A self-regulating hydrogen generator for micro fuel cells

The ever-increasing power demands and miniaturization of portable electronics, micro-sensors and actuators, and emerging technologies such as cognitive arthropods have created a significant interest in development of micro fuel cells. One of the major challenges in development of hydrogen micro fuel cells is the fabrication and integration of auxiliary systems for generating, regulating, and delivering hydrogen gas to the membrane electrode assembly (MEA). In this paper, we report the development of a hydrogen gas generator with a micro-scale control system that does not consume any power. The hydrogen generator consists of a hydride reactor and a water reservoir, with a regulating valve separating them. The regulating valve consists of a port from the water reservoir and a movable membrane with via holes that permit water to flow from the reservoir to the hydride reactor. Water flows towards the hydride reactor, but stops within the membrane via holes due to capillary forces. Water vapor then diffuses from the via holes into the hydride reactor resulting in generation of hydrogen gas. When the rate of hydrogen consumed by the MEA is lower than the generation rate, gas pressure builds up inside the hydride reactor, deflecting the membrane, closing the water regulator valve, until the pressure drops, whereby the valve reopens. We have integrated the self-regulating micro hydrogen generator to a MEA and successfully conducted fuel cell tests under varying load conditions.     

Design tool for estimating metal hydride storage system characteristics for light-duty hydrogen fuel cell vehicles

The U.S. Department of Energy (DOE) has developed the Framework model to simulate fuel cell-based light-duty vehicle operation for various hydrogen storage systems. This transient model simulates the performance of the storage system, fuel cell, and vehicle for comparison to DOE's Technical Targets using four drive cycles. Metal hydride hydrogen storage models have been developed for the Framework model. Despite the utility of this model, it requires that material researchers input system design specifications that cannot be easily estimated. To address this challenge, a design tool has been developed that allows researchers to directly enter physical and thermodynamic metal hydride properties into a simple sizing module that then estimates the systems parameters required to run the storage system model. This design tool can also be used as a standalone MS Excel model to estimate the storage system mass and volume outside of Framework and compare it to the DOE Technical Targets. This model will be explained and exercised with existing hydrogen storage materials.     

Comments on solid state hydrogen storage systems design for fuel cell vehicles

In recent years, significant research and development efforts were spent on hydrogen storage technologies with the goal of realizing a breakthrough for fuel cell vehicle applications. This article scrutinizes design targets and material screening criteria for solid state hydrogen storage. Adopting an automotive engineering point of view, four important, but often neglected, issues are discussed: 1) volumetric storage capacity, 2) heat transfer for desorption, 3) recharging at low temperatures and 4) cold start of the vehicle. The article shall help to understand the requirements and support the research community when screening new materials.     

Analyses of fuel utilization in microfluidic fuel cell

A microfluidic fuel cell is a miniature power source, which potentially could be used in micro electronic equipments, laptop computers, mobile phones and video cameras. In recent reports, the idea of a microfluidic fuel cell without using a polymer electrolyte membrane is proposed, whereby the laminar nature of the flow in the micro-channels is used to keep the anode and cathode streams separated such that adverse electrochemical reactions do not take place at the two electrode polarities. Since such cells are restricted by their size, improvement in fuel utilization would increase the cell efficiency by several degrees. In the present study, an improvement in fuel utilization is proposed by altering the design of the microfluidic fuel cell. In particular, a sulfuric acid stream is introduced between the fuel (HCOOH) and oxidizer (O₂ in H₂SO₄) streams to improve fuel utilization. Further improvement in fuel utilization is possible by changing the aspect ratio of the cell from 0.1 to 1. The fuel utilization of a cell with an aspect ratio of 0.1 is 14.1%, which increases to 16% when a sulfuric acid stream is introduced to prevent mixing of the fuel and oxidizer streams. The fuel utilization increases to 19% with the change in aspect ratio from 0.1 to 10, which further increases to 32% with the introduction of a sulfuric acid stream.     

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.     

