Fuel cell power plants for electric utilities and hydrogen

Phosphoric acid fuel cell power plants are nearing commercial status as multimegawatt electric power generators. These power plants are being designed to use a wide range of utility fuels that are converted to hydrogen within the power plant. This paper explores the status of fuel cell power plants and the prospects for pure hydrogen as the fuel.     

Observer designs for fuel processing reactors in fuel cell power systems

In control theory, an observer is a numerical algorithm which estimates unmeasured physical variables from those that are measured with the help of dynamical models that correlate them. This paper presents nonlinear observer designs for the catalytic partial oxidation (CPO), water gas shift (WGS) and preferential oxidation (PROX) reactors in fuel cell power systems. The observers make use of temperature measurements (and possibly one more variable, such as pressure) to estimate the mole fractions of each species in the reactors. Estimation of hydrogen and carbon monoxide fractions is particularly useful for monitoring and control of the fuel cell system. An advantage of our designs is that they are based on the reaction invariants and do not rely on a knowledge of reaction rate expressions.     

接近实用化的燃料电池发电技术

1前言 燃料电池以氢为燃料和空气中的氧反应后按照水电解为氢和氧的逆反应发电.早在六十年代,美国即在宇航方面应用液氢和液氧的燃料电池,效果甚好.1967年美国将它列入天然气转换计划,并吸收国内及日本的部份煤气公司参加开发.七十年代世界石油危机后更受到重视.特别是日本于1981年将它列入月光计划中大型节能技术开发项目后美日间的合作加强,促进了开发速度.     

Comparison and optimization of different fuel processing options for biogas-fed solid-oxide fuel cell plants

The biogas needs to be reformed before electro-chemical conversion in the solid-oxide fuel cell, which can be promoted efficiently with wise thermal management and reforming conditions. To ensure the system safety and catalysts durability, additional mineral-bearing water and carbon deposition should be avoided. This paper conducted a detailed biogas-SOFC CHP system analysis considering four layouts, featuring hot and cold recirculation of the anode off-gas, partial oxidation and complete internal reforming. The process optimization and sensitivity analysis are performed with the design variables including the recirculation ratio, and external reformer temperature. The anode supported SOFC operates at 800 °C and 0.4 A/cm² current density. The results show that pre-reforming with hot recirculation and cold recirculation schemes achieve the highest system efficiency between 56% and 63%. The pre-reforming with hot recirculation scheme has a broader self-sufficient water range eliminating the carbon deposition risk at the recirculation ratio of 42–78% and reforming temperature of 400–650 °C. The no pre-reforming with hot recirculation scheme achieves maximum system efficiency of 58% due to the fuel dilution. Moreover, the partial oxidation with hot recirculation scheme maximum efficiency is limited to 58.9%, given that the partial oxidation reaction is less efficient than steam and dry reforming reactions. The proposed system layout could demonstrate the feasibility of biogas-SOFC with different reforming options especially on small scale with high efficiency and optimal thermal integration opportunities.     

Coal gas as a feed fuel for phosphoric acid fuel cell power plants

Westinghouse Electric Corporation has assessed the effects of a number of coal gasification systems and their gas compositions on a 7.5 MW_(c) dc Phosphoric Acid Fuel Cell (PAFC) power plant. Both low and medium BTU synthesis gases were considered from various promising gasification processes. Degree of development or commercialization, technical complexity, availability and coal restrictions were taken into account. System studies performed on the 7.5 MW_(c) dc PAFC power plant for nonintegrated and integrated fuel conditioning systems are discussed for the various gasification processes. In all cases coal gas was assumed to be purchased “over-the-fence”. Representative characteristics for the derived coal gases and PAFC system performance data are presented. The direct capital costs of the PAFC power plant are evaluated for each system.     

Adaptive maximum power point tracking control of fuel cell power plants

A fuel cell's output power depends nonlinearly on the applied current or voltage, and there exists a unique maximum power point (MPP). This paper reports a first attempt to trace MPPs by an extremum seeking controller. The locus of MPPs varies nonlinearly with the unpredictable variations in the fuel cell's operation conditions. Thus, a maximum power point tracking (MPPT) controller is needed to continuously deliver the highest possible power to the load when variations in operation conditions occur. A two-loop cascade controller with an intermediate converter is designed to operate fuel cell power plants at their MPPs. The outer loop uses an adaptive extremum seeking algorithm to estimate the real-time MPP, and then gives the estimated value to the inner loop as the set-point, at which the inner loop forces the fuel cell to operate. The proposed MPPT control system provides a simple and robust control law that can keep the fuel cell working at MPPs in real time. Simulation shows that this control approach can yield satisfactory results in terms of robustness toward variations in fuel cell operation conditions.     

