Hossein Ghezel-Ayagh

An Explicit Dynamic Model for Direct Reforming Carbonate Fuel Cell Stack

A nonlinear, lumped-parameter mathematical model of direct reforming carbonate fuel cell stack is extended by deriving an explicit set of differential equations for computer simulation. The equilibrium assumption used for the water-gas shift reaction results in an implicit equation set, previously solved using numerical techniques. An explicit equation set is derived by eliminating a key variable associated with the water-gas shift reaction. In addition, results are improved by incorporating a fuel cell performance model to account for reversible cell potential and polarization losses. This requires determination of intermediate gas composition at the cell anode inlet, resulting in additional computations. All results and physical data used are specific to a lumped 16-stack 2-MW system design, a precursor to a demonstration plant that had been operated at Santa Clara, CA. Steady state results are validated for several load points over the upper region of operation and transient results are provided for sudden load change.

Modeling and cycling control of carbonate fuel cell power plants

A mathematical process model for an internal reforming molten carbonate fuel cell power plant is discussed in this paper. The dominant thermal and chemical dynamic processes are modeled for the cell stack array and balance-of-plant, including cathode gas preparation, heat recovery, and fuel processing. Physical data is obtained from a 2 MW system design that was a precursor to a demonstration plant operated at the City of Santa Clara, CA, USA. Steady state validation for several load points is provided for the cell stack array and a load cycling control system is described and tested under ramping operation between load points. r 2002 Elsevier Science Ltd. All rights reserved.

Control Design for a Bottoming Solid Oxide Fuel Cell Gas Turbine Hybrid System

A bottoming 275 kilowatt planar solid oxide fuel cell (SOFC) gas turbine (GT) hybrid system control approach has been conceptualized and designed. Based on previously published modeling techniques, a dynamic model is developed that captures the physics sufficient for dynamic simulation of all processes that affect the system with time scales greater than ten milliseconds. The dynamic model was used to make system design improvements to enable the system to operate dynamically over a wide range of power output (15 to 100% power). The wide range of operation was possible by burning supplementary fuel in the combustor and operating the turbine at variable speed for improved thermal management. The dynamic model was employed to design a control strategy for the system. Analyses of the relative gain array (RGA) of the system at several operating points gave insight into input/output (I/O) pairing for decentralized control. Particularly, the analyses indicate that for SOFC/GT hybrid plants that use voltage as a controlled variable it is beneficial to control system power by manipulating fuel cell current and to control fuel cell voltage by manipulating the anode fuel flowrate. To control the stack temperature during transient load changes, a cascade control structure is employed in which a fast inner loop that maintains the GT shaft speed receives its setpoint from a slower outer loop that maintains the stack temperature. Fuel can be added to the combustor to maintain the turbine inlet temperature for the lower operating power conditions. To maintain fuel utilization and to prevent fuel starvation in the fuel cell, fuel is supplied to the fuel cell proportionally to the stack current. In addition, voltage is used as an indicator of varying fuel concentrations allowing the fuel flow to be adjusted accordingly. Using voltage as a sensor is shown to be a potential solution to making SOFC systems robust to varying fuel compositions. The simulation tool proved effective for fuel cell/GT hybrid system control system development. The resulting SOFC/GT system control approach is shown to have transient load-following capability over a wide range of power, ambient temperature, and fuel concentration variations.Copyright

Operation and control of direct reforming fuel cell power plant

Computer simulation is used to analyze the operation and efficiency of a carbonate fuel cell power plant under load perturbations. The plant model is based on a 2 MW system design used in the Santa Clara Demonstration Project and includes: internal reforming carbonate fuel cell stack, cathode gas preparation system, heat recovery unit and fuel processing system. Model development for various processes is based on thermochemical principles and conservation of mass and energy. Overall plant efficiency is determined by net fuel consumption based on calculated gas compositions and auxiliary power consumption. During load maneuvering, several key operational constraints must be maintained. Among these are: allowable stack temperature deviation, baseline fuel utilization, steam/carbon ratio, and pressure difference between anode and cathode. Actual plant control schemes are used in the simulation and are evaluated for performance under load changes. The results of these simulations will be used as a benchmark and development tool for advanced intelligent controllers for autonomous and efficient operation of fuel cell systems.

