Fabian Mueller

Dynamic Simulation of an Integrated Solid Oxide Fuel Cell System Including Current-Based Fuel Flow Control

A two-dimensional dynamic model was created for a Siemens Westinghouse type tubular solid oxide fuel cell (SOFC). This SOFC model was integrated with simulation modules for other system components (e.g., reformer, combustion chamber, and dissipater) to comprise a system model that can simulate an integrated 25 kilowatt SOFC system located at the University of California, Irvine. A comparison of steady-state model results to data suggests that the integrated model can well predict actual system power performance to within 3 percent, and temperature to within 5 percent. In addition, the model predictions well characterize observed voltage and temperature transients that are representative of tubular SOFC system performance. The characteristic voltage transient due to changes in SOFC hydrogen concentration has a time scale that is shown to be on the order of seconds while the characteristic temperature transient is on the order of hours. Voltage transients due to hydrogen concentration change are investigated in detail. Particularly, the results reinforce the importance of maintaining fuel utilization during transient operation. The model is shown to be a useful tool for investigating the impacts of component response characteristics on overall system dynamic performance. Current-based flow control (CBFC), a control strategy of changing the fuel flow rate in proportion to the fuel cell current is tested and shown to be highly effective. The results further demonstrate the impact of fuel flow delay that may result from slow dynamic responses of control valves, and that such flow delays impose major limitations on the system transient response capability.Copyright

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

Parametric Thermodynamic Analysis of a Solid Oxide Fuel Cell Gas Turbine System Design Space

Author(s): Tarroja, B; Mueller, F; Maclay, J; Brouwer, J | Abstract: A parametric study of a solid oxide fuel cell-gas turbine (SOFC-GT) hybrid system design is conducted with the intention of determining the thermodynamically based design space constrained by modern material and operating limits. The analysis is performed using a thermodynamic model of a generalized SOFC-GT system where the sizing of all components, except the fuel cell, is allowed to vary. Effects of parameters such as pressure ratio, fuel utilization, oxygen utilization, and current density are examined. Operational limits are discussed in terms of maximum combustor exit temperature, maximum heat exchanger effectiveness, limiting current density, maximum hydrogen utilization, and fuel cell temperature rise. It was found that the maximum hydrogen utilization and combustor exit temperature were the most significant constraints on the system design space. The design space includes the use of cathode flow recycling and air preheating via a recuperator (heat exchanger). The effect on system efficiency of exhaust gas recirculation using an ejector versus using a blower is discussed, while both are compared with the base case of using a heat exchanger only. It was found that use of an ejector for exhaust gas recirculation caused the highest efficiency loss, and the base case was found to exhibit the highest overall system efficiency. The use of a cathode recycle blower allowed the largest downsizing of the heat exchanger, although avoiding cathode recycling altogether achieved the highest efficiency. Efficiencies in the range of 50-75% were found for variations in pressure ratio, fuel utilization, oxygen utilization, and current density. The best performing systems that fell within all design constraints were those that used a heat exchanger only to preheat air, moderate pressure ratios, low oxygen utilizations, and high fuel utilizations. Copyright

Design, Simulation and control of a 100 MW-class solid oxide fuel cell gas turbine hybrid system

