Outlet Hydrogen Research Articles

Protonic-Ceramic Supported Intermediate-Temperature Electrolysis Operations: From Unit-Cell to Utility-Scale

Advancing protonic ceramic electrochemical cell technology continues to be explored for a variety of energy applications, including novel membrane reactors, electrochemical compression, fuel cells, and electrolyzers. The present work focuses on hydrogen production using protonic ceramic electrolysis cells (PCECs) at cell- and system-scales. New cell models are developed, which overcome previous limitations, and are experimentally validated for purposes of scale-up, performance prediction, and system analyses.Computational modeling of PCECs is challenging due to tracking the movement of three charge carriers: protons, oxygen vacancies, and polarons to accurately predict the cell’s operating characteristics. The model-predictive cell performance is based on an experimentally validated, quasi-2D (1+1D) transient, dual-channel cell model capable of predicting the performance of a multiple-charged defect-conducting PCEC based on a BCZYYb electrolyte with the help of a Nernst-Planck formulation. The transient cell model is helpful in several aspects. First, it predicts the distribution of properties and key variables down the PCEC channel. Second, it estimates PCEC effectiveness in electrochemical steam splitting (i.e., faradaic efficiency) over a wide range of current densities. Third, it demonstrates that the PCEC’s actual thermal-neutral voltage decreases significantly as the faradaic efficiency decreases. Finally, it leads to the conclusion that operating a large adiabatic PCEC at its theoretical thermal-neutral voltage necessitates thermal management (e.g., convective heat loss) to regulate the cell temperature.A detailed parametric analysis that follows the transient model development quantifies convective heat losses required for PCEC thermal management and predicts PCEC performance under different operating conditions. This analysis predicts that operating the cell at high feedstock steam concentration (40%), low operating temperature (873K), low cell voltage (1.28V), and low steam utilization (50%) results in high average faradaic efficiency (and hence low convective heat losses), high cell energy efficiency, and increased amount of hydrogen produced per cell. The parametric analysis also generates guidance on gas channel inlet mass flow rates to obtain high purity hydrogen outlet stream.A wider perspective is then provided by examining the prospects for scale-up of PCEC technology to utility scale system design and techno-economics. An overview of a utility-scale PCEC electrolyzer capable of producing 50 ton dry hydrogen per day integrates stack with other balance of plant components, analyzes different design configurations for different feedstock steam concentrations, and identifies suitable steady-state operating conditions that improve the techno-economic performance of the system. This analysis shows that the feedstock steam concentration is the most critical factor in designing the utility-scale PCEC electrolyzer system. Operations at lower feedstock steam concentration results in lower system efficiency, high CAPEX and prohibitively high levelized cost of hydrogen (LCOH). Considering utility-scale PCEC electrolyzer system operations with stack cooling features, hydrogen can be produced in economical way ( $4.5/kg) at high feedstock steam concentration, lower operating temperature, and high steam utilization.

Hydrogen Concentration Prediction in a Passive Autocatalytic Recombiner Using Machine Learning Algorithms

Monitoring hydrogen levels within Nuclear Power Plants (NPPs) is crucial to mitigate potential risks during severe accidental scenarios. Passive Autocatalytic Recombiners (PARs), which operate passively through catalytic reactions, are installed in the containment to reduce hydrogen concentration. This study numerically investigates hydrogen behavior within PAR under various accident conditions. As the inlet temperature and hydrogen concentration increase, the reaction rate and maximum catalyst plate temperature rise. As the inflow velocity increases, the reaction amount rises, but the residence time for the reactions decreases, leading to an increase in the outlet hydrogen concentration. Leveraging the operational characteristics of the PAR, the present study develops a data-driven model to identify the correlations among the parameters associated with the PAR's performance and to predict hydrogen concentration at the PAR's outlet by adopting four machine learning algorithms: Artificial Neural Networks (ANN), k-nearest Neighbor (k-NN), Random Forest (RF), and Support Vector Regression (SVR). Using the PAR's inlet variables of flow velocity, temperature, hydrogen concentration, and outlet temperature as input parameters, the ANN model demonstrates good predictive performance with R2 values of about 0.99. The predictive performance of the ANN model remains robust even without the inlet hydrogen information.

