Récupération et Stockage d énergie Energétique 5A

Transcription

1 Récupération et Stockage d énergie Energétique 5A TP Evaluation théorique d un système pile à combustible Daniela CHRENKO 1. Introduction A fuel cell uses hydrogen and oxygen to create electricity by an electro-chemical process. A single fuel cell consists of an electrolyte sandwiched between an anode and a cathode. There are different types of fuel cells, the Proton Exchange Membrane (PEM) fuel cells being used in the CUTE trial operate in the following way: Hydrogen is fed to the anode where a catalyst separates the negatively-charged electrons in the hydrogen from the positively-charged protons Protons move through the membrane to the cathode The electrons from the anode side of the cells cannot pass through the membrane to the positively-charged cathode. They travel via an electrical circuit to reach the other side of the cell. This process produces the electrical current At the cathode, oxygen from the air combines with electrons and protons to produce water and heat To generate enough power to drive the bus, the many individual fuel cells are connected to each other and built up into "stacks". H2 Fuel Cell Bus: 2 stacks, 6 rows/stack; 160 cells/row The fuel cell voltage varies with the current as well as other operation conditions like the temperature and the membrane humidity. The variation of the voltage can be described by the open circuit voltage and a number of different types of losses: Open Circuit Voltage: The theoretic open circuit voltage consists can be calculated with the help of the Gibbs energy and the Faraday constant: With the free Gibbs energy of formation (in the case of reaction between hydrogen and oxygen: kj/mol at 25 C and kj/mol at 80 C. This value has to be adapted due to the partial pressues: 1/6

2 This leads to a theoretical maximum value of 1.14V. In reality, as it is often not possible to evaluate all losses and due to the fact that the open circuit voltage can be obtained easily, the measured value of the open circuit voltage is used. Activation losses: The activation losses are predominating in the steep voltage decrease at small currents. These are caused by the slowness of the reactions taking place on the surface of the electrodes. It can be described by the Tafel equation using: The crossover current density is usually around 3 ma/cm². Resistive losses: Actually two effects take place in the same time, which are predominating in the linear voltage fall at moderate currents. The fuel crossover and internal currents describe the effect that some electrons or hydrogen molecules pass through the membrane. A similar effect is caused by the ohmic losses, which is described by the membrane resistance, but also the resistance of the electrons and the various interconnectors. Concentration losses: The steep fall of the voltage at the high currents can be described by the phenomenon of the change in concentration of the reactants at the surface of the electrons as the fuel is used. The voltage as a function of current density can be calculated by: 2/6

3 2. Practical Work a. The Bahia Fuel Cell Simulator For this practical work you will use the offline version of the fuel cell module Bahia by Helion/Areva. Therefore you have to start the student version of the software. You do not need a login or a password. The homepage introduces the Bahia module; furthermore you have six buttons for different applications, namely: Procedure: Gives you the flow chart of the system. Additional information can be added using the buttons in affichages. Charge: Gives you the possibility to apply a charge on the system. The charge can be in the form of current, power or resistance. In this case only a current charge will be used. In the main window it is possible to give instructions consignes and do measurements measures. Furthermore it is possible to program rapid load changes using Définition du type d impact which has to be confirmed by pushing the button Transfert des données. On the very right of the window you can observe how the system answers to the instruction. Four different ways to programme a rapid load change can be used Echelon A step change at a given moment Ramp A ramp with a start and a final value as well as the delay before the start of the ramp and the delay at the end of the ramp Profil With different values and their associated delays And the possibility to use an external file containing a number of amplitudes and delays saved in a.txt file to run a given profile. Stack: The stack window gives the possibility to supervise the heart of the fuel cell, namely the stack. On the left you find momentary values, which value to take into consideration and different options (measure a point, save the result, load old results and do the interpretation of the measurement). Those options are linked to the polarization curve on the right side of the main window named courbe de polarisation. There you can find a curve generated from reference values and every time you do a measurement one point will be added to this curve. On the left side of the main window you find a histrogramm of the cells showing the instantaneous voltage of each one of the cells inside the stack. This plot helps you to supervise the state of health of the fuel cell stack. Finally you find the values that have been measured in the left hand table and you can analyse the acquired data. Courbes: Allows you to plot one or two plots using data of different parameters in real time or from a saved measurement. Système & Application: Gives you an overview of the consumptions and productions of the fuel cell system; this covers as well the auxiliary power from the net as the hydrogen consumed, an estimation of the produced water and an estimation of the rejected heat. Docs &TPs: Provides additional documents for the Bahia system in special but also for fuel cell systems in general. 1. Get familiar with the simulation of the Bahia system. Start the system in simulation mode and observe the starting procedure in the Procédé window. Add the additional information in Affichages /6

