Pile à combustible à oxyde solide

Les piles à combustibles à oxydes solides (ou SOFC selon l'acronyme anglais de Solid oxide fuel cells) sont prévues essentiellement pour les applications stationnaires avec une puissance de sortie allant de 1 kW à 2 MW. Elles fonctionnent à très hautes températures, typiquement entre et (723 et 1273 K). Leurs rejets gazeux peuvent être utilisés pour alimenter une turbine à gaz secondaire afin d'accroître le rendement électrique. Le rendement peut atteindre les 70 % dans les systèmes hybrides, appelés systèmes à chaleur et puissance combinées ou de cogénération (en anglais : combined heat and power device ou CHP). Dans ces piles, les ions oxygène sont déplacés au travers d'un oxyde solide pris comme électrolyte à haute température afin de réagir avec l'hydrogène du côté anode. En raison de la haute température de fonctionnement des SOFC, il n'y a pas besoin de catalyseur onéreux, ce qui est le cas des piles à combustible à membrane échangeuse de protons (du platine par exemple). Ce

Integrated Design, Optimization, and Experimental Realization of a Steam-Driven Micro Recirculation Fan for Solid Oxide Fuel Cell Systems

Patrick Hubert Wagner

This thesis presents the results of the design and experimental investigation of a patented 10 kWel solid oxide fuel cell (SOFC) system with a thermally-driven anode off-gas recirculation (AOR) fan, the so-called fan-turbine unit (FTU). The system has the advantage of higher global fuel utilization, and thus higher efficiencies and/or lower local fuel utilization, increasing the fuel cell stack lifetime. Electrical net DC efficiencies, based on the lower heating value (LHV) of 65 % for a global fuel utilization of 92 % are demonstrated with a system simulation and optimization. This correlates to an electrical gross DC efficiency based on the LHV of 69 %. Additionally, the waste heat of the SOFC stack can be used for local cogeneration in heating or cooling applications leading to a net utilization ratio (cogeneration of heat and electricity) of 90 %. Other advantages of this system design include the absence of a water supply, a simplified water treatment system, and the reduction of the evaporator and pump component size, which reduces the system's initial and maintenance costs. A first FTU proof-of-concept was designed, manufactured, and extensively experimentally tested. The radial inducer-less fan with a tip diameter of 19.2 mm features backward-curved prismatic blades with a constant height. Prior to coupling the recirculation fan with the SOFC, the fan was experimentally characterized with air at 200 °C. At the nominal operational point of 168000 rpm, the measured inlet mass flow rate was 4.9 kg/h with a total-to-total pressure rise of 55 mbar, an isentropic total-to-total efficiency of 55 %, and a power of 18 W. Although the consumed fan power is very low, this FTU can increase the net efficiency of a 10 kWel SOFC system by five percentage points. The fan and shaft are propelled by a radial-inflow, partial-admission (21 %), low-reaction (13 %) steam turbine with prismatic blades. This turbine has a diameter of 15 mm and consists of 59 rotor blades with a radial chord of 1 mm and a blade height of 0.6 mm. At the design point, it has a total-to-total pressure ratio of 1.9, an inlet mass flow rate of 2.1 kg/h, and an isentropic total-to-static efficiency of 38 %, yielding a power of 36 W. To the best of the author's knowledge, it is one of the smallest steam turbines in the world. The shaft features one single-sided spiral-grooved thrust and two herringbone-grooved journal gas film bearings. Nominally, these bearings operate with water vapor at temperatures up to 220 °C. The bearings were tested with ambient air, hot air, and water vapor to rotational speeds up to 220000 rpm, suggesting very stable operation (rotor orbit less than 0.002 mm). Within this work, the recirculation unit operated for more than 300 hours. In a final step, the FTU was coupled in-situ to a 6 kWel SOFC system, reaching electrical gross DC efficiencies, based on the LHV, of 66 % in part load (4.5 kWel) and 62 % in full load (6.4 kWel) for a global fuel utilization of 85 %. To the best of the author's knowledge, this was the first time that a steam-driven AOR fan was demonstrated in-situ with an SOFC system.

