High-temperature Fuel Cell Stack Research Articles

Development and investigation of a pollutants emission reduction process from a coal-gasification power plant integrated with fuel cell and solar energy

Abstract Even though coal resources are the most abundant among fossil fuels, coal-fired plants release large amounts of greenhouse gases into the atmosphere. In this regard, reducing environmental challenges and crises caused by coal burning can be a promising option to reduce today's crises in the energy field. The integration of coal-fired plants with renewable-driven energy systems can simultaneously improve thermodynamic performance and reduce pollutants emission rates. This article presents the thermodynamic and pollutant emission investigations of a new coal-fired plant coupled with a linear Fresnel solar collector (LFSC)-driven solar unit, a parabolic trough solar collector (PTSC)-driven solar unit, a high-temperature fuel cell stack (molten carbonate fuel cell stack [MCFCS]) and a heat recovery system (based on the steam turbine and gas turbine-based power cycles). The plant is able to produce electricity and hot water (HW). The main structure of the offered plant is based on coal, whereas, is coupled with renewables-based cycles to mitigate environmental impacts. The plant could generate ~ 207 MW of power and 3728 m3/h of HW. In such conditions, the energy efficiency of 73.1% and exergy efficiency of 44.18% could be achievable. Further, the emitted gas rates of the plant were nearly 403 tons/h. A comprehensive comparison is also presented for the plant's behavior under different types of coal (petcoke and anthracite). In addition, a two-function optimization is developed to determine the maximum value of exergy efficiency and the minimum value of total pollutants emission rate.

Characterizing SOFC Stack Electrochemical Performance at Elevated Pressure

Pressurized operation of high temperature fuel cell stacks is of increasing interest for a variety of applications, including high efficiency, hybrid solid-oxide fuel cell power generation systems. In this presentation, we report on our efforts to characterize the electrochemical performance of 1-kWeand 5-kWe Ceres Power SteelCell solid-oxide fuel cell stacks, with a focus on elevated pressure operation in the range of 2-5 barg. Increased pressure operation requires particular attention to pressure differentials across both the membrane electrode assembly and between the stack and surrounding atmosphere. The effort is part of a five-year U.S. DoE ARPA-E INTEGRATE program to combine the high efficiency of solid-oxide fuel cells with the high efficiency balance-of-plant equipment to demonstrate a 70%-efficient distributed power generator. In this architecture, the highly efficient solid-oxide fuel cells (SOFCs) are placed upstream of an internal combustion engine, where the anode tail gas, consisting of partially converted steam-reformed natural gas, is further utilized by the engine to produce electricity. The stacks are operated between 2-5 barg to maximize overall system efficiency and to increase SOFC power density.Few studies have been reported on SOFC stack operation at elevated pressures. The team at the Colorado School of Mines has designed and operated the pressurized stack test stand shown in the figure to characterize electrochemical performance of various kW-scale SOFC stack test modules developed by Ceres Power, Ltd. Minimizing pressure differentials across the anode and cathode during elevated-pressure operation is of primary concern. Excessive pressure on one side of the cell can cause substantial mechanical stress to the metal-supported membrane-electrode assemblies. Pressure differentials in this work are maintained below the manufacturer’s specifications of +/- 25 mbar at operating pressures of 0-5 barg. In addition to performance measurements at elevated pressures, studies have also been conducted on diluted fuel streams representative of high anode tail-gas recycle and various amounts of internal reforming. The stacks show steady, stable performance over all conditions tested.A fuel feed is produced with mass flow controllers that represents partially, steam-reformed natural gas with anode tail-gas recycle. A natural gas fuel processor will be incorporated into the stand at a later stage of the research program. Reactant gas feeds are preheated in multiple zones to properly control stack temperatures. The insulated stack test module is housed inside of a pressure vessel that has individual pressure control in order to minimize the pressure differentials between the internal flow channels of the stack and the surrounding environment. Exhaust gases exit the pressure vessel where heat exchangers are utilized to partially cool the 600°C gases before reaching back-pressure regulators.Stack operation is challenging because of the combination of a tight thermal operating window and the long system response time to changes in inlet conditions. These time constants become even larger at low air flow rates. Lower air flow (while maintaining required oxygen utilization) is desired in order to reduce the power required by the balance of plant, especially when operating at higher pressures. In order to mitigate this, a model predictive controller is being implemented. A dynamic model of the fuel cell was first tuned and validated against experimental transient response data. Then the resulting model was linearized at multiple operating points. The linearizations were then utilized within a gain-scheduled moving-horizon predictor that can take past input and output data, along with a proposed future input, and predict the future output response. The predictor performance has been validated, and then inserted into a model-predicative controller. Optimization algorithms select the input temperatures, flow rates, and electric current required to quickly reach a desired operating condition while meeting all operational constraints, including thermal constraints.In this talk, we will present our findings on the electrochemical performance of 1 and 5-kWe Ceres Power SteelCell stack test modules and the effects of pressure, fuel composition, anode recycle, and degree of internal reforming on stack performance. Results influence the design and net efficiency of the hybrid system. The trajectory of the research will be discussed. Figure 1