A flexible paper-based hydrogen fuel cell for small power applications

In this work, a paper-based hydrogen fuel cell is developed without the need for hydrogen storage. Instead, an embedded aluminum foil inside the paper is utilized for in-situ hydrogen generation. The electrodes and current collectors are also deposited on the paper, leading to a lightweight, compact and flexible hydrogen fuel cell with an OCV reaching 0.93 V and a peak power density of 4 mW cm⁻². Benefited from the impeded hydroxyl ion diffusion, the hydrogen generation rate is well controlled, leading to a high faradaic efficiency of 72%. In addition, the cell can be operated under different bending angles with negligible power loss. Furthermore, it can be conveniently stacked in the same piece of paper for higher voltage and power outputs. Such a novel fuel cell design is especially suitable for powering various flexible devices with small rated power.     

Control of grid-connected fuel cell plants for enhancement of power system stability

This paper presents a methodology for effective control of fuel cell devices connected to an electric utility distribution network. A controller is developed for a fuel cell power plant to assist the conventional generators to damp out oscillations, which is possible by utilizing the fast response characteristic of fuel cells. It achieves the objective by generating appropriate switching signals to the DC–AC inverters and modulating both active and reactive powers. Computer model of the controller is developed and its effectiveness is proved by a sample test. Fuel cell devices, therefore, can be used to improve power system stability when these are applied to a power distribution system.     

Fuel cell system modeling for solid oxide fuel cell/gas turbine hybrid power plants, Part I: Modeling and simulation framework

A sustainable future power supply requires high fuel-to-electricity conversion efficiencies even in small-scale power plants. A promising technology to reach this goal is a hybrid power plant in which a gas turbine (GT) is coupled with a solid oxide fuel cell (SOFC). This paper presents a dynamic model of a pressurized SOFC system consisting of the fuel cell stack with combustion zone and balance-of-plant components such as desulphurization, humidification, reformer, ejector and heat exchangers. The model includes thermal coupling between the different components. A number of control loops for fuel and air flows as well as power management are integrated in order to keep the system within the desired operation window. Models and controls are implemented in a MATLAB/SIMULINK environment. Different hybrid cycles proposed earlier are discussed and a preferred cycle is developed. Simulation results show the prospects of the developed modeling and control system.     

Performance evaluation of an energy recovery system for fuel reforming of PEM fuel cell power plants

This paper describes an energy recovery system that recovers waste thermal energy from a fuel cell stack and uses it for fuel reforming purposes. The energy recovery system includes a throttling valve, a heat exchanger, and a compressor, and is coupled with a coolant loop of the fuel cell stack. The feed stock of a fuel reformer, which is primarily a mixture of water and fuel, is vaporized in the heat exchanger and is compressed to a sufficiently high pressure before it is ducted into the fuel reformer. The performance of a fuel cell power plant equipped with the energy recovery system is evaluated. The results indicate that the power plant efficiency can be increased by more than 40% compared to that of a fuel cell power plant without the energy recovery system. Additionally, up to 90% of the waste heat generated in the fuel cell stack is recovered. As a result, the required heat dissipation capacity of the radiator that is used for cooling the fuel cell stack can be drastically reduced.     

Dynamic behavior of PEM fuel cell and microturbine power plants

This paper presents a comparison between the dynamic behavior of a 250 kW stand-alone proton exchange membrane fuel cell power plant (PEM FCPP) and a 250 kW stand-alone microturbine (MT). Dynamic models for the two are introduced. To control the voltage and the power output of the PEM FCPP, voltage and power control loops are added to the model. For the MT, voltage, speed, and power control are used. Dynamic models are used to determine the response of the PEM FCPP and MT to a load step change. Simulation results indicate that the response of the MT to reach a steady state is about twice as fast as the PEM FCPP. For stand-alone operation of a PEM FCPP, a set of batteries or ultracapacitors is needed in order to satisfy the power mismatch during transient periods. Software simulation results are obtained by using MATLAB^®, Simulink^®, and SimPowerSystems^®.     

