Michael D. Lukas

Development of a stack simulation model for control study on direct reforming molten carbonate fuel cell power plant

A nonlinear mathematical model of an internal reforming molten carbonate fuel cell stack is developed for control system applications to fuel cell power plants. The model is based on principles of energy and mass component balances and thermochemical properties. Physical data for this model is obtained from a 2 MW system design that is a precursor to a demonstration fuel cell power plant running on natural gas at the City of Santa Clara, CA. The model can be used to provide realistic evaluations of the responses to varying load demands on the fuel cell stack and to define transient limitations and control requirements. Simulation results are presented for a transient response to a power plant trip at full load.

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.

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.

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

Performance implications of rapid load changes in carbonate fuel cell systems

There are several key performance objectives of molten carbonate fuel cell systems operating under load perturbations. Among these include: regulation of stack temperature, regulation of differential pressure between anode and cathode, maintaining acceptable fuel utilization and steam-carbon ratio, and electrical load tracking. Utilities are interested in rapid load cycling of carbonate fuel cell systems while the military is interested in the response of carbonate fuel cell systems under sudden application and removal of electrical load. An integrated fuel cell system may endure these types of disturbances while satisfying performance goals, depending largely on the control system. In this paper, the authors examine two important performance variables: fuel utilization and steam-carbon ratio, under both ramping operation and sudden increase in load. Setpoint control laws are proposed for determining the proper steam and natural gas flows corresponding to steady state or transient conditions. In the case of a sudden load increase, they illustrate a trade-off between good load tracking and good utilization/steam-carbon ratio when considering rate constraints on valves.

Plant-wide simulation of direct reforming molten carbonate fuel cell systems

A plant-wide simulation model for a molten carbonate fuel cell power plant is outlined in this paper. The simulator is being developed for intelligent control applications to fuel cell systems as distributed generators. The dominant thermodynamics and chemical reactions are modeled for the cell stack and balance-of-plant, including heat recovery unit and anode exhaust oxidizer. The model is based on a 2 MW demonstration 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.

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.

Experimental transient validation of a Direct FuelCell(R) stack model

This paper compares the dynamic responses of a first principle model for internally reformed molten carbonate fuel cell stack with laboratory test results. ne dynamic model is currently being utilized in the design of a Ship Service Fuel Cell (SSFC) power plant. It is being employed to study the transient characteristics of the power plant and to aid in the design of the control system. The tests were performed on a nominally rated 20-kW, 30-cell laboratory test unit with full-scale cell area. Transient tests consisted of a set of step load changes it two different operating points. The predicted multi-time scale behavior is seen in the experimental results, which show a good agreement with the computer model.