Performance optimization study on the thermal management system of proton exchange membrane fuel cell based on metal hydride hydrogen storage

To compare the performance differences between proton exchange membrane fuel cell (PEMFC), metal hydride tank (MHT) and heat exchanger (HX) coupling configurations and optimize thermal management methods, this study investigates and compares three coupling systems, PEMFC-MHT-HX series, PEMFC-HX-MHT series, and PEMFC-MHT|HX parallel system. Results reveal that, under controlled load variables, all three systems achieve an energy efficiency of 48.1 % at the maximum net power of 41.99 kW, increased by 16.93 % compared to standalone PEMFC. Exergy efficiencies of the systems show little differences, with values of 43.65 %, 43.38 %, and 43.51 %, respectively. The PEMFC-MHT-HX system exhibits the highest hydrogen outlet pressure, which is suitable for high-pressure applications. Conversely, the PEMFC-MHT|HX system enables flexible hydrogen supply regulation through decoupled control of hydrogen outlet pressure and flow. Increasing the MHT heat transfer coefficient can enhance the outlet pressure, but this effect diminishes beyond 1500 W/(m2·K). Sensitivity analysis identifies current and stack temperature as pivotal factors, emphasizing the necessity of precise current regulation and effective thermal management strategies. Additionally, the proportion of PEMFC waste heat recovered by the MHT decreases from 36.8 % to 29.0 % as the current load rises from 100 A to 400 A, indicating potential for alternative waste heat recovery devices over HX.

Inlet baffle structural optimization and reforming performance of exhaust gas-fuel reformer for marine LNG engine

The structural and boundary conditions are pivotal factors influencing the performance of the reformer. However, previous research primarily relied on tailored theoretical boundary conditions specific to the research requirements, often conducted in low airspeed conditions ranging from 700 ℃ to 1000 ℃. There is a lack of research focused on the complex and dynamic process of reforming hydrogen production process in reformers that handle exhaust gas components with high flow rates, high dilution, complex compositions, and low to medium temperatures (300 °C–600 °C) as boundary conditions. The objective of this study is to optimize the structure of the reformation reactor while exploring its hydrogen reforming characteristics. Therefore, this study focused on the simulation-based optimization design of an EGR reformer for marine natural gas engines. A comprehensive model was developed by coupling fluid flow, heat transfer, and surface reactions on a nickel-based catalyst. The accuracy of the model was validated through experimental data from a single-tube fixed-bed catalytic reforming experiment. The results highlighted the importance of the reformer design, with the optimal configuration incorporating two baffle plates positioned at angles of 15° and 35°. This design achieved the most uniform flow field and minimal pressure drop among the six investigated configurations. Furthermore, the study emphasized the significance of reforming gas composition in achieving high performance. The optimal reforming gas composition was found to have an M/O value of approximately 2 and an S/M value of approximately 1. This resulted in an outlet hydrogen concentration of approximately 18.8%, hydrogen yield of approximately 46.1%, methane conversion rate of approximately 66.2%, and an energy efficiency of 71%. It was observed that increasing the EGR rate led to a decrease in performance indicators, with a more pronounced effect observed at an M/O value of 1. This study provides valuable insights into the performance optimization of EGR reformers for marine natural gas engines. The findings offer guidance on selecting operational parameters to enhance efficiency and hydrogen production.

Back propagation neural network based proportional-integral hybrid control strategy for a solar methane reforming reactor

To handle the impact of solar radiation fluctuations on the solar reactor, a new proportional-integral (PI) hybrid control strategy based on back propagation neural network (BPNN) is designed in this study to achieve stable operation. Firstly, the BPNN model optimized by genetic algorithm is trained using reactor model simulation data. Then, the BPNN model is combined with PI feedback control to obtain a BPNN-based PI feedforward-feedback hybrid control strategy for regulating the inlet flow rate. Finally, the effectiveness of the control strategy during transient operation under different disturbances is tested. The results show that during actual solar radiation disturbance, the integral square errors (ISE) of outlet hydrogen fraction and methane conversion for BPNN-based PI hybrid control are 7.03 × 10−6 and 1.05 × 10−4, respectively. Compared to traditional PI feedback control, the new hybrid control strategy represents a reduction of 94% and 91% in ISE, indicating that the strategy can significantly improve production stability.

Estimating the effect of the main design parameters on the effectiveness of high-purity hydrogen production from raw hydrocarbons in membrane catalytic devices

The paper presents the results of the application of a physically grounded mathematical model, verified through numerous practical examples, intended for estimating the effect of some design factors (membrane thickness and the system of high-purity hydrogen outlet from the under-membrane space of membrane elements) on the effectiveness and efficiency of the production of highly pure hydrogen from the products of steam conversion of hydrocarbons in advanced membrane catalytic devices.