Carbon dioxide separation from high temperature fuel cell power plants

High temperature fuel cell technologies, solid oxide fuel cells (SOFCs) and molten carbonate fuel cells (MCFCs), are considered for their potential application to carbon dioxide emission control. Both technologies feature electrochemical oxidisation of natural gas reformed fuels, avoiding the mixture of air and fuel flows and dilution with nitrogen and oxygen of the oxidised products; a preliminary analysis shows how the different mechanism of ion transport attributes each technology a specific advantage for the application to CO₂ separation. The paper then compares in the first part the most promising cycle configurations based on high efficiency integrated SOFC/gas turbine “hybrid” cycles, where CO₂ is separated with absorption systems or with the eventual adoption of a second SOFC module acting as an “afterburner”. The second part of the paper discusses how a MCFC plant could be “retrofitted” to a conventional fossil-fuel power station, giving the possibility of draining the majority of CO₂ from the stack exhaust while keeping the overall cycle electrical efficiency approximately unchanged.     

Fuel processors for fuel cell APU applications

The conversion of liquid hydrocarbons to a hydrogen rich product gas is a central process step in fuel processors for auxiliary power units (APUs) for vehicles of all kinds. The selection of the reforming process depends on the fuel and the type of the fuel cell.For vehicle power trains, liquid hydrocarbons like gasoline, kerosene, and diesel are utilized and, therefore, they will also be the fuel for the respective APU systems.The fuel cells commonly envisioned for mobile APU applications are molten carbonate fuel cells (MCFC), solid oxide fuel cells (SOFC), and proton exchange membrane fuel cells (PEMFC). Since high-temperature fuel cells, e.g. MCFCs or SOFCs, can be supplied with a feed gas that contains carbon monoxide (CO) their fuel processor does not require reactors for CO reduction and removal. For PEMFCs on the other hand, CO concentrations in the feed gas must not exceed 50 ppm, better 20 ppm, which requires additional reactors downstream of the reforming reactor.This paper gives an overview of the current state of the fuel processor development for APU applications and APU system developments. Furthermore, it will present the latest developments at Fraunhofer ISE regarding fuel processors for high-temperature fuel cell APU systems on board of ships and aircrafts.     

Parallel operation characteristics of PEM fuel cell and microturbine power plants

This paper reports on the dynamic behavior of a 250 kW proton exchange membrane fuel cell power plant (PEM FCPP) and a 250 kW microturbine (MT) when operating in parallel. A load sharing control scheme is used to distribute the load equally between the PEM FCPP and the MT. For stand alone operation of a PEM FCPP, a set of batteries or ultracapacitors are needed in order to satisfy the power mismatch during transient periods. Using MT in parallel with the PEM FCPP helps in eliminating the need for storage devices. Models for the PEM FCPP and the MT with power, voltage and speed controls are used to determine the dynamic response of the system to a step change in the load. Simulation results indicate viability of parallel operation of the PEM FCPP and the MT. These results are obtained using MATLAB^®, Simulink^®, and SimPowerSystems^®.     

Design of highly efficient coal-based integrated gasification fuel cell power plants

Integrated gasification fuel cell (IGFC) technology combining coal gasification and solid oxide fuel cell (SOFC) is believed to be the only viable solution to achieving U.S. Department of Energy (DOE)’s performance goal for next generation coal-based power plants, producing electricity at 60% efficiency (coal HHV-AC) while capturing more than 90% of the evolved CO₂. Achieving this goal is challenging even with high performance SOFCs; design concepts published to date have not demonstrated this performance goal. In this work an IGFC system concept consisting of catalytic hydro-gasification, proven low-temperature gas cleaning and hybrid fuel cell-gas turbine power block (with SOFC operating at about 10 bar) is introduced. The system is demonstrating an electricity efficiency greater than 60% (coal HHV basis), with more than 90% of the carbon present in the syngas separated as CO₂ amenable to sequestration. A unique characteristic of the system is recycling de-carbonized, humidified anode exhaust back to the catalytic hydro-gasifier for improved energy integration. Alternative designs where: (1) anode exhaust is recycled directly back to SOFC stacks, (2) SOFC stack operating pressure is reduced to near atmospheric and (3) methanation reactor in the reactor/expander topping cycle is removed, have also been investigated and the system design and performance differences are discussed.     

