Mathematical modelling and advanced control design applied to high-pressure electrolyzers for hydrogen production

Tesi en modalitat de cotutela: Universitat Politècnica de Catalunya i Instituto Tecnológico de Buenos Aires. Aplicat embargament des de la defensa de la tesi fins al dia 30 de setembre de 2022This thesis is mainly dedicated to the study of high-pressure alkaline electrolysis. Alkaline electrolysis is a well established technology and is commercially available. However, the operation at high pressure for dispensing compressors was not fully investigated. Moreover, there is a lack of dynamic models and publications related to control strategies. Therefore, this thesis contributes especially in the modelling and control of high pressure alkaline electrolyzers in order to improve purity of produced gases. The thesis is framed within a general idea about the renewed concern for the care of the environment, which involves reducing greenhouse gas emissions without sacrificing modern comforts. Widespread proposal focuses on energy produced from renewable sources and its subsequent storage and transportation based on hydrogen. Currently, this gas applies to the chemical industry and its production is based on fossil fuels. The introduction of this energy vector requires the development of environmental-friendly methods for obtaining it. Existing techniques are presented and the main focus is made on electrolysis, a mature procedure. In turn, some developed proposals as previous steps to the hydrogen economy are presented. Moreover, some lines of research to improve electrolysis technology are commented. Afterwards, a phenomenological-based semiphysical model for a self-pressurized alkaline electrolyzer is proposed. The model, based on mass and energy balances, represents the dynamic behavior of hydrogen and oxygen production using electrolysis. The model allows to anticipate operational variables as dynamic responses in the concentrations of the electrolytic cell, and variations in both, level and pressure, at the gas separation chambers due to the change in electric current. The model parameters have been adjusted based on experimental measurements taken from an available prototype and through a suitable identification process. Simulation results replicate the current dynamic response of the experimental self-pressurized electrolyzer assembly. This model proves to be useful in the improvement of the control of gas production rate in this kind of assemblies, both as a validated simulation platform and as a source of reduced order models for model-based control design. Later, this thesis presents two control strategies that mitigate the cross contamination of H2 and O2 in a high-pressure alkaline electrolyzer, which consequently increases the supplied purity of the gases: one based on a decoupled PI scheme and the other based on optimal control tools. In order to reduce the diffusion of gases through the membrane, the controllers establish the opening of two outlet valves based on the pressure of the system and the difference in liquid level between both separation chambers. Therefore, two multiple input - multiple output controllers are designed. For this purpose, the high-fidelity model previously mentioned was simplified in order to obtain a control-oriented model. The proposed controllers were evaluated in simulation using the high-fidelity nonlinear model in a wide operating range, which resulted in less than 1% impurity of gases. In addition, tests were carried out in the prototype electrolyzer where the operation of the PI control was verified, obtaining even better results, with a maximum contamination of 0.2%.Aquesta tesi es dedica principalment a l'estudi de l'electròlisi alcalina d'alta pressiò. L'electròlisi alcalina és una tecnologia ben establerta i està disponible comercialment. Tanmateix, no s'ha investigat completament el funcionament a alta pressiò per a la distribuciò de compressors. A més, falten models dinàmics i publicacions relacionades amb les estratègies de control. Per tant, aquesta tesi contribueix especialment en el modelatge i control d'electrolitzadors alcalins d'alta pressió per tal de millorar la puresa dels gasos produïts. La tesi s'emmarca dins d'una idea general sobre la renovada preocupació per la cura del medi ambient, que consisteix a reduir les emissions de gasos d'efecte hivernacle sense sacrificar les comoditats modernes. Una proposta generalitzada es centra en l'energia produïda a partir de fonts renovables i el seu posterior emmagatzematge i transport basat en hidrogen. Actualment, aquest gas s'aplica a la indústria química i la seva producció es basa en combustibles fòssils. La introduccio d'aquest vector energètic requereix el desenvolupament de mètodes respectuosos amb el medi ambient per obtenir-lo. Es presenten les tècniques existents i es centra principalment en l'electròlisi, un procediment madur. Al seu torn, es presenten algunes propostes desenvolupades com a passos previs a l'economia de l'hidrogen. A més, es comenten algunes línies de recerca per millorar la tecnologia d'electròlisi. Posteriorment, es proposa un model semifísic de base fenomenològica per a un electrolitzador alcalí auto-pressuritzat. El model, basat en els balanços de massa i energia, representa el comportament dinàmic de la producció d'hidrogen i oxigen mitjançant electròlisi. El model permet anticipar variables operatives com a respostes dinàmiques en les concentracions de la cèl·lula