Fuel sensor-less control of a liquid feed fuel cell under dynamic loading conditions for portable power sources (I)

This work presents a novel fuel sensor-less control scheme for a liquid feed fuel cell system that operates under dynamic loading conditions and is suitable for portable power sources. The proposed technique utilizes the operating characteristics of a fuel cell, such as voltage, current and power, to control the supply of liquid fuel and regulate its concentration. As verified by systematic experiments, this scheme controls effectively the supply of fuel under dynamic loading conditions and pushes the system toward higher power output. The primary features and advantages of sensor-less fuel control are as follows. When the fuel concentration sensor is excluded, the cost of a liquid feed fuel cell system is decreased and system volume and weight are reduced, thereby increasing specific energy density and design simplicity, and shortening system response time. Notably, temperature compensation for measurement data is unnecessary. With a decreased number of components, the control scheme improves durability and reliability of liquid feed fuel cells. These advantages will help commercialization of liquid feed fuel cells as portable power sources.     

Fuel sensor-less control of a liquid feed fuel cell under dynamic loading conditions for portable power sources (II)

This work presents a new fuel sensor-less control scheme for liquid feed fuel cells that is able to control the supply to a fuel cell system for operation under dynamic loading conditions. The control scheme uses cell-operating characteristics, such as potential, current, and power, to regulate the fuel concentration of a liquid feed fuel cell without the need for a fuel concentration sensor. A current integral technique has been developed to calculate the quantity of fuel required at each monitoring cycle, which can be combined with the concentration regulating process to control the fuel supply for stable operation. As verified by systematic experiments, this scheme can effectively control the fuel supply of a liquid feed fuel cell with reduced response time, even under conditions where the membrane electrolyte assembly (MEA) deteriorates gradually. This advance will aid the commercialization of liquid feed fuel cells and make them more adaptable for use in portable and automotive power units such as laptops, e-bikes, and handicap cars.     

Sensor-less control of methanol concentration based on estimation of methanol consumption rates for direct methanol fuel cell systems

Adequate control over the concentration of methanol is critically needed in operating direct methanol fuel cell (DMFC) systems, because performance and energy efficiency of the systems are primarily dependent on the concentration of methanol feed. For this purpose, we have built a sensor-less control logic that can operate based on the estimation of the rates of methanol consumption in a DMFC. The rates of methanol consumption are measured in a cell and the resulting data are fed as an input to the control program to calculate the amount of methanol required to maintain the concentration of methanol at a set value under the given operating conditions of a cell. The sensor-less control has been applied to a DMFC system employed with a large-size single cell and the concentration of methanol is found to be controlled stably to target concentrations even though there are some deviations from the target values.     

Critical challenges in the system development of direct alcohol fuel cells as portable power supplies: An overview

Direct alcohol fuel cells (DAFCs) are considered a reasonable alternative power source because alcohol has a much higher energy density than hydrogen. Most DAFC development has focused on small portable application by using passive systems. DAFCs with active feed systems have appeared as potential portable power sources for larger applications, as they are easily handled, simple systems with smaller volumes than polymer electrolyte membrane fuel cells (PEMFCs). A general active DAFC system consists of a fuel and oxidant supplying system, product management and fuel concentration control. However, system development and commercialization are constrained by various critical challenges. This paper highlights the critical challenges of the fuel cell system rather than fundamental problems in the membrane electrode assembly (MEA), including fuel feed fluctuation, contaminant poisoning, two-phase flow, low power density, and heat and water management.     

Performance assessment of a hybrid solid oxide and molten carbonate fuel cell system with compressed air energy storage under different power demands

As electricity demand can vary considerably and unpredictably, it is necessary to integrate energy storage with power generation systems. This study investigates a solid oxide and molten carbonate fuel cell system integrated with a gas turbine (GT) for power generation. The advanced adiabatic compressed air energy storage (AA-CAES) system is designed to enhance the system flexibility. Simulations of the proposed power system are performed to demonstrate the amount of power that can supply to the loads during normal and peak modes of operation under steady-state conditions. The pressure ratios of the GT and AA-CAES and the additional air feed are used to design the system and analyze the system performance. The results show that a small additional air feed to the GT is certainly required for the hybrid system. The GT pressure ratio of 2 provides a maximum benefit. The AA-CAES pressure ratio of 5 is recommended to spare some air in the storage and minimize storage volume. Moreover, implementation of the GT and AA-CAES into the integrated fuel cell system allows the system to cope with the variations in power demand.     

