Renewable Energy Part 3 Professor Mohamed A. El-Sharkawi

Fuel Cell

Hydrogen and Hydrogen Gas Electron Shell Proton H H2

Generation of Hydrogen CO conversion H2O Water (H2O) Reformer CH2 O2 Hydrocarbon fuel CO2 Methylene H2 CO2 CO H2 Fuel Cell

Fuel Cell (Electrochemical Process) - Load I Anode Electrolyte Cathode - - Oxygen (Air) O2 Hydrogen 2H2 Ions 4H+ Water 2 H2O

Chemical Reaction - Anode reaction: Cathode Reaction: Electrolyte Load - Hydrogen 2H2 Ions 4H+ Oxygen (Air) O2 Water 2 H2O I Anode reaction: Cathode Reaction: Overall Reaction: Hydrogen ions electrons

Fuel Cell Produces power without combustion or rotating machinery. Makes electricity by combining hydrogen ions, drawn from a hydrogen- containing 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.

Types of Fuel Cells Fuel Cell Electrolyte Anode Gas Cathode Gas Approximate Temperature Typical Efficiency Proton Exchange Membrane (PEM) Solid polymer membrane Hydrogen Pure or atmospheric oxygen 80°C 35–60% Alkaline (AFC) Potassium hydroxide Pure oxygen 65-220°C 50–70% Phosphoric Acid (PAFC) Phosphorous Atmospheric oxygen 150-210°C 35–50% Solid Oxide (SOFC) Ceramic Oxide Hydrogen, methane 600–1000°C 45–60% Molten Carbonate (MCFC) Alkali-Carbonates 600-650°C 40–55% Direct Methanol (DMFC) Methanol solution in water 50-120°C 35–40%

Hydrogen Economy Renewable energy system Hydrogen Generation Hydrogen Distribution Infrastructure FC System Co-Gen Industrial use Air conditioning etc. Water for drinking, agriculture, etc. Electric Load Hydrogen Oxygen Steam Water Electricity Utility Grid

Process of Ideal 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) H is the enthalpy of the process Thermodynamic potential energy in the fuel For hydrogen, it is the energy at the Anode (INPUT ENERGY) At one atmospheric pressure and 298o K, H = 285.83 kJ/mole Q is the entropy of the process entropy is the wasted heat during the process (LOSSES) At one atmospheric pressure and 298o K, Q= 48.7 kJ/mole The mole is a unit of measurement in chemistry A mole is the amount of elementary entities (atoms, molecules, ions, electrons) in 12 grams of pure carbon

Thermal Process Gibbs free energy G (generated energy) At 298o K H = 285.83 kJ/mole Q= 48.7 kJ/mole

Electrical Process Amount of electric charge qe in a mole of electrons q: the charge of a single electron (1.602*10-19 coulomb) NA is the Avogadro number (6.002*1023 Hydrogen molecules/mole) For each hydrogen molecule, 2 electrons are released, then the number of electrons Ne released by one mole of H2

Electrical Process The charge of electrons released by one mole of H2 Coulomb’s law I: current t: time Electric Energy Output voltage

Example Assume ideal conditions; compute the output voltage of a PEM fuel cell. Solution

Modeling of FC: Losses Activation loss (electrode kinetic) Ohmic loss 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)

Polarization Characteristics of FC Activation Ohmic Mass Transport Voltage and Power 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 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. Hydrogen cannot be found free in nature, it is often extracted by reformers