electrolítica i variacions en el nivell i la pressió de les cambres de separació de gas a causa del canvi de corrent elèctric. Els paràmetres del model s'han ajustat en funció de mesures experimentals obtingudes en d'un prototip disponible i mitjançant un procés d'identificació adequat. Els resultats de la simulació repliquen la resposta dinàmica actual del conjunt experimental d'electrolitzador auto-pressuritzat. Aquest model demostra ser útil per millorar el control de la taxa de producció de gas en aquest tipus d'assemblatges, tant com a plataforma de simulació validada com a font de models d'ordre reduït per al disseny de control basat en models. Posteriorment, aquesta tesi presenta dues estratègies de control que mitiguen la contaminació creuada de H2 i O2 en un electrolitzador alcalí d'alta pressió, que en conseqüència augmenta la puresa subministrada dels gasos: una basada en un esquema de PI desacoblat i l'altra basada en un esquema de control òptim. Per tal de reduir la difusió de gasos a través de la membrana, els controladors estableixen l'obertura de dues vàlvules de sortida en funció de la pressió del sistema i de la diferència de nivell de líquid entre les dues cambres de separació. Per tant, es dissenyen dos controladors d'entrada i sortida múltiple. Amb aquest propòsit, el model d'alta fidelitat esmentat anteriorment s'ha simplificat per obtenir un model orientat al control. Els controladors proposats han estat avaluats en simulació mitjançant el model no lineal d'alta fidelitat en un ampli rang operatiu, el qual ha resultat en una impuresa de gasos inferior a 1%. A més, es van realitzar assajos al electrolitzador prototip on es va constatar el funcionament de l'control PI, obtenint inclusivament millors resultats, amb una contaminació màxima de 0,2%.Esta tesis está dedicada principalmente al estudio de la electrólisis alcalina de alta presión. La electrolisis alcalina es una tecnología bien establecida y está disponible comercialmente. Sin embargo, la operación a alta presión para dispensar el uso de compresores no ha sido investigada completamente. Además, hay una falta de modelos dinámicos y publicaciones relacionadas con las estrategias de control. Por tanto, esta tesis contribuye especialmente en el modelado y control de electrolizadores alcalinos de alta presión para mejorar la pureza de los gases producidos. La tesis se enmarca dentro de una idea general sobre la renovada preocupación por el cuidado del medio ambiente, que pasa por reducir las emisiones de gases de efecto invernadero sin sacrificar las comodidades modernas. La propuesta generalizada se centra en la energía producida a partir de fuentes renovables y su posterior almacenamiento y transporte a base de hidrogeno. Actualmente, este gas se utiliza en la industria química y su producción se basa en combustibles fósiles. La introducción de este vector energético requiere el desarrollo de métodos amigables con el medio ambiente para su obtención. Se presentan las técnicas existentes y se hace hincapié en la electrolisis, un procedimiento maduro. A su vez, se presentan algunas propuestas desarrolladas como pasos previos a la economía del hidrogeno. Además, se comentan algunas líneas de investigación para mejorar la tecnología de electrolisis. Posteriormente, se propone un modelo semifísico de base fenomenológica para un electrolizador alcalino autopresurizado. El modelo, basado en balances de masa y energía, representa el comportamiento dinámico de la producción de hidrogeno y oxígeno mediante electrolisis. El modelo permite anticipar variables operativas como respuestas dinámicas en las concentraciones de la celda electrolítica y variaciones tanto de nivel como de presión en las cámaras de separación de gases debido al cambio de corriente eléctrica. Los parámetros del modelo se han ajustado en base a medidas experimentales tomadas de un prototipo disponible y mediante un proceso de identificación adecuado. Los resultados de la simulación replican la respuesta dinámica actual del conjunto electrolizador autopresurizado experimental. Este modelo demuestra ser útil en la mejora del control de la tasa de producción de gas en este tipo de montajes, tanto como plataforma de simulación validada como fuente de modelos de orden reducido para el diseño de control basado en modelos. Después, esta tesis presenta dos estrategias de control que mitigan la contaminación cruzada de H2 y O2 en un electrolizador alcalino de alta presión, lo que consecuentemente aumenta la pureza suministrada de los gases: una basada en un esquema de PI desacoplado y otra basada en herramientas de control óptimo. Para reducir la difusión de gases a través de la membrana, los controladores establecen la apertura de dos válvulas de salida en función de la presión del sistema y la diferencia de nivel de líquido entre ambas cámaras de separación. Por lo tanto, se diseñan dos controladores de múltiples entradas y múltiples salidas. Para ello, se simplificó el modelo de alta fidelidad anteriormente mencionado para obtener un modelo orientado al control. Los controladores propuestos se evaluaron en simulación utilizando el modelo no lineal de alta fidelidad en un amplio rango operativo, lo que resultó en menos de 1% de impureza de gases. Además, se realizaron ensayos en el electrolizador prototipo donde se constató el funcionamiento de los controles PI y H1, obteniendo inclusive mejores resultados, con una contaminación máxima de 0;2 %.Postprint (published version