Portable DMFC system with methanol sensor-less control

This work develops a prototype 20 W portable DMFC by system integration of stack, condenser, methanol sensor-less control and start-up characteristics. The effects of these key components and control schemes on the performance are also discussed. To expedite the use of portable DMFC in electronic applications, the system utilizes a novel methanol sensor-less control method, providing improved fuel efficiency, durability, miniaturization and cost reduction. The operating characteristics of the DMFC stack are applied to control the fuel ejection time and period, enabling the system to continue operating even when the MEAs of the stack are deteriorated. The portable system is also designed with several features including water balance and quick start-up (in 5 min). Notably, the proposed system using methanol sensor-less control with injection of pure methanol can power the DVD player and notebook PC. The system specific energy and energy density following three days of operation are 362 Wh kg⁻¹ and 335 Wh L⁻¹, respectively, which are better than those of lithium batteries (∼150 Wh kg⁻¹ and ∼250 Wh L⁻). This good energy storage feature demonstrates that the portable DMFC is likely to be valuable in computer, communication and consumer electronic (3C) markets.     

A strategy of estimating fuel concentration in a direct liquid-feed fuel cell system

Fuel control is one of the most pressing topics to achieve a self-sustainable direct liquid-feed fuel cell system, such as a direct methanol fuel cell (DMFC), and enhance its overall efficiency. In a DMFC system, sensing the methanol concentration generally serves as the basis of the fuel control strategies. This paper proposes a three-dimensional measurement space and constant concentration surfaces (CCS) to develop an algorithm of estimating fuel concentration in a liquid-feed system, which embraces the following merits: (1) it measures only three quantities or indices that correlate with the fuel concentration. The indices can be chosen as current, voltage, temperature, or other quantities that are easily acquired in an operating fuel cell system. The estimation can be accomplished without interrupting the operation of the system, (2) it estimates the fuel concentration in a three-dimensional measurement space; hence it is suitable for situations when one or more operating conditions are varying, (3) it can be performed as a sensor-less approach that requires no additional methanol sensors, thus consuming the less system power, and (4) it is particularly suitable for small and hand-held applications.     

A direct methanol fuel cell system with passive fuel delivery based on liquid surface tension

The existing direct methanol fuel cell (DMFC) systems are fed with a fixed concentration of fuel, which are either a diluted methanol solution or an active fuel delivery driven by an attached active pump. Both approaches limit the power conversion density or degrade the overall efficiency of the DMFC system significantly. Such disadvantages become more severe in small-scale DMFCs, which require a high conversion efficiency and a small physical space suitable for portable electronics. In this paper, passive fuel delivery based on a surface tension driving mechanism was designed and integrated in a laboratory-made prototype to achieve consumption depending on fuel concentration and power-free fuel delivery. Unidirectional methanol-to-water smooth flow is achieved through the capillaries of a Teflon PTFE (polytetrafluoroethylene) membrane based on the difference in liquid surface tension. The prototype was demonstrated to exhibit a better polarization performance and to last for an extended operating time compared to conventional DMFCs. Its high efficiency and load regulation performance were also demonstrated in contrast to an active DMFC supplied with a constant concentration fuel. The fuel delivery driven by the liquid surface tension effect demonstrated here is believed to be more applicable for future small-scale DMFCs for portable electronics.     

