Fuel Cell Technology

Transcription

1 Fuel Cell Technology 1. Technology overview 2. Fuel cell performance 3. Fuel cell systems 4. Sample calculations 5. Experiment using PEM cell Goal: To provide a better understanding of the fuel cell technology, its benefits and systems issues that influence its application. Reference: Fuel Cell Handbook, 5 th Edition, US DOE, October 2000.

2 Fuel Cell Technology Overview Fuel cell: A device for directly converting the chemical energy of a fuel into electrical energy in a constant temperature process. Besides offering high theoretical efficiency, especially at low temperatures, fuel cells emit low or zero levels of pollutants. They can run on wide variety of fuels ranging from gaseous fuels such as hydrogen and natural gas to liquid fuels such as methanol and gasoline. Main applications: Stationary power generation Transportation Battery replacement.

3 Ballard 250 kw PEMFC power system

4 Vehicle Fuel Cell System

5 Solar Hydrogen System

6 Operation of a Solid Polymer Fuel Cell (SPFC) Under load a single cell produces about 0.7 volts.

7 Fuel Cell Stack In order to achieve a useful output power individual cells are connected together in a stack using an interconnect called bipolar plate. MEA: Membrane Electrode Assembly: A combination of electrolyte and electrodes

8 Fuel Cell Efficiency Carnot efficiency: The maximum efficiency of a heat engine is subject to the Carnot efficiency limitation, which defines the maximum efficiency that any heat engine can have if its temperature extremes are known. η C =1 T L T H Where T H and T L are the absolute high and low temperatures respectively. Fuel cell efficiency: The theoretical efficiency of a fuel cell is related to the ratio of two thermodynamic properties, namely the chemical energy, represented by Gibbs Free Energy ( G o ) and the total heat energy, represented by the Enthalpy ( H o ) of the fuel. η FC = ΔGo ΔH o

9 Fuel Cell Efficiency Efficiency vs Temperature Fuel cell operating at low temperatures emit low levels of pollutants such as SO x and NO x when using methanol and natural gas. Comparison of theoretical efficiency of a fuel cell running on hydrogen as a function of temperature with that of the Carnot efficiency at the same temperature, assuming T L = 25 o C

10 Fuel Cell Efficiency Under Load

11 Fuel Cell Types Two classes: Low temperature High temperature

12 Fuel Cell Types PEFC: Polymer electrolyte fuel cell; AFC: Alkaline fuel cell; PAFC: Phosphoric acid fuel cell; MCFC: molten carbonate fuel cell; ITSOFC: Intermediate temperature solid oxide fuel cell; TSOFC: Tubular solid oxide fuel cell

13 Low temperature fuel cell characteristics: Incorporation of precious metal electrocatalysts to improve performance Fast dynamic response and short start-up times Available commercially and near commercialization Pure supply of hydrogen as a fuel (e.g. catalysts are poisoned by CO) High temperature fuel cell characteristics: Fuel flexibility - use of hydrocarbon fuels Reduces the need for expensive electrocatalysts Generate useful waste heat and well suited for cogeneration and combined cycle applications Long start-up times and are sensitive to thermal transients Expensive construction materials - reliability and durability is of concern At the demonstration stage Fuel Cell Characteristics

14 Fuel Cell Fuel All fuel cell can run on hydrogen as a fuel. High temperature fuel cells can also run directly on other fuels, especially hydrogen rich gases such as methane. Low temperature systems can run on specific liquid fuels such as methanol. The application of the fuel cell system often determines on which fuel it will run, and whether that fuel first needs to be processed into hydrogen-rich reformate, possibly also containing carbon dioxide, nitrogen or other nondetrimental products.