Energy and exergy analysis of microfluidic fuel cell

Microfluidic fuel cell (MFC) is a promising fuel cell type because its membraneless feature implies great potential for low-cost commercialization. In this study, an energy and exergy analysis of MFC is performed by numerical simulation coupling computational fluid dynamics (CFD) with electrochemical kinetics. MFC system designs with and without fuel recirculation are investigated. The effects of micropump efficiency, fuel flow rate and fuel concentration on the MFC system performance are evaluated. The results indicate that fuel recirculation is preferred for MFC to gain higher exergy efficiency only if the efficiency of the micropump is sufficiently high. Optimal cell operating voltage for achieving the highest exergy efficiency can be obtained. Parasitic effect will cause a significant reduction in the exergy efficiency. An increase in the fuel concentration will also lead to a reduction in the exergy efficiency. Increasing the fuel flow rate in a MFC with fuel recirculation will cause a fluctuating variation in the exergy efficiency. On the other hand, in a one-off MFC system, the exergy efficiency decreases with increasing fuel flow rate. The present work enables better understanding of the energy conversion in MFC and facilitates design optimization of MFC.     

A flexible on-fiber H2O2 microfluidic fuel cell with high power density

To promote the simplification and integration of membraneless microfluidic fuel cell (MMFC) system and combine with flexible portable devices, a flexible on-fiber MMFC exploiting H₂O₂ as sole reactant is presented, eliminating the separation requirement of fuel and oxidant. Nickel (Ni) nano-particles and Prussian blue with multiwalled carbon nanotube (PB-MWCNT) are coated on hydrophilic braided carbon fibers (BCFs) to serve as the anode and cathode, respectively. The three-dimensional (3D) flow-through anode and cathode with a wealth of exposed electroactive sites improve reactant mass transfer. The anode and cathode are respectively wound on both sides of the middle cotton thread-based flow channel for separation. Under the combination of capillary force and gravity, reactants flow continuously through the fiber-based microchannels without external pumps. Importantly, the H₂O₂ MMFC achieves the highest maximum power density (MPD) of 14.41 mW cm^?2 so far in one-chamber or single-stream H₂O₂ fuel cells. Besides, no serious deterioration in the power-generation performance is observed in complex practical operating conditions including bending with various angles, repeated folding and dropping. Three presented flexible MMFCs are connected to power a handheld calculator, indicating the tremendous potential of developing micro power supplies based on abundant flexible materials as well as green and sustainable energy.     

Fully-integrated micro PEM fuel cell system with NaBH4 hydrogen generator

A fully-integrated micro PEM fuel cell system with a NaBH₄ hydrogen generator was developed. The micro fuel cell system contained a micro PEM fuel cell and a NaBH₄ hydrogen generator. The hydrogen generator comprised a NaBH₄ reacting chamber and a hydrogen separating chamber. Photosensitive glass wafers were used to fabricate a lightweight and corrosion-resistant micro fuel cell and hydrogen generator. All of the BOP such as a NaBH₄ cartridge, a micropump, and an auxiliary battery were fully integrated. In order to generate stable power output, a hybrid power management operating with a micro fuel cell and battery was designed. The integrated performance of the micro PEM fuel cell with NaBH₄ hydrogen generator was evaluated under various operating conditions. The hybrid power output was stably provided by the micro PEM fuel cell and auxiliary battery. The maximum power output and specific energy density of the micro PEM fuel cell system were 250 mW and 111.2 W h/kg, respectively.     

Distributed generation system with PEM fuel cell for electrical power quality improvement

In this paper, a physical model for a distributed generation (DG) system with power quality improvement capability is presented. The generating system consists of a 5 kW PEM fuel cell, a natural gas reformer, hydrogen storage bottles and a bank of ultra-capacitors. Additional power quality functions are implemented with a vector-controlled electronic converter for regulating the injected power.     

