1,721,068 research outputs found
Advanced control approach for hybrid systems based on solid oxide fuel cells
No full textThis paper shows a new advanced control approach for operations in hybrid systems equipped with solid oxide fuel cell technology. This new tool, which combines feed-forward and standard proportional–integral techniques, controls the system during load changes avoiding failures and stress conditions detrimental to component life. This approach was selected to combine simplicity and good control performance. Moreover, the new approach presented in this paper eliminates the need for mass flow rate meters and other expensive probes, as usually required for a commercial plant. Compared to previous works, better performance is achieved in controlling fuel cell temperature (maximum gradient significantly lower than 3 K/min), reducing the pressure gap between cathode and anode sides (at least a 30% decrease during transient operations), and generating a higher safe margin (at least a 10% increase) for the Steam-to-Carbon Ratio. This new control system was developed and optimized using a hybrid system transient model implemented, validated and tested within previous works. The plant, comprising the coupling of a tubular solid oxide fuel cell stack with a microturbine, is equipped with a bypass valve able to connect the compressor outlet with the turbine inlet duct for rotational speed control. Following model development and tuning activities, several operative conditions were considered to show the new control system increased performance compared to previous tools (the same hybrid system model was used with the new control approach). Special attention was devoted to electrical load steps and ramps considering significant changes in ambient conditions
Comparison Between Uncontrolled and Controlled Solid Oxide Fuel Cell Hybrid Systems
No full textBecause of the high complexity of Solid Oxide Fuel Cell hybrid systems, a transient analysis is mandatory to implement a control system able to maintain safe operation during disturbances or regular operational load variations. In fact, several parameters, such as the turbine rotational speed, the surge margin, the temperatures within the fuel cell, the turbine inlet temperature, the differential pressure between the anodic and the cathodic side and 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. To develop a control strategy for these cycles, the work starts from the implementation of a transient model necessary to simulate a hybrid system based on the tubular SOFC technology. In fact, the first goal of this work is the analysis of the response to a step decrease in the fuel mass flow rate of the uncontrolled system, focusing the attention on the time scales of the transient phenomena and discussing the results from electrochemical, fluid dynamic and thermal point of view. The simulation shows that while the cathodic side is driven only by the temperature variation, because of the rotational speed is assumed to be constant, the anodic side is characterized by three different time-scale phenomena. In fact, all the plant properties show a negligible fluid dynamic delay, a depressurization time delay and a thermal long time-scale effect mainly due to the high thermal inertia of the cell. The considerations, carried out with the uncontrolled system, are used in the second part of the work to develop a control strategy to follow the power demand over time avoiding malfunctions or risk situations. The paper focuses the attention on a detailed presentation of the control system layout based on the by-pass valve between the compressor outlet and the turbine inlet, necessary to overcome the difficulty due to the difference between the small mechanical inertia of the microturbine shaft and the very high thermal inertia of the fuel cell stack. The simulations, carried out with a load step decrease, show the transient behaviour of the controlled SOFC hybrid system, presenting, over time, the values of the main critical parameters. Finally, the paper presents the results obtained with a power step increase to investigate the limitations of this control strategy focusing the attention on the fuel cell average temperature and the fuel utilization factor
Cathode–anode side interaction in SOFC hybrid systems
No full textCathode-anode interaction, mainly based on cathode versus anode volume influence, recirculation performance, and turbomachinery integration, is an important issue for pressurised SOFC hybrid systems, and this aspect must be carefully considered to prevent fuel cell ceramic material failures through a reliable control system. Over the last 10 years, several theoretical analyses of this issue have been carried out at the University of Genoa. These interaction studies have been analysed and an experimental approach (for model validation, system development and prototype design activities) has been applied using emulator facilities or real plants. In particular, general hybrid system layouts based on the coupling of pressurized SOFC stacks of different geometries (planar, tubular, etc.) with a gas turbine bottoming cycle have been investigated using the hybrid system emulator facility of the University of Genoa. The experimental results are focused on the interaction between gas turbine and anodic circuit and on cathode-anode differential pressure behaviour for design, off-design and transient hybrid system operative conditions. The information obtained in these tests is essential to understand the main features of the variables that drive the phenomena and to design a suitable control system that can mitigate the differential pressure values during all plant operating conditions
Solid oxide fuel cell hybrid system: Control strategy for stand-alone configurations
No full textThe aim of this study is the development and testing of a control system for solidoxidefuelcellhybrid systems through dynamic simulations. Due to the complexity of these cycles, several parameters, such as the turbine rotational speed, the temperatures within the fuelcell, the differential pressure between the anodic and the cathodic side and the Steam-To-Carbon Ratio need to be monitored and kept within safe limits. Furthermore, in stand-alone conditions the system response to load variations is required to meet the global plant power demand at any time, supporting global load variations and avoiding dangerous or unstable conditions. The plant component models and their integration were carried out in previous studies. This paper focuses on the controlstrategy required for managing the net electrical power from the system, avoiding malfunctions or damage. Once the control system was developed and tuned, its performance was evaluated by simulating the transient behaviour of the whole hybrid cycle: the results for several operating conditions are presented and discussed
Flexible Micro Gas Turbine Rig for Tests on Advanced Energy Systems
No full textThe Thermochemical Power Group (TPG) of the University of Genoa, Italy, has developed a new flexible laboratory to study advanced energy systems based on micro gas turbine technology. In the laboratory a general-purpose experimental rig, based on a modified commercial 100 kW recuperated micro gas turbine, was installed and fully instrumented.