Catalytically activated stainless steel plates for the dehydrogenation of perhydro dibenzyltoluene

Hydrogen storage and transport via Liquid Organic Hydrogen Carriers (LOHC) is gaining increasing attention. In this study, we present catalytically activated stainless steel plates as a promising alternative to the commonly used pellet catalysts for the dehydrogenation of perhydro dibenzyltoluene (H18-DBT). These plate catalysts promise better heat transport to the active sites. For improved performance, we modified our Pt/alumina plate catalysts by using i) platinum sulfite impregnation and ii) post-treatment with (NH4)2SO4. Post-treatment with (NH4)2SO4 resulted in a less active catalyst with lower formation of high-boiling side products compared to the S-free plate catalyst. Catalysts prepared with platinum sulfite showed both >35% higher activities and 90% reduction in high-boiler formation compared to the S-free plate catalysts. Our findings pave the way for the development of catalytically activated heat transfer plates that would allow the incorporation of LOHC dehydrogenation units into the geometry of future high temperature fuel cell stacks.

Computational fluid dynamics analysis of an innovative start-up method of high temperature fuel cells using dynamic 3d model

Abstract The article presents a numerical analysis of an innovative method for starting systems based on high temperature fuel cells. The possibility of preheating the fuel cell stacks from the cold state to the nominal working conditions encounters several limitations related to heat transfer and stability of materials. The lack of rapid and safe start-up methods limits the proliferation of MCFCs and SOFCs. For that reason, an innovative method was developed and verified using the numerical analysis presented in the paper. A dynamic 3D model was developed that enables thermo-fluidic investigations and determination of measures for shortening the preheating time of the high temperature fuel cell stacks. The model was implemented in ANSYS Fluent computational fluid dynamic (CFD) software and was used for verification of the proposed start-up method. The SOFC was chosen as a reference fuel cell technology for the study. Results obtained from the study are presented and discussed.

Flexible micro temperature, voltage and current sensors for local real-time microscopic diagnosis inside high temperature proton exchange membrane fuel cell stack

The local temperature, voltage and current nonuniformity in the high temperature proton exchange membrane fuel cell (HT-PEMFC) stack can accelerate the aging of membrane electrode assembly (MEA) and the failure of overall fuel cell stack.This study used micro-electro-mechanical systems (MEMS) technology to integrate micro temperature, voltage and current sensors into a 40 μm thick stainless steel substrate successfully, and selected the temperature tolerant and electrochemical corrosion resistant polyimide (PI) as protective layer. This flexible micro sensor is characterized by multiple functions, compactness, corrosion resistance, good temperature tolerance, quick response, real-time measurement and optional location. Flexible micro sensor embedded in the high temperature fuel cell stack, the effect on the performance of fuel cell stack was less than 1%.This study used NI PXI 2575 data acquisition unit for real-time microscopic diagnosis of local temperature, voltage and current information at 150 °C operating temperature and constant current (5, 15, 30 A) inside the high temperature fuel cell stack, and found that the nonuniform temperature distribution in the fuel cell stack resulted in nonuniform voltage and current distributions and hot stack.

Investigation of Advanced Components in a High Pressure Single-Cell Electrolyser for the Development of a HP-PEM-ELY Stack as Part of a Regenerative Fuel Cell System

The objective of the presented work, done under current ESA activity (Contract No. 4000109578/13/NL/SC), is the performance and tolerance evaluation of selected components and materials for the development of a High Pressure, Polymer Electrolyte Membrane (PEM) Electrolyser (HP-PEM-ELY) Stack, aiming to operate at 80 bar with a performance output of 0.3 A/cm 2 at 1.6 V. An extensive study was performed on a single-cell high pressure PEM electrolyser manifold, leading to a list of materials with suitable properties and engineering solutions towards operation in space environment. This investigation provided the necessary feedback for the design of a HP-PEM-ELY stack, which is also discussed. The ultimate target of the current ESA activity is to implement research findings, develop and operate a complete regenerative fuel cell system, comprising of a High Temperature Fuel Cell Stack and the HP-PEM-ELY stack. System aspects are briefly discussed.