15 Fuels for Stationary Power Generation Common fuel: Natural gas - reformed by separate steam reformer before resulting hydrogen is fed into the fuel cell stack. High temperature fuel cells are able to operate directly to reform fuels such as methane directly on the anode of the fuel cell (DIRECT). This process causes severe temperature gradients across the stack. Use a separate catalyst bed within the system to split the hydrogen and carbon (INDIRECT). Typical fuel cell type: Phosphoric Acid Fuel Cell (PAFC)

16 Fuels for Transportation Common fuel: Hydrogen - severe size, weight, cost and performance constraints Methanol is being suggested to be a good compromise fuel. It is easy to process into hydrogen using autothermal reforming. Reformer-based systems have longer response times and are less efficient and more polluting than direct hydrogen systems. Typical fuel cell type: Direct Methanol Fuel Cell (PAFC)

17 Fuels for Battery Replacement Common fuel: Propane or butane - severe size, weight, cost and performance constraints Fuel cell type: Solid Oxide Fuel cell (SPFC). The electrolyte is a solid, nonporous metal oxide, usually Y 2 O 3 -stabilized ZrO 2. High power density for miniaturization. Also uses most easily handled electrolyte of the low temperature fuel cells.

18 Fuel Cell Stability Suitability of different fuel cell types for applications

19 Fuel Cell System Costs The low temperature fuel cells can cost as little as $30/kW for transport and $300/kW for stationary power in mass production. Fuel cell system for large scale power generation < $1,500/kW Automobile fuel cell system < $50/kW (1 kw = hp)

20 Fuel Cell State of the Art 1. Battery replacement for mobile phones/ laptops 2. Portable power systems of several hindered watts for replacement of small diesel generators. 3. Small power or cogeneration systems 4. Transport based fuel cell systems

21 TSOFC with a Gas Turbine Engine Siemens Westinghouse Concept

22 Distributed Power Generating Systems 13.5

23 System Performance Requirements

24 Subsystems and Components

25 Electrochemical reactions in fuel cells See LECTURES 18 &19 of EML 4450, FALL 2006 Reference: Fuel Cell Handbook, 5 th Edition, US DOE, October 2000.

26 Nernst Equation The Nernst equation and open circuit: The electrochemical work, which is done by the movement of electrons through a difference in a electrical potential, is denoted as W e or W cell. In electrical terms, the work done by electrons with the charge nf (n is the number of electrons transferred per mole of fuel and F is the charge carried by a mole of electrons, which is Faraday s constant - 96,485C/mole -1 ) moving through a potential difference, E ( voltage difference across electrodes) is W e = nfe ΔG = nfe ΔG o = nfe o ΔG =ΔG o + RT lnq E = ΔGo nf RT nf lnq E = E o RT nf lnq E o is the standard electrode potential We also assume here that a complete reversible oxidation of a mole of fuel Electrical work done = charge x voltage

27 Hydrogen Fuel Cell For a hydrogen-oxygen fuel cell, the overall reaction stoichiometry is H O 2 H 2 O The electrons transferred in this reaction, n = 2. Using the partial pressures of water, hydrogen and oxygen in the reaction coefficient, we then have E = E o RT nf ln P H 2O 1/2 P H 2 P O2 Diluting the reactant gases will lower the maximum voltage that the cell can produce.

28 Fuel Cell Reactions Fuel cell reactions and the corresponding Nernst equation a the cell reactions are obtained from the anode and cathode reactions shown in an earlier table.

29 Ideal Standard Potential, E o : Ideal Standard Potential Standard conditions: one atmosphere and 25 o C The ideal cell voltage for H 2 /O 2 fuel cell E o = 1.229V with liquid water as product E o = 1.18V with gaseous water as product E o is a strong function of the cell temperature Ideal voltage, E for the oxidation of hydrogen

30 Irreversible Losses Fuel cell Voltage/Current plot Activation Polarization: Related to the rates of electrochemical reactions at an electrode surface. Ohmic Polarization: Losses due to resistance to the flow of ions in the electrolyte and resistance to flow of electrons through the electrode materials Concentration Polarization: Due to inability of the surrounding reactant material to maintain the initial concentration. Cell voltage, V = E- Losses

31 Activation Polarization It is customary to express the voltage drop due to chemical polarization by strictly empirical equation, called the Tafel equation as ΔV chem = RT αnf ln i i o ~ mv Where α is the electron transfer coefficient of the reaction at the electrode and i o is the exchange current density Gas diffusion electrode reduces the chemical polarization by maximizing the three-phase interface of gas-electrode-electrolyte. The small pores create large reactive surface areas per unit geometrical area and allow free entrance to reactants and exit to products. Increases in pressure and temperature will also generally decrease chemical polarization. Tafel plot (η = V chem )

32 The ohmic losses can be expressed by the equation ΔV ohm = ir Ohmic Polarization Where i is the current flowing through the cell and R is the total cell resistance, which includes electronic, ionic and contact resistance. These losses can be reduced by decreasing the electrode separation and enhancing the ionic conductivity of the electrolyte. Hydrogen-oxygen fuel cells employing concentrated solutions of potassium or sodium hydroxide as electrolytes show that resistance polarization is negligibly low even at fairly high current densities.

33 The concentration polarization loss is given by the following equation: ΔV conc = RT nf i ln 1 Where i is given by the Frick s first law of diffusion (the rate of mass transport to an electrode surface) Concentration Polarization i L i = nfd ( C C B S) δ i L = nfdc B δ Where D is the diffusion coefficient of the reacting species, C B is its bulk concentration, C S is its surface concentration and δ is the thickness of the diffusion layer. The limiting current i L is a measure of the maximum rate at which a reactant can be supplied to an electrode and occurs when C S = 0.

34 Electrode Polarization Activation and concentration polarizations can exist at both the positive (cathode) and negative (anode) electrodes in fuel cells. The total polarization at these electrodes is then the sum of V act and V conc. and V anode = V act,a + V conc, a V cathode = V act,c + V conc, c V anode = E anode + V anode V cathode = E cathode - V cathode PAFC fuel cell

35 Cell Voltage V cell = V cathode -V anode - ir V cell = E cathode - V cathode -(E anode + V anode ) - ir V cell = E e - V cathode - V anode - ir The goal is to minimize the polarization so that V cell E e

36 Fuel Cell Performance Variables Operating variables: Temperature, pressure, gas composition, reactant utilizations and current density System requirements: Power level, voltage or system weight Stationary power plant operation Vehicle application Selection of a cell design point is dependant upon the system design.

37 Temperature and pressure: Fuel Cell Performance Variables The changes in Gibbs free energy with temperature and pressure will effect the ideal potential, E. E T or E P P T = ΔS nf = ΔV nf An increase in operating pressure has beneficial effects - reactant partial pressure, gas solubility, higher mass transfer rates and reduced electrolyte loss by evaporation. Adverse effects: parasite power costs and additional hardware costs

38 Fuel Cell Performance Variables Reactant utilization and gas concentration: Utilization, U refers to the fraction of the total fuel or oxidant introduced into a fuel cell that reacts electrochemically. When H 2 is the fuel, because it is only reactant involved in the electrochemical reaction (assuming no gas cross-over or leakage out of the cell), we have U f = H 2,in H 2,out H 2,in = H 2,consumed H 2,in Where H and H 2,in 2,out are the hydrogen mass flow rates at the inlet and outlet of the fuel cell respectively. Similar type of calculation is done to determine the oxidant utilization

39 Molten Carbonate Fuel Cell: Water gas shift reaction High Temperature Fuel Cell CO + H 2 O H 2 + CO 2 The fuel utilization is defined by U f = H 2,consumed H 2,in + CO in The Nernst equation in terms of the mole fraction of the gases (X i ) at the fuel cell outlet is MCFC Where P is the cell gas pressure.

40 Fuel Cell Performance Variables Gas Composition: H 2 /air fuel cell: E o T=25 o C = 60 mv E o T=1200 o C = 300 mv At high current densities, there is an inability to diffuse enough reactants to the reaction sites so the cell experiences a sharp performance decrease through reactant starvation. ir also increase with current density. Kinetic Overpotential Ohmic Overpotential Concentration Overpotential

41 Fuel Cell Performance Variables Heat Transfer: In cells with high current densities, it is often important to calculate the heat transfer within a fuel cell. 1) The electrochemical reaction producing the current in the cell is not adiabatic which gives rise to a reversible heat transfer whose magnitude is TΔS. 2) Some of the fuel reacts chemically with the oxidizer rather than electrochemically to generate an irreversible heat transfer. 3) The cell operates at some voltage less than the theoretical open circuit voltage with the difference manifesting itself as I 2 R and I ΔV heat in the cell (I is the current drawn and R and ΔV represent irreversible resistances and voltage drops). [ ( )] Q Ý t = Q Ý rev + Q Ý chem(irr) + Q Ý ΔV = 1 nf TΔS ++nf V V ac Generally small

