Early start-up of solid oxide fuel cell hybrid systems with ejector cathodic recirculation: experimental results and model verification

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1 627 Early start-up of solid oxide fuel cell hybrid systems with ejector cathodic recirculation: experimental results and model verification M L Ferrari, A Traverso, M Pascenti, and A F Massardo Thermochemical Power Group (TPG), Dipartimento di Macchine, Sistemi Energetici e Trasporti (DiMSET), Università di Genova, Italy The manuscript was received on 13 March 2007 and was accepted after revision for publication on 19 April DOI: / JPE438 Abstract: A new test rig, founded in part by Rolls-Royce Fuel Cell Systems Limited, by the EU integrated project FELICITAS, and by the national project PRIN-2005, has been designed and built at the University of Genova to study different early start-up layouts and strategies for solid oxide fuel cell (SOFC) hybrid systems, using ejector-based cathodic recirculation. The test rig, which is flexible enough to analyse different emulated turbomachine layout (single shaft, two shafts, regenerated cycles or not, and so forth), different ejector configurations, and different stack arrangements, is unique. It also allows the early start-up performance to be fully investigated with particular attention to the analysis of the hybrid system fluid dynamic behaviour when the stack is off and the control system operates at critical conditions. The experiments have been carried out using a patented start-up combustor emulator (outlet temperature higher than 950 C), and monitoring the pressure and temperature quick rise to prevent mechanical and thermal stress to the fuel cell stack and to maintain turbomachine stability. The experimental results have also been used to verify the time-dependent hybrid system models based on TPG-TRANSEO tool. The results show good agreement with experimental data. This is essential to increase confidence with predictive time-dependent simulation tools during the start-up phase, where experimental data are hardly available and fuel cell materials may operate under risky conditions. Keywords: hybrid system, fuel cell, cathodic recirculation, emulator, early start-up, experimental validation 1 INTRODUCTION Pressurized hybrid systems based on the coupling of micro gas turbines with high temperature fuel cells, both molten carbonate fuel cells [1] and solid oxide fuel Cells (SOFCs) [2, 3], are expected to play an important role in the high efficiency distributed power generation market in the near future. The main reasons that make these systems attractive are: (a) a very high Corresponding author: Thermochemical Power Group (TPG), Dipartimento di Macchine, Sistemi Energetici e Trasporti (DiM- SET), University of Genova, via Montallegro 1, Genova 16145, Italy. mario.ferrari@unige.it JPE438 IMechE 2007 efficiency (over 60 per cent as a mid-term perspective) even in small size plants, (b) ultra low emissions, and (c) exhaust gases at high temperature conditions which is useful for co-generative applications. However, fuel cell hybrid systems are not ready for wide commercialization because of: (a) their too high specific costs at component level (i.e. the fuel cell), (b) technological problems that are not completely solved mainly related to the integration of fuel cell system with turbomachinery technology, and (c) the long time (hours) required for start-up and shut-down. Furthermore, since hybrid systems use risky components [2, 4, 5], such as the stack and the rotating machine, wide and detailed theoretical analyses are mandatory to better investigate critical properties and Proc. IMechE Vol. 221 Part A: J. Power and Energy

2 628 M L Ferrari, A Traverso, M Pascenti, and A F Massardo to prevent malfunctions or damage during the main critical operating conditions, that are: start-up, idle operation (i.e. zero net power), load changes, load rejection, and shut-down. Even if the hybrid system transient behaviour and safe management are currently under investigation mainly through theoretical approaches [1, 3, 5], the experimental support is essential for both model verifications and critical phase management. A practical solution to avoid very expensive rigs comes from the development of simplified experimental plants, named emulators. They are able to generate the same effects of a real system, in terms of pressures, temperatures, and mass flow rates, without the fuel cell stack and sometimes without turbomachines. This new experimental approach, that generates reliable results as a complete test rig, also allows to investigate high-risk situations with more flexibility without serious and expensive consequences to the equipment and at very low cost compared to real hybrid configurations. Studying such operating conditions and gaining practical experience on hybrid system emulators is a good strategy to develop the necessary skills for the proper operation of real hybrid systems as well as minimizing the research and development costs. An important activity with emulators is under development at National Energy Technology Laboratory (NETL) of Morgantown (WV), where an innovative physical