A 100 MW-class planar solid oxide fuel cell, synchronous gas turbine hybrid system has been designed, modeled and controlled. The system is built of 70 functional fuel cell modules each containing 10 fuel cell stacks, a blower to recirculate depleted cathode air, a depleted fuel oxidizer and a cathode inlet air recuperator with bypass. The recuperator bypass serves to control the cathode inlet air temperature while the variable speed cathode blower recirculates air to control the cathode air inlet temperature. This allows for excellent fuel cell thermal management without independent control of the gas turbine, which at this scale will most likely be a synchronous generator. In concept the demonstrated modular design makes it possible to vary the number of cells controlled by each fuel valve, power electronics module, and recirculation blower, so that actuators can adjust to variations in the hundreds of thousands of fuel cells contained within the 100 megawatt hybrid system for improved control and reliability. In addition, the modular design makes it possible to take individual fuel cell modules offline for maintenance while the overall system continues to operate. Parametric steady state design analyses conducted on the system reveal that the overall fuel-to-electricity conversion efficiency of the current system increases with increased cathode exhaust recirculation. To evaluate and demonstrate the conceptualized design, the fully integrated system was modeled dynamically in Matlab–Simulink®. Simple proportional feedback with steady state feed-forward controls for power tracking, thermal management, and stable gas turbine operation were developed for the system. Simulations of the fully controlled system indicate that the system has a high efficiency over a large range of operating conditions, decent transient load following capability, fuel and ambient temperature disturbance rejection as well as the capability to operate with a varying number of fuel cell modules. The efforts here build upon prior work and combine the efforts of system design, system operation, component performance characterization and control to demonstrate hybrid transient capability in large-scale coal synthesis gas-based applications through simulation. Furthermore, the use of a modular fuel cell system design, the use of blower recirculation, and the need for integrated system controls are verified.Copyright

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 Simulation of a Stationary PEM Fuel Cell System

A dynamic model of a stationary PEM fuel cell system has been developed in Matlab-Simulink®. The system model accounts for the fuel processing system, PEM stack with coolant, humidifier with anode tail-gas oxidizer (ATO), and an enthalpy wheel for cathode air. For the fuel processing system four reactors were modeled: (1) an auto thermal reactor (ATR) (2) a high temperature shift (HTS) reactor, (3) a low temperature shift (LTS) reactor, and (4) a preferential oxidation (PROX) reactor. Chemical kinetics for ATR that describe steam reformation of methane and partial oxidation of methane were simultaneously solved to accurately predict the reaction dynamics. Chemical equilibrium of CO with H2 O was assumed at HTS and LTS reactor exits to calculate CO conversion corresponding to the temperature of each reactor. A quasi-two dimensional unit PEM cell was modeled with five control volumes for solving the dynamic species and mass conservation equations and seven control volumes to solve the dynamic energy balance and to capture the details of MEA behavior, such as water transport, which is critical to accurately determine polarization losses. The dynamic conservation equations, primary heat transfer equations and equations of state are solved in each bulk component and each component is linked together to represent the complete system. A comparison of steady-state model results to experimental data shows that the system model well predicts the actual system power and catalytic partial oxidation (CPO) temperature. Transient simulation of DC power is also well matched with the experimental results to within a few percent. The model predictions well characterized the observed dynamic CPO temperature, voltage, and temperature of stack coolant outlet observations that are representative of a generic PEM stationary fuel cell system performance. The model is shown to be a useful tool for investigating the effects of inlet conditions and for the development of control strategies for enhancing system performance.Copyright

Dynamic Simulation of a Stationary Proton Exchange Membrane Fuel Cell System

A dynamic model of a stationary proton exchange membrane (PEM) fuel cell system has been developed in MATLAB-SIMULINK®. The system model accounts for the fuel processing system, PEM stack with coolant, humidifier with anode tail-gas oxidizer, and an enthalpy wheel for cathode air. Four reactors are modeled for the fuel processing system: (1) an autothermal reformation (ATR) reactor, (2) a high temperature shift (HTS) reactor, (3) a low temperature shift (LTS) reactor, and (4) a preferential oxidation reactor. Chemical kinetics for ATR that describe steam reformation of methane and partial oxidation of methane were simultaneously solved to accurately predict the reaction dynamics. The chemical equilibrium of CO with H2O was assumed at HTS and LTS reactor exits to calculate CO conversion corresponding to the temperature of each reactor. A quasi-onedimensional PEM unit cell was modeled with five control volumes for solving the dynamic species and mass conservation equations and seven control volumes to solve the dynamic energy balance. The quasi-one-dimensional cell model is able to capture the details of membrane electrode assembly behavior, such as water transport, which is critical to accurately determine polarization losses. The dynamic conservation equations, primary heat transfer equations and equations of state are solved in each bulk component, and each component is linked together to represent the complete system. The model predictions well matched the observed experimental dynamic voltage, stack coolant outlet temperature, and catalytic partial oxidation (CPO) temperature responses to perturbations. The dynamic response characteristics of the current system are representative of a typical stationary PEM fuel cell system. The dynamic model is used to develop and test a proportional-integral (PI) fuel flow controller that determines the fuel flow rate to maintain the uniform system efficiency. The dynamic model is shown to be a useful tool for investigating the effects of inlet conditions, load, and fuel flow perturbations and for the development of control strategies for enhancing system performance. Copyright