Numerical investigation of hydrogen production via methane steam reforming in a novel packed bed reactor integrated with diverging tube

Hydrogen is one of essential renewable energy sources to effectively solve the climate challenge and methane steam reforming accounts for 60.0% of global hydrogen supply. In this study, to improve the efficiency, the methane steam reforming in a packed bed reactor integrated with diverging tube was investigated via numerical method. To quantify the performance of flow and heat transfer, the flow disturbance analysis and the thermal resistance analysis of different inclination angle diverging tube were conducted. The results showed that, the integration of diverging tube lead to the reduction of average temperature, the decreasing of pressure drop and the increment of outlet mass flow. The flow disturbance and the thermal resistance increase with the inclination angle rising. As for the overall efficiency of flow and heat transfer, the improvement of flow has more effects than the weakness of heat transfer. As the inclination angle is 3°, there is the highest overall heat transfer coefficient, which is increased by 9.0 % compared with the normal packed bed reactor. Since hydrogen production mainly depends on the flow and heat transfer, the integration of diverging tube improves the hydrogen yield and the hydrogen yield become larger with the inclination angle rising. Considering the usage of catalyst, the flow loss and outlet hydrogen mass flow, the efficiency of hydrogen production is the highest as the inclination angle is 3°, which is increased by 34.0%.

Steam methane reforming over a preheated packed bed: Heat and mass transfer in a transient process

The primary method of large scale hydrogen production is steam methane reforming (SMR) in the packed bed reactors filled with the catalysts. In these reactors, heat is mostly supplied through the reactor wall to compensate for the endothermic effect of SMR reactions. In this paper, an alternative route of steam methane reforming process over a preheated packed bed filled with Ni-based catalyst particles was studied. Consequent packed bed heating with hot flue gas and cooling with H2O - CH4 endothermically reacting mixture was investigated. Numerical model has been developed for the cooling and heating processes and realized via Ansys Fluent. The analyses were performed for various steam-to-methane ratios and different initial temperature distributions. Results are indicating that the packed bed volume averaged temperature increases by 98 °C after 15 s of heating with flue gas. Packed bed cools down by more than 125 °C after 15 s of the SMR process. Mixture species distribution was also analysed along the reformer length. Hydrogen outlet molar fraction constantly decreases from 0.38 to 0.25 at the process beginning and end, respectively. The highest hydrogen production was observed on the half of the packed bed closer to the reformer outlet.

Efficiency Maximization of a Direct Internal Reforming Solid Oxide Fuel Cell in a Two-Layer Self-Optimizing Control Structure.

Control system configuration is essential for the efficiency performance of a solid oxide fuel cell (SOFC). In this paper, we aim to report a novel two-layer self-optimizing control (SOC) system for the efficiency maximization of a direct internal reforming SOFC, where the efficiency index is defined as the profit of generated electricity penalized by carbon (CO2) emission. Based on the lumped-parameter model of the SOFC, comprehensive evaluations are carried out to identify the optimal controlled variables (CVs), the control of which at constant set-points can optimize the efficiency, in spite of operating condition changes. In the lower SOC layer, we configure single variables as the CVs. The results show that the stack temperature is the active constraint which should be controlled to maintain the cell performance. In addition, the outlet hydrogen composition is identified as the optimal CV. This result differs from several previous proposals, such as methane composition. In the presence of operating condition changes, the set-point of hydrogen composition is further automatically adjusted by the upper SOC layer, where a linear combination of the SOFC measurements is configured as the CV, giving negligible efficiency losses. The cascaded two-layer SOC structure is able to maximize the SOFC efficiency and reduce carbon emission without using online optimization techniques; meanwhile, it allows for smooth and safe operations. The validity of the new scheme is verified through both static and dynamic evaluations.

MULTIPHYSICS SIMULATION OF SUPERCRITICAL CO2 GASIFICATION FOR HYDROGEN PRODUCTION

Among the gasification technologies being considered, supercritical carbon dioxide (sCO<sub>2</sub>) gasification offers certain advantages for direct fired systems and is the subject of this work. A reacting computational fluid dynamics (CFD) model was developed using Ansys Fluent in order to investigate the impact of various operating parameters on syngas composition. The model was validated using available data in the literature. Simulations were conducted over a wide range of operating parameters with the goal of optimizing hydrogen production for the considered reactor. The results from the simulations identified a positive correlation between slurry loading and hydrogen production, with slurry loadings of 80% resulting in an average outlet hydrogen mole percent of 25.4%. Reactor temperature and pressure were found to have a limited impact on H<sub>2</sub> production at the conditions of interest. Additionally, the simulations indicated that oxygen mass flowrate has less of an impact on hydrogen production at higher slurry loadings. Hydrogen content at conditions relevant to sCO<sub>2</sub> gasification is comparable to other slurry-fed gasifiers, while CO/CO<sub>2</sub> ratios far exceed those of widely employed dry or slurry-fed gasifiers, highlighting the need for experimental investigation.

