82 research outputs found
Performance indicators for the dynamics modeling and control of PEMFC systems
Society is gradually becoming aware that the current energy industry, based on the
use of fossil fuels, is inefficient, highly polluting and has a finite supply. Within the
scientific community, there are indications that hydrogen (H2) as an energy vector,
obtained from renewable energy sources, can represent a viable option to mitigate
the problems associated with hydrocarbon combustion. In this context, the change
from the current energy industry to a new structure with a significant involvement of
H2 facilitates the introduction of fuel cells as elements of energy conversion. Polymer
Electrolyte Membrane Fuel Cells (PEMFC) are gaining increased attention as viable
energy conversion devices for a wide range of applications from automotive,
stationary to portable. In order to optimize performance, these systems require active
control and thus in-depth knowledge of the system dynamics which include fluid
mechanics, thermal dynamics and reaction kinetics. One of the main issues, with
respect to proper control of these systems, is the understanding of the water
transport mechanisms through the membrane and the liquid water distribution. The
thesis is based on the publication of nine international journal articles that are divided
into 4 sub-topics: Dynamic fuel cell modeling, fuel cell system control-oriented
analysis, identification of parameters and performance indicators and finally, fault
and failure detection and system diagnosis. In the sub-topic of Dynamic Fuel cell
modeling, experimentally validated Computational Fluid Dynamics (CFD) modeling is
used to relate the effects of the physical phenomena associated with fluid mechanics
and thermal dynamics, that occur inside the fuel cell [Alonso, 2009][Strahl, 2011], to
water distribution. However, since these CFD models cannot be directly used for
control, control-oriented models [Kunusch, 2008][Kunusch, 2011] have been
developed in parallel. As well, another study is done in [Serra, 2006] which includes
a controllability analysis of the system for future development and application of
efficient controllers. The results of the above mentioned studies are limited because
either they do not incorporate an electrochemical model or the model is not experimentally validated. Moreover, these models do not take into account the
voltage losses due to liquid water inside the fuel cell. Therefore, there is a need to
properly relate the relevant effects of fluid mechanics and thermal dynamics,
including liquid water, to the fuel cell voltage. Primarily, methodologies are needed to
determine the relevant indicators associated to the effect of water on the fuel cell
performance. The works published in [Husar, 2008] and [Husar, 2011] treats
experimental parameter identification, mainly focused on water transport through the
membrane and fuel cell voltage loss indicators respectively. The implementation of
the indicators indirect measurement methodology provides an experimental way for
the isolation of three main types of voltage losses in the fuel cell: activation, mass
transport and ohmic losses. Additionally since these voltage loss indicators relate the
fuel cell operating conditions to the fuel cell voltage, they can be utilized to calibrate
and validate CFD models as well as employed in novel control strategies. On the
other hand, to develop reliable systems, the controller should not only take into
account performance variables during standard operation but should also be able to
detect failures and take the appropriate actions. A preliminary study on failure
indicators is presented in [Husar 2007] and fault detection methodologies are
described in [de Lira 2011]. As a whole, the compilation of articles represented in this
thesis applies a comprehensive experimental approach which describes the
implementation of novel methodologies and experimental procedures to characterize
and model the PEMFC and their associated systems taking into consideration
control oriented goals.La societat s'està adonant que la indústria energètica actual, basada en l'ús de
combustibles fòssils, és ineficient, molt contaminant i té un subministrament limitat.
Dins de la comunitat científica, hi ha indicis que el hidrogen (H2) com vector
energètic, obtingut a partir de fonts d'energia renovables, pot representar una opció
viable per a mitigar els problemes associats amb la combustió d'hidrocarburs. En
aquest context, el canvi de la indústria energètica actual a una nova estructura amb
una important participació de el hidrogen exigeix la introducció de les piles de
combustible com elements de conversió d'energia. Les piles de combustible de
membrana polimèrica (PEMFC) estan tenint cada vegada més atenció com a
dispositius viables de conversió d'energia per a una àmplia gamma d'aplicacions
com automoció, estacionàries o portàtils. Amb la finalitat d'optimitzar el seu
rendiment, les piles PEM requereixen un control actiu i per tant un coneixement
profund de la dinàmica del sistema, que inclou la mecànica de fluids, la dinàmica
tèrmica i la cinètica de les reaccions. Un dels temes principals relacionat amb el
control adequat d'aquests sistemes és la comprensió dels mecanismes de transport
d'aigua a través de la membrana i la distribució d'aigua líquida. Aquesta tesi es basa
en nou articles publicats en revistes internacionals que es divideixen en 4 subtemes:
la modelització dinàmica de piles de combustible, l'anàlisi orientada al control
del sistema, la identificació de paràmetres i d’indicadors de funcionament i,
finalment, la detecció de fallades i la diagnosi dels sistemes. En el sub-tema de la
modelització dinàmica de piles PEM, la modelització basada en la Dinàmica de
Fluids Computacional (CFD) amb validació experimental s'ha utilitzat per a
relacionar els efectes dels fenòmens físics de la mecànica de fluids i de la dinàmica
tèrmica que es produeixen dintre de la pila [Alonso, 2009] [ Strahl, 2011] amb la
distribució d'aigua. No obstant això, com aquests models CFD no poden ser utilitzats
directament per al control, s'han desenvolupat models orientats a control [Kunusch,
2008] [Kunusch, 2011] en paral·lel. A més, en un altre estudi [Serra, 2006] s'inclou una anàlisi de control·labilitat del sistema per al desenvolupament i aplicació futurs
de controladors eficaços. Però els resultats dels estudis esmentats anteriorment són
limitats, ja sigui perquè no incorporen un model electroquímic o bé perquè no han
estat validats experimentalment. A més, cap dels models té en compte les pèrdues
de tensió degudes a l'aigua líquida dins de la pila de combustible. Per tant, hi ha una
necessitat de relacionar adequadament els efectes rellevants de la mecànica de
fluids i de la dinàmica tèrmica, incloent l'aigua líquida, amb el voltatge de la pila de
combustible. Principalment, són necessàries metodologies per a determinar els
indicadors rellevants associats a aquest efecte de l'aigua sobre el rendiment de la
pila de combustible. Els treballs publicats en [Husar, 2008] i [Husar, 2011] tracten la
identificació experimental de paràmetres, centrada en el transport d'aigua a través
de la membrana i els indicadors de pèrdua de tensió, respectivament. L'aplicació
d'una proposta de metodologia de mesura indirecte dels indicadors permet
l'aïllament dels tres tipus principals de pèrdues de voltatge en la pila de combustible:
l'activació, el transport de massa i les pèrdues ohmiques. Aquests indicadors de
pèrdua de tensió relacionen les condicions d'operació amb el voltatge de la pila de
combustible i per tant poden ser utilitzats per a calibrar i validar models CFD, així
com per a definir noves estratègies de control. D'altra banda, per a aconseguir
sistemes fiables, el controlador no només ha de considerar els indicadors de
funcionament de l'operació normal, sinó que també ha de detectar possibles fallades
per a poder prendre les accions adequades en cas de fallada. Un estudi preliminar
sobre indicadors de fallades es presenta en [Husar 2007] i una metodologia de
detecció de fallades completa es descriu en [Lira de 2011]. En el seu conjunt, el
compendi d'articles que formen aquesta tesi segueix un enfocament experimental i
descriu la implementació de noves metodologies i procediments experimentals per a
la caracterització i el modelatge de piles PEM i els sistemes associats amb objectius
orientats al control eficient d'aquests sistemes.La sociedad se ésta dando cuenta de que la industria energética actual, basada en
el uso de combustibles fósiles, es ineficiente, muy contaminante y tiene un
suministro limitado. Dentro de la comunidad científica, hay indicios de que el
hidrógeno (H2) como vector energético, obtenido a partir de fuentes de energía
renovables, puede representar una opción viable para mitigar los problemas
asociados con la combustión de hidrocarburos. En este contexto, el cambio de la
industria energética actual a una nueva estructura con una importante participación
de H2 exige la introducción de pilas de combustible como elementos de conversión
de energía. Las pilas de combustible de membrana polimérica (PEMFC) están
ganando cada vez más atención como dispositivos viables de conversión de energía
para una amplia gama de aplicaciones como automoción, estacionarias o portátiles.