The main objectives of the laboratory is to perform experimental activities related to gas turbine based cycles in both steady-state and transient conditions. The rig layout was defined to include the effects of interaction between the turbomachines (especially the compressor) and further components. This approach is extremely significant for innovative cycle analyses, such as recuperated, humid air, and hybrid (with high temperature fuel cells) configurations.
The facility was partially funded by two Integrated Projects of the EU VI Framework Program (Felicitas and Large-SOFC) and the Italian Government (PRIN project). It was designed with a high flexibility approach including: flow control management, co-generative applications, downstream compressor volume variation, grid-connected or stand-alone operations, recuperated or simple cycles, and room temperature control. In the new EU VII Framework (E-HUB Project), the test rig has been improved with the installation of an absorption cooler to operate the system in tri-generative configuration.
The layout of the whole system, including connection pipes, valves, and instrumentation (in particular mass flow meter locations) was carefully designed to measure all the significant properties with high accuracy performance. Particular attention was devoted to component design, using CFD tools (Fluent), to perform emulation tests on high temperature fuel cell hybrid systems. For this reason, the facility was equipped with a modular cathodic vessel, an anodic recirculation loop (including a vessel and an ejector), and a steam injection system for chemical composition emulation.
To compare tests affected by a significant influence of the ambient temperature variation, such as the performance tests on the machine maximum electrical power and electrical efficiency or on the recuperator effectiveness, the rig was integrated with a compressor inlet temperature control system. This equipment is composed of three air/water heat exchangers located at the air intake, controlled valves and a variable speed pump operating in a closed loop. This circuit was designed to couple the machine air inlet with the absorption cooler.
The large number of experimental data available for the high flexibility test rig design is also used to validate both steady-state (design and off-design) and transient (also real-time) theoretical models. A good level of consistency can be achieved thanks to the complete knowledge of the test rig dimensions, volumes, masses, shaft inertia, thermal capacitances, and operating procedure. Such completeness is difficult to obtain in industrial plants, where details about equipment are often missing or confidential.
This facility is also essential to introduce undergraduate students to micro gas turbine technology, and Ph.D.s to advanced experimental activities in the same field. With this experimental rig, in addition to learning about the thermodynamic cycles and plant layouts, students can also become familiar with their materials, piping, gaskets, technology for auxiliaries, and instrumentation
Transient Analysis of Solid Oxide Fuel Cell Hybrids. Part B: Anode Recirculation Model
No full textThe 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
Laboratory Tests in Cyber-Physical Mode for an Energy Management System Including Renewable Sources and Industrial Symbiosis
No full textThe aim of this paper regards the laboratory validation of an energy management system (EMS) for an industrial site on the Eigerøy island (Norway). It will be the demonstration district in the ROBINSON project, for a consequent concept replication. This activity in cyber-physical mode is an innovative approach to finalize the EMS tool with real measurement data with prime movers available at laboratory level, considering the necessary EMS robustness and flexibility for replication on other industrial islands. This EMS was designed and developed to minimize variable costs, producing on/off and set-point signals that, through a Model Predictive Control (MPC) software, establish the system status. This smart grid includes renewable sources (e.g., solar panels, a wind turbine, and syngas) and traditional prime movers, such as a steam boiler for the industry needs. Moreover, an energy storage device is installed composed of an electrolyzer with a hydrogen pressure vessel.
