Download Renewable Energy, Part 3

Renewable Energy
Part 3
Professor Mohamed A. El-Sharkawi
Fuel Cell
Hydrogen and Hydrogen Gas
Electron
Shell
Proton
H
H2
Generation of Hydrogen
Hydrocarbon fuel
H2O
CO2
Water
(H2O)
O2
Reformer
Methylene
H2
CO2
CO
CO conversion
CH2
Fuel
Cell
H2
- --
Fuel Cell (Electrochemical Process)
Load
I
Oxygen (Air)
Ions
Anode
Hydrogen
O2
4H+
Cathode
---
2H2
- --
Electrolyte
Water
2 H2O
electrons
 Anode reaction:
 Cathode Reaction:
• Overall Reaction:
-- --- -4H+
Ions
Oxygen (Air)
Electrolyte
2 H 2  4 H   4 e
4 H   4 e   O2  2 H 2O
2 H 2  O2  2 H 2O
O2
Cathode
Hydrogen ions
I
Anode
2H2
Hydrogen
Load
---
Chemical Reaction
Water
2 H2 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.
Types of Fuel Cells
Fuel Cell
Proton Exchange
Membrane (PEM)
Electrolyte
Anode Gas
Solid polymer Hydrogen
membrane
Alkaline (AFC)
Potassium
hydroxide
Phosphoric Acid (PAFC) Phosphorous
Hydrogen
Solid Oxide (SOFC)
Hydrogen,
methane
Hydrogen,
methane
Methanol
solution in
water
Molten Carbonate
(MCFC)
Direct Methanol
(DMFC)
Ceramic
Oxide
AlkaliCarbonates
Solid polymer
membrane
Hydrogen
Cathode Gas Approximate Typical
Temperature
Efficiency
Pure or
atmospheric
oxygen
Pure oxygen
80°C
35–60%
65-220°C
50–70%
Atmospheric
oxygen
Atmospheric
oxygen
Atmospheric
oxygen
Atmospheric
oxygen
150-210°C
35–50%
600–1000°C
45–60%
600-650°C
40–55%
50-120°C
35–40%
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)
G  H Q
• 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)
G  H Q
• At 298o K
– H = 285.83 kJ/mole
– Q= 48.7 kJ/mole
G  H  Q  285.83 - 48.7  237.13 kJ/mole
output energy G 237.13
t 


 83%
input energy
H 285.83
Electrical Process
• Amount of electric charge qe in a mole of electrons
qe  N A * q
• 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
Ne  2N A
Electrical Process
• The charge of electrons released by one mole of H2
qm  N e * q
• Coulomb’s law
• Electric Energy
qm
I
t
I: current
t: time
E  V I t  V qm
• Output voltage
E
G
V

qm qm
Example
• Assume ideal conditions; compute the output
voltage of a PEM fuel cell.
• Solution
N e  2 N A 1.2004 *10 24
qm  N e * q  1.2004 *10 24 *1.602 *10 19  1.9288 *105 C/mole
E
G 237.13 *103
V


 1.23 V
5
qm qm 1.9288 *10
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)
Polarization Characteristics of FC
Voltage and Power
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 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