1. Renewable Energy Part 3 Professor Mohamed A. El-Sharkawi

2. Fuel Cell

3. Hydrogen and Hydrogen Gas
Electron
Shell
Proton H H2

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

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

6. 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

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

8. 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.

9. 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 °C
35–50%
Solid Oxide (SOFC)
Ceramic Oxide
Hydrogen, methane
600–1000°C
45–60%
Molten Carbonate (MCFC)
Alkali-Carbonates °C
40–55%
Direct Methanol (DMFC)
Methanol solution in water
50-120°C
35–40%

11. 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

12. 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.

13. 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 = 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

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

15. 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

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

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

18. 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)

19. Polarization Characteristics of FC
Activation
Ohmic
Mass Transport
Voltage and Power
Power
Voltage
Current

20. 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%.

21. 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