Efficient hydrogen recovery and purification from industrial waste hydrogen to high-purity hydrogen based on metal hydride powder

Flow-through reaction model of metal hydride powder for hydrogen recovery and purification from industrial by-product hydrogen to high-purity hydrogen is proposed. The hydriding mechanism of hydrogen storage alloy in the flow-through reactor is expounded. The results show that the evolution of bed temperature field leads to a unique increase in outlet hydrogen mole fraction compared to the case of pure hydrogen. In addition, the effects of operating parameters and reactor geometrical parameters are systematically investigated for performance optimization. The indexes of hydrogen absorption efficiency and effective metal hydride utilization ratio are introduced to evaluate the hydrogen recovery and purification performance of reactor. The optimum hydrogen absorption efficiency of 65 % with the effective metal hydride utilization ratio of about 0.8 could be achieved at the cooling water temperature of 373 K. This work contributes to the design and development of advanced hydrogen separation and purification technology.

The case for high-pressure PEM water electrolysis

Hydrogen compression is a key part of the green hydrogen supply chain, but mechanical compressors are prone to failure and add system complexity and cost. High-pressure water electrolysis can alleviate this problem through electrochemical compression of the gas internally in the electrolyzer and thereby eliminating the need for an external hydrogen compressor. In this work, a detailed techno-economic assessment of high-pressure proton exchange membrane-based water electrolysis (PEMEL) systems was carried out. Electrolyzers operating at 80, 200, 350, and 700 bar were compared to state-of-the-art systems operating at 30 bar in combination with a mechanical compressor. The results show that it is possible to achieve economically viable solutions with high-pressure PEMEL-systems operating up to 200 bar. These pressure levels fit well with the requirements in existing and future industrial applications, such as e-fuel production (30–120 bar), injection of hydrogen into natural gas grids (70 bar), hydrogen gas storage (≥200 bar), and ammonia production (200–300 bar). A sensitivity analysis also showed that if the cost of electricity is sufficiently low (<0.1 €kWh−1), it may even be economical to operate PEMEL systems with hydrogen outlet pressures up to 350 bar.

Modeling and Economic Operation of Energy Hub Considering Energy Market Price and Demand

This paper discusses the economic operation strategy of the energy hub, which is being established in South Korea. The energy hub has five energy conversion devices: a turbo expander generator, a normal fuel cell, a fuel cell with a hydrogen outlet, a small-scale combined heat and power device, and a photovoltaic device. We are developing the most economically beneficial operation strategy for the operators who own the hub, without making any systematic improvements to the energy market. First, sixteen conversion efficiency matrices can be achieved by turning each device (except the PV) on or off. Next, even the same energy must be divided into different energy flows according to price. The energy flow is controlled to obtain the maximum profit, considering the internal load of the energy hub and the price fluctuations of the energy market. Using our operating strategy, the return on investment period is approximately 9.9 years, which is three years shorter than that without the operating strategy.

Improving chiller performance and energy efficiency in hydrogen station operation by tuning the auxiliary cooling

In a hydrogen station that operates with direct fueling through the use of a 700 bar boost compressor, the outlet hydrogen temperature can significantly increase, stressing the chiller system. This paper evaluates improvements that can be made to the auxiliary cooling system integrated with the compressor. Both theoretical modeling and experiments were performed at Cal State LA Hydrogen Research and Fueling Facility. The findings suggest that adjusting the auxiliary closed-loop cooling system from 15 °C to 10 °C reduced the station energy consumption and decreased the demand on the station chiller that needed to provide −20 °C hydrogen at the hose. The overall energy consumption for a single fueling reduced by between 2.86 and 9.43% for the set of experiments conducted. After the temperature of the closed-loop cooling system was reduced by 5 °C, the boost compressor outlet temperature dropped from 46-50 °C–40 °C and consequently at the hose the hydrogen temperature declined by 3 °C. Results were scaled up with a forecast on the number of daily refueling events. With a low number of daily fuelings, the proposed set-up showed a minor influence on the overall station energy consumption. However, the benefits were more pronounced for a connector station with sales at 180 kg/day, where the energy efficiency improved by between 1.4 and 5.5%, and even more so for a higher capacity station at 360 kg/day, where the improvement was between 2.9 and 8%.