Scale-up and study of the BioGenerator

For the time being transfer from the fossil fuel powered electricity generation technologies to renewable sources is facing a great deal of challenges, because of their intermittent nature. Efficient ways of electricity storage are essential to make it happen, and our electro-biotechnology - the BioGenerator - may be a potential solution, due to its uniqueness consisting in employing iron oxidizing microorganisms. This work presents a scale-up of the BioGenerator from 1W to 300W capacity in a stepwise manner. It involved the design, study and scale-up of the airlift bioreactor from 1.4 to 600 L, and electrochemical cells with different catholyte flow patterns from 4x4 cm (1.6 W) to a stack of 20x20 cm cells (271 W). An impact of different operating regimes and the catholyte characteristics on the electrochemical cell performance was studied. Based on the experimental results collected over the course of this Ph.D. research project, the largest and most advanced system to date - 10 kW BioGenerator - was designed and currently is under construction. Oxygen mass transfer and microbial dynamics in the large-scale bioreactors (400 and 600 L) were studied and extraordinary resilience of L. ferriphilum dominated culture was observed. It was found that it takes ~5.5. days for the bacteria to recover and resume their iron oxidizing ability even after 5 months of starvation. An array of commercially available proton exchange membranes was tested in terms of their suitability for the use in the BioGenerator, and Selemion HSF was found to be the best amongst them. The straight forward techniques for the synthesis of Nafion- and polyvinyl alcohol (PVA)-based membranes were proposed. The proton conductivity, water transport and ferric ion diffusion through the synthesized membranes were measured. Testing in the Fe3+/H2 electrochemical cell revealed that the most promising, in terms of both performance and economics, amongst them is a phosphorylated PVA membrane. A mathematical model describing the operation of the BioGenerator system was developed and successfully tested during validation experiments. The data predicted was in a fairly good agreement with the experimental

Thermodynamic analysis, modelling and control of a novel hybrid propulsion system