Carbonate fuel cells: Milliwatts to megawatts

The carbonate fuel cell power plant is an emerging high efficiency, ultra-clean power generator utilizing a variety of gaseous, liquid, and solid carbonaceous fuels for commercial and industrial applications. The primary mover of this generator is a carbonate fuel cell. The fuel cell uses alkali metal carbonate mixtures as electrolyte and operates at 650 °C. Corrosion of the cell hardware and stability of the ceramic components have been important design considerations in the early stages of development. The material and electrolyte choices are founded on extensive fundamental research carried out around the world in the 60s and early 70s. The cell components were developed in the late 1970s and early 1980s. The present day carbonate fuel cell construction employs commonly available stainless steels. The electrodes are based on nickel and well-established manufacturing processes. Manufacturing process development, scale-up, stack tests, and pilot system tests dominated throughout the 1990s. Commercial product development efforts began in late 1990s leading to prototype field tests beginning in the current decade leading to commercial customer applications. Cost reduction has been an integral part of the product effort. Cost-competitive product designs have evolved as a result. Approximately half a dozen teams around the world are pursuing carbonate fuel cell product development. The power plant development efforts to date have mainly focused on several hundred kW (submegawatt) to megawatt-class plants. Almost 40 submegawatt units have been operating at customer sites in the US, Europe, and Asia. Several of these units are operating on renewable bio-fuels. A 1 MW unit is operating on the digester gas from a municipal wastewater treatment plant in Seattle, Washington (US). Presently, there are a total of approximately 10 MW capacity carbonate fuel cell power plants installed around the world. Carbonate fuel cell products are also being developed to operate on coal-derived gases, diesel, and other logistic fuels. Innovative carbonate fuel cell/turbine hybrid power plant designs promising record energy conversion efficiencies approaching 75% have also emerged. This paper will review the historical development of this unique technology from milliwatt-scale laboratory cells to present megawatt-scale commercial power plants.     

Vapor feed direct methanol fuel cells with passive thermal-fluids management system

The present paper describes a novel technology that can be used to manage methanol and water in miniature direct methanol fuel cells (DMFCs) without the need for a complex micro-fluidics subsystem. At the core of this new technology is a unique passive fuel delivery system that allows for fuel delivery at an adjustable rate from a reservoir to the anode. Furthermore, the fuel cell is designed for both passive water management and effective carbon dioxide removal. The innovative thermal management mechanism is the key for effective operation of the fuel cell system. The vapor feed DMFC reached a power density of 16.5 mW cm⁻² at current density of 60 mA cm⁻². A series of fuel cell prototypes in the 0.5 W range have been successfully developed. The prototypes have demonstrated long-term stable operation, easy fuel delivery control and are scalable to larger power systems. A two-cell stack has successfully operated for 6 months with negligible degradation.     

A semi-empirical model for efficiency evaluation of a direct methanol fuel cell

Energy density and power density are two of the most significant performance indices of a fuel cell system. Both the indices are closely related to the operating conditions. Energy density, which can be derived from fuel cell efficiency, is especially important to small and portable applications. Generally speaking, power density can be easily obtained by acquiring the voltage and current density of an operating fuel cell. However, for a direct methanol fuel cell (DMFC), it is much more difficult to evaluate its efficiency due to fuel crossover and the complex architecture of fuel circulation. The present paper proposes a semi-empirical model for the efficiency evaluation of a DMFC under various operating conditions. The power density and the efficiency of a DMFC are depicted by explicit functions of operating temperature, fuel concentration and current density. It provides a good prediction and a clear insight into the relationship between the aforementioned performance indices and operating variables. Therefore, information including power density, efficiency, as well as remaining run-time about the status of an operating DMFC can be in situ evaluated and predicted. The resulting model can also serve as an important basis for developing real-time control strategies of a DMFC system.     

Development of a proton exchange membrane fuel cell cogeneration system

A proton exchange membrane fuel cell (PEMFC) cogeneration system that provides high-quality electricity and hot water has been developed. A specially designed thermal management system together with a microcontroller embedded with appropriate control algorithm is integrated into a PEM fuel cell system. The thermal management system does not only control the fuel cell operation temperature but also recover the heat dissipated by FC stack. The dynamic behaviors of thermal and electrical characteristics are presented to verify the stability of the fuel cell cogeneration system. In addition, the reliability of the fuel cell cogeneration system is proved by one-day demonstration that deals with the daily power demand in a typical family. Finally, the effects of external loads on the efficiencies of the fuel cell cogeneration system are examined. Results reveal that the maximum system efficiency was as high as 81% when combining heat and power.     