Persistent effect test for high temperature resistant integrated microsensor embedded in high temperature proton exchange membrane fuel cell stack

The chemical reaction process of high temperature proton exchange membrane fuel cell stack (HT-PEMFC stack) results in poor membrane material durability and nonuniform fuel distribution and influences the fuel cell stack performance and life. The local temperature, voltage and flow inside the high temperature fuel cell stack are three major factors, so this study aims at the three physical phenomena.This study used micro-electro-mechanical systems (MEMS) technology to develop a high temperature electrochemical environment resistant integrated (temperature, voltage and flow) microsensor on a 40μm thick stainless steel substrate, the protective layer was made of polyimide (PI) with better temperature tolerance. This study also captured the internal information of local temperature and voltage and flow inside the high temperature fuel cell stack instantly. The 100-h persistent effect test at operating temperature of 160°C and current 6.28A validated the reliability of the high temperature electrochemical environment resistant integrated microsensor. The experimental results showed the nonuniform temperature and flow distributions in the high temperature fuel cell stack resulted in high temperature contrast and nonuniform voltage distribution, and there was hot stack.

Real-time Monitoring of Internal Temperature and Voltage of High-temperature Fuel Cell Stack

The nonuniform local temperature and voltage in the chemical reaction process of high-temperature proton exchange membrane fuel cell (HT-PEMFC) stack can affect the reaction of membrane electrode assembly (MEA) and the performance and life of fuel cell stack. The effectiveness and internal information of fuel cell stack can be discussed by using external measurement, invasive, theoretical modeling, and single temperature, or voltage measurement. But there are some problems, such as mm scale sensor, inaccurate measurement, influencing the fuel cell stack performance, and failing to know internal actual reactive state instantly.This study uses micro-electro-mechanical systems (MEMS) technology to develop a new generation flexible micro temperature and voltage sensors applicable to high-temperature electrochemical environment.Micro sensors have embedded in the cathode channel plate of HT-PEMFC stack. At the operating temperature of 170°C and constant current (2, 10, 20A), the curvilinear trends of local temperature and voltage inside the fuel cell stack measured by flexible micro sensors are consistent, proving the reliability of micro sensors. The test result also shows that the heat distribution in the fuel cell stack is nonuniform.

Parameter identification and observer-based control for distributed heating systems – the basis for temperature control of solid oxide fuel cell stacks

The control of high-temperature fuel cell stacks is the prerequisite to guarantee maximum efficiency and lifetime under both constant and varying electrical load conditions. Especially, for time-varying electrical load demands, it is necessary to develop novel observer-based control approaches that are robust against parameter uncertainties and disturbances that cannot be modelled a priori. Since we aim at real-time applicability of these control procedures, classical high-dimensional models – which result from a discretization of mathematical descriptions given by the partial differential equations for heat and mass transfer – cannot be applied. Furthermore, these models have to be linked to the electrochemical properties of the fuel cell. To reduce the order of these models to a degree that allows us to use them in real-time, information on both the temperature distribution in the fuel cell stack and the heat flow into its interior due to electrochemical reactions is required. However, a direct temperature measurement is not possible from a practical point of view. For that reason, it is essential to reliably estimate the temperature distribution and the heat flow on the basis of easily available measured data. These data have to be available not only during development stages but also in future series products. For such products, it is desirable to reduce the number of sensors to improve the system's reliability and to decrease the operating costs. The basic strategies that are applicable for model-based open-loop and closed-loop control of heating systems as well as for the identification of parameters, operating conditions and disturbances as well as for state monitoring are summarized in this article. They are demonstrated for exemplary set-ups in both simulation and experiment.

Interval Methods for Control-Oriented Modeling of the Thermal Behavior of High-Temperature Fuel Cell Stacks

Solid oxide fuel cells (SOFCs) can be used as decentralized energy supply devices for providing electricity and heat directly by converting chemical energy. In such applications of SOFCs, the electric power demand is commonly varying over time. Therefore, all processes in fuel cell systems are typically instationary. For example, the heating and cooling phases for starting up and shutting down the fuel cell system as well as the response to varying electrical load demands characterize the instationarity of the operating conditions for the thermal subprocess. In contrast to most existing control approaches, which only cover stationary operating strategies, our work aims at controlling SOFC systems in instationary operating regions. This means that a mathematical system model for these regions is necessary. Such control-oriented models for the temperature distribution in a fuel cell stack module can be obtained by the method of finite volume discretization. On the basis of the first law of thermodynamics, local energy balances are derived for each volume element, which leads to a system of coupled nonlinear ordinary differential equations. In this paper, parameter identification routines are compared which are based both on classical floating point techniques and on verified interval arithmetic approaches. In particular, interval techniques are employed to deal with imperfect system knowledge expressed by bounded parameter uncertainties and to search for globally instead of locally optimal system parameterizations.