(Invited) Progress in Nafion™ Membrane Development for Proton Exchange Membrane Water Electrolyzers

Proton exchange membrane water electrolyzers (PEMWE) have received significant recent attention in order to address the burgeoning need for low/zero-carbon sources of hydrogen at scale. PEMWE devices are an attractive source of “green” hydrogen due to their relatively compact size, scalable construction, high efficiency, and proven durability. They also integrate well with intermittent electricity sources and can deliver elevated hydrogen outlet pressures to reduce parasitic efficiency losses associated with hydrogen compression.1 Membranes made from Nafion™ perfluorosulfonic acid (PFSA) polymer have been the leader in PEMWE device application for decades, due to their high proton conductivity and chemical durability in the hot, acidic PEMWE environment. Traditional Nafion™ membranes in this application, however, comprise thick films (>100 µm) where ohmic-driven voltage losses due to proton transport resistance are high.To maximize the electrical efficiency and hydrogen production of PEMWE applications, and help drive hydrogen costs below $2/kg, new membranes – specifically engineered for low proton transport resistance and high durability in a PEMWE environment – are required. A multigenerational plan has been established to improve the performance of Nafion™ membranes while maintaining their best-in-class durability. One major contributor to membrane improvement is a reduced proton transport resistance inherent in thinner membranes. However, as membranes become thinner, they invite higher gas crossover, especially in a differential pressure environment. Novel mitigation strategies are required to ensure safe operation and long lifetimes of the thinnest membranes in advanced PEMWE membrane concepts.2-3 In this presentation, an overview of the activities and strategic developments within Nafion™ membranes for water electrolyzers will be summarized. The multigenerational product development program will be discussed, highlighting the significant contribution possible from Nafion™ membrane development to green hydrogen proliferation. Ayers, K.; Danilovic, N.; Ouimet, R.; Carmo, M.; Pivovar, B.; Bornstein, M., Perspectives on Low-Temperature Electrolysis and Potential for Renewable Hydrogen at Scale. Annual Review of Chemical and Biomolecular Engineering 2019, 10 (1), 219-239.Klose, C.; Trinke, P.; Böhm, T.; Bensmann, B.; Vierrath, S.; Hanke-Rauschenbach, R.; Thiele, S., Membrane Interlayer with Pt Recombination Particles for Reduction of the Anodic Hydrogen Content in PEM Water Electrolysis. J. Electrochem. Soc. 2018, 165 (16), F1271-F1277.Baker, A. M.; Babu, S. K.; Mukundan, R.; Advani, S. G.; Prasad, A. K.; Spernjak, D.; Borup, R. L., Cerium Ion Mobility and Diffusivity Rates in Perfluorosulfonic Acid Membranes Measured via Hydrogen Pump Operation. J. Electrochem. Soc. 2017, 164 (12), F1272-F1278. Figure 1

Optimal design and thermodynamic analysis on the hydrogen oxidation reactor in a combined hydrogen production and power generation system based on coal gasification in supercritical water

The novel power generation system based on coal gasification in supercritical water (SCW) holds great promise to meet the increasing energy demands in China, with less emissions in comparison to the traditional coal-fired power generation. The stable and complete oxidation of hydrogen occurring in the supercritical water oxidation (SCWO) reactor is paramount to the safety and overall energy efficiency of the system. In order to better understand and improve the performance of this reactor, optimal design and thermodynamic analysis were conducted using Aspen Plus. The outlet hydrogen concentration and the internal temperature of the reactor were taken as the two key indicators of the optimal design. The optimized results for the reactor were found to be the reactor diameter of 90 mm, axial length of 10.75 m, oxygen inlet number of 3 with the oxygen ratio of 11.0%/39.9%/49.1%, under the given inlet working fluid components and flow rates. The three-stage reactor design was confirmed to have good robustness when different types of coals are adopted for generating syngas. In addition, by analyzing the enthalpy and exergy flow distributions in the reactor, it was revealed that the total exergy destruction for the whole three-stage reactor is only about 6.29%, and that the exergy destruction is consistent with the oxidation process. These findings could be used as guidelines for the design and implementation of the SCWO reactor towards higher safety margin and energy efficiency for this new coal-boiled power generation technique.

