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Degradation phenomena of FC components
FC components degradation can be divided into three categories: mechanical, chemical, and thermal degradations. In practice, these three modes of degradation are coupled. Mechanical degradation is caused by the pressure difference between the hydrogen and oxygen, and by stresses that occur due to non-uniform clamping pressure at the channel -rib scale and moisture cycling. Chemical degradation is related to different components of the cell: variation in hydrophobicity of the gas diffusion layer, corrosion of bipolar plates, catalyst poisoning, and polymer decomposition. This category of degradation has many consequences such as a decrease in conductivity, loss of catalyst activity, weakening of the membrane, etc. Excessive temperature and insufficient gas humidity caused thermal degradation. The mechanical and thermal degradation usually can be considered as an irreversible degradation. The degradation of material during the time is also irreversible and cannot be avoided [17] but reduced thanks to adapted system management.
Bipolar plates
Bipolar plates provide the flow fields for incoming reactants and outgoing products. Three degradation mechanisms occur in these components: corrosion, the formation of a resistive surface layer, and fractures or deformation of it. The corrosion affects the catalyst layer and the membrane durability due to multivalent cations production. The resistive surface layer formation increases the ohmic resistance [27].
Gas diffusion layer (GDL)
GDL is made of a Microporous Layer (MPL) and a Gas Diffusion Backing (GDB). GDL allows the reactants to reach the catalyst layer near the membrane with mixed vapor water. The hydrophobicity of the MPL prevents flooding of the electrode by liquid water coming from the channels or the GDB. The degradation of this component leads to an increase in electrical resistivity and decrease the protection against liquid water [27].
Electrodes
The nanoparticle of platinum, the carbon black as the support of these nanoparticles, and ionomer form the porous electrodes. This component of the FC allows the reactants to reach platinum and protons and electrons to transport. According to three parts of the electrodes, four main degradation phenomena can be considered: carbon support oxidation, catalyst dissolution, ionomer attack, and degradation of the topology constituted by the assembly of these three ingredients (loss of porosity). These degradations lead to a decrease in the active area. Platinum dissolution in ionomer, platinum agglomeration, oxide formation on the carbon surface, reactive species generation causing the membrane degradation, and adsorption of contaminants are the main of these phenomena. The dissolution, oxidation, and agglomeration of platinum mainly concern the cathode for standard operating conditions [27].
Propagation of cell degradation
The cells inside a stack are thermally coupled. Therefore, the over-temperature of one cell can influence the adjacent cells. This coupling is schematically depicted in Figure 1-7. As seen in this figure, the second cell has a problem and its temperature has increased. Because of coupling, the anode temperature of the third cell and the cathode temperature of the first cell increase. Thus, a temperature gradient between the anode and cathode appears for the first and third cells. Under this condition, as the produced water is evacuated at the colder side, water droplets are produced where the temperature is lower [20], [28]. Therefore, the water droplets appear on the anode side of the previous cell (the first cell) and the cathode side of the next cell (the third cell). In this condition, the hydrogen starvation and the Oxygen starvation are quietly possible for the previous cell and the next cell respectively. Once the temperature of the adjacent cell has increased because of gas starvation, these cells themselves can propagate this problem to their adjacent cells. Hence, a faulty cell can influence the whole stack and an FC management system can deal with such faults and in the worst condition, electrically separate the faulty cells.
Durability enhancement tool
Lack of direct links between phenomena and FC model parameters and not taking into account all phenomena that occur in an FC are the basic problems of these methods. In this thesis, an electrochemical model is proposed. This model can consider almost all the phenomena and dynamic behavior of the FC. Besides, many parameters have a direct link with those phenomena. This model must run at the same time as the real system. Therefore, this model should be simple and should have a small time constant. Based on these reasons, a one-dimensional transient model is proposed in this thesis. Heat, mass, and charge transfers are coupled in this model. This model is introduced in the second chapter. Based on this model, it is possible to detect the occurred faults in the cells of one stack. However, a topology is required to manage the cells and supply the load. This topology must allow the cells inside a stack to be managed separately.
Multi-stack can be considered as an attempt to increase the lifetime of the fuel cell system at the cost of compactness loss. As seen in Figure 1-10, four basic topologies are used for the multi-stack or segmented FCs: series, parallel, cascade, series-parallel [3], [69], [70]. The series topology requires a low voltage ratio converter. In such a connection, the failure of a single cell means losing the whole system. Furthermore, there is no freedom degree in controlling the cells separately. The second topology, which the cells or stacks separately connect to the dc link by individual converters, provides the freedom degree in controlling the cells [71]. However, the high conversion ratio converters, which are required to increase the output voltage, provide higher stress on the semiconductor devices. This architecture is the most expensive topology due to a great requirement of the passive energy storage components. The cascade topology resolves the problem of the parallel topology. In this topology, the DC-link voltage is divided between the cells. This leads to lower stress on semiconductor devices. The series-parallel topology is as same as the parallel topology except that more cells connect to each converter. This topology inherits the advantages and disadvantages of series and parallel topologies. In such a topology, the converters with a lower voltage ratio can be used. Considering the ability of separately controlling the cells or stacks, the cascade topology can be used to manage the cells. However, connecting the cells to the high voltage DC-link while maintaining the controllability of the converters is challenging.