The main results reported in this work regard 26-hour tests performed in cyber-physical mode thanks to the real-time interaction of hardware and software. So, a real microturbine and real photovoltaic panels were managed by the EMS in conjunction with software models for components not physically present in the laboratory. Although the optimization target was cost minimization, significant improvement was also obtained in terms of efficiency increase and CO2 emission decrease
Emulatore celle a combustibile: controllo dinamico temperature e pressione stack, monitoraggio remoto impianto ibrido con microturbina
No full textQuesto articolo presenta l'utilizzo di NI Compact FieldPoint e LabVlEW per il monitoraggio ed il controllo di un impianto emulatore di sistemi ibridi con microturbina a gas recuperata (Turbec T100) da 100 kW elettrici, accoppiata con un volume modulare di 4 m3 per l'emulazione di sisterni ibridi con cella a combustibile ad alta temperatura. L'impianto sperimentale, installato presso il laboratorio dell'Università di Genova, DIMSET-TPG, a Savona è il primo esempio in Europa ed il secondo al mondo del suo tipo, in quanto si propone di studiare sperimentalmente l'accoppiamento di una microturbina a gas con un volume modulare, al fine di verificare il funzionamento della macchina e del suo controllo in condizioni di forte off-design tipiche di tali sistemi ibridi e durante i transitori
Ejector Model for High Temperature Fuel Cell Hybrid Systems: Experimental Validation at Steady-State and Dynamic Conditions
No full textThe aim of this work is the experimental validation of a steady-state and transient ejector model for high temperature fuel cell hybrid system applications. This is a mandatory step in performing the steady state and the transient analysis of the whole plant to avoid critical situations and to develop the control system. The anodic recirculation test rig, developed at TPG-University of Genoa, and already used in previous works to validate the ejector design models (0D and computational fluid dynamics), was modified and used to perform tests at transient conditions with the aim of ejector transient model validation. This ejector model, based on a “lumped volume” technique, has been successfully validated against experimental data at steady-state and transient conditions using air or CO2 at room temperature and at 150°C in the secondary duct inlet. Then, the ejector model was integrated with the models of the connecting pipes, and with the volume simulation tool, equipped with an outlet valve, in order to generate an anodic recirculation model. Also in this case, the theoretical results were successfully compared with the experimental data obtained with the test rig. The final part of the paper is devoted to the results obtained with square wave functions generated in the ejector primary pressure. To study the effects of possible fast pressure variations in the fuel line (ejector primary line), the test rig was equipped with a servo-controlled valve upstream of the ejector primary duct to generate different frequency pressure oscillations. The results calculated with the recirculation model at these conditions were successfully compared with the experimental data too
Smart polygeneration grids: experimental performance curves of different prime movers
No full textThis paper shows the performance curves obtained with an experimental campaign on the following different prime movers: a 100 kW microturbine, a 20 kWinternal combustion engine, a 450 kW SOFC-based hybrid system and a 100 kW absorption chiller. While the size related to the microturbine and the engine are actual electrical power values, the hybrid system size is an electrical virtual power (an emulator rig was used for this plant) and the chiller value is a cooling thermal power. These experimental results were obtained with a smart polygeneration facility installed in the Innovative Energy Systems Laboratory by the Thermochemical Power Group of the University of Genoa. This facility was designed to perform tests on smart grids equipped with different generation technologies to develop and improve innovative control and optimization tools. The performance curves were obtained with two different approaches: tests on real prime movers (for the microturbine, the engine and the chiller) or measurements on an emulator rig (for the hybrid system). In this second case, the tests were carried out using an experimental facility based on the coupling of a second microturbine with a modular vessel. A real-time simulation software was used for components not physically present in the experimental plant. These results are a significant improvement in comparison with the available data, because experimental results are presented for different prime movers in different operative conditions (both design and part-load operations). Moreover, since both manufacturers and users are not usually able to control air inlet temperature, special attention was devoted to the ambient temperature impact on the 100 kW microturbine because this property has a strong influence on the performance of this machine. For this reason, empirical correlations on the ambient temperature effect were obtained from the experiments with the objective to perform an easy implementation of the optimization tools. Experimental performance curves (including several off-design conditions) are essential for smart grid management because (if they are implemented in optimization tools) they allow to find real optimal solutions (while tools based on linear or calculated correlations can obtain results affected by significant errors)
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