Hybrid Fuel Cell / Gas Turbine Systems Auxiliary Power Unit

Recent interest in fuel cell-gas turbine hybrid applications for the aerospace industry has led to the need for accurate computer simulation models to aid in system design and performance evaluation. To meet this requirement, solid oxide fuel cell (SOFC) and fuel processor models have been developed and incorporated into the Numerical Propulsion Systems Simulation (NPSS) software package. The SOFC and reformer models solve systems of equations governing steady-state performance using common theoretical and semi-empirical terms. An example hybrid configuration is presented that demonstrates the new capability as well as the interaction with pre-existing gas turbine and heat exchanger models. Finally, a comparison of calculated SOFC performance with experimental data is presented to demonstrate model validity. Introduction Fuel cell technology continues to mature due to innovations from industry, government, and academia. Electric drive-trains for automotive applications are evolving from early pure battery-powered vehicles to commercially viable combustion engine/battery hybrids, with pure fuel cell buses and automobiles undergoing on-road demonstrations. Stationary power fuel cell systems continue to be installed, proving environmental sensitivity and becoming more capable with regard to reliability, availability and user friendliness. New types of fuel cells, such as the direct methanol fuel cell, are potentially creating new market applications for fuel cells including portable power for lap[top of page] computers and other compact electronics. This continued progress towards more reliable and cost-effective fuel cells establishes a basis to consider fuel cells in aerospace applications. These applications include electrical power units for commercial aircraft and uninhabited aerial vehicles (UAVs) and also propulsion power for UAVs and other small aircraft. NASA has been using fuel cells for the manned space program since its inception and is currently assessing the feasibility of proton exchange membrane (PEM) fuel cells for its next-generation space launch vehicle. Other space applications for fuel cells such as electrical power for satellites and even planetary in-situ-based electrical power units are also being examined. As in other applications, aerospace fuel cells may offer reduced criteria pollutant (e.g., NOX, CO, hydrocarbons) and CO2 emissions when compared to current aviation electrical power production methods. In addition, noise may be diminished as a result of the lower gas velocities and smaller rotating components of fuel cell systems compared to gas turbine combustion engines. Another benefit is that the thermal efficiency of small fuel cell systems is typically much higher than similarly sized aeronautical gas turbines. Any fuel weight saved due to improved efficiency will thus counteract the weight increase due to the lower specific power (power/weight) of fuel cell system compared to the gas turbine. Based on initial analyses, this balance of increased hardware weight versus increased fuel efficiency appears to be one of the primary issues for the design of the system. Moreover, the relative value of hardware weight or fuel efficiency is entirely mission-dependent. The longer the mission, the more fuel weight is saved due to efficiency improvement. While this type of design trade-off is relevant for all forms of transportation, its importance is greatest for aerospace applications. As a result, aerospace fuel cells may ultimately require fuel cell and system designs that differ from that of the ground-based applications. Fuel cells may also enable new aerospace missions that were previously not possible. An example of such is the reversible-regenerative fuel cell power system, where a complete energy cycle is created to enable extremely long-duration missions. For this system, a fuel cell is combined with an electrolyzer such that hydrogen and oxygen react to form water and electricity in the fuel cell while water is electrolyzed to re-form hydrogen and oxygen in the reverse process. The power required to electrolyze the water comes from an external source such as solar power, a technology that is often used in the aerospace industry. NASA, along with the fuel cell and aerospace industries, the U.S. Departments of Energy and Defense, and academia (including the National Fuel Cell Research Center at the University of California, Irvine), has begun investigating fuel cells for both UAVs and commercial aircraft. PEM fuel cells are attractive for these applications due to their high specific power when compared to other fuel cell types. In addition, with PEM fuel cells being the primary choice for automotive applications, there is substantial research both in the public and private sectors on this technology resulting in high expectations for commercially viable technology development. However, the PEM fuel cell is very sensitive to fuel impurities (especially carbon monoxide) and when integrated into systems using hydrocarbon fuel sources typically has a lower efficiency than other fuel cell types. The higher temperature ceramic-based SOFC tolerates fuel impurities and can utilize carbon monoxide as a fuel. In addition, the high temperature operation allows systems design that well uses the fuel cell thermal output, which leads to higher system efficiency than comparable PEM fuel cell systems. For a SOFC, liquid hydrocarbon fuels, including the aeronautical fuels, are much easier to use due to the relaxed constraints on the fuel cell anode inlet gas. The liquid fuels are more energy dense per unit volume than hydrogen, which is also important, especially for longer missions and larger aircraft. Due to the importance of efficiency and due to the need to operate fuel cells at altitude, the hybrid SOFC/gas turbine cycle is a potentially attractive option for aerospace applications. The myriad of potential fuel cell system designs, configurations, and operating conditions must be analyzed systematically for each mission, as the best system is likely to be different depending on the application. These complexities reinforce the need for systems analysis and optimization tools, such as those presented in this paper, as an element of the overall design process. Part of the challenge of modeling fuel cell systems is the multidisciplinary nature of the physics and chemistry that governs their performance. Electrochemistry, fluid mechanics, materials science, and electrical engineering, among other disciplines, are all critical to understanding and designing fuel cell systems. In addition, few commercial software packages contain the requisite models for simulating all components