1,721,006 research outputs found
Surge Prevention Techniques for a Turbocharged Solid Oxide Fuel Cell Hybrid System
No full textPressurized solid oxide fuel cell (SOFC) systems are one of the most promising technologies to achieve high energy conversion efficiencies and reduce pollutant emissions. The most common solution for pressurization is the integration with a micro gas turbine, a device capable of exploiting the residual energy of the exhaust gas to compress the fuel cell air intake and, at the same time, generating additional electrical power. The focus of this study is on an alternative layout, based on an automotive turbocharger, which has been more recently considered by the research community to improve cost effectiveness at small size (<100 kW), despite reducing slightly the top achievable performance.
Such turbocharged SOFC system poses two main challenges. On one side, the absence of an electrical generator does not allow the direct control of the rotational speed, which is determined by the power balance between turbine and compressor. On the other side, the presence of a large volume between compressor and turbine, due to the fuel cell stack, alters the dynamic behavior of the turbocharger during transients, increasing the risk of compressor surge. The pressure oscillations associated with such event are particularly detrimental for the system, because they could easily damage the materials of the fuel cells.
The aim of this paper is to investigate different techniques to drive the operative point of the compressor far from the surge condition when needed, reducing the risks related to transients and increasing its reliability. By means of a system dynamic model, developed using the TRANSEO simulation tool by TPG, the effect of different anti-surge solutions is simulated: (i) intake air conditioning, (ii) water spray at compressor inlet, (iii) air bleed and recirculation, and (iv) installation of an ejector at the compressor intake. The pressurized fuel cell system is simulated with two different control strategies, i.e. constant fuel mass flow and constant turbine inlet temperature. Different solutions are evaluated based on surge margin behavior, both in the short and long terms, but also monitoring other relevant physical quantities of the system, such as compressor pressure ratio and turbocharger rotational speed
Emulation tests of dynamics and control for a turbocharged SOFC system
No full textThis work regards experimental emulation activities for a Solid Oxide Fuel Cell (SOFC) pressurized by a turbocharger, focusing attention on the control system validation. The SOFC-based plant considered here was developed to couple high efficiency (due to pressurization) with reasonable capital costs. In detail, significant cost decrease (against SOFC hybrid plants including a microturbine) can be obtained with a turbocharger, due to large mass manufacturing process for these machines. Moreover, to pursue the zero emission target, the system was sized to operate with a renewable source fuel (biogas). Since this system has integration problems due to critical dynamic and control aspects, the University of Genoa designed and installed an experimental facility based on the coupling between a pressure vessel with a commercial turbocharger. The fuel cell is emulated equipping the vessel with a burner (to obtain the SOFC temperature range) and with inert ceramic material (to generate the same dynamic response). The tests presented in this work were obtained with this emulation rig operated in cyber-physical mode: the hardware interacted in real-time mode with previously validated software for components not physically included in the rig. The results demonstrated the system feasibility for load changes and validated the proposed control system, showing robustness and good prevention of critical conditions, such as SOFC thermal stress
Robust design of a fuel cell - Turbocharger hybrid system
No full textPressurized solid oxide fuel cell systems are a particularly attractive conversion technology for their high electric efficiency, potential for cogeneration applications, low carbon emissions and high performance at part-load. In this work an innovative biofueled hybrid system is considered, where the fuel cell stack is pressurized with a turbocharger, resulting in a system with improved cost effectiveness than a microturbine-based one at small scales. In a previous work, a detailed steady state model of the system, featuring components validated with industrial data, was developed to simulate the system and analyze its behavior in different conditions. The results obtained from this model were used to create response surfaces capable of evaluating the impact of the main operating parameters (fuel cell area, stack current density and recuperator surface) on the performance and the profitability of the plant considering system uncertainties. In this paper, similar but extended response surfaces will be used to perform a multi-objective optimization of the system considering the capital costs of the plant and the net power produced as objectives (turbocharger is fixed in geometry). The impact of the energy market scenario on the optimal design of such a system will be investigated considering its installation in three different countries. Finally, the Pareto front produced by optimization will be used to evaluate the robustness of the top performance solutions