EPFL2019

Modelling-Assisted Investigation of Degradation Phenomena in LSM-based SOFC Composite Cathodes

Pietro Tanasini

This thesis focuses on the degradation pathways occurring in composite solid oxide fuel cell (SOFC) cathodes based on lanthanum-strontium manganite (LSM), combining modeling at the electrode level with experimental data. LSM composite electrodes are one of the most promising candidates for SOFCs cathodes, yet life-time is currently one of the limiting factors for the deployment of this technology. The combination of modelling tools and analysis of experimental data permitted to achieve a better comprehension of the mechanisms controlling the performance loss during operation and a quantification of the degradation. By acquisition and formalization of knowledge of degradation processes, this thesis offers new tools to predict the life-time of a cell, hypothesize technological solutions for limiting the performance losses, and evaluate the impact of such solutions. A new experimental strategy for SOFC button cell testing has been developed and validated during this work, namely the simultaneous operation of several cathode segments on the same cell support. This allowed, on the one hand, the production of reproducible and reliable data for the evaluation and investigation of degradation processes and, on the other hand, the possibility of rapid identification of experimental problems affecting one segment. The approach has been validated both experimentally and theoretically – through finite element calculations. The vast majority of the experimental results contained in this work has been obtained using this testing configuration. Furthermore, investigation techniques with unprecedented resolution in the SOFC research field have been developed in collaboration between EPFL and external partners, namely a fast Cr quantification in operated cells, and 3D non-destructive reconstruction of cathode microstructures by X-ray computed tomography, giving new instruments for the study of SOFC performance and degradation. Finally, a set of data has been gathered concerning the reactivity of LSM-based systems, aging composite pellets for different times and temperatures and analyzing them with Rietveld-XRD, in order to map and assess the loss of performance caused by the formation of insulating phases in the electrode. A steady-state electrode model present in literature has been selected and improved for simulating the performance of composite cathodes in presence of degradation phenomena, converting it into a time-dependent model. The first phenomenon that has been integrated is the variation of the microstructure of the composite electrode, allowing the prediction of variation in performance for a number of case studies: coarsening of only one phase, coarsening of both phases, coarsening of both phases with variation of porosity. Experimental results obtained by microstructural analysis and electrochemical characterization of cells aged for different times were compared to the simulations of the correspondent case study, showing that the morphological variation of the anode electrode in the early cell operation (first few hundreds hours) is well predicted by the time-dependent model. No cathode morphological variation on this time scale could be detected. Another degradation pathway analyzed was poisoning by Cr species. The phenomenon has been modelled assuming an overpotential-driven deposition of Cr blocking species on the electrode active sites. Validation has been performed comparing the simulations to Cr pro les obtained experimentally from cell tested for relatively short time (1000 h); one conclusion reached was that increased electrode thickness has a beneficial effect in limiting Cr poisoning. The simulation was extended to operation times on the order of several 10 kh, difficult to achieve experimentally, describing the progressive deactivation of the electrode material. The model provides an estimation of the expected life-time of an electrode and predicts quantitatively the performance loss during time; this allows individuating technological solutions to hinder Cr poisoning, and the evaluation of the impact of such solutions. In particular, it has been found that the conductivity and amount of the electrolyte phase are critical, and that a three-fold decrease of the degradation related to Cr poisoning is expected while passing from 50%LSM/50%YSZ (standard composition for most LSM based cathodes) to 42%LSM/58%ScSZ electrodes (new composition). Finally, it has been found that cathode composition may have an indirect effect on the amount of deposited Cr: experimental evidences supporting a Mn-catalyzed deposition of Cr species have been found, suggesting that the presence of excess Mn in the cathode system can be detrimental.

EPFL2011

Optimal Process Design for Power Production from Wet Waste Biomass by Hydrothermal Gasification coupled to a Solid Oxide Fuel Cell

This report presents a system analysis of the production of electricity from wet waste biomass. A system of Hydrothermal Gasification is coupled with a Hybrid Cycle composed by a SOFC and two Gas Turbines, driven in an inverted Brayton--‐Joule cycle. A thermodynamic optimization approach, based on the system energy integration, is used to analyze several configuration options. The influence of the process design on performances is discussed for the most representative scenario that highlights the key aspects of the system. Optimization results prove that a systematic system design may reaches efficiencies higher than 60%.

2011