Estimation of intrinsic and relative permeability
The three unknown parameters and functions that must be estimated for a given channel geometry are the intrinsic permeability, the gas relative permeability f g S , and the liquid water relative permeability f liq S . To identify the intrinsic permeability and these two functions, a method is proposed in this section. This method is realized by imposing the gas flows (hydrogen and humid air) in an initially dry cell, then recording the variation of the pressure drop as a function of time following a current step. To have a cell with its channels initially in dry conditions, the cell should be supplied by dry gases for a long time before running the experiment. The intrinsic permeability of the gas is obtained from (2.33) when the channels are dry (S=0, f g S =1). Relative permeability of the gas and liquid water are obtained by the evolution of the pressure drop over time before the liquid flow starts and thanks to the pseudo-steady state.
Response to a current step for an imposed pressure condition
As mentioned before (Figure 2-6), the transfer of water in the perpendicular direction to the membrane is highly dependent on the temperature field. If the anode plate is hotter than the anode electrode, all the produced water flow will be discharged to the cathode. This particular thermal boundary condition is used to determine the flow of water discharged on the cathode side. As in the parameter estimations of the diphasic flow model, the temperatures of the anode and cathode plates are imposed to 67.5°C and 62.5°C respectively. Under these conditions, all the produced water flow is evacuated from the cathode side ( N el ch H 2Ovap ILW 2F ).
Figure 2-10 (c) shows the time evolution of the experimental stoichiometry of the air Stair after imposing a current step for a boundary condition of imposed pressure. The pressure drop is experimentally regulated by a pressure controller. This device was mounted before the membrane humidifier to avoid condensation problems. The cathode compartment is initially dry (S0=0). It can be seen that the pressure drop P changes slightly over time (Figure 2-10 (a)).
This can be explained by the fact that the pressure controller imposes a constant pressure before the membrane humidifier. As the flow rate decreases due to the appearance of the liquid water in the cell, the pressure drop in the humidifier slightly decreases (~1.5 mbar), and therefore the pressure at the inlet of the fuel cell P increases. For this reason, the experimental measurement of P(t), which is shown in Figure 2-10 (a), is considered as an input of the model. Figure 2-10 (c) shows ten experimental results (in black) that have been obtained under the same conditions. Because of the stochastic nature of the airflow, it is useful to perform the same experiment several times. Considering the imposed pressure drop condition as Figure 2-10 (a), the experimental results are compared to the simulation results in Figure 2-10 (c). The simulation result is obtained by the numerical resolution of (2.47)-(2.51). The model also predicts the evolution of the saturation over time (Figure 2-10 (b)).
DC bus voltage regulator
To regulate the DC bus voltage, an SC is connected to the DC bus by a bidirectional boost converter as depicted in Figure 3-1. The state-space model of the super-capacitor with a boost converter is as follows: L di v 1 d V r i SC dt SC sc SC dc SC SC dv i SC C SC dt SC (3.28). where Lsc is the inductance connected to the boost converter of the SC and rsc is its resistance, vsc is the SC voltage, and isc is its current. Therefore, the injected average current to the DC bus by the SC can be calculated as follows: i 1 d SC i a SC (3.29).
In this thesis, a hierarchical control method with two loops is used to regulate the DC bus voltage. This method includes an outer loop (energy loop) and an inner loop.
Table of contents :
Chapter 1 Thesis Context – Presentation of the studied Fuel cell management systems
1.1. Introduction
1.1.1. Problem statement
1.1.2. Methodology
1.2. Polymer Electrolyte Membrane Fuel Cell
1.3. Degradation
1.3.1. Operational conditions
1.3.2. Degradation phenomena of FC components
1.3.3. Degradation and snowball effects
1.4. Durability study
1.4.1. Non-model based methods
1.4.2. Model-based methods
1.4.3. Durability enhancement tool
1.5. Conclusions
Chapter 2 Polymer Electrolyte Membrane Fuel cell
2.1. Introduction
2.2. Heat transfer model
2.3. Mass transfer
2.3.1. Oxygen transfer
2.3.2. Water transfer
2.4. Charge transfer
2.5. Hydrogen starvation detection
2.6. Simulation Results
2.6.1. Simulation of one cell
2.6.2. Stack simulation
2.7. Conclusions
Chapter 3 Power electronics Structure
3.1. Introduction
3.2. Proposed power electronics structure
3.2.1. Operation Modes
3.2.2. Design consideration
3.3. Control Method
3.3.1. Equalizer controller
3.3.2. DC bus voltage regulator
3.3.3. SC voltage controller
3.4. Simulation results
3.5. Experimental results
3.6. Stability analysis
3.7. Conclusions
Chapter 4 Management system
4.1. Introduction
4.2. Proposed management system
4.3. Simulation results
4.4. Energy management system
4.5. Conclusions
Bibliography