Optimum operating strategies for liquid-fed direct methanol fuel cells

This study focuses on optimum operating strategies for liquid-fed direct methanol fuel cells (DMFCs) to minimize methanol consumption. A mathematical model is developed and verified with experimental data from the literature using the parameter estimation method. The model consists of a set of differential and algebraic equations and makes it possible to describe zero initial hold-up conditions. Based on the model, steady-state simulation results are obtained and explain the dependence on the feed concentration of key variables such as cell voltage, cell power density, overpotentials of both electrodes, and methanol crossover ratio. Dynamic simulation results are also presented to check the transient behaviour of a DMFC operated from start-up to shut-down. Dynamic optimization allows determination of the optimum transient strategies of feed concentration required to maximize the fuel efficiency. With six scenarios of power density load, it is demonstrated that the optimum transient strategies depend heavily on both the load of power density and the number of control actions. The main advantage of these approaches is to reduce fuel consumption and, ultimately, to enable DMFCs to be operated more efficiently.     

Analysis of a solid oxide fuel cell and a molten carbonate fuel cell integrated system with different configurations

A solid oxide fuel cell with internal reforming operation is run at partial fuel utilization; thus, the remaining fuel can be further used for producing additional power. In addition, the exhaust gas of a solid oxide fuel cell still contains carbon dioxide, which is the primary greenhouse gas, and identifying a way to utilize this carbon dioxide is important. Integrating the solid oxide fuel cell with the molten carbonate fuel cell is a potential solution for carbon dioxide utilization. In this study, the performance of the integrated fuel cell system is analyzed. The solid oxide fuel cell is the main power generator, and the molten carbonate fuel cell is regarded as a carbon dioxide concentrator that produces electricity as a by-product. Modeling of the solid oxide fuel cell and the molten carbonate fuel cell is based on one-dimensional mass balance, considering all cell voltage losses. Primary operating conditions of the integrated fuel cell system that affect the system efficiencies in terms of power generation and carbon dioxide utilization are studied, and the optimal operating parameters are identified based on these criteria. Various configurations of the integrated fuel cell system are proposed and compared to determine the suitable design of the integrated fuel cell system.     

Membrane‐less micro fuel cell system design and performance: An overview

Researchers interest in using fuel cells as a power source has grown because fuel cells are environmentally friendly. However, fuel cells still present challenges due to their performance and cost. This limits the commercialization of fuel cell systems, particularly in liquid fuel cells. One of the major obstacles is the Nafion membrane. The Nafion membrane is extremely expensive and causes the “fuel crossover phenomenon.” Therefore, researchers have proposed a membrane‐less fuel cell that eliminates the need of a membrane in the system mainly in micro fuel cells. Membrane‐less fuel cell has shown an improvement on power density by approximately 12% compared with conventional type of proton electrolyte membrane fuel cell. However, there still a lack of information on system design and performance. Therefore, the main objective of this review is to present an extensive study focusing on the geometrical system design and performance of a membrane‐less fuel cell system. It also presents the different types of membrane‐less fuel cell systems. Lastly, it highlights the current problems and potentials to improve the performance of the system. Finally, it is observed that the cost of a membrane fuel cell can be reduced by 20% to 40% compared with the conventional type of fuel cell.     

One-dimensional and non-isothermal model for a passive DMFC

Passive direct methanol fuel cells (DMFCs) are promising energy sources for portable electronic devices. Different from DMFCs with active fuel feeding systems, passive DMFCs with nearly stagnant fuel and air tend to bear comparatively less power densities. A steady state, one-dimensional, multi-component and thermal model is described and applied to simulate the operation of a passive direct methanol fuel cell. The model takes into consideration the thermal and mass transfer effects, along with the electrochemical reactions occurring in the passive DMFC. The model can be used to predict the methanol, oxygen and water concentration profiles in the anode, cathode and membrane as well as to estimate the methanol and water crossover and the temperature profile across the cell. Polarization curves are numerically simulated and successfully compared with experiments for different methanol feed concentrations. The model predicts with accuracy the influence of the methanol feed concentration on the cell performance and the correct trends of the current density and methanol feed concentration, on methanol and water crossover. The model is rapidly implemented and is therefore suitable for inclusion in real-time system level DMFC calculations. Due to its simplicity the model can be used to help seek for possibilities of optimizing the cell performance of a passive DMFC by studying impacts from variations of the design parameters such as membrane thickness, catalyst loading, diffusion layers type and thicknesses.     

