MIE 517 Final Celebration of Learning Wednesday, April 19, 2017, 2-4:30pm

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1 MIE 517 Final Celebration of Learning Wednesday, April 19, 2017, 2-4:30pm Instructions: Answer all questions and show all work for which you wish to receive credit in the answer booklets. You may use one 8.5 x 11" sheet of paper with handwritten notes (no typed or photocopied notes) on both sides and a non-graphing calculator. State any assumptions that you need to make in answering the questions. Good luck! Some data that may be helpful: Molar masses: H: 1.01 g/mol, 0: g/mol, N: 14.0 g/mol, C: g/mol Content of dry air: assume 79 mol% N2 and 21 mol% 02 (unless otherwise indicated) Ideal gas constant: J/(mol K); Faraday's constant: C/mol e Ideal gas law: PV=nRT; assume all gases are ideal at the operating conditions used 1 atm = Pa 1 L = 1000 cm L= 1 m3 Density of liquid ethanol = g/ml; density of liquid water = 1 g/ml Part I (20 pts total) A square PEMFC (cell A) has an active area of 900 cm2. Another has an active area of 100 cm (cell B). Both cells operate at the same operating conditions (25 C, 1 atm H2 at anode, 1 atm air at cathode, water formed in liquid state at cathode). Both cells use the same materials, and their electrolytes each have a thickness of 100 tm. The area-specific polarization resistances of both anodes are 0.1 Qcm2, and the ASR values of the cathodes are both 0.2 Q cm2 between OCV and 0.7 V. How much total power can each cell produce at 0.7 V? How much fuel will each cell consume in g per hour when operating at 0.7V? If both cells would be operated at a new choice of conditions such that the same total amount of power were produced by each cell, which cell would operate more efficiently, and why? Page 1 of

2 Part II You normally operate 2 high temperature fuel cell stacks, one based on SOFCs, and one based on MCFCs, at T1000 K. Both stacks typically operate on a fuel-side mixture of 97 %H2 and 3% H20 by volume. The SOFC uses air at the cathode, while the MCFC uses a mixture of 85% air and 15% CO2 by volume. Both stacks have their anode and cathode sides open to atmospheric pressure (total pressures = 1 atm at all electrodes) 2. (25 pts total) (a) What is the ideal electrical efficiency based on the oxidation of hydrogen at standard activities of all reactants and products at 1000 K (in other words, not at the compositions given above, but with all partial pressures of reactants and products equal to I atm)? When an extended interruption to the hydrogen supply occurs, you decide to use some locally-available vodka (a liquid solution of 40 vol% ethanol and 60 vol% water) to continue producing electricity. The vodka is fed as a liquid to the stacks, where it rapidly evaporates by the time it reaches the cells, and then undergoes complete reaction by steam reforming and water gas shift until only CO2, H2, and H20 remain at the anodes. Calculate the ideal voltage for the SOFC stack on the vodka fuel. Calculate the ideal voltage for the MCFC stack on the vodka fuel. If the sum of activation, ohmic, and mass transport losses is 0.3V and there are no OCV losses, calculate the electrical efficiency of the SOFC operating on the vodka fuel and also on the original humidified hydrogen fuel, assuming complete fuel utilization. For comparison purposes (to contrast with part (a)), what would be the ideal electrical efficiency if dry ethanol were directly oxidized at the anode (rather than reformed and water gas shifted to hydrogen followed by electrochemical oxidation of the hydrogen)? 3. (30 pts total) If instead of vodka or hydrogen, you used methane (from desulfurized natural gas supplied to the building) in either of the SOFC or MCFC stacks, consider a situation where no extra steam is supplied, but instead, the methane dissociates slightly, releasing hydrogen that electrochemically oxidizes to steam. The steam thus produced then continues to react with the methane, reforming it and water-gas-shifting it internally. Page 2 of 7

3 Calculate the enthalpy change involved in the steam reforming and water gas shift combined reactions to fully convert the methane to CO2 plus H2. Calculate the higher heating value of both methane and the reformate resulting from the stoichiometric steam reforming plus water gas shift of one mole of methane. If the electrochemical operation of the fuel cell stacks is to produce only enough waste heat to reform and water gas shift the fuel, what would the electrical efficiency have to be? If methane is directly electrochemically oxidized (rather than undergoing reforming and water gas shift), what operating voltage would result in the efficiency listed above, assuming all reactant and product activities = I? If hydrogen is the electrochemically oxidized species, what is the minimum amount of additional waste heat that would be released externally from the fuel cell stack, once some of the waste heat from the electrochemical reaction is used to reform and water gas shift the methane to hydrogen? Assume all reactant and product activities = 1. What is the maximum electrical and thermal combined efficiency from part (e) for the combination of the hydrogen production from methane followed by electrochemical oxidation of the hydrogen in situ within the fuel cell anode? Part III 4. (loptstotal) Sketch a parametric degradation graph of R /R vs. R /R, for five fuel cells, each experiencing only I predominant degradation mode. Cell A is experiencing partial electrode delamination of the cathode from the electrolyte. Cell B is experiencing partial detachment of the interconnect from the cathode. Cell C is experiencing oxidation of a metallic bi-polar plate interconnect. Cell D is experiencing sulfur poisoning of the anode. Cell E is also experiencing sulfur poisoning of the anode, and it has an anode thickness half that of cell D. Indicate the points corresponding to time t=o, and sketch approximate trajectories of the curves for each cell. Comment briefly (approx. 1 sentence each) on the expected time scales of the degradation modes mentioned in part (a). Page 3 of 7