Con el fin de optimizar su rendimiento, las pilas PEM requieren un control activo y
por lo tanto un conocimiento profundo de la dinámica del sistema, que incluye la
mecánica de fluidos, la dinámica térmica y la cinética de las reacciones. Uno de los
temas principales relacionado con el control adecuado de estos sistemas, es la
comprensión de los mecanismos de transporte de agua a través de la membrana y
la distribución de agua líquida. Esta tesis se basa en la publicación de nueve
artículos en revistas internacionales que se dividen en 4 sub-temas: el modelado
dinámico de pilas de combustible, el análisis orientado a control del sistema, la
identificación de parámetros e indicadores de desempeño y, por último, la detección
de fallos y la diagnosis. En el sub-tema de la modelización dinámica de pilas PEM,
el modelado basado en Dinámica de Fluidos Computacional (CFD) con validación
experimental se ha utilizado para relacionar los efectos de los fenómenos físicos de
la mecánica de fluidos y la dinámica térmica que se producen dentro de la pila
[Alonso, 2009] [ Strahl, 2011] con la distribución de agua. Sin embargo, como estos modelos CFD no pueden ser utilizados directamente para el control, modelos
orientados a control [Kunusch, 2008] [Kunusch, 2011] se han desarrollado en
paralelo. Además, en otro estudio [Serra, 2006] se incluye un análisis de
controlabilidad del sistema para el futuro desarrollo y aplicación de controladores
eficaces. Pero los resultados de los estudios mencionados anteriormente son
limitados, ya sea porque no incorporan un modelo electroquímico o bien porque no
son validados experimentalmente. Además, ninguno de los modelos tiene en cuenta
las pérdidas de tensión debidas al agua líquida dentro de la pila de combustible. Por
lo tanto, hay una necesidad de relacionar adecuadamente los efectos relevantes de
la mecánica de fluidos y la dinámica térmica, incluyendo el agua líquida, con la
tensión de la pila de combustible. Principalmente, son necesarias metodologías para
determinar los indicadores relevantes asociados al efecto del agua sobre el
rendimiento de la pila de combustible. Los trabajos publicados en [Husar, 2008] y
[Husar, 2011] tratan la identificación experimental de parámetros, centrada en el
transporte de agua a través de la membrana y los indicadores de pérdida de tensió,
respectivamente. La aplicación de una metodología propuesta de medición indirecta
de los indicadores permite el aislamiento de los tres tipos principales de pérdidas de
tensión en la pila de combustible: la activación, el transporte de masa y las pérdidas
óhmicas. Éstos indicadores de pérdida de tensión relacionan las condiciones de
operación con la tensión de la pila de combustible y por lo tanto pueden ser
utilizados para calibrar y validar modelos CFD, así como para definir nuevas
estrategias de control. Por otro lado, para conseguir sistemas fiables, el controlador
no sólo debe considerar los indicadores de desempeño de la operación regular, sino
que también debe detectar posibles fallos para poder tomar las acciones adecuadas
en caso de fallo. Un estudio preliminar sobre indicadores de fallos se presenta en
[Husar 2007] y una metodología de detección de fallos completa se describe en [Lira
de 2011]. En su conjunto, el compendio de artículos que forman esta tesis sigue un
enfoque experimental y describe la implementación de nuevas metodologías y
procedimientos experimentales para la caracterización y el modelado de pilas PEM
y los sistemas asociados con objetivos orientados al control eficiente de estos
sistemas
Load profile effect on durability of proton exchange membrane fuel cells
Proton Exchange Membrane Fuel Cells (PEMFC) are a promising technology for substitution of hydrocarbon powerdrives and battery based electrical supply systems. However, below-required durability is still an important factor that hinders its widespread use, thus, novel operating approaches are required to surpass this shortcoming. The present abstract presents a line of research directed towards the of analysis and definition of load profile characteristics in order to design controllers and operation strategies that optimize cell efficiency and durability. Degradation of the catalyst layer, commonly composed of platinum supported on carbon (Pt/C), mainly due to platinum loss by oxidation/dissolution and the resulting reduction of Electrochemical Active Surface Area (ECSA), is the main issue reducing the durability of PEMFC.Peer ReviewedPostprint (author's final draft
Water transport study in high temperature fuel cell stack
This work presents analysis of water transport phenomena in the 120 Wel high temperature PEM fuel cell stack. Phosphoric acid doped PBI membranes are known for their abilities to work with dry gases; however water in the electrolyte still plays an integral role in the proton conduction mechanism. The presence of water in the membrane increases proton conductivity, however at high operating temperatures in the anhydrous environment phosphoric acid can dehydrate consequently lowering its conductivity. Therefore, understanding of water transport in the PBI- H3PO4 membranes is important as it can explain certain phenomena inside the cell. Tests have shown that water transport from cathode to anode due to water accumulation rises almost linearly with current density while decreases with cathode stoichiometry. Water transport seems to be independent of operating temperature. Also, in some cases, reverse flow water transport (from anode to cathode) appears to take place near the outlet. Ohmic resistance was also found to decrease slightly with an increase in current density and lower stoichiometries.Peer ReviewedPostprint (author’s final draft