Stringent emission regulations imposed by governments and depleting fossil fuel reserves have promoted the development of the automotive industry towards novel technologies. Various types of hybrid power plants for transport and stationary applications have emerged. The methodology of design and development of such power plants varies according to power producing components used in the systems. The practical feasibility of such power plants is a pre-requisite to any further development. This work presents thermodynamic analysis and modelling of such a novel power plant, assesses its feasibility and further discusses the development of a suitable control system. The proposed system consists of a hybrid configuration of a solid oxide fuel cell and IC engine as the main power producing components. A reformer supplies fuel gas to the fuel cell while the IC engine is supplied with a liquid fuel. The excess fuel from the fuel cell anode and the oxygen-depleted air from cathode of the fuel cell are also supplied to the engine. This gas mixture is aspirated into the engine with the balance of energy provided by the liquid fuel. The fuel cell exhaust streams are used to condition the fuel in the engine to ensure minimum pollutants and improved engine performance. Both, fuel cell and engine share the load on the system. The fuel cell operates on a base load while the engine handles majority of the transient load. This system is particularly suitable for a delivery truck or a bus cycle. Models of the system components reformer, solid oxide fuel cell, IC engine and turbocharger were developed to understand their steady state and dynamic behaviour. These models were validated against sources of literature and used to predict the effect of different operating conditions for each component. The main control parameters for each component were derived from these models. A first law analysis of the system at steady state was conducted to identify optimum operating region, verify feasibility and efficiency improvement of the system. The results suggested reduced engine fuel consumption and a 10 % improvement in system efficiency over the conventional diesel engines. Further, a second law analysis was conducted to determine the key areas of exergy losses and the rational efficiency of the system at full load operating conditions. The results indicate a rational efficiency of 25.4 % for the system. Sensitivity to changes in internal exergy losses on the system work potential was also determined. The exergy analysis indicates a potential for process optimisation as well as design improvements. This analysis provides a basis for the development of a novel control strategy based on exergy analysis and finite-time thermodynamics. A dynamic simulation of the control oriented system model identified the transient response and control parameters for the system. Based on these results, control systems were developed based on feedback control and model predictive control theories. These controllers mainly focus on air and fuel path management within the system and show an improved transient response for the system. In a hierarchical control structure for the system, the feedback controllers or the model predictive controller can perform local optimisation for the system, while a supervisory controller can perform global optimisation. The objective of the supervisory controller is to determining the load distribution between the fuel cell and the engine. A development strategy for such a top-level supervisory controller for the system is proposed. The hybrid power plant proposed in this thesis shows potential for application for transport and stationary power production with reduced emissions and fuel consumption. The first and second law of thermodynamics can both contribute to the development of a comprehensive control system. This work integrates research areas of powertrain design, thermodynamic analysis and control design. The development and design strategy followed for such a novel hybrid power plant can be useful to assess the potential of other hybrid systems as well

An analytical, control-oriented state space model for a PEM fuel cell system

If fuel cell technology – with its inherent benefits of high efficiency and low emissions – is to be used in decentralised power sources, in mobile or transportation applications, the systems have to be able to adapt to fast load changes and varying operating conditions. In order to achieve such performance, the balance of plant systems – typically governed by an on-board system controller – need to dynamically supply the fuel cell stack with reactant gases at the right flow rates, pressures and humidities while keeping the fuel cell at its correct operating temperature. Since best overall system performance is achieved by using model-based controllers, an appropriate model is required to implement such controllers. This thesis provides a control-oriented state space model for a PEM fuel cell system. The model describes the effects of a user interaction with any of the balance of plant actuators on overall system performance. The system model is elaborated in a two-step process. In a first step, an analytical, steady state, cell-averaged, isothermal fuel cell stack model is developed. The model predicts the fuel cell voltage and membrane water content as a function of the stack's operating conditions – i.e. reactant flow rates, pressures and humidities as well as cell temperature. It provides an analytical expression to the overall water transport within the fuel cell stack. In the second step, dynamic state space models are developed for the balance of plant systems. They link the effects of the auxiliary systems' actuators to the evolution of the operating conditions for the fuel cell stack. In the context of this thesis, state space models for a non-pressurised air supply subsystem, for a recirculating, pressurised hydrogen supply subsystem and for a liquid cooled thermal management subsystem are elaborated. A dedicated fuel cell test bench has been developed that was used to experimentally validate the proposed models

Modeling, Parameter Identification, and Degradation-Conscious Control of Polymer Electrolyte Membrane (PEM) Fuel Cells

