Rory A. Roberts

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

Development of controls for dynamic operation of carbonate fuel cell-gas turbine hybrid systems

Hybrid fuel cell/gas turbine (FC/GT) systems have been shown through experiment and simulation to be highly efficient technologies with low emissions. Maintaining efficient, low emission, and safe operation, whether during disturbances or regular operational transients, is a challenge to both understand and address. Some likely disturbances can arise from changes in ambient temperature, fuel flow variations induced by supply pressure disturbances, fuel composition variability, and power demand fluctuations. To gain insight into the dynamic operation of such cycles and address operating challenges, dynamic modeling tools have been developed at two different laboratories. In this paper these models are used to simulate the dynamic operation of an integrated MCFC/GT hybrid system and to subsequently develop and test control strategies for the hybrid power plant. Two control strategies are developed and tested for their ability to control the system during various perturbations. Predicted fuel cell operating temperature, fuel utilization, fuel cell and GT power, shaft speed, compressor mass flow and temperatures throughout the FC/GT system are presented for the controlled response to a fuel cell voltage increase in order to show the effect of a load decrease.Copyright

Gas turbine assessment for air management of pressurized SOFC/GT hybrid systems

This paper analyzes and compares transient and steady-state performance characteristics of different types of single-shaft turbo-machinery for controlling the air through a pressurized solid oxide fuel cell (SOFC) stack that is integrated into a SOFC/GT pressurized hybrid system. Analyses are focused on the bottoming part of the cycle, where the gas turbine (GT) has the role of properly managing airfiow to the SOFC stack for various loads and at different ambient conditions. Analyses were accomplished using two disparate computer programs, which each modeled a similar SOFC/GT cycle using identical generic gas turbine performance maps. The models are shown to provide consistent results, and they are used to assess: (1) the influence of SOFC exhaust composition on expander behavior for on-design conditions, (2) the off-design performance of the bypass, bleed, and variable speed controls for various part-load conditions and for different ambient conditions; (3) the features of such controls during abrupt transients such as load trip and bypass/bleed valve failure. The results show that a variable speed micro-turbine is the best option for off-design operation of a SOFC/GT hybrid system. For safety measures a bleed valve provides adequate control of the system during load trip. General specifications for a radial GT engine for integration with a 550 kW pressurized SOFC stack are identified, which allow operation under a wide range of ambient conditions as well as several different cycle configurations. Copyright

Modeling Techniques for a Computational Efficient Dynamic Turbofan Engine Model

A transient two-stream engine model has been developed. Individual component models developed exclusively in MATLAB/Simulink including the fan, high pressure compressor, combustor, high pressure turbine, low pressure turbine, plenum volumes, and exit nozzle have been combined to investigate the behavior of a turbofan two-stream engine. Special attention has been paid to the development of transient capabilities throughout the model, increasing physics model, eliminating algebraic constraints, and reducing simulation time through enabling the use of advanced numerical solvers. The lessening of computation time is paramount for conducting future aircraft system-level design trade studies and optimization. The new engine model is simulated for a fuel perturbation and a specified mission while tracking critical parameters. These results, as well as the simulation times, are presented. The new approach significantly reduces the simulation time.

Deriving Function-Failure Similarity Information for Failure-Free Rotorcraft Component Design

Performance and safety are the top concerns of high-risk aerospace applications at NASA. Eliminating or reducing performance and safety problems can be achieved with a thorough understanding of potential failure modes in the design that lead to these problems. The majority of techniques use prior knowledge and experience as well as Failure Modes and Effects as methods to determine potential failure modes of aircraft. The aircraft design needs to be passed through a general technique to ensure that every potential failure mode is considered, while avoiding spending time on improbable failure modes. In this work, this is accomplished by mapping failure modes to certain components, which are described by their functionality. In turn, the failure modes are then linked to the basic functions that are carried within the components of the aircraft. Using the technique proposed in this paper, designers can examine the basic functions, and select appropriate analyses to eliminate or design out the potential failure modes. This method was previously applied to a simple rotating machine test rig with basic functions that are common to a rotorcraft. In this paper, this technique is applied to the engine and power train of a rotorcraft, using failures and functions obtained from accident reports and engineering drawings.

