NTU ME H.K. Ma Department of Mechanical Engineering National Taiwan University, Taipei, Taiwan November, 2009 台灣大學機械工程系能源環境實驗室.

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Presentation transcript:

NTU ME H.K. Ma Department of Mechanical Engineering National Taiwan University, Taipei, Taiwan November, 2009 台灣大學機械工程系能源環境實驗室

 Maximum electric work (W el ) at constant temp. and pressure is given by Gibbs free energy: n is no. of electrons F is Farady’s constant E is ideal potential of the cell

 Besides, Gibbs’ free energy can be written as: ∆H is enthaply change ∆S is entropy change T∆S represents the unavailable energy resulting from the entropy change

At constant pressure, the specific entropy at temperature T is given by It than follows that,

 The Gibbs’ free energy can be expressed by the equation For the generation reaction, Substituting the equation into then, This is general form of the Nernest Equation

NTU ME

 The ideal efficiency of a fuel cell, operating reversibly, is then, At standard condition (298K, 1atm), thermal energy (∆H ) in the hydrogen/oxygen reaction is 285.8KJ/mole, and the free energy for useful work is 237KJ/mole, therefore,

 The thermal efficiency of a hydrogen/oxygen can be written in term of the actual cell voltage

 Activation losses are caused by sluggish electrode kinetics. It is possible to approximate the voltage drop by a semi-empirical equation, called the Tafel equation. α is the electron transfer coefficient of the reaction at the electron being addressed i 0 is the exchange current density

 Ohmic losses occur because of resistance to the flow in the electrolyte and resistance to the flow electrons through the electrode.  Because the electrolyte and fuel cell electrodes obey Ohm’s law, the ohmic losses can be expressed by the equation,

 The total resistance, R, which includes electronic, ionic, and contact resistance

 The combined effect of the losses for a given cell and given operating conditions can be expressed as polarizations. The total polarization at the electrode is the sum of anode and cathode.

 The cell voltage includes the contribution of the anode and cathode potentials and ohmic polarization When and are substituted in then,

19 AFCPEMFCDMFCPAFCMCFCSOFC IonOH - H+H+ H+H+ H+H+ CO -2 3 O -2 Temperature 50~200 ℃ 30~120 ℃ 20~90 ℃ ~220 ℃ ~650 ℃ 500~1000 ℃ FuelH2H2 H2H2 CH 3 OHH2H2 NG, H 2 … OxidesO2O2 O2O2 O2O2 O2O2 O2O2 O2O2 Output (W)1KW~10K1~100K1~10010K~1M1M~10M1K~10M 2015/12/25

20 Water productT (K)E (V) Liquid Liquid Gas Gas Gas Gas Gas /12/25

21 Volume flow rate H2H cc/min. O2O cc/min. Air cc/min. 2015/12/25

22 Water productT (K)Efficiency limit (%) Liquid29883 Liquid35380 Gas37379 Gas Efficiency of fuel cell = electric energy produced per mole of fuel / enthalpy of formation Limit of fuel cell efficiency = Carnot Limit= Thermodynamic efficiency 2015/12/25

Reactant Product Reactant Product Energy Barrier Reaction Rate 2015/12/2523

Activation losses: Ohmic losses: Concentration losses: 2015/12/2524