42 Fuel Cell Efficiency The ideal efficiency is simply the change in free energy, which is the maximum useful work we can obtain from any system, divided by the heat of reaction η i = ΔG ΔH =1 TΔS ΔH = nfe ΔH = ItE ΔH Where I is the current and the time for which the current flows. In a fuel cell under load, the actual electromotive force that drives the electrons through the external circuit will fall below E to some lower value, we will call E ac. The reasons for this drop are: a) An undesirable reaction may be taking place at the electrodes or else where in the cell; b) Something may be hindering the reaction at anode or cathode; c) a concentration gradient may become established in the electrolyte or in the reactants; d) Joule heating associated with the IR drop occurs in the electrolyte. The actual efficiency is η ac = nfe ac ΔH

43 Hydrogen Fuel Cell Quantity H 2 0.5O 2 H 2 O Change Enthalpy kj ΔH = kj Entropy J/K 0.5 x J/K J/K TΔS = kj Pressure: 1 atmosphere Temperature: 298K W = PΔV = (101.3 kpa)(1.5 moles)(-22.4 x 10-3 m 3 /mol)(298k/273k) = J ΔU = ΔH - PΔV = kj kj = kj ΔG = ΔH - TΔS = kj kj = kj η = 237.1/ = 0.83 (83%)

44 Actual Performance η ac = V actual V ideal Then the thermal efficiency of the fuel cell can then be written as η = Useful Energy/ H = Useful Power/( G/0.83) = 0.83 (V actual /V ideal ) The cell voltage operating reversibly on pure hydrogen and oxygen at 1 atm pressure and 25 o C is 1.229V. The thermal efficiency of an actual fuel cell operating at a voltage of V cell, based on HHV of hydrogen is given by η ideal = 0.83 (V cell /V ideal ) = 0.83 (V cell /1.229) = V cell Current density Cell voltage Fuel cell efficiency Then active cell area must be increased to obtain the requisite amount of power.

45 Polymer Electrolyte Fuel Cell Characteristics: High power density, low weight, low cost and low volume. Operates at low temperature, allowing faster start ups and immediate response to changes in the demand for power. Water management has a significant impact on the cell performance. Nafion membranes - DuPont electrolytes - fully fluorinated polymers

46 Low platinum loaded electrodes PEFC Performance

47 Dow Membrane PEFC Performance

48 Effect of CO PEFC Performance

49 PEFC Performance Effects of Pressure and Temperature

50 PEFC Performance Effect of oxygen pressure

51 Direct Methanol Proton Exchange Fuel Cell Single cell DMPEFC

52 Fuel Cell Calculations Sample calculations for the development of a fuel cell power system We will determine for a simple hydrogen-air fuel cell: Oxygen usage rate Air inlet flow rate Air exit flow rate Hydrogen usage and the energy content of hydrogen Rate of water production Heat production References: Fuel Cell Handbook, 5 th Edition, US DOE, October Fuel Cell systems explained by Larminie & Dicks, Wiley, 2003

53 Stoichiometry: In the simple fuel cell reaction would be provided for each mole of oxygen. exactly two moles of hydrogen Since two electrons are transferred for each mole of hydrogen, the reaction would produce exactly 4 F of charge. Where F is faraday constant, the charge on one mole of electrons, 96,485 Colulombs. One ampere of current is defined as 1 C/s. Hydrogen and oxygen are often supplied at greater than stoichiometric rate. The stoichiometry can be used as a variable and is denoted by the symbol λ. If the rate of use of a chemical in a reaction is Ýn (moles/second) Rate of supply : λýn Fuel Cell Calculations (moles/second) 2H 2 + O 2 2H 2 O

54 Electrical Power of the Whole Fuel Cell Stack, P e : Average voltage of each cell in the stack : V c Fuel Cell Calculations The electrical power of the fuel cell system is either given or known. If V c is not given, it can be assumed to be between 0.6 and 0.7 V Polarization curve: The fuel cell stack is assumed to run under constant temperature (70 o C) and pressure (3 bar).

