Fuel Cell Electric Prime Movers P M V Subbarao Professor Mechanical Engineering Department A Single Stage Energy Conversion Systems .....

The Future of Vehicles – Various Technologies for Automobiles Vehicle weight challenges 2nd generation biofuels high transportation Long range public transport Fuel cell vehicles Public transport Economy of fuel Plug-in hybrids City LDVs Everyday use Battery vehicle 2nd car Economy of propulsion system acceptance Energy density Electro cycle safety low distance Short trips (city) Long trips (highway)

Fuel cell Electric Prime Mover

Fuel Cells “Fuel cells are the electrochemical device that converts chemical energy of the fuel directly into the electrical energy". H2 O2 H2O 2e- Electrolyte (ion conductor) Cathode (positive electrode) Anode (negative electrode) Air flow channel Fuel flow channel Heat + Fuel in Depleted air Negative Ion Load Air Positive ion or Depleted fuel and product gases out Electrolyte layer is in contact with a porous anode and cathode on sides. Hydrogen flows along anode side. Oxygen flows along cathode. O2 H2 H2O

Electrochemical Reaction Electrochemical reaction produces electricity At cathode ½ O2 + 2e-  O2- At anode H2 + O2-  H2O + 2e- Overall reaction H2+ ½ O2  H2O Emits steam and heat The chemical reaction in a fuel cell is similar to that in a chemical battery. The thermodynamic voltage of a fuel cell depends on the energy released and the number of electrons transferred in the reaction. The energy released by the battery cell reaction is given by the change in Gibbs free energy, ΔG. The change in Gibbs free energy in a chemical reaction can be expressed as

Ideal Fuel Cell Capacity The “ideal” efficiency of a reversible galvanic cell is related to the enthalpy for the cell reaction by ηid will be 100% if the electrochemical reaction involves no change in the number of gas moles, that is, when ΔS is zero.

Fuel Cell Technologies There are six major types of fuel cells depending on the type of their electrolyte. Proton exchange membrane (PEM) or Polymer exchange membrane fuel cells (PEMFCs) Alkaline fuel cells (AFCs) Phosphoric acid fuel cells (PAFCs) Molten carbonate fuel cells (MCFCs), Solid oxide fuel cells (SOFCs) Direct methanol fuel cells (DMFCs)

Classification based on electrolyte used V AFC PEMFC PAFC MCFC SOFC H2 OH - Fuel H2O H + DMFC O 2- CO3 2- 600C 100 kW 600C 250 kW 1900C 11 MW 6500C 2 MW 10000C 1 MW 800C 2 MW O2 CO2 CH3OH Anode Electrolyte Cathode Oxygen e Max attained production

Ideal Thermodynamic Potential The maximum thermodynamic work output obtained from the above reaction is related to the free-energy change of the reaction. The chemical reactions occur at a constant temperature and pressure. The above reaction is spontaneous and thermodynamically favoured because the free energy of the products is less than that of the reactants. The standard free energy change of the fuel cell reaction is indicated by the equation Where ΔG is the free energy change, n is the number of moles of electrons involved, E is the reversible potential, and F is Faraday’s constant

The Nernst Equation The Nernst Equation enables the determination of cell potential under non-standard conditions. It relates the measured cell potential to the reaction quotient and allows the accurate determination of equilibrium constants. General form of Nernst Expression Ideal cell potential

Other Electro-chemical Reactions C+1/2O2 CO CO+1/2O2 CO2 C+O2 CO2 Reforming Reactions for Methane : CH4 + H2O 3H2+CO This is an endothermic reaction

Electrode Potential and Current–Voltage Curve

Actual Fuel Cell Efficiency Actual cell potential (Vactual ) = Ideal cell potential (E) – losses or polarization

Polarization Distinctiveness Actual potential of the cell is less than the equilibrium potential due to irreversible losses or polarization. Losses or polarizations in Actual Performance Vact Vohm Vcon

Polarization Distinctiveness Actual potential of the cell is less than the equilibrium potential due to irreversible losses or polarization. Losses or polarizations in Actual Performance V-I Characteristics of the Fuel cell Activation over potential (Vact) Flow of ions should overcome the electronic barrier. Ohmic over potential (Vohm) Resistance offered by the total cell components to the flow. Concentration over potential (Vcon) Gas transport losses, dilution of fuel as the reactions progress.

A hydrogen–air fuel cell system