Small direct methanol fuel cells with passive supply of reactants

Small, stand-alone, direct methanol fuel cells (DMFCs) that have no auxiliary liquid pumps and gas blowers/compressors are known as passive DMFCs. The devices are ideal for powering portable electronic devices, as this type of fuel cell uniquely has a simple and compact system and no parasitic power losses. This article provides a comprehensive review of experimental and numerical studies of heat and mass transport in passive DMFCs. Emphasis is placed on the mechanisms and key issues of the mass transport of each species through the fuel cell structure under the influence of passive forces. It is shown that the key issue regarding the methanol supply is how to feed high-concentration methanol solution but with minimum methanol crossover through the membrane so that both the system specific energy and cell performance can be maximized. The key issue regarding the oxygen supply is how to enhance the removal of liquid water from the cathode under the air-breathing condition. For water transport, the aim is to transport the water produced on the cathode through the membrane to the anode by optimizing the design of the membrane electrode assembly so that the fuel cell can be operated with pure methanol and with minimum flooding at the cathode. The heat loss from a passive DMFC is usually large and it is therefore critically important to reduce this feature so that the fuel cell can be operated at a sufficiently high temperature, which critically affects the cell performance.     

Analysis and control of a fuel delivery system considering a two-phase anode model of the polymer electrolyte membrane fuel cell stack

The fuel delivery system using both an ejector and a blower for a PEM fuel cell stack is introduced as a fuel efficiency configuration because of the possibility of hydrogen recirculation dependent upon load states.A high pressure difference between the cathode and anode could potentially damage the thin polymer electrolyte membrane. Therefore, the hydrogen pressure imposed to the stack should follow any change of the cathode pressure. In addition, stoichiometric ratio of the hydrogen should be maintained at a constant to prevent a fuel starvation at abrupt load changes.Furthermore, liquid water in the anode gas flow channels should be purged out in time to prevent flooding in the channels and other layers. The purging control also reduces the impurities concentration in cells to improve the cell performance.We developed a set of control oriented dynamic models that include a anode model considering the two-phase phenomenon and system components The model is used to design and optimize a state feedback controller along with an observer that controls the fuel pressure and stoichiometric ratio, whereby purging processes are also considered. Finally, included is static and dynamic analysis with respect to tracking and rejection performance of the proposed control.     

基于AspenPlus的兆瓦级燃料电池分布式发电系统建模及仿真分析

目的   燃料电池分布式发电技术是适应未来能源低碳化、清洁化、高效化发展趋势的重要应用方向。国内燃料电池电站项目较少,缺乏实际项目经验积累。为了推进燃料电池分布式电站技术的应用,文章概述了国内外应用现状,总结了高温燃料电池的优势与不足,调研了国内燃料电池建设应用案例,并建立了固体氧化物燃料电池与熔融碳酸盐燃料电池发电系统流程。 方法   经过文献调研与实地调研,确定了两种适合建设大型电站的燃料电池分布式发电技术,并利用AspenPlus化工模拟软件建立燃料电池系统流程模型、电化学模型和能量分析模型,并开展系统的性能仿真分析。 结果   分析结果与实际运行结果相吻合,分析预测的系统性能趋势与已有研究相一致。 结论   该仿真方法可用于兆瓦级高温燃料电池分布式发电系统的研究,可为扩大燃料电池应用规模提供数据支持。     

Modelling and analysis of a direct ascorbic acid fuel cell

l-Ascorbic acid (AA), also known as vitamin C, is an environmentally-benign and biologically-friendly compound that can be used as an alternative fuel for direct oxidation fuel cells. While direct ascorbic acid fuel cells (DAAFCs) have been studied experimentally, modelling and simulation of these devices have been overlooked. In this work, we develop a mathematical model to describe a DAAFC and validate it with experimental data. The model is formulated by integrating the mass and charge balances, and model parameters are estimated by best-fitting to experimental data of current–voltage curves. By comparing the transient voltage curves predicted by dynamic simulation and experiments, the model is further validated. Various parameters that affect the power generation are studied by simulation. The cathodic reaction is found to be the most significant determinant of power generation, followed by fuel feed concentration and the mass-transfer coefficient of ascorbic acid. These studies also reveal that the power density steadily increases with respect to the fuel feed concentration. The results may guide future development and operation of a more efficient DAAFC.