Boosting cell performance and fuel utilization efficiency in a solar assisted methanol microfluidic fuel cell

Solar generated hydrogen from an optimized P25 thin film of 3.2 mg/cm² with 0.25% of platinum as co-catalyst improves the peak power output of a methanol microfluidic fuel cell operated with a methanol to water ratio of 1:1 almost ninefold, from 22 mW/cm² to 213 mW/cm². Different methanol to water ratios in the fuel tank generate similar amounts of hydrogen, but the cell performance has large variations due to the different oxidation kinetics of hydrogen and methanol in the fuel breathing anode, resulting in a mixed-potential anodic performance. The trade-off between power output and fuel utilization diminishes in this system. The methanol utilization efficiency at peak power operation increases from 50% (for 0.2 V) to 78% (for 0.5 V) for methanol to water ratio of 1:1. The result indicates that in-situ generation of hydrogen by solar light can be applied to both portable and large-scale stationary fuel cells.     

Feedforward-feedback control of a solid oxide fuel cell power system

One of the main challenges for wide-spread utilization of the solid oxide fuel cell (SOFC) power systems is how to achieve high electrical efficiency without increasing the degradation rate of the fuel cells. To run the SOFC power system at high efficiency over a long period of time, properly designed controllers are indispensable.Although a number of various approaches to control SOFC have been proposed so far, it seems that the design of control system, along with simple tuning procedure, has not been treated in a consistent manner. This issue is addressed in the present paper resulting in a feedforward-feedback control structure. The feedforward part is based on the stoichiometry of electro-oxidation, reforming and combustion reactions, which allow immediate response to variable current demand. The feedback part performs additional fine adjustment of fuel and air supply in order to minimize the undesired system temperatures variations. The selection of pairings of manipulated and controlled variables for control is based on physical knowledge of the system. Input/output pairing for single-loop feedback control is assessed by the relative gain analysis. An efficient procedure for tuning the parameters of the feedback controllers is suggested, relying on simple open-loop step responses of the system.The proposed low-level control is assessed on a detailed physical model of a 2.5 kW SOFC power system by simulating two nonstationary load regimes. Simulations show that the control provides a robust operation under large load variations while meeting the operating constraints. Due to its simplicity, the control is feasible for implementation on a real SOFC system.     

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.     

Fuzzy control of hydrogen pressure in fuel cell system

Precise control of hydrogen pressure is crucial for the performance and durability of fuel cell systems. With the widely used common-rail injection system, traditional PID controller still dominates. For a long time, the hydrogen pressure fluctuates acutely when hydrogen purge valve switches or load sharply changes with using PID controller. In recent studies, several new control strategies are presented. However, mostly of them are theoretical and experimental. In this study, an improved common-rail injection system, hydrogen injector/ejector assembly is introduced. Based on a real fuel cell system, a Mamdani fuzzy controller is designed to regulate the hydrogen pressure. The algorithm of fuzzy controller is explained in detail. A comparative study is carried out between fuzzy controller and PID controller. According to the results, the stability of hydrogen pressure with using fuzzy controller is better than using PID controller. This research could be useful for the control of fuel cell system.     

A novel direct ethanol fuel cell with high power density

A new type of direct ethanol fuel cell (DEFC) that is composed of an alkaline anode and an acid cathode separated with a charger conducting membrane is developed. Theoretically it is shown that the voltage of this novel fuel cell is 2.52 V, while, experimentally it has been demonstrated that this fuel cell can yield an open-circuit voltage (OCV) of 1.60 V and a peak power density of 240 mW cm⁻² at 60 °C, which represent the highest performance of DEFCs that has so far been reported in the open literature.     

A natural-gas fuel processor for a residential fuel cell system

A system model was used to develop an autothermal reforming fuel processor to meet the targets of 80% efficiency (higher heating value) and start-up energy consumption of less than 500 kJ when operated as part of a 1-kWe natural-gas fueled fuel cell system for cogeneration of heat and power. The key catalytic reactors of the fuel processor – namely the autothermal reformer, a two-stage water gas shift reactor and a preferential oxidation reactor – were configured and tested in a breadboard apparatus. Experimental results demonstrated a reformate containing ∼48% hydrogen (on a dry basis and with pure methane as fuel) and less than 5 ppm CO. The effects of steam-to-carbon and part load operations were explored.     