4 2. Change the charge while adding a reference current of 40A and observe the changes in the Procédé window. 3. Go to the Stack window and take some measurement points. 4. Now change the current reference using a current slope between 0A and 79A with duration of 80s. Apply this slope on the system and try to do one measurement every second. Why is your graph different from the reference graph? Give possible answers. Export the measured data. Analyse the polarization curve using Interpréter la courbe and note the coefficients, as well as the open circuit voltage. 5. Using the software EXCEL (or equivalent): a. Plot the measured points b. Establish the theoretic polarisation curve with a precision of 0.5A using the parameters given by the integrated analyse as well as the equation (U=f(A pente de Tafel, j0 densité d échange, r résistance ohmique, B coefficient de diffusion, jl densité limite)) using??? c. Compare your simulation with the measured results (if they have already been obtained). b. Simulation of Hydrogen Electric Vehicle In the following your task is to evaluate theoretically the hydrogen consumption of a fuel cell bus in two different configurations. It is based on the fuel cell bus CUTE and the fuel cell hybrid bus Citaro FuelCELL-Hybrid, the technical data have to be verified by means of modelling. Their technical data are given hereafter: Fuel cell bus (CUTE) Citaro FuelCell Hybrid Power [kw] 205 kw < sec 220 kw < sec Autonomy [km] >250 Battery - Li-Ion, 180kW permanent H2 Consumption [kg/100km] Maximum Efficiency 48 % 58 % Passenger Capacity 23+49= =76 Front area [mm x mm] x Weight (empty full) [kg] Drag coefficient 0.7 Traction efficiency 75% Energy recovery efficiency 25% Rolling resistance /6

5 The vehicle fulfils the following driving cycle: It accelerates form 0 km/h to 50 km/h in 30s, stays on this speed for 40s and decelerates to stop during 20s, it is at rest during 20s. During a working day it runs a distance of 200 km. 1. Calculate the power needed to run this driving cycle using EXCEL (or equivalent). Which power has to be delivered by the fuel cell? 2. Ramp down the power demand proportionally so that the maximum power corresponds with the maximum power available at the Bahia system (1200W). 3. Program the power values of power and delay in a.tex file. 4. Load the file in the Bahia simulation. Run the power profile and visualize the variation in current and voltage. 5. Rerun the power profile and evaluate the hydrogen consumption using the window système appli. (raz: deletes the former calculated hydrogen). Estimate the hydrogen which has to be stored inside the reference vehicle, so that is able to run all day and keep a buffer of 10%. Estimate the size of the hydrogen storage, knowing that the hydrogen is stored in pressurized form with a pressure of 300bar. Observe the system efficiency rendément matière and take into account the transmission efficiency. Now you can compare the energy provided by the hydrogen driven fuel cell system with the energy needed to run the system. Do the numbers agree? 6. Now assume that the reference vehicle is a hybrid vehicle that has been designed for this particular application. Thus the size of the fuel cell system represents the mean power needed during the cycle. Simulate the driving cycle under this conditions. Evaluate the hydrogen consumption in this mode. Compare the results of the hybrid reference vehicle to the original reference vehicle. What do you observe? Try to explain your observation /6

6 c. Influence of Fuel Cell Conditions Run system with: no purge, different Lambda, different temperature. Validate consumption, but also if the desired power could be delivered In the system simulation you can try the influence of different parameter without the risk of damaging the system. This is why the influence of different parameters is testes in the simulation hereafter. For every measurement the following procedure has to be effectuated (maybe you have to restart the Bahia software for every measurement in order to save only the measurement points of this experience): 1. Change the parameter. 2. Prepare the measurement of a polarization curve 0A to 79A in 80s. 3. Effectuate measurements every second. 4. Analyse the system behaviour using the analysing mode proposed by the system and note down the parameters. 5. Plot the polarization curve against the reference polarization curve. 6. Explain the differences. The parameters that have to be modified are the following: Stoichiometry: 1.2, 2, 3 Purge: with (avec) and without (sans) Temperature: 30 C, 50 C and 70 C, the temperature constraint limits the maximum temperature, if the system is below this working temperature the constraint has no influence. References: CUTE and Highfleet Cute Project ( Fuel Cell Systems Explained, James Larminie and Andrew Dicks, Wiley and sons Piles à combustible, Principes, modélisation, applications avec exercises et problèmes corrigés, Benjamin Blunier, Abdellatif Miraoui, ellipses, /6