Methane steam reforming with axial variable diameter particle structures in grille-sphere composite packed bed: A numerical study of hydrogen production performance

Methane steam reforming with packed bed is an important approach to get hydrogen in industrial production. Compared with the conventional packed bed, the grille-sphere composite packed bed has the advantages of lowering the pressure drop, increasing the uniformity of the radial temperature distribution and improving overall efficiency. In the present paper, the axial variable diameter particle structure of grille-sphere composite packed bed has been proposed, and four different structures are studied to compare their performances. Moreover, two simulation methods are used in study: (a) the solid particle method which can show details of the simulation, (b) the equivalent medium method which can show the macro situation of simulation. Through these two methods, performances of four structures in different parameters are compared and analyzed. It is found that the axial variable diameter particle structures have better performance than the typical structures in different parameters cases. Inlet temperature, inlet velocity and inlet steam-carbon ratio are set to parameters respectively in simulation. According to the simulations, higher inlet temperature and inlet steam-carbon ratio will facilitate the reactions, however, higher inlet velocity will hinder the reactions. Through the solid particle method, the reasons of better performance have been found, and two methods difference are also studied. Compared with typical structures, the specific axial variable diameter structure brings 4.5% higher outlet hydrogen mass flow and 5.2% higher outlet hydrogen selectivity. Analysis of this study can guide the design of packed beds in industrial production and the results of this study have significance in industrial hydrogen production.

Simulation analysis of hydrogen recirculation rates of fuel cells and the efficiency of combined heat and power

The objective of this study was to simulate a proton-electrolyte membrane fuel cell (PEMFC) system, namely a PEMFC stack, an anode gas supply subsystem, an anode gas-recovery subsystem, a cathode gas supply subsystem, and a tail gas exhaustion subsystem. In addition, this paper presents an analysis of the efficiency of combined heat and power (CHP) systems. MATLAB and Simulink were employed for dynamic simulation and statistical analysis. The rates of active and the passive anode hydrogen recirculation were considered to elucidate the mechanism of hydrogen circulation. When recovery involved diverse recovery mechanisms, the recirculation rate was affected by the pressure at the hydrogen outlet of the PEMFC system. The greater the pressure was at that outlet, the higher the recovery rate was. In the hydrogen recovery system, when the temperature of the hydrogen supply end remained the same, increasing the temperature of the gas supply end increased the efficiency of the fuel cells; fixing the flow of the hydrogen supply end and increasing the temperature of the hydrogen supply end increased the efficiency of the PEMFC system. A calculation of the efficiency of the recovery system indicated that the thermal efficiency of the fuel cells exceeded 35%, the power generation efficiency exceeded 45%, and the efficiency of the CHP system exceeded 80%.

Experimental Study on Performance of Anode Humidification System of Large Power Fuel Cell Stack

The heat and mass transfer characteristics of the hydrogen membrane humidification system of a 70kW atmospheric pressure proton exchange membrane fuel cell stack were experimentally studied. The shell-and-tube membrane humidifier is applied to the high-power fuel cell stack hydrogen humidification system, which has the advantages of fast humidification rate and good wettability of humidified hydrogen. The liquid hydrogen is humidified by liquid water. After the humidification in the experiment, the hydrogen can always reach the supersaturation state. When the hydrogen flow rate is constant, the temperature difference between the membrane humidifier humidification water inlet and outlet decreases with the increase of the water flow rate, and the hydrogen outlet The temperature approaches the humidification water inlet temperature as the liquid water flow rate increases; increasing the humidification water flow rate can reduce the temperature difference before and after the humidification water passes through the membrane humidifier. When the temperature and flow rate of the humidified water are constant, the fuel cell stack load increases, and the relative humidity of the humidified hydrogen outlet does not change significantly.

Experimental behaviour of a three-stage metal hydride hydrogen compressor

A three-stage metal hydride hydrogen compressor (MHHC) system based in AB2-type alloys has been set-up. Every stage can be considered as a Sieverts-type apparatus. The MHHC system can work in the pressure and temperature ranges comprised from vacuum to 250 bar and from RT to 200 °C, respectively. An efficient thermal management system was set up for the operational ranges of temperature desired. It drops temperature shifts due to hydrogen expansion during stage coupling and hydrogen absorption/desorption in the alloys. Each reactor consists of a single and thin stainless-steel tube to maximize heat transfer. These were filled with similar amount of AB2 alloy. The MHHC system was able to produce a compression ratio as high as 84.7 for inlet and outlet hydrogen pressures of 1.44 and 122 bar for a temperature span of 23 °C – 120 °C.