Robust Design of a Fuel Cell - Turbocharger Hybrid System
No full textPressurized solid oxide fuel cell (SOFC) systems are a particularly attractive conversion technology for their high electric efficiency, potential for cogeneration applications, low carbon emissions, and high performance at part-load. In this work, an innovative biofueled hybrid system is considered, where the fuel cell stack is pressurized with a turbocharger, resulting in a system with improved cost effectiveness than a microturbine-based one at small scales. In a previous work, a detailed steady-state model of the system, featuring components validated with industrial data, was developed to simulate the system and analyze its behavior in different conditions. The results obtained from this model were used to create response surfaces capable of evaluating the impact of the main operating parameters (fuel cell area, stack current density, and recuperator (REC) surface) on the performance and the profitability of the plant considering system uncertainties. In this paper, similar but extended response surfaces will be used to perform a multi-objective optimization of the system considering the capital costs of the plant and the net power produced as objectives (turbocharger is fixed in geometry). The impact of the energy market scenario on the optimal design of such a system will be investigated considering its installation in three different countries. Finally, the Pareto front produced by optimization will be used to evaluate the robustness of the top performance solutions
Dynamic Effect of Cold-Air Bypass Valve for Compressor Surge Recovery and Prevention in Fuel Cell Gas Turbine Hybrid Systems
No full textA large volume between compressor and turbine is present in fuel cell gas turbine hybrid systems. The substantially larger compressor plenum volume modifies the dynamic behaviour of these systems, increasing the risk of compressor surge during transients and subsequent destruction of both turbomachinery and fuel cell components. Diverting part of the compressor inlet flow directly to the turbine inlet through a cold-air bypass valve, bypassing the fuel cell stack, has been proven to be an effective method to increase the surge margin during normal operation and also to recover the machine from fully developed surge. This study investigates the dynamic effect of different cold-air bypass valve opening/closing procedures, especially steps and ramps changing the valve fractional opening. This analysis was carried out with reference to the Hybrid Performance (Hyper) facility: a hybrid system emulated using hardware and a cyber-physical fuel cell system at the National Energy Technology Laboratory (NETL), U.S. Department of Energy (DOE). Simulations performed on a Matlab®-Simulink® dynamic model of the system based on Greitzer's theory showed a different behaviour varying the valve fractional opening with steps or ramps. Many experimental tests were performed on the Hyper facility to confirm the trends obtained from the simulations results. From the outcomes of this study, it has been possible to determine how to maximize the surge recovery effect of the cold-air bypass valve opening and to minimize surge related risks during the valve closure
Uncertainty quantification analysis of a pressurised fuel cell hybrid system
No full textPressurised solid oxide fuel cell (SOFC) systems are a sustainable opportunity for improvement over conventional systems, featuring high electric efficiency, potential for cogeneration applications and low carbon emissions. Such systems are usually analyzed in deterministic conditions. However, it is widely demonstrated that such systems are affected significantly by uncertainties, both in component performance and operating parameters. This paper aims to study the propagation of uncertainties related both to the fuel cell (ohmic losses, anode ejector diameter and fuel gas composition) and the gas turbine cycle characteristics (compressor and turbine efficiencies, recuperator pressure losses). The analysis is carried out on an innovative hybrid system layout, where a turbocharger is used to pressurise the fuel cell, promising better cost effectiveness then a microturbine-based hybrid system, at small scales. Due to plant complexity and high computational effort required by uncertainty quantification methodologies, a response surface is created. To evaluate the impact of the aforementioned uncertainties on the relevant system outputs, such as overall efficiency and net electrical power, the Monte Carlo method is applied to the response surface. Particular attention is focused on the impact of uncertainties on the opening of the turbocharger wastegate valve, which is aimed at satisfying the fuel cell constraints at each operating condition
Uncertainty Quantification Analysis of a Pressurized Fuel Cell Hybrid System