4 Part IV 5. (l5ptstotal) An alkaline fuel cell is operating at 25 C. Humidified air (with 3 mol% water vapour and 97 mol% dry air) is delivered to the cell, corresponding to air that is bubbled through water at room temperature. The cell is observed to have a limiting current density of 0.2 A/cm2 at this condition. Both electrodes are open to atmosphere (i.e. have a total pressure of I atm). Assume that product water condenses to form a liquid at the anode. Estimate the cathode mass transport overpotential when the cell is operated at 0.15 A/cm2. Calculate the ideal voltage for the condition described above. At the same operating current density, the cathode feed is now replaced by a mixture of 15 mol% water vapour and 85 mol% dry air. Estimate the new cathode overpotential. END OF QUESTIONS THERMODYNAMIC TABLES: H2[g] HYDROGEN (GAS) Phase T C S -(G--H298)/T H H-H298 G ih1 AG I log K1 1K] [,J/(Kmol) ) [ kj/mol I [ - GAS , ODO , Page 4 of 7

5 31999 OXYGEN (GAS) 02[g] Phase T C, S -(G-H298)/i H H AK1 AG, log K1 [KI [ J/(Krnol) ] [ k,j/mot '-'-'-- [ - GAS : , , , METHANE (GAS) CH4[g] Phase T C, S -(G-H298)IT H H-H298 0 AK1 AG, log K1 [K] [ Jf(Kmot) I [ ' k.j/mot ] I - GAS , , , , , , , , , ETHANOL (GAS) C2H60[9] Phase T C, S -(G-H298)/T H H AK1 AG( log K1 [K] [ J/(Kmot) [- kjfmot J [ - GAS ' Page 5 of 7

6 Enthalpy of formation, Gibbs function of formation, and absolute entropy at 25 C, 1 atm Substance Formula ho kj/kmol 0 kilkmol s ki/kmol- K Carbon C(s) Hydrogen 142(g) Nitrogen N2(g) Oxygen 02(g) Carbon monoxide CO(g) -110, , Carbon dioxide CO2(g) -393, , Water vapor H20(9) -241, , Water H20( ) -285, , Hydrogen peroxide H202( -136, , Ammonia NH3(g) -46,190-16, Methane CH4g) -74,850-50, Acetylene C2H2(9) +226, , Ethylene C2H4 g) +52, , Ethane C2H6(g) -84,680-32, Propylene C3H6(g) +20, , Propane C3H8(g) -103,850-23, n-butane C4H10(th -126,150-15, n-octane C8H 8(g) -208, , n-octane C8H18(e) -249,950 +6, n-dodecane C12H( -291, , Benzene C6H6(g) +82, , Methyl alcohol CH30H(g) -200, , Methyl alcohol CH39H(e) -238, , Ethyl alcohol C2H5OH(g) --235, , Ethyl alcohol C2H50H( ) -277, , Oxygen O(g) +249, , Hydrogen H(g) +218, , Nitrogen N(g) +472, , Hydroxyl OH(g) +39, , Source: From JANAF, Thermochemicai Tables (Midland, MI: Dow Chemical Co., 1971); Selected Values of Chemical Thermodynamic Propetties. NBS Technical Note ; and API Research Project 44 (CarnegIe Press, 1953). Thermodynamics; An Engineering Approach, 0 Ed. YA. Cengal and M.A. Boles, McGraw-Hill, Page 6 of 7

7 28010 CARBON MONOXIDE (GAS) co[g] phase T C, S -(G--H298)/T H H G AH, AG, log K [K] J!(Krrol) kj/mol - I - GAS , , , , , , CO2[g] CARBON DIOXIDE (GAS) Phase T C S -(G-H298)/T H H-H298 G LHf AG, log Kr [K] [-J/(K ] 1 kj/mol I - GAS , , , , , , O.V) ')70'q71 90 q , H20[g] WATER (GAS) Phase 7 CO S -(G )/T H H AHi AG, log K [K] [ JI(Kmo1) ] [ kjlmol. - I GAS 298, , , , , , , l Ann frfl 46, , ,334 43, Page 7 of 7