Thermodynamic Design Analysis of a Solid Oxide Fuel Cell Gas Turbine Hybrid System for High-Altitude Applications

A systems-level analysis and design iteration of a liquid hydrogen fueled 50 kW class solid oxide fuel cell gas turbine (SOFC/GT) hybrid auxiliary power unit for high-altitude unmanned aerial vehicles is performed with the intention of investigating system configuration, system parameters and system design constraints on system efficiency. The analysis is performed using thermodynamic models where the sizing of components is allowed to vary. Effects of system pressure ratio, oxygen utilization, and current density on the system design are examined. It was found that an optimal effective pressure ratio exists, indicating that a SOFC/GT hybrid system operating at high altitude should not necessarily be pressurized to one standard atmosphere or higher. It was discovered that the major design constraints for SOFC/GT system operating at high altitude is the large preheating requirement of the liquid hydrogen and cold ambient air at high altitude. To alleviate preheating challenges and increase system efficiencies, five SOFC/GT hybrid systems were developed for high altitude operation. The most efficient system achieved a thermodynamic efficiency greater than 65% at the design power of 50 kW, indicating that SOFC/GT hybrid systems can provide substantial efficiency increases for long endurance unmanned aerial vehicles.

Design, Simulation, and Control of a 100 Megawatt-Class Solid Oxide Fuel Cell Gas Turbine Hybrid System

A 100 MW-class planar solid oxide fuel cell, synchronous gas turbine hybrid system has been designed, modeled and controlled. The system is built of 70 functional fuel cell modules each containing 10 fuel cell stacks, a blower to recirculate depleted cathode air, a depleted fuel oxidizer and a cathode inlet air recuperator with bypass. The recuperator bypass serves to control the cathode inlet air temperature while the variable speed cathode blower recirculates air to control the cathode air inlet temperature. This allows for excellent fuel cell thermal management without independent control of the gas turbine, which at this scale will most likely be a synchronous generator. In concept the demonstrated modular design makes it possible to vary the number of cells controlled by each fuel valve, power electronics module, and recirculation blower, so that actuators can adjust to variations in the hundreds of thousands of fuel cells contained within the 100 megawatt hybrid system for improved control and reliability. In addition, the modular design makes it possible to take individual fuel cell modules offline for maintenance while the overall system continues to operate. Parametric steady state design analyses conducted on the system reveal that the overall fuel-to-electricity conversion efficiency of the current system increases with increased cathode exhaust recirculation. To evaluate and demonstrate the conceptualized design, the fully integrated system was modeled dynamically in Matlab–Simulink®. Simple proportional feedback with steady state feed-forward controls for power tracking, thermal management, and stable gas turbine operation were developed for the system. Simulations of the fully controlled system indicate that the system has a high efficiency over a large range of operating conditions, decent transient load following capability, fuel and ambient temperature disturbance rejection as well as the capability to operate with a varying number of fuel cell modules. The efforts here build upon prior work and combine the efforts of system design, system operation, component performance characterization and control to demonstrate hybrid transient capability in large-scale coal synthesis gas-based applications through simulation. Furthermore, the use of a modular fuel cell system design, the use of blower recirculation, and the need for integrated system controls are verified.Copyright