In situ monitoring of high-temperature proton exchange membrane fuel cell stack using flexible micro temperature and voltage sensors

Internal local temperature and voltage not only affect reactions that occur in a membrane electrode assembly (MEA); they can also cause dehydration of the MEA in the electrochemical process that occurs in a proton exchange membrane fuel cell (PEMFC) stack, affecting its performance. In this work, micro-electro-mechanical systems (MEMS) are utilized to develop novel integrated micro sensors for use in high-temperature proton exchange membrane fuel cell stack, and integrated micro temperature and voltage sensors on stainless steel foil as a flexible substrate. These micro sensors can measure temperature and voltage data simultaneously and have the following advantages over other sensors: (1) they are small, (2) they are highly sensitive, (3) they can be batch-produced, (4) they can be placed anywhere to make measurements in situ.In this work, six micro sensors are embedded in the flow field of a high-temperature fuel cell stack and used to monitor local temperature and voltage in situ given various operating parameters. The results demonstrate that embedding micro sensors do not affect on cell performance and that the temperature and voltage distribution are non-uniform in a high-temperature fuel cell stack.

Design and Experimental Characterization of a High-Temperature Proton Exchange Membrane Fuel Cell Stack

Proton exchange membrane (PEM) fuel cells based on polybenzimidazole (PBI) polymers and phosphoric acid can be operated at temperature between 120 °C and 180 °C. Reactant humidification is not required and CO content up to 1% in the fuel can be tolerated, only marginally affecting performance. This is what makes high-temperature PEM (HTPEM) fuel cells very attractive, as low quality reformed hydrogen can be used and water management problems are avoided. From an experimental point of view, the major research effort up to now was dedicated to the development and study of high-temperature membranes, especially to development of acid-doped PBI type membranes. Some studies were dedicated to the experimental analysis of single cells and only very few to the development and characterization of high-temperature stacks. This work aims to provide more experimental data regarding high-temperature fuel cell stacks, operated with hydrogen but also with different types of reformates. The main design features and the performance curves obtained with a three-cell air-cooled stack are presented. The stack was tested on a broad temperature range, between 120 and 180 °C, with pure hydrogen and gas mixtures containing up to 2% of CO, simulating the output of a typical methanol reformer. With pure hydrogen, at 180 °C, the considered stack is able to deliver electrical power of 31 W at 1.8 V. With a mixture containing 2% of carbon monoxide, in the same conditions, the performance drops to 24 W. The tests demonstrated that the performance loss caused by operation with reformates, can be partially compensated by a higher stack temperature.

High Power Internal-Reforming Direct Carbonate Fuel Cell Stack Development Through Mathematical Modeling and Engineering Optimization

Fuel cell power generation has evolved from the laboratory and aerospace applications, and moved onto practical applications of stationary power generation and automotive propulsion, driven by its high-energy efficiency and low emissions. The success of the fuel cell technology depends on its performance, cost, and reliability in commercial applications. Fuel Cell Energy Inc. (Danbury, CT) has been developing its direct fuel cell (DFC™) technology for power generation based on internal-reforming carbonate fuel cells. The DFC technology integrates the reforming reaction within the carbonate fuel cell stack. The integration of the reforming process inside the high temperature fuel cell stack simplifies the fuel cell power plant system and makes the fuel cell technology more accessible to the practical usage with low cost and high efficiency. The internal-reforming direct carbonate fuel cell technology has progressed steadily with improvement in performance and success in precommercialization applications. Modeling and simulation of the fuel cell performance played an important role in the fuel cell development. This paper will illustrate improved mathematical model for the direct carbonate fuel cell with the internal-reforming process and complete fuel cell physical and chemical descriptions for the simulation. The model has been validated with data from real-scale fuel cell stacks and applied to fuel cell stack design. More powerful and reliable DFC stack with improved performance has been developed with the assistance of this model. This paper will present progress in developing high performance stack designs aided by modeling efforts, its impact on power increase, and cost reduction in the DFC product.