed at high temperature (130 °C). Metal hydride reactors can have interesting applications in thermal storage systems, refrigeration and heat pumps. The experimental setup is described, as well as the governing equations of the model. Correlations are used for the relationship between the equilibrium pressure, hydrogen concentration and temperature. An innovative approach is used for the modelling of hysteresis. The simulation results are compared and validated with experimental measurements during dynamic refrigeration cycles.     

Thermodynamic simulation of hydrogen based thermochemical energy storage system

The increasing energy demand needs the attention for energy conservation as well as requires the utilisation of renewable sources. In this perspective, hydrogen provides an eco-friendly and regenerative solution toward this matter of concern. Thermochemical energy storage system working on gas-solid interaction is a useful technology for energy storage during the availability of renewable energy sources. It provides the same during unavailability of energy sources. This work presents a performance analysis of metal hydride based thermal energy storage system (MH-TES), which can transform the waste heat into useful high-grade heat output. This system opens new doors to look at renewable energy through better waste heat recovery systems. Experimentally measured PCIs of chosen metal hydride pairs, i.e. LaNi_4.6Al_0.4/La_0.9Ce_0.1Ni₅ (A-1/A-3; pair 1) and LaNi_4.7Al_0.3/La_0.9Ce_0.1Ni₅ (A-2/A-3; pair 2) are employed to estimate the thermodynamic performance of MH-TES at operating temperatures of 298 K, 373 K, 403 K and 423 K as atmospheric temperature (T_atm), waste heat input temperature (T_m), storage temperature (T_s) and upgraded/enhanced heat output temperature (T_h) respectively. It is observed that the system with alloy pair A-1/A-3 shows higher energy storage density of 121.83 kJ/kg with a higher COP of 0.48 as compared to A-2/A-3 pair. This is due to the favourable thermodynamic properties, and the pressure differential between coupled MH beds, which results in higher transferrable hydrogen. Besides, the effect of operating temperatures on COP is studied, which can help to select an optimum temperature range for a particular application.     

Seasonal energy storage system based on hydrogen for self sufficient living

SELF is a resource independent living and working environment. By on-board renewable electricity generation and storage, it accounts for all aspects of living, such as space heating and cooking as well as providing a purified rainwater supply and wastewater treatment, excluding food supply. Uninterrupted, on-demand energy and water supply are the key challenges. Off-grid renewable power supply fluctuations on daily and seasonal time scales impose production gaps that have to be served by local storage, a function normally fulfilled by the grid. While daily variations only obligate a small storage capacity, requirements for seasonal storage are substantial.The energy supply for SELF is reviewed based on real meteorological data and demand patterns for Zurich, Switzerland. A battery system with propane for cooking serves as a reference for battery-only and hybrid battery/hydrogen systems. In the latter, hydrogen is used for cooking and electricity generation. The analysis shows that hydrogen is ideal for long term bulk energy storage on a seasonal timescale, while batteries are best suited for short term energy storage. Although the efficiency penalty from hydrogen generation is substantial, in off-grid systems, this parameter is tolerable since the harvesting ratio of photovoltaic energy is limited by storage capacity.     

Design and optimization of single particle electrodes for the kinetic analysis of hydrogen evolution and absorption on hydrogen storage alloys

In the literature, there is a large discrepancy between reported values of electrochemical, kinetic and transport parameters of hydrogen storage alloys. These discrepancies arise, because in most cases, electrodes are prepared with the powdered alloy supported within a porous matrix, constituted by carbon and additive binders such as PTFE. The main drawback, of this preparation technique, for the identification of kinetic parameters, is the uncertainty in the specific active area value, where the hydrogen evolution and absorption processes take place. To overcome the disadvantages described, a new type of electrode, was designed, using a single particle of AB₅ and AB₂ hydride forming alloys. The data obtained from electrochemical impedance measurements were adjusted in terms of a physicochemical model that takes into account the processes of hydrogen evolution and absorption coupled to hydrogen diffusion. From the study it can be concluded that the differences in the behavior of the AB₅ and AB₂ alloys, presenting the first best performance during the activation and operation at high discharge currents, are mainly due to higher values of the exchange current density and the diffusion coefficient of H for the AB₅ alloy.     

Numerical simulation of the hydrogen storage with reaction heat recovery using metal hydride in the totalized hydrogen energy utilization system

Numerical simulation of a hydrogen storage tank of a Totalized Hydrogen Energy Utilization System (THEUS) for application to commercial buildings was done to verify the practicality of THEUS. THEUS consists of a fuel cell, water electrolyzer, hydrogen storage tank and their auxiliary machinery. The hydrogen storage tanks with metal hydrides for load leveling have been previously experimentally investigated as an important element of THEUS. A hydrogen storage tank with 50 kg AB5 type metal hydride was assembled to investigate the hydrogen-absorbing/desorbing process, which is exothermic/endothermic process. The goal of this tank is to recover the cold heat of the endothermic process for air conditioning, and thus improve the efficiency of THEUS. To verify the practical effectiveness of this improved system, we developed a numerical simulation code of hydrogen storage tank with metal hydride. The code was validated by comparing its results with experimental results. In this code the specific heat value of the upper and lower flanges of the hydrogen storage tank was adjusted to be equal to the thermal capacity of the entire tank. The simulation results reproduce well the experimental results.     

Model-based design of an automotive-scale,metal hydride hydrogen storage system

Sandia and General Motors have successfully designed, fabricated, and experimentally operated a vehicle-scale hydrogen storage demonstration system using sodium alanates. The demonstration system module design and the system control strategies were enabled by experiment-based, computational simulations that included heat and mass transfer coupled with chemical kinetics. Module heat exchange systems were optimized using multi-dimensional models of coupled fluid dynamics and heat transfer. Chemical kinetics models were coupled with both heat and mass transfer calculations to design the sodium alanate vessels. Fluid flow distribution was a key aspect of the design for the hydrogen storage modules and computational simulations were used to balance heat transfer with fluid pressure requirements.     

Study of heat and mass transfer in MmNi4.6Al0.4 during desorption of hydrogen

In this paper, a numerical investigation of two-dimensional coupled heat and mass transfer during desorption of hydrogen in a cylindrical metal hydride reactor containing MmNi_6.4Al_0.4 is presented. By considering the variation in heat transfer fluid temperature along the axial direction (variable wall temperature boundary condition), the changes in hydride bed temperature at different axial locations are presented. The effect of variable wall temperature boundary condition on hydrogen desorption rate for different hot fluid temperatures and hydride bed thicknesses is investigated. The rate of hydrogen desorption at different hot fluid temperatures showed good agreement with the experimental data reported in the literature. As the desorption progresses, the change in heat transfer fluid temperature along the axial direction is found to decrease with time and becomes unchanged at the end of the process. The effect of variable wall temperature boundary condition on desorption time is found to be significant for the hydride bed thicknesses of about 7.5 mm and more. For a given bed thickness of 17.5 mm, the maximum difference in desorption time between variable wall and constant wall temperature convective boundary conditions is about 375 s at 303 K.