of a fuel cell system. Even fewer modeling programs include robust and fundamentally sound strategies for simulation of fuel cells. As a result, new models must be developed. There have been multiple published approaches to SOFC/gas turbine hybrid models. Massardo and Lubelli [1] developed a primarily theoretical model for a SOFC/gas turbine hybrid that can accommodate equilibrium-based internal methane reforming as well as more typical external fuel reforming (also at equilibrium). The model is used to examine several valuable parametric trends including fuel cell pressure and gas stream temperatures as well as four overall configurations. Later this model is expanded, improved and applied to SOFC/micro-gas turbine [2] and SOFC/personal turbine [3] hybrid systems. Both design and off-design characteristics are included in [2] and [3], with an off-design scheme that takes advantage of the fuel cell performance at off-design and a variable speed gas turbine. In another approach, Burer et al. [4] presents a SOFC/gas-turbine hybrid as one subset of a larger power, heating, and cooling cogeneration system model. Palsson, et al. [5] developed a two-dimensional planar SOFC model to add further fidelity and flexibility to the system and proceeded to evaluate several novel concepts, including networked SOFC stacks [6]. Even ground-based mobile applications using hybrid SOFC/gas turbines have been examined, presented by Winkler and Lorenz [7]. Few published models have been validated at the system level because of the lack of available systems-level data. One study, produced by Yi et al. [8], has compared the results of a SOFC/gas turbine hybrid model to an operating 220 kW hybrid system. While these models have been recently developed and used to simulate various applications, no model has been developed with aerospace applications as its focus. Data such as atmospheric conditions at various altitudes and the need for propulsor models require specialized tools that differ from those used in the automotive and stationary power industries. The current effort therefore develops a separate SOFC/gas turbine hybrid model within an existing aerospace code, known as the Numerical Propulsion Systems Simulation (NPSS). This software package is a NASA and U.S. aerospace industry-developed tool used primarily for gas turbine and rocket engine simulation [9,10] . NPSS is mainly a steady-state zero-dimensional thermodynamic analysis code that was created so that the aerospace community would have a similar tool for aerospace propulsion analysis. In addition to the altitude data and propulsor models, the NPSS code was chosen for this work because of its familiarity and regular use within the aerospace industry. NPSS also contains other aerospace-specific features including a form of the JANNAF thermodynamic data and the Chemical Equilibrium with Applications (CEA) code, developed by Gordon and McBride [11]. CEA is based on the concept of Gibb's Free Energy minimization and can contain a large thermodynamic database of chemical species including thermodynamic information on various types of jet fuel. NPSS also contains built-in gas turbine and heat exchanger components that are readily applicable to the SOFC/gas turbine hybrid system. The compressor and turbine components are capable of both design and off-design analysis with the use of performance maps or other correlations. The heat exchanger is a gas-phase model based on either an input effectiveness or total heat flow. NPSS also includes a nonlinear system of equations solver that is based on the modified NewtonRaphson method. The solver gives the code remarkable flexibility to balance a system in a number of different ways. Examples include the ability to equalize the compressor and turbine work or, in the case of a reformer, to create an autothermal enthalpy condition. Nomenclature Symbol Units Description A kΩ*cm2 Pre-exponential factor in resistance equation E K Exponential factor in resistance equation F C/mol Faraday's constant H2O/C Molar steam-to-carbon ratio HX Heat Exchanger i mA/cm2 Current density iL mA/cm2 Limiting current density in mA/cm2 Internal current density io mA/cm2 Exchange current density n Number of electrons transferred per mole of reactant O/C Molar oxygen-to-carbon ratio P Watts Power p kPa Pressure pp (as a prefix) partial pressure Watts Heat r kW*cm2 Bulk internal area specific resistance term (combines both ionic and electronic resistances) R J/K*mol Universal gas constant T K Temperature U Overall stack utilization V Voltage ∆fG° J/mol Change in Gibb's energy of formation at standard pressures, a function of temperature. J/sec Enthalpy flow (enthalpy times mass flow) mol/sec Molar flow rate α Empirically derived charge transfer coefficient η Efficiency Model development Solid Oxide Fuel Cell (SOFC) The SOFC model developed in this study is a bulk model that predicts overall performance of a SOFC and utilizes fundamental understanding of fuel cell electrochemistry to predict these outputs. There are no model features that relate to geometry except for the physical cell area, and there are no constraints on the size or physical configuration. This provides a widely adaptable model without the expense of detailed internal flow field, heat flux, and electrochemistry information, each of which is highly dependent on the geometrical configuration of an individual fuel cell stack. The accuracy of the model therefore comes directly from the integrity of the multiple input values as determined by comparison to experimental results. Default values for the parameterization of the model come from literature sources as accepted generic SOFC performance parameters [12, 13]. The user can retain these default values and run simulations for the purpose of seeing the performance of a typical SOFC stack in various hybrid systems and flight situations. If a user has a detailed SOFC model or an actual SOFC stack, the model parameterization (input values) can be changed to represent the outputs of the detailed model or the measured performance values. This implies that with the correct parameterization (inputs) this model can be used to accurately predict the performance of a system that uses any of the different types and configurations of actual SOFCs being studied today, including both tubular and planar designs. Many of the thermodynamic and flow calculations (i.e., mass and energy balances) that govern fuel cell operation as predicted by the current model are solved by the NPSS software utilizing the built-in CEA equilibrium model and the numerical solver. The electrochemical and purely electrical equations are added to the built-in structure and are presented below. The reversible (Nernst) voltage is: