Mario L. Ferrari

Design and Testing of Ejectors for High Temperature Fuel Cell Hybrid Systems

Our goal in this work is the improvement of the ejector performance inside hybrid systems supporting the theoretical activity with experimental tests. In fact, after a preliminary ejector design, an experimental rig has been developed to test single stage ejectors for hybrid systems at different operative conditions of mass flow rates, pressures, and temperatures. At first, an open circuit has been built to perform tests at atmospheric conditions in the secondary duct. Then, to emulate a SOFC anodic recirculation device, the circuit has been closed, introducing a fuel cell volume in a reduced scale. This configuration is important to test ejectors at pressurized conditions, both in primary and secondary ducts. Finally, the volume has been equipped with an electrical heater and the rig has been thermally insulated to test ejectors with secondary flows at high temperature, necessary to obtain values in similitude conditions with the real ones. This test rig has been used to validate simplified and CFD models necessary to design the ejectors and investigate the internal fluid dynamic phenomena. In fact, the application of CFD validated models has allowed us to improve the performance of ejectors for hybrid systems optimizing the geometry in terms of primary and secondary ducts, mixing chamber length, and diffuser. However, the simplified approach is essential to start the analysis with an effective preliminary geometry.

Hybrid Simulation Facility Based on Commercial 100 kWe Micro Gas Turbine

A new high temperature fuel cell-micro gas turbine physical emulator has been designed and installed in the framework of the European Integrated Project “FELICITAS” at the Thermochemical Power Group (TPG) laboratory located at Savona. The test rig is based on a commercial 100 kWe recuperated micro gas turbine (mGT) (Turbec T100) modified to be connected to a modular volume designed for physical emulation of fuel cell stack influence. The test rig has been developed starting with a complete theoretical analysis of the micro gas turbine design and off-design performance and with the definition of the more flexible layout to be used for different hybrid system (molten carbonate fuel cell or solid oxide fuel cell) emulation. The layout of the system (connecting pipes, valves, and instrumentation, in particular mass flow meter locations) has been carefully designed, and is presented in detail in this paper. Particular attention has been focused on the viscous pressure loss minimization: (i) to reduce the unbalance between compressor and expander, (ii) to maintain a high measurement precision, and (iii) to have an effective plant flexibility. Moreover, the volume used to emulate the cell stack has been designed to be strongly modular (different from a similar system developed by U.S. Department Of Energy-National Energy Technology Laboratory) to allow different volume size influence on the mGT rig to be easily tested. The modular high temperature volume has been designed using a computational fluid dynamics (CFD) commercial tool (FLUENT ). The CFD analysis was used (i) to reach a high level of uniformity in the flow distribution inside the volume, (ii) to have a velocity field (m/s) similar to the one existing inside the emulated cell stack, and (iii) to minimize (as possible) the pressure losses. The volume insulation will also allow to consider a strong thermal capacity effect during the tests. This paper reports the experimental results of several tests carried out on the rig (using the mGT at electrical stand-alone conditions with the machine control system operating at constant rotational speed) at different load values and at both steady-state and transient conditions.

Transient Modeling of the NETL Hybrid Fuel Cell/Gas Turbine Facility and Experimental Validation

This paper describes the experimental validation of two different transient models of the hybrid fuel cell/gas turbine facility of the U.S. DOE-NETL at Morgantown. The first part of this work is devoted to the description of the facility, designed to experimentally investigate these plants with real components, except the fuel cell. The behavior of the SOFC is obtained with apt volumes (for the stack and the off-gas burner) and using a combustor to generate similar thermal effects. The second part of this paper shows the facility real-time transient model developed at the U.S. DOE-NETL and the detailed transient modeling activity using the TRANSEO program developed at TPG. The results obtained with both models are successfully compared with the experimental data of two different load step decreases. The more detailed model agrees more closely with the experimental data, which, of course, is more time consuming than the real-time model (the detailed model operates with a calculation over calculated time ratio around 6). Finally, the TPG model has been used to discuss the importance of performance map precision for both compressor and turbine. This is an important analysis to better understand the steady-state difference between the two models.

Micro Gas Turbine Recuperator: Steady-State and Transient Experimental Investigation

The aim of this work is the experimental analysis of a primary-surface recuperator, operating in a 100 kW micro gas turbine, as in a standard recuperated cycle. These tests, performed in both steady-state and transient conditions, have been carried out using the micro gas turbine test rig, developed by the Thermochemical Power Group at the University of Genova, Italy. Even if this facility has mainly been designed for hybrid system emulations, it is possible to exploit the plant for component tests, such as experimental studies on recuperators. The valves installed in the rig make it possible to operate the plant in the standard recuperated configuration, and the facility has been equipped with new probes essential for this kind of tests. A wide-ranging analysis of the recuperator performance has been carried out with the machine, operating in stand-alone configuration, or connected to the electrical grid, to test different control strategy influences. Particular attention has been given to tests performed at different electrical load values and with different mass flow rates through the recuperator ducts. The final section of this paper reports the transient analysis carried out on this recuperator. The attention is mainly focused on thermal transient performance of the component, showing the effects of both temperature and flow steps. [DOI: 10.1115/1.3156822].

Generic Real-Time Modeling of Solid Oxide Fuel Cell Hybrid Systems

Real-time (RT) modeling is a recognized approach to monitor advanced systems and to improve control capabilities. Applications of RT models are commonly used in the automotive and aerospace fields. Starting from existing components and models developed in TRANSEO[REF] , a new approach, called the multipurpose RT approach, is developed for the solid oxide fuel cell hybrid system application. Original C-based models have been reprogrammed into embedded MATLAB functions for direct use within MATLAB-SIMULINK . Also, models in TRANSEO have been simplified to improve execution time. Using MATLAB ’s Real-Time Workshop application, the system model is able to be translated into an autogenerated C-code, and run as an application specific RT executable.

Transient Analysis of Solid Oxide Fuel Cell Hybrids: Part C — Whole-Cycle Model

A Solid Oxide Fuel Cell-Hybrid System is mainly composed of three parts: the stack, the anodic recirculation system with fuel feeding, and the cathodic side (air side) where turbomachinery and heat exchangers are installed. In Part A of this work the transient models of the fuel cell are described, while in Part B the anodic side is investigated. Many previous studies have been carried out on the cathodic side at the Thermochemical Power Group facility to simulate the transient behavior of the main components such as compressors, expanders and heat exchangers. In this paper attention is focused on the integration of the transient models of the hybrid system components. Following the on and off-design analysis of the SOFC-HS the transient response of the system from an electrochemical, fluid dynamic and thermal point of view has been studied at several operating conditions.Copyright

Micro Gas Turbine Based Test Rig for Hybrid System Emulation

The Thermochemical Power Group (TPG) is building at the laboratory of the University of Genoa, Italy, a new high temperature fuel cell - micro gas turbine physical emulator based on commercial machine technology. The aim of this new test rig is the experimental analysis of the coupling of commercial machines with fuel cell stacks focusing the attention on the critical phases of start-up, shutdown and load changes. The experimental facility is composed of a Turbec T100 micro gas turbine package modified for the fuel cell emulator connection, a set of pipes designed for by-pass, measurement or bleed reasons, and a high temperature volume designed for the RRFCS stack dimension physical emulation. This experimental approach is essential for model validations, and to test different transient operative procedures and control systems without any risk for an expensive real fuel cell stack. This paper shows the preliminary experimental data obtained with the machine in stand alone configuration, focusing the attention on the comparison of these results with the tests performed with the external pipes. Furthermore, a theoretical transient model of this new experimental facility has been developed with the TRANSEO tool. It is essential for the rig design and to perform preliminary results necessary to prevent dangerous conditions during the tests. This paper reports a preliminary verification of this model performed with the facility.Copyright

Pressurized SOFC Hybrid Systems: Control System Study and Experimental Verification

In this paper, two different advanced control approaches for a pressurized solid oxide fuel cell (SOFC) hybrid system are investigated and compared against traditional proportional integral derivative (PID). Both advanced control methods use model predictive control (MPC): in the first case, the MPC has direct access to the plant manipulated variables, in the second case, the MPC operates on the setpoints of PIDs which control the plant. In the second approach, the idea is to use MPC at the highest level of the plant control system to optimize the performance of bottoming PIDs, retaining system stability and operator confidence. Two MIMO (multi-input multi-output) controllers were obtained: fuel cell power and cathode inlet temperature are the controlled variables; fuel cell bypass flow, current and fuel mass flow rate (the utilization factor kept constant) are the manipulated variables. The two advanced control methods were tested and compared against the conventional PID approach using a SOFC hybrid system model. Then, the MPC controller was implemented in the hybrid system emulator test rig developed by the thermochemical power group (TPG) at the University of Genoa. Experimental tests were carried out to compare MPC against classic PID method: load following tests were carried out. Ramping the fuel cell load from 100% to 80% and back, keeping constant the target of the cathode inlet temperature, the MPC controller was able to reduce the mismatch between the actual and the target values of the cathode inlet temperature from 7 K maximum of the PID controller to 3 K maximum, showing more stable behavior in general.

Control System for Solid Oxide Fuel Cell Hybrid Systems

The aim of this work is the development and testing of a control system for solid oxide fuel cell hybrid systems through dynamic simulation. Because of the complexity of these cycles, several parameters, such as the turbine rotational speed, the temperatures within the fuel cell, the differential pressure between the anodic and the cathodic side, the Steam-To-Carbon Ratio, need to be monitored and kept within safe limits. On the other hand, the system response to load variations is required to be as quick as possible in order to meet the energy demand. The plant component models and their integration were carried out in previous works. This paper focuses on the control strategy to regulate the net electrical power from the hybrid system, avoiding malfunctions or damage. Once the control system had been developed, its performance was evaluated simulating the transient behavior of the whole hybrid cycle: the results for several operating conditions are presented and discussed.Copyright

Transient Analysis of Solid Oxide Fuel Cell Hybrids: Part B — Anode Recirculation Model

The aim of this work is the transient analysis of hybrid systems based on high-temperature Solid Oxide Fuel Cells (SOFC). The cell models were presented and discussed in Part A of this work. In this part attention is focused on the anode recirculation system. In a SOFC hybrid system it is necessary to recirculate part of the exhaust gas in order to maintain a proper value for the Steam-To-Carbon Ratio and to support the reforming reactions. This is carried out with an ejector, which exploits the pressure energy of the fuel to recirculate part of the anodic exhausts to fuel cell anodic side. Initially, a “dynamic” stand-alone ejector model is presented and validated for the analysis of unsteady flows. Particular attention was paid to the effect of time variation in the mixture composition, creating a general model for the unsteady simulation of flows with variable composition. To analyze the whole anodic circuit the “dynamic” model was simplified to the “lumped volume” model, which, even if it cannot properly analyze supersonic flows and shock waves, considerably reduces calculation time. So, it is suitable for transient system simulations, generally longer than a few minutes. The “lumped volume” model has been tested with the “dynamic” model and it has been used for the anodic recirculation system time-dependent simulations.Copyright