COMPREHENSIVE DEPICTION OF A SUCCESSFUL CFD MODELING COMPREHENSIVE DEPICTION OF A SUCCESSFUL CFD MODELING FOR A REAL GEOMETRY SOLID OXIDE FUEL CELLS FOR A REAL GEOMETRY SOLID OXIDE FUEL CELLS P M V Subbarao P M V Subbarao Associate Professor Mechanical Engineering Department Indian Institute of Institute of Technology Technology – – New Delhi New Delhi Indian More You Can See Through ….. More Confidence…
Research Interests Research Interests • Use of high end experimental and computational fluid flow and heat transfer methods for solving Real Time Problems. • CFD simulation of In-cylinder flow in I.C. Engine to predict mixture preparation in advanced engines. • Experimental mapping of temperature and heat transfer coefficient in advanced fin and tube heat exchangers. • Experimental and Computational analysis of fluid flow with shocks in Super Sonic Ejectors. • CFD analysis of SOFC Stacks for Distributed Power Generation.
Reference SOFC- Reference SOFC -GT system GT system – – Regenerative Brayton cycle Regenerative Brayton cycle Air in Combustor Exhaust Air Filter Excess air Gas Turbine Compresso r DC SOFC G AC Generator Power conditioning system Fuel Air Recuperator Compressor Natural Gas
Motivation for hybridization Motivation for hybridization � Due to the polarization losses � Due to the polarization losses,100% utilization of hydrocarbon fuel is not 100% utilization of hydrocarbon fuel is not possible in the fuel cell stack possible in the fuel cell stack � Hydrocarbon fuels used in SOFC that produce power also produce Hydrocarbon fuels used in SOFC that produce power also produce � rejected heat. (Reversible reaction, Process irreversibility). (Reversible reaction, Process irreversibility). rejected heat. � The heat must be rejected in order to maintain its temperature a The heat must be rejected in order to maintain its temperature at a t a � desired level. desired level. US energy studies
CFD Modeling of SOFC Stacks CFD Modeling of SOFC Stacks • Solid Electrolyte. • High temperature and high pressure fluid flow with heat generation. • Complex flow geometry: Flow through a system of micro channels. • Stack of such geometries. • Many fuel options. • Huge scope for optimization and improvements.
Overall Research Objective Overall Research Objective � To Investigate / develop SOFC based power generation processes To Investigate / develop SOFC based power generation processes � � Achieve high electricity generation efficiencies Achieve high electricity generation efficiencies � � � Key Issues to be Investigated: Key Issues to be Investigated: • fuel composition (Especially H2 and CO mixtures), • fuel composition (Especially H2 and CO mixtures), • • utilization factor, utilization factor, • • Temperature & pressure, Temperature & pressure, • operating cell voltage or current density, • operating cell voltage or current density, • efficiency (stack and hybrid cycle) and • efficiency (stack and hybrid cycle) and • cost (if possible) • cost (if possible)
Classification based on electrolyte used Classification based on electrolyte used V Max attained e production H 2 OH - O 2 60 0 C 100 kW AFC H 2 O O 2 H + 60 0 C 250 kW PEMFC H 2 H 2 O O 2 CH 3 OH H + 80 0 C 2 MW DMFC H 2 O CO 2 O 2 H + 190 0 C 11 MW PAFC H 2 H 2 O H 2 O 2 2- 650 0 C 2 MW MCFC CO 3 H 2 O CO 2 H 2 O 2- 1000 0 C 1 MW SOFC O 2 H 2 O Fuel Oxygen Anode Cathode Electrolyte
Solid Oxide Fuel Cells Solid Oxide Fuel Cells � Electrolyte is solid and conducts oxygen ion at 650 o o C C � Electrolyte is solid and conducts oxygen ion at 650 � High operating temperature and solid electrolyte permits flexibility in choosing the fuel lity in choosing the fuel � High operating temperature and solid electrolyte permits flexibi � Ideal fuel is hydrogen but any hydrocarbon fuel can be used after reformation r reformation � Ideal fuel is hydrogen but any hydrocarbon fuel can be used afte � Fuel reformation can be done internally or externally � Fuel reformation can be done internally or externally Chemical Reactions Chemical Reactions Reformation Reaction Reformation Reaction CH 4 + H 2 O 3H 2 + CO CH 4 + H 2 O 3H 2 + CO (Endothermic) (Endothermic) CO + H 2 O CO 2 CO 2 + H + H 2 Shift Gas Reaction CO + H 2 O Shift Gas Reaction 2 Electrochemical Reactions Electrochemical Reactions H 2 H 2 O + 2e O + 2e - - H 2 H 2 + O + O 2 2- - At anode At anode CO 2 CO 2 + 2e + 2e - - CO + O 2 CO + O 2- - Exothermic Exothermic ½ O ½ O 2 2 + 2e + 2e - - O 2 O 2- - At cathode At cathode