Nonlinear observation in fuel cell systems: a comparison between disturbance estimation and High-Order Sliding-Mode techniques
© . This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/This paper compares two Nonlinear Distributed Parameter Observers (NDPO) for the observation of a Proton Exchange Membrane Fuel Cell (PEMFC). Both NDPOs are based on the discretisation of distributed parameters models and they are used to estimate the state profile of gas concentrations in the anode and cathode gas channels of the PEMFC, giving detailed information about the internal conditions of the system. The reaction and water transport flow rates from the membrane to the channels are uncertainties of the observation problem and they are estimated throughout all the length of the PEMFC without the use of additional sensors. The first observation approach is a Nonlinear Disturbance Observer (NDOB) for the estimation of the disturbances in the NDPO. In the second approach, a novel implementation of a High-Order Sliding-Mode (HOSM) observer is developed to estimate the true value of the states as well as the reaction terms. The proposed observers are tested and compared through a simulation example at different operating points and their performance and robustness is analysed over a given case study, the New European Driving Cycle.Peer ReviewedPostprint (author's final draft
Modelling liquid water effect on proton-exchange membrane fuel cells
This work introduces our ongoing research in developing accurate models for predicting and analyzing Proton-Exchange Membrane Fuel Cells performance. An initial model, whose response is compared with experimental data is first implemented. The simulated (and theoretically expected) cell voltage deviates from real behavior when the operating conditions change. Then an improved model, aimed to better approximate instantaneous voltage response to changing operating conditions by including liquid water dynamics, is developed. The results show that the new model achieves a better approximation of the voltage profile. The model is validated through experiments on a real fuel cell test-bench.Peer ReviewedPostprint (author's final draft
Study of hydrogen purge effects on performance and efficiency of a PEM fuel cell system
Experimental analysis and CFD modeling is used in this work to analyze system efficiency related to hydrogen purge based water management in an open-cathode PEM fuel cell system. Excess water in a deadended
anode decreases hydrogen concentration at the active catalyst surface and thus causes fuel cell performance losses. Purging the anode with hydrogen removes water and nitrogen that diffused through the membrane but also means wasting energy and thus decreasing overall system efficiency. Experiments with a 100W open-cathode stack have revealed that the need for a hydrogen purge strongly depends on the operation
conditions and the state-of-health of the fuel cell and therefore the decision to perform a purge has to be evaluated online. A dynamic 2D CFD model of a single cell within the stack is used to investigate water
distribution and transport within the cell before, during and after performing a purge at different operating conditions, linked to cell performance. Moreover, the model is capable of studying water transfer dynamics across the membrane and along the channel, including liquid water saturation. Altogether, the presented experimental
and modeling work helps to improve the understanding of water transport in a PEM fuel cell and thus facilitates the development of strategies for increasing system efficiency and optimizing the water management by properly controlling the hydrogen purge.Peer ReviewedPostprint (published version
Experimental characterization and identification of the voltage losses in an open cathode PEM fuel cell stack
For the past 20 years remarkable progress has been made in PEM fuel cell materials, component design, production, and system power density im-provements. However, there is still a lot to be done in the field of fuel cell system control, which makes it essential to understand the different physical phenomena within a working fuel cell and how they need to be controlled in order to improve efficiency, operating range and durability. This experimental study analyses the effects of ambient conditions, through the use of an environmental chamber, on a Horizon® H-100 20 cell stack, 22cm2 active area, open cathode, where the only active control mech-anism employed is a single fan that both, cools and provides the oxygen needed for the reaction. All the other control mechanisms are disconnected and a constant dry pure hydrogen flow rate of 1.8 SLPM is supplied to the stack.