Polymer electrolyte membrane (PEM) fuel cells are touted as zero-emission alternatives to internal combustion engines for automotive applications. However, high cost and durability issues have hindered their commercialization. Therefore, significant research efforts are underway to better understand the scientific aspects of PEM fuel cell operation and engineer its components for improved lifetime and reduced cost. Most of the research in this area has been focused on material development. However, as demonstrated by Toyota's fuel cell vehicle, intelligent control strategies may lead to significantly improved durability of the fuel cell stack even with existing materials. Therefore, it seems that the outstanding issues can be better resolved through a combination of improved materials and effective control strategies. Accordingly, this dissertation aims to develop a model-based control strategy to improve performance and durability of PEM fuel cell systems for automotive applications. To this end, the dissertation first develops a physics-based and computationally efficient model for online estimation purposes. The need for such a model arises from the fact that detailed information about the internal states of the cell is required to develop effective control strategies for improved performance and durability, and such information is rarely available from direct measurements. Therefore, a software sensor must be developed to provide the required signals for a control system. To this end, this work utilizes spatio-temporal decoupling of the underlying problem to develop a model that can estimate water and temperature distributions throughout an operating fuel cell in a computationally efficient manner. The model is shown to capture a variety of complex physical phenomena, while running at least an order of magnitude faster than real time for dynamically changing conditions. The model is also validated with extensive experimental measurements under different operating conditions that are of interest for automotive applications. Furthermore, the dissertation extensively explores the sensitivity of the model predictions to a variety of parameters. The sensitivity results are used to study the parameter identifiability problem in detail. The challenges associated with parameter identification in such a large-scale physics-based model are highlighted and a model parameterization framework is proposed to address them. The proposed framework consists of three main components: (1) selecting a subset of model parameters for identification, (2) optimally designing experiments that are maximally informative for parameter identification, and (3) designing a multi-step identification algorithm that ensures sufficient regularization of the inverse problem. These considerations are shown to lead to effective model parameterization with limited experimental measurements. Finally, the dissertation uses a version of the proposed model to develop a degradation-conscious model-predictive control (MPC) framework to enhance the performance and durability of PEM fuel cell systems. In particular, a reduced-order model is developed for control design, which is then successively linearized about the current operating point to enable use of linear control design techniques that offer significant computational advantages. A variety of constraints on system safety and durability are considered and simulation case studies are conducted to evaluate the framework's utility in maximizing performance while respecting the durability constraints. It is also shown that the linear MPC framework employed here can generate the optimal control commands faster than real time. Therefore, the proposed framework is expected to be implementable in practical applications and contribute to extending the lifetime of fuel cell systems.PHDMechanical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttps://deepblue.lib.umich.edu/bitstream/2027.42/155288/1/goshtasb_1.pd

Hierarchical Model Predictive Control for the Dynamical Power Split of a Fuel Cell Hybrid Vehicle