Fuzzy Fault Diagnosis and Accommodation System for Hybrid Fuel-Cell/Gas-Turbine Power Plant

The concept of hybrid fuel-cell power plants has shown its potential for applications and is already under commercialization. In a hybrid fuel-cell and turbine power plant, the control system is an essential component that guarantees reliable and efficient operations. However, due to the limited information possessed by particular local controllers, the plant may become degraded or unstable during system failures. To regulate the power plants under such abnormal situations, a fault diagnosis and accommodation (FDA) system based on fuzzy logic has been developed as an effective complement for the local control scheme. Being a quantitative approach, the fuzzy FDA system can be implemented with considerably lower complexities than an analytical fault regulator. The system structures and design methods are discussed in this paper. Three types of controllers are implemented and investigated. Simulation results are presented to verify the performance of the overall system.

Ultra High Efficiency Hybrid Direct Fuel Cell/Turbine Power Plant

FuelCell Energy, Inc has developed a design for a hybrid powerplant which combines the company’s carbonate Direct Fuel Cell (DFC) with a gas turbine cycle in an ultra high efficiency system. Under a recently completed program supported by the U.S. Department of Energy through the Federal Energy Technology Center (FETC, Morgantown WV), the system design was optimized to eliminate high-temperature developmental components and reduce the overall complexity of the cycle. Modeling studies indicate that the system is capable of operating with fuel-to-electrical LHV efficiencies in the low 70’s with near term fuel cell performance levels. Longer term fuel cell performance gains may provide system efficiencies near 80 percent.© 2000 ASME

Linear quadratic regulator for a bottoming solid oxide fuel cell gas turbine hybrid system

The control system for fuel cell gas turbine hybrid power plants plays an important role in achieving synergistic operation of subsystems, improving reliability of operation, and reducing frequency of maintenance and downtime. In this paper, we discuss development of advanced control algorithms for a system composed of an internally reforming solid oxide fuel cell coupled with an indirectly heated Brayton cycle gas turbine. In high temperature fuel cells it is critical to closely maintain fuel cell temperatures and to provide sufficient electrochemical reacting species to ensure system durability. The control objective explored here is focused on maintaining the system power output, temperature constraints, and target fuel utilization, in the presence of ambient temperature and fuel composition perturbations. The present work details the development of a centralized linear quadratic regulator (LQR) including state estimation via Kalman filtering. The controller is augmented by local turbine speed control and integral system power control. Relative gain array analysis has indicated that independent control loops of the hybrid system are coupled at time scales greater than 1 s. The objective of the paper is to quantify the performance of a centralized LQR in rejecting fuel and ambient temperature disturbances compared with a previously developed decentralized controller. Results indicate that both the LQR and decentralized controller can well maintain the system power to the disturbances. However, the LQR ensures better maintenance of the fuel cell stack voltage and temperature that can improve high temperature fuel cell system durability.

Dynamic Modeling and Simulation of a Hybrid Fuel Cell/Gas Turbine Power Plant for Control System Development

This paper presents dynamic modeling and simulation results for a Fuel Cell/Turbine Hybrid Power System and describes the overall use of modeling/simulation as a tool in the design of advanced controllers for Fuel Cell/Turbine systems. The simulation includes representation of the fuel cell stack integrated with balance-of-plant, including microturbine generator and heat recovery. A conventional control system based on PID controllers is also represented. Motivation for this work is to help sustain and enhance commercial viability of hybrid systems by operating them at maximum possible reliability, efficiency, and load range.Copyright

Reduced-order dynamic model of carbonate fuel cell system for distributed generation control

Internally reformed carbonate fuel cell-based power plants have the capability of rapid load cycling provided that operational constraints are met during load perturbations. These constraints include acceptable deviations in stack temperature and stack pressure, both of which exhibit slow dynamics due to a large stack thermal time constant. Fuel cell stack dynamics exhibit multi-time scale behavior, however, when considering fast electrochemical reactions that occur. Therefore, in grid transient studies involving fuel cells, the slower dynamics can be neglected. This results in a simpler, reduced-order dynamic model. In this paper we present a complete model for direct reforming carbonate fuel cell stack and then simplify the equation set under the condition of constant temperature. A comparison is made between the full order model and reduced-order model by examining gas composition and system DC voltage under a severe transient.

Modeling, Simulation and Control of Direct Reforming Molten Carbonate Fuel Cell Power Plant

Abstract A mathematical model, control description, and simulation results for internal reforming molten carbonate fuel cell power plant are discussed in this paper. The dominant thermodynamics and chemical reactions are modeled for the cell stack and balance-of-plant, including anode exhaust oxidizer and simplified heat recovery unit. Physical data is obtained from a 2-MW system design that is a precursor to a demonstration fuel cell power plant that had been running on natural gas at the City of Santa Clara, CA, USA. The fuel cells in this design utilize direct reforming of methane gas through placement of internal reforming catalysts within the cells. The control loops and operation in the actual plant have been retained in the simulation model and experimental transient results are provided for a sudden increase in power demand on the fuel cell stack.