simulator of SOFC hybrid system cathode side (turbomachine, recuperators, fuel cell, and off-gas burner vessel) was built up [4, 6] and is operating in order to examine fundamental issues related to component integration and control strategies. The absence of an actual fuel cell stack allowed the system to run into surge several times without causing serious (and very expensive) damages to the equipment. However, at the present time, a limitation of such test rig is the absence of thermal capacitance of the fuel cell stack. This problem was bypassed with the use of a real-time SOFC model [4]. Similar experimental activity is under development at the Thermochemical Power Group (TPG) laboratory of the University of Genova [7] where: (a) a hybrid system emulator based on a recuperated modified micro gas turbine (Turbec T100) is under construction in the frame of the EU integrated project Felicitas [7]; (b) a simplified physical simulator of a hybrid system with ejector cathodic recirculation (Fig. 1) with the aim to study the hybrid system early start-up phase has been designed and installed. In this paper, the second rig is presented and analysed in depth: the rig is fully emulative since it operates without the real turbomachinery, whose whole effect Fig. 1 RRFCS 250 kw generator module [2] (in bold green the section emulated with the rig) is obtained with a turbomachinery patented emulator [8]. The rig also incorporates an original start-up combustor emulator constituted by a patented system using a ceramic electrical heater [9], and a cathode recirculation based on a single-stage ejector [2]. The emulated system in the presented configuration is similar to the Rolls-Royce Fuel Cell Systems Limited (RRFCS) hybrid system under development. Since the main goal of this work is the study of the early start-up, the cell electrochemical reactions are not required because, during this phase, this component is off. Hence, it is emulated with a pressurized vessel not filled with active ceramics (real cell support) to generate the apt volume and thermal capacitance. Summarizing, the main objectives of this work are as follows: (a) design and development of the start-up test rig; (b) the experimental evaluation of mass flow rates, temperatures, and pressures in all the hybrid system points during the early start-up phase, with particular attention paid to recirculation devices; (c) the layout performance experimental evaluation during the early start-up; (d) the theoretical simulation of complex transient phenomena under operative conditions that are very different from design point; (e) the model experimental verification to produce a reliable simulation tool for further start-up and control configuration analysis and optimization. 2 START-UP OF FUEL CELL HYBRID SYSTEMS The start-up of a hybrid system is a critical phase which can last several hours, mainly depending on the acceptable average temperature ramp for the fuel cell stack. Assuming an average operating temperature of an SOFC stack equal to 900 C, and a maximum temperature ramp of 2 K deg/min, the minimum start-up Proc. IMechE Vol. 221 Part A: J. Power and Energy JPE438 IMechE 2007

3 Early start-up of SOFC hybrid systems 629 time from cold conditions is about 7 h and a half. However, these types of system will require to start-up not more than 2 4 times a year. For this reason, only customers with continuous power demand over the year and over the hours can benefit from this technology. During such a significant period of start-up time, the main targets to be pursued are as follows. 1. Time to full power as short as possible. This is the main target of the overall start-up procedure. 2. Achieving turbomachinery self-sustaining conditions as fast as possible. This is desirable for minimizing the use of external energy sources, such as grid, battery, or compressed air. Moreover, once idle conditions are reached for the turbomachinery, useful power can start to be produced in order to feed all the hybrid system auxiliaries. 3. Reducing the fuel cell pressurization and preheating time without generating stress conditions. 4. Preserving system durability. Start-up phase, if performed too quickly, may affect the overall system durability by causing thermal fatigue to system components, in particular to stack ceramics. Hence, the proper conditions in terms of temperature ramp and turbomachinery safe operation (i.e. avoidance of surge) have to be ensured during all the start-up time. 5. High level of safety. Managing gaseous fuel mixtures at intermediate temperatures may cause the system to run under dangerous situations such as: (a) an excessive temperature or temperature gradient in the fuel cell, (b) too high pressure difference between the cathodic and the anodic sides, (c) too low steam-to-carbon-ratio value in the reformer, (d) too high microturbine rotational speed, (e) an operating condition too close to the compressor surge line (reduced surge margin), or (f) excessive thermal stress in the heat exchanger and the cell. From the layout perspective, start-up may require modifications to the hybrid system piping and configuration. The following highlights must be considered: (a) minimum number of hot valves; (b) minimum number of in-lined components (e.g. start-up combustor); (c) minimum amount of technical gases (e.g. bottles of H 2, steam generator, etc.). Examples of general hybrid layouts are reported in Figs 2 and 3: they show two alternative ways of carrying out the start-up phase. Figure 2 illustrates a possible configuration according to the philosophy of inlining the start-up combustor : one start-up combustor (1) is placed upstream from the fuel cell stack in order to provide the required temperature ramp. A second start-up combustor (2) is placed downstream the fuel JPE438 IMechE 2007 Fig. 2 Fig. 3 Start-up layout with inlined start-up combustors Start-up layout with the main start-up combustor on a bypass branch cell stack in order to quickly rise turbine inlet temperature (TIT) and achieve turbomachinery self-sustaining conditions. A bypass branch (3) can be included to allow turbomachinery-only operation, excluding the fuel cell stack. Such a layout allows hot valves to be avoided, with evident advantages in terms of cost and reliability. On the other hand, it causes permanent pressure drops on the cathode line, due to the presence of the two inlined combustors, which affect performance of the turbine cycle. In particular, for SOFC-based systems, the second one may have thermal stress problems due to the uncooled operation. Moreover, the second start-up combustor receives, during normal operation of the hybrid system, hot exhaust gases from the fuel cell, which can decrease its durability. Figure 3 shows the hybrid system layout following the philosophy of bypassing the start-up combustor : one start-up combustor is placed on a bypass branch and its exhaust gases are directed to the turbine. Such hot exhausts must not flow back through valve 3 into the fuel cell system, in order to avoid thermal shocks. As a result, a second combustor must be present within the fuel cell stack vessel in order to ensure fuel cell lighting-on conditions. This configuration avoids permanent pressure drops during normal operation due to inlined start-up combustors but, on the other hand, Proc. IMechE Vol. 221 Part A: J. Power and Energy

4 630 M L Ferrari, A Traverso, M Pascenti, and A F Massardo requires the use of one hot valve (3) at the fuel cell system outlet. Moreover, the heat source within the fuel cell may represent a serious issue in terms of safety. Once the start-up layout is decided, a possible procedure for starting-up the hybrid system can be outlined as follows: (a) conventional start-up of the recuperated microturbine cycle reaching idle condition; (b) turbine load ramp, in order to produce useful power from turbomachinery; (c) fuel cell vessel pressurization, if not already completed during the previous phases; (d) fuel cell heating up till to lighting-on conditions (fuel cell average temperature > 800 C); (e) fuel cell load ramp (e.g. 10 per cent maximum power step every 10 min). 3 TEST RIG DESCRIPTION To provide experimental insights into the initial part of the hybrid system start-up, a fully emulated test rig has been designed and built at University of Genoa: the test rig tries to retain high flexibility to operate in a simple way with several different start-up configurations, like those presented in Figs 2 and 3, or similar. The test rig is a physical simulator of a fuel cell hybrid system cathode side [4] equipped with a recirculation carried out by an ejector (Fig. 1). The anode loop start-up [10] has been already studied inside the PIP-SOFC European contract (NNE ) coordinated by RRFCS Ltd. The rig is scaled in order to consider a fraction of the total hybrid system mass flow rate. However, it has been designed in order to emulate the dynamics and performance of the actual plant, that is a 250 kw electrical power hybrid system. The main goal of the rig is the physical simulation of the real operative conditions during the initial phase of the start-up (first quarter of hour). The fuel cell system is chemically inactive (the fuel cell may be defined off ). It is already integrated with the turbine air flow path and, in particular, with the cathodic recirculation: the turbomachinery still needs to achieve self-sustaining conditions and the flows around the fuel cell system have to be carefully controlled in order to avoid thermal shocks or backflows (mainly difficulties to be controlled when ejector is used for cathodic recirculation). Since during the early start-up phase the fuel cell is off, in order to emulate the stack a thermal capacitance (a plenum filled with ceramics without the electrochemical reaction materials) has been adopted. Another important aspect is that the turbomachine is not actually present in the rig, but its behaviour and influence is simulated by a compressor turbine model [8] based on performance curves and rotational Fig. 4 Start-up test rig layout (see scheme in Fig. 3) inertia. In this presentation, a two shaft turbomachine has been considered since this machine is used in the RRFCS system [2]. The information pattern works out as follows: the rig transducers supply to the turbomachine model compressor outlet pressure, expander inlet