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).     

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.     

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.     

Metal hydride actuator for a rescue jack driven by hydrogen desorption

This paper presents a metal hydride (MH) actuator rescue jack that uses hydrogen-absorbing alloy as its pressure source. The MH actuator is attached to a thin and flexible fiber-reinforced rubber bag end effector, enabling it to pry open narrow and rough gaps. The proposed MH rescue jack was prepared with different amounts of alloy (6–15 g). The results showed that, with only 6 g of alloy, the rescue jack was able to jack up a 100 kg weight to a height of 10 mm within 1 min by heating the alloy container to 50 °C. Moreover, the jack-up speed increased as the amount of alloy increased. A mathematical model was derived from the jack-up operation tests to estimate the jack-up height, with the aim of designing appropriate amounts of hydrogen-absorbing alloys for rescue jack development. According to the experimental results, the response of the jack-up height was modeled as a first order system in time domain. The model was defined as a function of alloy amount, and its validity was assessed by comparing the experimental and simulation results. The results were in good agreement which confirmed the potential of the proposed model as a design tool for jacks from the viewpoint of time constant.     

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 dynamic modeling of a real embedded metal hydride hydrogen storage system

In this work, the dynamic responses of the on board hydrogen storage system with commercially available metal hydride tanks are investigated based on the database collected from fuel cell electric vehicles operation experiments. A mathematical model considering the heat transfer measured by the temperature control system is developed to analyze the absorption and desorption reaction during operation. As a seal container filled with unknown metal hydride, the practical used hydrogen storage tank is a gray box in the embedded storage system. Without the information about characteristics of material, current operation state and degradation degree, the parameters of the model are uncertain. Particle swarm optimization (PSO) algorithm is applied to search for the optimal parameter set. The simulation results of the hydrogen storage system model using these parameters show great agreements with the real operating data under different temperature conditions; the maximal error is lower than 9%.     

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.     

Hybrid nickel-metal hydride/hydrogen battery

High capacity, high efficiency and resource-rich energy storage systems are required to store large scale excess electrical energy from renewable energy. We proposed “Hybrid Nickel-Metal Hydride/Hydrogen (Ni-MH/H₂) Battery” using high capacity AB₅-type hydrogen storage alloy and high-pressure H₂ gas as negative electrode active materials. It was experimentally confirmed that hydrogen gas can be utilized as an active material of negative electrode by the presence of the AB₅-type hydrogen storage alloy. The experimental average cell voltage suggested that H₂ gas passed through the alloy in the form of atoms. The calculated gravimetric energy density of this hybrid battery increased up to 1.5 times of the conventional Ni-MH battery with low content of rare-earth element which is 32 wt% of the Ni-MH battery.     

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.     

Construction and operation of hydrogen energy utilization system for a zero emission building

A bench-scale stationary hydrogen energy utilization system with renewable energy (RE) that realizes a zero emission building (ZEB) is presented. To facilitate compactness, safety, and mild operation conditions, a polymer electrolyte membrane (PEM) electrolyzer for hydrogen production (5 Nm³/h), PEM fuel cells (FC) for hydrogen use (3.5 kW), and metal hydride (MH) tanks for hydrogen storage (80 Nm³) are incorporated. Each hydrogen apparatus and Li-ion batteries (20 kW/20 kWh) are installed in a 12-ft. container and 20-kW photovoltaic panels provide power. A building energy management system (BEMS) controlled these system components in an integrated manner. The PEM Ely and FC have fast start-up and high efficiency under partial load operations, indicating suitability for daily start-stop operations. An AB-type TiFe-based alloy (520 kg) is used as the MH (not an AB₅-type rare earth alloy that has been commonly used in bench-scale hydrogen store) because, in addition to being low-cost, it is non-hazardous material under Japanese regulations. The results of a 24-h operation experiment verify ZEB attainment. PEM FC and TiFe-based tanks thermal integration results indicate that hydrogen use operation is achievable without external heat sources.     

Techno-economic modeling of zero-emission marine transport with hydrogen fuel and superconducting propulsion system: Case study of a passenger ferry

This paper proposes a techno-economic model for a high-speed hydrogen ferry. The model can describe the system properties i.e. energy demand, weight, and daily operating expenses of the ferry. A novel aspect is the consideration of superconductivity as a measure for cost saving in the setting where liquid hydrogen (LH₂) can be both coolant and fuel. We survey different scenarios for a high-speed ferry that could carry 300 passengers. The results show that, despite higher energy demand, compressed hydrogen gas is more economical compared with LH₂ for now; however, constructing large-scale hydrogen liquefaction plants make it competitive in the future. Moreover, compressed hydrogen gas is restricted to a shorter distance while LH₂ makes longer distances possible, and whenever LH₂ is accessible, using a superconducting propulsion system has a beneficial impact on both energy and cost savings. These effects strengthen if the operational time or the weight of the ferry increases.     

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.