Fuel Cell Systems Explained - from and for SET students

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The chemical reactions have to take place at the surface of the electrodes, due to removing orsupplying electrons. These reactions of the fuel with the electrolyte at the electrode is called a threephase contact. The reaction rate is propotional to the effective surface area of the electrodes.Requirements for the electrodes: high effective surface area (highly porous material) incoporate with a catalyst endure high temperatures in a corrosive environment1.3 Connecting cells in series – bipolar plateA single fuel cell generates only a small voltage drop (0.7V). In order to generate a useful voltageseveral cells are connected into series. Such a connection is called a stack. The simplest way tocreate a series connection would be to connect the edge of each anode to the cathode of the nextcell. The problem of such a system is, that the electrons would have to travel through the wholeelectrode, thus the ohmic resistance would be relative high.A better alternative are bipolare plates. These plates are made of a conducting material (Graphite,stainless steel, some ceramic materials) with connections areas over the surface as well as gassupply channels for the electrodes. The bipolare plates are complex parts and difficult tomanufacture, thus the most expensive parts of a fuel cell.In order to optimize the design of bipolare plates several aspects counteracts each other: good electric contact requires a large contact area and a thin plate, to reduce the ohmicresistance. This lead to small narrow gas channels thus bad gas distribution over the electrode and lowgas flow rates or high operation pressueres.1.4 Gas supply and coolingA major problem of manufacture fuell cells is leakage of the reactant gases. In order to seperate thegases strictly from each other are the electrrodes sealed at the edges with a gasket.External manifolds:gas is supplied through an external lid, that covers the whole side of the stack.+ simple structure- cooling of the system. Needs a higher air supply rate for cooling. Makes the cell more inefficient- higher leakage probability. The gasket is not pressed evenly onto the electrodes.Internal manifolds:gas is supplied into the stack through additional supply channels in the bipolare plates.- That requires larger and more complex bipolare plates with extra channels for the reactants supply.These channels are connected to the the gas channels along the electrodes.+ solid stack with the gases fed in at the same ends of the electrical connections.+ additional cooling channels along the bipolare plates.+ lower leakage probability
1.5 Fuel cell typesBesides the practical issuses fuel cells have to fundamental technical problems: slow reaction rate, low current and power hyrogen is not readily available as fuelThere are six viable systems of fuel cells:TypeAlkalineAFCProton exchangemembranPEMFCDirect methanolDMFCPhosporic acidPAFCMoltencarbonateMCFCMobileIonOperatingtemp. [°C]Anode and cathodereactionsApplication and notesOH - 50-200 Space vehiclesH + 30-100 2H 2 →4H + +4e - (a)O 2 +4H + +4e - →2H 2 O (c)Vehicles and mobileapplications, low power CHPH + 20-90 Portable electronic systems,low power but long runningtimeH + ca. 220 For small CHP plants(hundreds of kW), firstcommercial oneCO 32-ca. 650For medium to large CHP (upto MW)Solid OxideSOFCO 2- 500-1000 For all sizes of CHP (from kWto multi MW)Advantages and disadvantages are discussed later.1.6 Other cells – Some fuel cells, some not1. Biological fuel cellsuse an organic fuel, like methanol or ethanol, and enzymes to produce energy. Not yetcommercial2. Metal air cells (batteries)at the negative electrode the metal reacts with an alkaline electrolyte to form metal oxide orhydroxide. The released eelctrons pass though an external electric circuit to the air cathode,where water and oxygen reacts to hydroxyl ions (see alkaline fuel cell). Metal oxide isdissolved into the electrolyte, which have to be renewed after some time. Consumption ofanode and electrolyte.Very good energy densitiy: commercial for low power applications with long running time.Research for higher power for electric vehicles.3. Reodx flow cells or regenerative fuel cellsFor charging the reactants are removed from the electrodes and stored in a tank. Fordischarging the electrolytes are supplied back to the electrodes. Is used for very largecapacity rechargeable batteries. The electrocolyte changes during operation and the systemcannot work idenfinitely.
Page 1: -Fuel Cell Systems Explained-(Summa
Page 5 and 6: Fuel cells can be used successfully
Page 7 and 8: to the HHV or LHV. If the informati
Page 9 and 10: The products are three parts of H 2
Page 11 and 12: Figure 3.2 shows the performance of
Page 13 and 14: In order to get the current instead
Page 15 and 16: Assumption:Introduction a limiting
Page 17 and 18: To summarize the irreversibilities
Page 19 and 20: 4.4 Water management in the PEMFCWt
Page 21 and 22: With Pe: electrical Power of the st
Page 23 and 24: 4.4.5 External humidification - pri
Page 25 and 26: 4.6 PEMFC connection - the bipolare
Page 27 and 28: with T 1 : entry temperatureη c :
Page 29 and 30: thus leads to higher efficiency. He
Page 32 and 33: minimum difference. This so-called
Page 34: converted into hydrogen at operatin

Fuel Cell Systems Explained - from and for SET students

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