pressure, and temperature; the turbomachine model supplies to the rig compressor outlet flow and temperature, and turbine exhaust mass flow [8]. As reported in Fig. 4, the compressor simulation is obtained with a mass flow meter-regulator (point 1) and with an electric heater (point 2) to achieve the right outlet temperature and pressure. Therefore, the rig presents a very high level of flexibility because it allows not only to change the start-up layout, but also to change the virtual turbine by simply changing the characteristic curves of the real-time model (and adapting the valves). The main flow can be divided into three paths, regulated by three mid-temperature automatic on/off valves, apt controlled. The first path passes through the start-up combustor (point 3), simulated by a ceramic electrical heater: its position allows to simulate the start-up combustor placed on a bypass branch or just upstream from the expander. Such electrical heater has a bypass circuit (point 4), that represents the second path: it is used to simulate the bypass branch without start-up combustors or with the start-up combustor off. The third path is directed to the cathodic ejector primary nozzle (point 6): this choice allows the rig to also simulate the start-up of hybrid systems that present such cathodic recirculation [2]. The test rig includes some physical emulators of distributed thermal and volume capacitance [11]. The plenum upstream from the cathodic primary nozzle (point 5) represents the capacitance between the compressor outlet and the ejector primary inlet; the plenum at the start-up combustor outlet (point 7) represents a possible mixing device (in case the cathodic recirculation is employed) and its capacitance; the ejector plenum (point 8) represents the capacitance between the ejector outlet and stack inlet; the fuel cell Proc. IMechE Vol. 221 Part A: J. Power and Energy JPE438 IMechE 2007

5 Early start-up of SOFC hybrid systems 631 plenum (point 9) represents the fuel cell stack and its capacitance. The flow exits the system through a servo controlled valve, that simulates the expander (point 10). No representation of anodic side is currently included in the test rig because the cell is off during the analysis of early start-up. The start-up combustor is formed by a stainless steel tube surrounded by two ceramic semi-shells in which two 3.5 kw electrical heaters (point 3) [8] are embedded. This system is thermally insulated with ceramic fibres and rock wool to reduce thermal losses. The internal configuration of such device allows the experiments to reach around 1000 C air outlet temperature, which is a good simulation of a real start-up combustor. This approach have been used since no actual fuel combustors could be employed for safety reasons at the laboratory. However, the real combustor dynamic behaviour is correctly simulated through the use of the bypass branch. In fact, the electrical heater is warmed up to the operative temperature before the test and the valves upstream the heater and the bypass line are used to obtain the real behaviour over time of the combustor outlet temperature [12]. Figure 5 shows the plenum simulating the fuel cell (point 2): such plenum was filled with about 200 kg of non-active ceramic material used to build RRFCS stacks, which provides a correct representation of thermal capacitance of the fuel cell stack. The rig is equipped with a data acquisition system constituted by a conventional PC equipped with the LabVIEW TM software, which interacts via local area network with the FieldPoint TM, which collects all the transducer signals and provides control outputs for the actuators. Figure 6 shows the final test rig layout, whose scheme, developed using the DSC LabVIEW TM module, is reported in Fig. 7 with the main transducers used to perform the tests. Fig. 5 Simulator of fuel cell stack thermal and volume capacitance (point 9 in Fig. 4) JPE438 IMechE 2007 Fig. 6 4 THE MODEL Test rig ready for operation The test rig transient model has been implemented using the TRANSEO tool [11, 12] developed at TPG using the MATLAB R -Simulink R environment. It is a validated visual, user-friendly, modular program, organized in an easy-access library, implemented for the off-design, transient, and dynamic analyses of advanced energy systems based on microturbine technology [5]. The models of the components (ejector [13], volumes, thermal capacitances, and pipes) have been validated through experimental measurements: one of the latest achievements is reported in references [11] and [14]. 5 EXPERIMENTAL RESULTS AND MODEL VERIFICATION Several start-up configurations were tested in order to prove the test rig capabilities and to gain operational experience and technical understating of the features of early start-up procedure. The results reported here refer to the full capability of the rig employing cathode recirculation. In the first start-up case (Fig. 4), the largest flow fraction passes through the start-up combustor (point 3), while the remaining flow rate feeds the cathodic ejector primary nozzle (point 6) downstream the plenum representing the capacitance of the compressed air duct (point 5). The flow through the start-up combustor is mixed in the mixing plenum (point 7) with the cold flow coming from the fuel cell (point 8) and partly recirculated by the ejector, to slowly heat up the fuel cell. The flow exits the system through the expander, represented by the servo controlled valve (point 10 in Fig. 4). The results presented here have been obtained with generic characteristic maps of the turbomachinery [12]. Such maps were used to obtain the inlet (compressor) and outlet (turbine) mass flow rates versus the compressor outlet pressure, necessary to perform the tests controlling the inlet and outlet valves. Although different tests have been carried out at different temperature and mass flow rate conditions [11], Proc. IMechE Vol. 221 Part A: J. Power and Energy