55 Efficiency: Fuel Cell Calculations If the efficiency is given, then V c can be calculated using the equation: η = U f V c % Where U f is fuel utilization coefficient (mass of fuel reacted in cell/mass of fuel input). E = 1.48 V if using the HHV E = 1.35 V if using the LHV For 100% efficient system

56 Oxygen and Air Usage: Fuel Cell Calculations At the cathode, oxygen reacts with electrons taken from the electrode and H + ions from the electrolyte, to form water and four electrons are transferred for each mole of oxygen. O 2 + 4e + 4H + 2H 2 O charge = 4F x amount of O 2 O 2 usage = (I/4F) moles/s I: current For a stack of n cells O 2 usage = (ni/4f) moles/s If the voltage of each cell in the stack is V C then Power, P e = V c I n I = P e / (nv c ) O 2 usage = P e / (4V c F) moles/s O 2 usage = 32 x 10-3 P e / (4V c F) kg/s = 8.29 x 10-8 (P e /V c ) kg/s

57 Oxygen and Air Usage: continued Fuel Cell Calculations The oxygen used will normally be derived from air, so we need to develop an expression for air usage. The molar proportion of air that is oxygen is 0.21 and the molar mass of air is x 10-3 kg/mole. Air usage = (28.97 x 10-3 /0.21) (P e / (4V c F)) kg/s = 3.57 x 10-7 (P e /V c ) kg/s If the air was used at this rate, then as it left the cell it would be completely devoid of any oxygen. Therefore, in practice, the air flow is typically twice as much. If the stoichiometry is λ, then we have Air usage = 3.57 x 10-7 λ (P e /V c ) kg/s Note: 1 kg/s = 1795 SCFM 1 kg/s = 5.1 x 10 4 standard L/minute

58 Air Exit Flow Rate: Fuel Cell Calculations Exit air flow rate = Air inlet flow rate - oxygen usage Exit air flow rate = 3.57 x 10-7 λ (P e /V c ) x 10-8 (P e /V c ) kg/s = (3.57 x 10-7 λ x 10-8 )(P e /V c ) kg/s e.g.: Given P e = 5 kw operating for I h. Assume V c = 0.7, λ = 2 air usage = 3.57 x 10-7 λ (P e /V c ) = 5.1 x 10-3 kg/s = SCFM ~ 10 SCFM

59 Hydrogen Usage: Fuel Cell Calculations Note that there are two electrons from each mole of hydrogen H 2 usage = (ni/2f) moles/s H 2 usage = P e / (2V c F) moles/s The molar mass of hydrogen is 2.02 x 10-3 kg/mole, therefore H 2 usage = 2.02 x 10-3 (P e / (2V c F)) kg/s = 1.05 x 10-8 (P e / V c ) kg/s ~ SCFM for 5 kw fuel cell Effective energy content of hydrogen fuel: 2H 2 4H + + 4e Specific enthalpy (HHV) * = 1.43 x 10 8 k/kg = 39.7 kwh/kg Specific electrical energy = 26.8 V c kwh/kg Energy density at STP = 3.20 kwh/m 3 = 3.2 Wh/SL * LHV = HHV x 0.846

60 Hydrogen Usage: continued Fuel Cell Calculations The utilization of the fuel is defined as U f = H 2,consumed H 2,in Therefore the required fuel flow rate can be calculated H 2,in = H 2,consumed U f If U f = 80% then for 5 kw fuel cell H 2 usage = 0.17 SCFM

61 Fuel Cell Calculations Water Production: Water is produced at the rate of one mole for every two electrons. Water production = P e /(2V c F) moles/s The molecular mass of water is x 10-3 kg/mole, therefore Water production = 9.34 x 10-8 (P e /V c ) kg/s = x 10-3 kg/s for 5kW fuel cell Since the density of water is 1.0 g/cm 3 Water production = cm 3 /s In one hour : 2401 cm 3 = 2.4 L

62 Fuel Cell Calculations Heat Produced: If all the enthalpy of reaction of hydrogen fuel cell is converted into electrical energy, then the output voltage would be 1.48V if the water product is in liquid form or 1.35 V if the water product is in vapor form. The difference between the actual cell voltage and this voltage represents the energy that is not converted into electricity - it is converted into heat. For a stack of n cells the heat generated is then Heating rate = n I (1.25-V c ) Watts = P e [(1.25/V c ) -1]

63 Fuel Cell System

64 Fuel Cell Calculations PAFC operating on reformed natural gas