Modeling,control and simulation of an autonomous wind turbine/photovoltaic/fuel cell/ultra-capacitor hybrid power system

This paper focuses on the combination of wind turbine (WT), photovoltaic (PV), fuel cell (FC) and ultra-capacitor (UC) systems for grid-independent applications. The dynamic behavior of the proposed hybrid system is tested under various wind speed, solar radiation and load demand conditions. The developed model and its control strategy exhibit excellent performance for the simulation of a complete day. In the simulation, the solar radiation and power demand data are based on real world measurements, while the wind speed data are quasi-real because it is simulated based on special wind speed generation algorithms.     

Modeling and control of a portable proton exchange membrane fuel cell-battery power system

A portable proton exchange membrane (PEM) fuel cell-battery power system that uses hydrogen as fuel has a higher power density than conventional batteries, and it is one of the most promising environmentally friendly small-scale alternative energy sources. A general methodology of modeling, control and building of a proton exchange membrane fuel cell-battery system is introduced in this study. A set of fuel cell-battery power system models have been developed and implemented in the Simulink environment. This model is able to address the dynamic behaviors of a PEM fuel cell stack, a boost DC/DC converter and a lithium-ion battery. To control the power system and thus achieve proper performance, a set of system controllers, including a PEM fuel cell reactant supply controller and a power management controller, were developed based on the system model. A physical 100 W PEM fuel cell-battery power system with an embedded micro controller was built to validate the simulation results and to demonstrate this new environmentally friendly power source. Experimental results demonstrated that the 100 W PEM fuel cell-battery power system operated automatically with the varying load conditions as a stable power supply. The experimental results followed the basic trend of the simulation results.     

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.     

Analysis of ammonia decomposition reactor to generate hydrogen for fuel cell applications

In this paper, reaction engineering principles are utilized to analyze process conditions for producing sufficient hydrogen in an ammonia decomposition reactor for generating net power of 100 W in a fuel cell. It is shown that operating the reactor adiabatically results in a sharp decrease in temperature due to endothermic reaction, which results in low conversion of ammonia. For this reason, the reactor is heated electrically to provide heat for the endothermic reactions. It is observed that when the reactor is operated non-adiabatically, it is possible to get over 99.5% conversion of ammonia. The weight of absorbent to reduce ammonia to ppb levels is calculated. An energy balance on the reactor exit gas indicates that there is sufficient heat available to vaporize enough water to achieve 100% relative humidity in the fuel cell. A suitable fuel cell stack is designed and it is shown that this stack is able to provide the necessary power to electrically heat the reactor and produce net power of 100 W.     

Optimum geometrical design for improved fuel utilization in membraneless micro fuel cell

In membraneless micro fuel cells, the mixing and depletion widths are major factors that determine the cell performance. Cells in which these widths are too large exhibit severely reduced fuel utilization and, hence, less electrochemical reaction especially in the downstream region of the channel. For cells with conventional rectangular geometry, increasing the aspect ratio reduces the mixing width but reduces the effective electrode area. This work proposes a trident-shaped geometrical design for membraneless micro fuel cells in which the anode fluid, cathode fluid and proton-conducting fluid are introduced through three distinct inlets. The anode and cathode fluids are interconnected by the proton-conducting fluid channel. In addition, the anode fluid and proton-conducting fluid are connected by a small narrow passage, and the cathode fluid and proton-conducting fluid channel are also connected by a small narrow passage. Numerical simulations, including the effects of electrochemical reaction and fluid flow, are carried out to investigate reactant distributions in the downstream region of the channel and to study fuel utilization. A fuel utilization of around 51% is achieved when the two opposite walls are used as reaction surfaces and the inlet velocity is set at 0.01 m s⁻¹. By varying the cell length and expanding the reaction surface areas by including additional surfaces within the cell, simulations show that the fuel utilization can be improved to around 86%, which is much higher than has been achieved in previous studies. The present numerical results are validated by comparison with available literature data.