Fuel Cell/Gas Turbine Hybrid System Control for Daily Load Profile and Ambient Condition Variation

Fuel Cell/Gas Turbine (FC/GT) hybrid technology is promising, but introduces challenges in system operation and control. For base-load applications, changes in ambient conditions perturb the system and it becomes difficult to maintain constant power production by the FC/GT system. If the FC/GT hybrid system is load-following then the problem becomes even more complex. In the current study, a dynamic model of a FC/GT power plant is developed with system controls. Two cases are evaluated: (1) system controls are developed to maintain constant power and process control within acceptable constraints, (2) the FC/GT power plant is set in power following mode connected in parallel to the grid for a daily load profile scenario. Changing ambient conditions are employed in the dynamic analysis for both cases. With appropriate attention to design of the system itself and the control logic, the challenges for dynamic system operation and control can be addressed.© 2006 ASME

A Function-Based Exploration of JPL’s Problem/Failure Reporting Database

This paper presents the exploration of a failure database derived from a Problem and Failure Reporting (P/FR) database for NASA space missions. The overall goal is to analyze an existing database of problems and anomalies recorded at the Jet Propulsion Laboratory (JPL) for multiple missions, and classify a subset of them into distinct failure modes from a predetermined set of failure modes. Results are presented to describe the failure modes and functional descriptions derived from the PF/R database, using observed in-flight failure reports for six unmanned space missions. The challenges and issues in mining this information are presented, as well as comments on the adequacy of the existing failure mode and function taxonomies to describe the subset of observed failures reported in the PF/R database, and on the utility of such large databases.Copyright

Generic Aircraft Thermal Tip-to-Tail Modeling and Simulation

A system-level thermal management aircraft model has been developed in a multidisciplinary modeling and simulation environment. Individual subsystem models developed exclusively in MATLAB/Simulink, representing the vehicle dynamics, the propulsion, electrical power, and thermal systems, and their associated controllers, are combined to investigate the thermal management issues of a typical long range strike platform. A thermal tip-to-tail model allows conceptual design trade studies of various subsystems and can quantify performance gains across the aircraft. The final result is an aircraft that is thermally optimized at the system-level, rather than at the subsystemlevel. In addition, the model has been built without the aid of proprietary data, thereby allowing the distribution of the tool to a variety of conceptual design groups and researchers. Special attention has been paid to the development of transient component models within the thermal management systems, including the Integrated Power Package, heat exchangers, fuel and oil pumps, and the engine oil heat rejection. As a result, the thermal and power challenges that face modern aircraft can be addressed, potentially increasing the performance capabilities of future aircraft. Preliminary simulation results are discussed with a specific focus on the thermal challenges encountered during reduced engine power mission segments.

Inter-Laboratory Dynamic Modeling of a Carbonate Fuel Cell for Hybrid Application

A detailed comparison of dynamic models developed for carbonate fuel cells used in hybrid fuel cell gas turbine systems is presented. The two models are nearly similar in that both treat the bulk behavior of the system (e.g., through lumped or one-dimensional solutions of the fundamental equations. However, both models are implemented independently by different research groups using disparate simulation software programs. As a test case for the comparison, a generic molten carbonate hybrid fuel cell gas turbine system is identified. Such comparison-work benefits all parties by ensuring sub-model reliability prior to integration into a complete hybrid system model. Detailed results for the carbonate fuel cell models are presented. For a generic planar design, voltage and current behavior are shown following step changes in load resistance and fuel flow. The time scales for thermal dynamic response are much greater than those required for the initial electrochemical dynamic response as is expected. These results provide understanding of some of the operational characteristics of fuel cells and indicate the complexity of the dynamic response of fuel cell hybrid components. The results from the two models are not identical, but compare sufficiently well to provide confidence in each of the model’s reliability, enabling them to be integrated for hybrid system simulation. Results from the integrated simulations will provide guidance on future hybrid technology development needs.Copyright

Implementation of a Non-Equilibrium Exergy Analysis for an Aircraft Thermal Management System

System optimization and design of an aircraft is required to achieve multiple objectives. Often one of the main objectives is system efficiency for reduction in fuel use for a given mission. System efficiency can be quantified by either a 1st or 2nd law thermodynamic analysis. A 2nd law exergy analysis can provide a more robust means of accounting for all of the energy flows within and in between subsystems. These energy flows may be thermal, chemical, electrical, pneumatic, etc. The incorporation of a transient system analysis in the design process of an aircraft can provide untapped opportunities for gains in energy efficiency of the aircrafts operation. In order to quantify the efficiency gains utilizing a 2nd law exergy analysis, the non-equilibrium term of exergy generation must be accounted for in the analysis. This paper demonstrates the implementation of a non-equilibrium exergy analysis of a heat exchanger.