Single cell - - dismantle view dismantle view Single cell Cathode ( LSM –YSZ ) Interconnects ( Electro ceramic family ) Anode ( Ni-YSZ ) Electrolyte Seals ( YSZ )
Stack assemble Stack assemble Individual fuel cells must be combined to produce appreciable voltage levels ltage levels Individual fuel cells must be combined to produce appreciable vo Array of cells Single cell assemble Stack assemble Array of cells with insulation plate
Stack modeling Stack modeling � Single cell geometry is analyzed for the � Single cell geometry is analyzed for the given inlet fuel given inlet fuel Top most cell Top most cell � Stack effect is evaluated by multiplying Stack effect is evaluated by multiplying � the number of cells with single cell’s the number of cells with single cell’s result result ∑ = � � Stack Power Stack Power Intermediate cell Intermediate cell P N IV cell � � Effluent heat Q = Q Effluent heat Q = Q elect elect + Q + Q s s - - Q Q r r - - Q Q surr surr ∑ = η + ∆ Electrochemical process Electrochemical process Q i T S Bottom cell Bottom cell elect Heat consumed by reforming Q Heat consumed by reforming Q r r Heat associated with shift gas reaction Q s Heat associated with shift gas reaction Q s
System Modeling - - Macroscopic approach Macroscopic approach System Modeling � Thermodynamic modeling of a single cell and a stack � Polarization modeling for a fuel cell � Performance analysis of SOFC-GT hybrid power generation cycle
Cell Potential Calculation Cell Potential Calculation [ ] [ ] δ c C D ∆ =∆ + General form of Nernst Expression 0 G G RT ln [ ] [ ] α β A B [ ] ∏ reactant activity RT = + ∆ = − 0 Ideal cell potential E E ln ( . .) i e G nFE ∏ [ ] nF product activity kJ mol − + → ∆ = − 1 H 1/ 2 O H O H 241 2 2 2 298 ⎛ ⎞ 1/ 2 P P RT = + ⎜ H O ⎟ o E E ln 2 2 ⎜ ⎟ 2 F P ⎝ ⎠ H O 2 Solid oxide fuel cell kJ mol − kJ mol − + → ∆ = − + → ∆ = − 1 1 H 1/ 2 O H O H 241 CO 1/ 2 O CO H 283 2 2 2 298 2 2 298 ⎛ ⎞ ⎛ ⎞ 1/ 2 1/ 2 P P P P RT RT = + ⎜ ⎟ CO O = + ⎜ ⎟ o H O o ln E E 2 E E ln 2 2 ⎜ ⎟ ⎜ ⎟ CO H 2 2 F P ⎝ ⎠ 2 2 F P ⎝ ⎠ CO H O 2 2 Actual cell potential (V V actual ) = Ideal cell potential (E) – – losses or polarization losses or polarization Actual cell potential ( actual ) = Ideal cell potential (E)
Fuel Cell Efficiency Fuel Cell Efficiency 237.1 η = = , O , At STP H 2 + ½ O 2 H 2 At STP H 2 + ½ O H 2 O 0.83 2 285.8 useful energy η = ∆ ideal H useful power = ∆ G / 0.83 × ( volts ) current 0.83 V = = actual actual × ( volts ) current / 0.83 V ideal ideal
Polarization Distinctiveness 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 Losses or polarizations in Actual Performance ( ) η � Activation over potential Activation over potential – Flow of ions Flow of ions � – act should overcome the electronic barrier. should overcome the electronic barrier. ( ) η � Ohmic over potential Ohmic over potential – Resistance Resistance � – ohm offered by the total cell components to the offered by the total cell components to the flow. flow. ( ) η � Concentration over potential Concentration over potential - Gas - Gas � con transport losses, dilution of fuel as the transport losses, dilution of fuel as the reactions progress. reactions progress. V- -I Characteristics of the Fuel cell I Characteristics of the Fuel cell V
Summation of electrode polarization Summation of electrode polarization Activation and concentration polarizations – – both Anode and Cathode both Anode and Cathode Activation and concentration polarizations η = η + η η = η + η anode act a , con a , cathode act c , con c , Polarizations increase Anode potential and decrease Cathode potential ntial Polarizations increase Anode potential and decrease Cathode pote = + η = − η V E V E anode anode anode cathode cathode cathode Cell potential Cell potential = − − V V V iR actual cathode anode ( ) ( ) = − η − + η − V E E iR actual cathode cathode anode anode
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