No full textPressurized solid oxide fuel cell (SOFC) systems are a sustainable opportunity for improvement over conventional systems, featuring high electric efficiency, potential for cogeneration applications, and low carbon emissions. Such systems are usually analyzed in deterministic conditions. However, it is widely demonstrated that such systems are affected significantly by uncertainties, both in component performance and operating parameters. This paper aims to study the propagation of uncertainties related both to the fuel cell (ohmic losses, anode ejector diameter, and fuel gas composition) and the gas turbine cycle characteristics (compressor and turbine efficiencies, recuperator pressure losses). The analysis is carried out on an innovative hybrid system layout, where a turbocharger is used to pressurize the fuel cell, promising better cost effectiveness then a microturbine-based hybrid system, at small scales. Due to plant complexity and high computational effort required by uncertainty quantification methodologies, a response surface (RS) is created. To evaluate the impact of the aforementioned uncertainties on the relevant system outputs, such as overall efficiency and net electrical power, the Monte Carlo method is applied to the RS. Particular attention is focused on the impact of uncertainties on the opening of the turbocharger wastegate valve, which is aimed at satisfying the fuel cell constraints at each operating condition
MODELLING AND CONTROL SYSTEM DEVELOPMENT OF A TURBOCHARGED PROTON-EXCHANGE MEMBRANE FUEL CELL SYSTEM
No full textPressurized Polymeric Electrolyte Membrane Fuel Cell (PEMFC) systems are growing among electrical power generation systems, as a zero-emissions technology. In this framework, the University of Genoa have been modelling an innovative concept for the application of low-temperature fuel cells. A turbocharged proton exchange membrane fuel cell (TC-PEMFC) plant layout with design output power higher than 200 kW is investigated. Pressurizing air before entering the stack can increase the stack efficiency and recovering the exhaust air within a turbocharger leads to better system performance. A crossflow flat pack membrane humidifier has been introduced between the air turbocharger and the PEMFC to guarantee proper operating condition into the stack. Finally, a Gas-to-Gas (GtG) heat exchanger enables cooling of the charging air while recovering the thermal energy through the turbine. In this paper, the authors present a dynamic model of an innovative turbocharged PEMFC system developed in Matlab/Simulink environment. The “RealTime” library, built by the University of Genoa, has been adopted: most of the models have been previously validated and a few new models have been customized for this application. The Matlab/Simulink model shown in this paper is used to aid the design process of the future plant, to evaluate the performance of the plant in many operating conditions and to design and test its control system. At the current stage, the model is ready to be used for on-design, off-design and transient analyses. It can be used to investigate the effect of different design choices, as well as to determine the specifications of the plant components. Some preliminary steady-state simulations are presented: the performance of the system in a wide operative range (cell currents between 150 A and 500 A), with net power higher than 300 kW and system efficiency up to 58%
Dynamic Performance and Control Analysis of a Supercritical CO2 Recuperated Cycle
No full textSupercritical CO2 (sCO2) power cycles represent a promising technology for driving the energy transition. In fact, various research projects around the world are currently studying the possible applications of this technology, which is characterized by high efficiency, competitive costs, compact machinery, and enhanced flexibility with respect to competing systems, such as steam-based power cycles. Within this context, the European Union (EU)-funded SOLARSCO2OL project aims to build a MW-scale sCO2 pilot facility coupled with a concentrated solar power (CSP) plant. A transient model of the demonstration plant was previously developed in the transeo simulation tool by the Thermochemical Power Group (TPG) of University of Genoa to study the operational envelope of the cycle. In the present work, the model is upgraded to take into account all the relevant fluid-dynamic and thermodynamic phenomena affecting the transient behavior of the plant. In particular, a detailed crossflow sCO2-air cooler model is now included, which is crucial for assessing the compressor inlet temperature (CIT) behavior and controllability. The system has to comply with several constraints, such as compressor surge margin, turbomachinery inlet temperatures, and compressor inlet pressure (CIP). The desired net power output should also be guaranteed. The dynamic responses of the system to step variations in various input variables were recorded and used to design and tune the main operational controls. The input variables considered include: (1) compressor rotational speed, (2) anti-surge valve (ASV) fractional opening, (3) mass flowrate of air through the cooler, (4) mass flowrate of the molten salts through the heater, and (5) CO2 inventory for injection and extraction of working fluid. The implemented control structure includes proportional-integral-derivative (PID) controllers, feedforward action, and their combinations. The controllers are tuned using a mix of established methods, such as Cohen-Coon response-based PID tuning and adjustments from feedforward controls. The feedforward controls were designed taking into account the steady-state values from off-design simulations, as well as the interactions between each controller and the other controlled variables. The final control setup is tested on various power ramps to assess the capability of the prototype cycle in load following and disturbance rejection, showing very good performance in set-point tracking
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