The objective is to isolate and determine the dif-ferent voltage losses with respect to the ambient conditions and currents.Postprint (author’s final draft
A multi-timescale modeling methodology for PEMFC performance and durability in a virtual fuel cell car
The durability of polymer electrolyte membrane fuel cells (PEMFC) is governed by a nonlinear cou-pling between system demand, component behavior, and physicochemical degradation mechanisms, occurring on timescales from the sub-second to the thousand-hour. We present a simulation methodol-ogy for assessing performance and durability of a PEMFC under automotive driving cycles. The simu-lation framework consists of (a) a fuel cell car model converting velocity to cell power demand, (b) a 2D multiphysics cell model, (c) a flexible degradation library template that can accommodate physi-cally-based component-wise degradation mechanisms, and (d) a time-upscaling methodology for ex-trapolating degradation during a representative load cycle to multiple cycles. The computational framework describes three different time scales, (1) sub-second timescale of electrochemistry, (2) minute-timescale of driving cycles, and (3) thousand-hour-timescale of cell ageing. We demonstrate an exemplary PEMFC durability analysis due to membrane degradation under a highly transient load-ing of the New European Driving Cycle (NEDC).Peer ReviewedPostprint (author's final draft
Water transport study in high temperature proton exchange membrane fuel cell stack
A study of water transport in a high temperature phosphoric acid doped polybenzimidazole (PBI) membrane fuel cell stack is reported. Tests with different stoichiometries of dry cathode and different humidity levels of anode are performed. It is found that water transport across the membrane electrode assembly (MEA) is noteworthy and that water vapor partial pressure on the anode outlet is almost always higher than on the cathode outlet, even when using dry hydrogen. The water transport is a strong function of current density but it also depends on stoichiometry and humidity level. In a series of tests with dry nitrogen on one side and humid nitrogen on the other side, the membrane's water permeability coefficient is determined to be 2.4 × 10-13 mol s-1 cm-1 Pa-1 at 160 °C which is more than an order of magnitude higher than the values previously reported in the literature. Also, the results indicate that the permeability coefficient might be relative humidity dependent and could even be somewhat higher than the value reported here, but further investigation is needed. The experimental findings are reproduced and explained with a 2D steady state computational fluid dynamics (CFD) model. Internal water transport profiles across the membrane and along the gas flow channels are presented and discussed.Peer ReviewedPostprint (author’s final draft
Box-Jenkins autoregressive models for PEMFC operating under dynamical conditions
The objective of the present work is to explore and validate autoregressive, control oriented models models Proton Exchange Membrane Fuel Cells coperatinf under dynamic condditions. Autoregressive models have several advantages: they are obtained solely from input-output signals, have low computational cost, simple structure and a small number of parameters. Four datasets from experiments in static and dynamic operating conditions are used to estimate an validate the models, each dataset is divided into estimation and prediction subsets. The Box-Jenkins system identification method is used to built the model structure. The models are validated through analysis of the correlation of residuals by the Box-Ljung test and through calculation of the root mean squared error (RMSE).Peer ReviewedPostprint (published version
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