In order to reduce emissions of the transport sector, fuel cell hybrid vehicles (FCHVs) constitute a promising alternative as they have zero local emissions and overcome the limited range of electric vehicles. The power management of the propulsion system poses many challenges since it is a highly nonlinear, constrained, strongly coupled, multiple-input multiple-output (MIMO) system. The control objectives aim at dynamic power delivery, minimization of hydrogen consumption and charge sustainability of the battery. This thesis presents a hierarchical model predictive control (MPC) with three levels approaching the control problem on different time scales. The high-level control (HLC) implemented as a nonlinear MPC optimizes the static power split between battery and fuel cell system. The intermediate-level control (ILC) uses static optimization to determine the optimal operating point of the air supply. The lowlevel control (LLC) is a nonlinear MPC and tracks the reference trajectories received from the higher levels. The hierarchical MPC is evaluated on a detailed model of an FCHV using the worldwide harmonized light vehicles test cycle. Utilizing predictive information about the power demand, the HLC provides a power split that assures charge sustainability of the battery and only deviates by 0.2% from the optimal solution in terms of hydrogen consumption. Due to the predictive behavior and inherent decoupling capability of an MPC, the LLC achieves dynamic power delivery while explicitly considering the system constraints caused by prevention of oxygen starvation and limited operating range of the compressor. Moreover, the actual hydrogen consumption deviates only by 1% from the hydrogen consumption that is predicted by the HLC. Even for uncertain power demand prediction, the LLC attains dynamic power delivery by deviating from the reference trajectories to relieve the fuel cell system when operating under system constraints.In order to reduce emissions of the transport sector, fuel cell hybrid vehicles (FCHVs) constitute a promising alternative as they have zero local emissions and overcome the limited range of electric vehicles. The power management of the propulsion system poses many challenges since it is a highly nonlinear, constrained, strongly coupled, multiple-input multiple-output (MIMO) system. The control objectives aim at dynamic power delivery, minimization of hydrogen consumption and charge sustainability of the battery. This thesis presents a hierarchical model predictive control (MPC) with three levels approaching the control problem on different time scales. The high-level control (HLC) implemented as a nonlinear MPC optimizes the static power split between battery and fuel cell system. The intermediate-level control (ILC) uses static optimization to determine the optimal operating point of the air supply. The lowlevel control (LLC) is a nonlinear MPC and tracks the reference trajectories received from the higher levels. The hierarchical MPC is evaluated on a detailed model of an FCHV using the worldwide harmonized light vehicles test cycle. Utilizing predictive information about the power demand, the HLC provides a power split that assures charge sustainability of the battery and only deviates by 0.2% from the optimal solution in terms of hydrogen consumption. Due to the predictive behavior and inherent decoupling capability of an MPC, the LLC achieves dynamic power delivery while explicitly considering the system constraints caused by prevention of oxygen starvation and limited operating range of the compressor. Moreover, the actual hydrogen consumption deviates only by 1% from the hydrogen consumption that is predicted by the HLC. Even for uncertain power demand prediction, the LLC attains dynamic power delivery by deviating from the reference trajectories to relieve the fuel cell system when operating under system constraints

Energy, Science and Technology 2015. The energy conference for scientists and researchers. Book of Abstracts, EST, Energy Science Technology, International Conference & Exhibition, 20-22 May 2015, Karlsruhe, Germany

We are pleased to present you this Book of Abstracts, which contains the submitted contributions to the "Energy, Science and Technology Conference & Exhibition EST 2015". The EST 2015 took place from May, 20th until May, 22nd 2015 in Karlsruhe, Germany, and brought together many different stakeholders, who do research or work in the broad field of "Energy". Renewable energies have to present a relevant share in a sustainable energy system and energy efficiency has to guarantee that conventional as well as renewable energy sources are transformed and used in a reasonable way. The adaption of existing infrastructure and the establishment of new systems, storages and grids are necessary to face the challenges of a changing energy sector. Those three main topics have been the fundament of the EST 2015, which served as a platform for national and international attendees to discuss and interconnect the various disciplines within energy research and energy business. We thank the authors, who summarised their high-quality and important results and experiences within one-paged abstracts and made the conference and this book possible. The abstracts of this book have been peer-reviewed by an international Scientific Programme Committee and are ordered by type of presentation (oral or poster) and topics. You can navigate by using either the table of contents (page 3) or the conference programme (starting page 4 for oral presentations and page 21 for posters respectively)

Conceptual Design Tool to Analyze Electrochemically-Powered Micro Air Vehicles

A multi-fidelity conceptual design tool was developed to assess electrochemically-powered micro air vehicles(MAVs). The tool utilizes four areas of contributing analyses (CAs): aerodynamics, propulsion, power management, and power sources to determine the endurance duration of a given mission. The low-fidelity aerodynamic CA consisted of drag polar calculations and the high-level CA used a vortex theory code called Athena Vortex Lattice (AVL). The propulsion CA employed QPROP and a MATLAB code that used experimental propeller data and motor constants to predict propeller-motor combination performance for the low- and high-fidelity tracks, respectively. The power management CA determined the percentage of required power the power sources needed to provide by a user-defined split or an optimization to maximize endurance duration for the two fidelity options. The power source CA used specific energy and specific power calculations for the low-fidelity track and polarization curves and Ragone plots for the high-fidelity track. Model Center software allowed for integration of each of these CAs into one model. Based on the current state of the art battery and fuel cell technology, the model predicted endurance durations ranging from 88.5 to 107.3 min. The mission simulations that led to these durations used a generic MAV (GenMAV) configuration and the complete spectrum of fidelity combinations