6 632 M L Ferrari, A Traverso, M Pascenti, and A F Massardo Fig. 7 Control panel in LabVIEW TM during operation this paper shows the main results during the first 15 min of start-up (from cold conditions) obtained at ambient temperature and with the start-up combustor temperature at 950 C. 5.1 Ambient temperature test This preliminary test was carried out without electrical heaters, in order to verify the operation of the rig and to perform an initial experimental verification of model prediction. It was assumed that an electrical motor would start the turbomachinery from the zero speed. The servo controlled valves emulate the compressor and turbine flows through a machine model [8]. Figure 8 shows the behaviour of compressor delivery pressure during an early start-up phase operated at ambient temperature. As expected, the initial pressure rise (about 1 bar in less than 1 min) could be quite stressing for the fuel cell and it should probably be minimized by reducing the power supplied to the microturbine alternator. After such initial pressure ramp, the increase in pressure becomes smoother and it basically follows the increase in rotational speed of the turbine (Fig. 9). The fuel cell volume dimension mainly determines the curve trend during the initial pressure rise: hence, lower cathode volumes would enhance the mechanical stress in this phase. Experimental and theoretical results are shown to be in good agreement in Figs 8 and 10. The 5 per cent difference between the experimental and calculated results (Fig. 10) in the initial part of the ejector differential pressure is in agreement with the ejector model performance at low mass flow rate values presented Fig. 8 Compressor outlet pressure: ambient temperature and 950 C tests Fig. 9 Rotational speed of both shafts referred to the design conditions [2, 7] Proc. IMechE Vol. 221 Part A: J. Power and Energy JPE438 IMechE 2007

7 Early start-up of SOFC hybrid systems 633 Fig. 10 Ambient temperature test: mass flow rates and ejector pressure rise results in previous works [14]. As a general comment, the good consistency was achieved mainly thanks to the complete knowledge about the test rig dimensions, volumes, masses, and operating procedure, unlike industrial plants, where details about equipment are often missing. Figure 10 shows the mass flow rates and the nondimensional pressure rise across the ejector. These properties match the experimental data with good accuracy showing errors in agreement with the measurement performance. Comparing the mass flow at compressor outlet and the mass flow through the ejector primary nozzle, it is evident that about half the compressed air, apart from storage phenomena, passes through the start-up combustor line (which is off in this test): this is also similar in the other tests and it is mainly due to the relatively high pressure drop through the ejector primary nozzle. It is important to highlight that the flow splitting between ejector primary duct and start-up combustor line is only driven by viscous pressure losses not by control valves. 5.2 Start-up combustor outlet at 950 C In the 950 C test, the start-up combustor, represented by the special electrical heater in Fig. 4, during the preparation time, is heated up to the temperature required; when the target temperature is achieved the test can start. Any temperature ramp of the start-up combustor can then be obtained by modulating the flows through the paths in points 3 and 4 of Fig. 3. In this test, the start-up combustor is assumed to ignite at time zero and the delivery temperature is stepped from ambient to the operative start-up combustor outlet temperature. By comparing this case, whose results are reported in Figs 8 and 11 to 13, with the previous one, a general JPE438 IMechE 2007 Fig. 11 Start-up combustor at 950 C: temperature results around the (inactive) fuel cell and ejector increase in temperatures is evident: this may not be desirable during this initial phase of start-up, even if a higher TIT could bring the turbomachinery to self-sustaining conditions in a short time. The system pressures increase strongly, in comparison with the ambient temperature test, due to the high temperature effect (Fig. 8). The temperature trends reported in Figs 11 and 12 show the effect of the high thermal Fig. 12 Start-up combustor at 950 C: temperature results at the mixing plenum (point 7 in Fig. 4) Proc. IMechE Vol. 221 Part A: J. Power and Energy