A practical algorithm for optimal operation management of distribution network including fuel cell power plants

Fuel cell power plants (FCPPs) have been taken into a great deal of consideration in recent years. The continuing growth of the power demand together with environmental constraints is increasing interest to use FCPPs in power system. Since FCPPs are usually connected to distribution network, the effect of FCPPs on distribution network is more than other sections of power system. One of the most important issues in distribution networks is optimal operation management (OOM) which can be affected by FCPPs. This paper proposes a new approach for optimal operation management of distribution networks including FCCPs. In the article, we consider the total electrical energy losses, the total electrical energy cost and the total emission as the objective functions which should be minimized. Whereas the optimal operation in distribution networks has a nonlinear mixed integer optimization problem, the optimal solution could be obtained through an evolutionary method. We use a new evolutionary algorithm based on Fuzzy Adaptive Particle Swarm Optimization (FAPSO) to solve the optimal operation problem and compare this method with Genetic Algorithm (GA), Particle Swarm Optimization (PSO), Differential Evolution (DE), Ant Colony Optimization (ACO) and Tabu Search (TS) over two distribution test feeders.     

Effects of fuel processing methods on industrial scale biogas-fuelled solid oxide fuel cell system for operating in wastewater treatment plants

The performance of three solid oxide fuel cell (SOFC) systems, fuelled by biogas produced through anaerobic digestion (AD) process, for heat and electricity generation in wastewater treatment plants (WWTPs) is studied. Each system has a different fuel processing method to prevent carbon deposition over the anode catalyst under biogas fuelling. Anode gas recirculation (AGR), steam reforming (SR), and partial oxidation (POX) are the methods employed in systems I-III, respectively. A planar SOFC stack used in these systems is based on the anode-supported cells with Ni-YSZ anode, YSZ electrolyte and YSZ-LSM cathode, operated at 800 °C. A computer code has been developed for the simulation of the planar SOFC in cell, stack and system levels and applied for the performance prediction of the SOFC systems. The key operational parameters affecting the performance of the SOFC systems are identified. The effect of these parameters on the electrical and CHP efficiencies, the generated electricity and heat, the total exergy destruction, and the number of cells in SOFC stack of the systems are studied. The results show that among the SOFC systems investigated in this study, the AGR and SR fuel processor-based systems with electrical efficiency of 45.1% and 43%, respectively, are suitable to be applied in WWTPs. If the entire biogas produced in a WWTP is used in the AGR or SR fuel processor-based SOFC system, the electricity and heat required to operate the WWTP can be completely self-supplied and the extra electricity generated can be sold to the electrical grid.     

Fuel cell power trains for road traffic

Legal regulations, especially the low emission vehicle (LEV) laws in California, are the driving forces for more intensive technological developments with respect to a global automobile market. In the future, high efficient vehicles at very low emission levels will include low temperature fuel cell systems (e.g., polymer electrolyte fuel cell (PEFC)) as units of hydrogen-, methanol- or gasoline-based electric power trains. In the case of methanol or gasoline/diesel, hydrogen has to be produced on-board using heated steam or partial oxidation reformers as well as catalytic burners and gas cleaning units. Methanol could also be used for direct electricity generation inside the fuel cell (direct methanol fuel cell (DMFC)). The development potentials and the results achieved so far for these concepts differ extremely. Based on the experience gained so far, the goals for the next few years include cost and weight reductions as well as optimizations in terms of the energy management of power trains with PEFC systems. At the same time, questions of fuel specification, fuel cycle management, materials balances and environmental assessment will have to be discussed more intensively. On the basis of process engineering analyses for net electricity generation in PEFC-powered power trains as well as on assumptions for both electric power trains and vehicle configurations, overall balances have been carried out. They will lead not only to specific energy demand data and specific emission levels (CO₂, CO, VOC, NO_x) for the vehicle but will also present data of its full fuel cycle (FFC) in comparison to those of FFCs including internal combustion engines (ICE) after the year 2005. Depending on the development status (today or in 2010) and the FFC benchmark results, the advantages of balances results of FFC with PEFC vehicles are small in terms of specific energy demand and CO₂ emissions, but very high with respect to local emission levels.     

Fuel economy of hydrogen fuel cell vehicles

On the basis of on-road energy consumption, fuel economy (FE) of hydrogen fuel cell light-duty vehicles is projected to be 2.5–2.7 times the fuel economy of the conventional gasoline internal combustion engine vehicles (ICEV) on the same platforms. Even with a less efficient but higher power density 0.6 V per cell than the base case 0.7 V per cell at the rated power point, the hydrogen fuel cell vehicles are projected to offer essentially the same fuel economy multiplier. The key to obtaining high fuel economy as measured on standardized urban and highway drive schedules lies in maintaining high efficiency of the fuel cell (FC) system at low loads. To achieve this, besides a high performance fuel cell stack, low parasitic losses in the air management system (i.e., turndown and part load efficiencies of the compressor–expander module) are critical.