Module 11: Fuel Cell H H 2. Generation of Hydrogen. Types of Fuel Cells. Hydrogen and Hydrogen Gas 5/16/2009. Professor Mohamed A. El-Sharkawi.

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1 and Gas Module 11: Fuel Cell Shell Electron Professor Mohamed A. ElSharkawi Proton H H 2 Generation of Hydrocarbon fuel CH 2 (H 2 O) O 2 former Re H 2 O H 2 CO 2 CO CO co onversion CO 2 H 2 Fuel Cell Types of Fuel Cells Fuel Cell Gas Cathode Gas Approximate Typical Temperature Efficiency Proton Exchange Membrane (PEM) Solid polymer membrane Pure or atmospheric oxygen 80 C 35 60% Pure oxygen C 50 70% Alkaline (AFC) Potassium hydroxide Phosphoric Acid (PAFC) Phosphorous Atmospheric oxygen Solid Oxide (SOFC) Ceramic, Atmospheric Oxide methane oxygen Molten Carbonate Alkali, Atmospheric (MCFC) Carbonates methane oxygen Direct Methanol Solid polymer Atmospheric (DMFC) membrane oxygen Methanol solution in water C 35 50% C 45 60% C 40 55% C 35 40% 1

2 Proton Exchange Membrane (PEM) Fuel Cell O 2 2H 2 4H + Hd ons (Air) Cathode 2 H 2 O Chemical Reaction ions reaction: Cathode Reaction: Overall Reaction: O 2 2H 2 4H + ons (Air) electrons 2 H H e 4 H 4e O 2 H O H H O2 2 H 2O Cathode 2 H 2 O Fuel Cell Produces power without combustion or rotating machinery. Makes electricity by combining hydrogen ions, drawn from a hydrogencontaining fuel, with oxygen atoms. Fuel Cell The current is proportional to the size (area) of the electrodes. The voltage is limited electrochemically to about 1.23 volts per electrode pair, or cell. Cells can be stacked until the desired power level is reached. 2

3 Alkaline fuel cell (AFC) (2H 2 ) Hydroxyl ons (4OH ) + (O 2 ) Cathode Phosphoric acid fuel cell (PAFC) Molten carbonate fuel cell (MCFC) (2H 2 ) ons (4H + ) + (O 2 ) (2H 2 ) Carbonate ons (2CO 3 2 ) + (O 2 ) Cathode Carbon Dioxide (2CO 2 ) e Cathode 3

4 Economy Methanol (CH 3 OH) Carbon Dioxide (CO 2 ) (H 2 O) ons (6H + ) Cathode + (H 2 O) (1.5O 2 ) CoGen ndustrial use Air conditioning i i etc. for drinking, agriculture, etc. Steam FC System Electricity Electric Renewable energy system Electricity Generation Distribution nfrastructure Electricity FC System Electricity Electric Steam CoGen ndustrial use Air conditioning i i etc. for drinking, agriculture, etc. Utility Grid Process of deal Fuel Cells Fuel cell has two processes Thermal process Tell us how much energy can be produced by the fuel cell Electrical processes. Gives the value of the voltage and current. Thermal Process Gibbs free energy G (generated energy) G H Q H is the enthalpy of the process energy in the fuel (hydrogen) at the (NPUT ENERGY) At one atmospheric pressure and 298 o K, H = kj/mole Q is the entropy of the process. entropy is the wasted heat during the process (LOSSES) At one atmospheric pressure and 298 o K, Q= 48.7 kj/mole. G H Q t output energy input energy G H % kj/mole 4

5 Electrical Process Amount of electric charge q e in a mole of electrons q N * q e A * q: the charge of a single electron (1.602*10 19 coulomb) N A is the Avogadro number (0.6002*10 24 molecules/mole) For each hydrogen molecule, 2 electrons are released, then the number of electrons N e released by one mole of H 2 N e 2N A Electrical Process The charge of electrons released by one mole of H n electric process Electric Energy Output voltage qm Ne * q q m t E V t V q m V E q G m q m : current t: time Example Assume ideal conditions; compute the output voltage of a PEM fuel cell. Solution 24 N e 2N *10 q m N A e * q *10 *1.602*10 V E q G m q m * * V *10 5 Modeling of FC: Losses Activation loss (electrode kinetic) due to the anode and cathode reactions at low currents or when the cell is activated (oxygen are not fully diffused at starting) Ohmic loss due to the resistances of the electrolyte and electrodes Mass transport loss When the input reaction is less than the output reaction (when the output current is very high and the input reaction cannot match the needed demand) 5

6 Voltage and Power Polarization Characteristics of FC Activation Ohmic Mass Transport Power Voltage Current Evaluation of FC FCs have great potential in transportation, household use and utility size generation. Several generations of fuel cell automobiles and buses are already roaming city streets. Fuel cells are used as backup systems or independent source of energy. Several sensitive installations such as hospitals, satellites, and military installations i are using fuel cells as backup systems. The efficiency of the fuel cell including reformer is 26% 40%. Evaluation of FC High temperature fuel cells produce enough heat that can be used in industrial processes A single fuel cell produces a dc voltage < 1.5V. For higher voltage, fuel cells are stacked in series FCs have relatively short lifetime. Their various components can suffer from pollution and corrosions. Pure hydrogen is a volatile gas, and requires special storage and transportation. cannot be found free in nature, it is often extracted by reformers Generation of Reforming Electrolysis of water energy intensive process that reduces the overall efficiency of the entire system to less than 10%. Sodium hydride (NaH) can be liberated from a solution of sodium hydride (NaH) in the presence of certain catalyst without using electric energy. Old algae Genetically altered to produce hydrogen instead of oxygen during their photosynthesis process. Hydride alloys Some metal hydrides absorb hydrogen and store it between their molecules when they are cooled and release the hydrogen when heated 6

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