8 634 M L Ferrari, A Traverso, M Pascenti, and A F Massardo start-up, when higher temperatures and pressures are achieved. Future plans of development of the test rig could be the extension to long time start-up, possibly up to the first hour of the start-up procedure. This would permit to increase the understanding of the best configurations for starting the hybrid system up and would enhance the confidence with predictive dynamic simulation tools. Fig. 13 Start-up combustor at 950 C: mass flow and ejector pressure rise (non-dimensional) results capacity due to the ceramics inside the cell vessel. Although temperature increase is very low for the ejector and the fuel cell inlet, the SOFC average temperature is almost flat in this first start-up minutes and it increases with the allowed maximum gradient during the following hours not emulated with the rig yet (but theoretically evaluated through the use of model). Also for this test, the calculated results match the measured data with good accuracy completing the model verification approach. 6 CONCLUSIONS Start-up is a critical operating phase for hightemperature fuel cell hybrid systems, because several safety issues and practical targets have to be managed together in order to avoid thermal shocks to the fuel cell stack and to minimize the time to full load of the whole system. To study this critical phase, the following activities have been carried out. 1. A start-up test rig at TPG laboratory of the University of Genoa, Italy, has been designed and built to physically simulate different start-up layout and procedures for hybrid systems including cathodic recirculation. 2. The results have shown that the quick rise of pressure and temperature during the early phase of start-up, which could represent an issue for the stack durability and mechanical stress. 3. TheTRANSEO model of the test rig has been verified against the experimental results for temperature, pressure, and mass flow rate values. 4. Such a start-up test rig represents a first attempt to study, from both theoretical and practical points of view, the initial period of the early start-up of a hybrid system. Future developments of the test rig may include the extension to longer periods of ACKNOWLEDGEMENTS The authors wish to thank Dr G. Agnew of Rolls-Royce Fuel Cell Systems for the support to the activities described here. This work has been partly supported by EU-IP FELICITAS and by National project PRIN 2005 ( Theoretical and Experimental Analysis of Second Generation High Temperature Fuel Cell Hybrid Systems with Very High Performance ), national coordinator A F Massardo. REFERENCES 1 Roberts, R. A., Brouwer, J., Liese, E., and Gemmen, R. S. Development of controls for dynamic operation of carbonate fuel cell-gas turbine hybrid systems. ASME paper GT , Agnew, G. D., Bozzolo, M., Moritz, R. R., and Berenyi, S. The design and integration of the Rolls-Royce fuel cell systems 1MW SOFC. ASME paper GT , Litzinger, K. P., Veyo, S. E., Shockling, L. A., and Lundberg, W. L. Comparative evaluation of SOFC/Gas turbine hybrid system options. ASME paper GT , Tucker, D., Lawson, L., and Gemmen, R. S. Characterisation of air flow management and control in a fuel cell turbine hybrid power system using hardware simulation. ASME paper PWR , Ferrari, M. L., Magistri, L., Traverso, A., and Massardo A. F. Control system for solid oxide fuel cell hybrid systems. ASME paper 2005-GT-68102, Tucker, D., Lawson, L., Smith, T. P., and Haynes, C. Evaluation of cathode air flow transients in a hybrid system using hardware simulation. The 4th International Conference on Fuel Cell Science, Engineering and Technology, 2006, FUELCELL FELICITAS European Project TIP4-CT First year report, May Traverso, A., Ferrari, M. L., Massardo, A. F., and Pascenti, M. Turbovir: hardware simulation of virtual turbomachinery. Italian Patent, deposit n.ge2005a00092, Traverso, A., Ferrari, M. L., Massardo, A. F., and Pascenti, M. Non intrusive heater for fluids. Italian Patent, deposit n. TO2005A000912, PIP-SOFC European Project NNE Technical meeting, Røskilde, May Proc. IMechE Vol. 221 Part A: J. Power and Energy JPE438 IMechE 2007

9 Early start-up of SOFC hybrid systems Ferrari, M. L. Transient analysis of solid oxide fuel cell hybrid plants and control system development. PhD Thesis, TPG-DiMSET, University of Genoa, Traverso, A. TRANSEO: a new simulation tool for transient analysis of innovative energy systems. PhD Thesis, DiMSET, University of Genoa, Genoa, Italy, Ferrari, M. L., Traverso, A., Magistri, L., and Massardo, A. F. Influence of the anodic recirculation transient behaviour on the SOFC hybrid system performance. J. Power Sources Elsevier, 2005, 149C, 22 32, JPS-BS Ferrari, M. L., Pascenti, M., and Massardo, A. F. Experimental validation of an unsteady ejector model for hybrid systems. ASME paper GT , APPENDIX Notation N rotational speed (r/min) p pressure (Pa) T temperature (K) TIT turbine inlet temperature (K) p pressure rise (Pa) Subscripts sco start-up combustor outlet 0 on design JPE438 IMechE 2007 Proc. IMechE Vol. 221 Part A: J. Power and Energy

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