Pmd,IRE, Fuel Cell

7/31/2019 Pmd,IRE, Fuel Cell

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Fuel Cell: Chemical Energy Sources

Fuel Cells

Introduction

Principle

ClassificationsPerformance Analysis of fuel cell

Heat generated by fuel cells

Advantages of the fuel cellsLimitations of fuel cells

Application of fuel cells


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Fuel Cell: Introduction

An electrochemical device that converts chemical energy directly into

electricity and heat..

Fuel cell operates on pure H2 andO2 (air) to produce electricity with water and

heat as by-products.

It differs from conventional electric cell in two respects: (1) reactants hydrogen

and oxygen are supplied from outside which are consumed; electrodes and

electrolyte are not consumed. (2) electrical energy stored in the form ofchemical energy after discharge needs to be recharged so that chemical energy

is built up for reconversion into electrical energy to cater to demand. But fuel

cell needs no recharging ; as long as reactants are fed it continues to provide

electricity to the load through an external circuit.

Its efficiency does not depend on size; according to power output requirement,

size varies . It is modular in construction. .

As conversion from chemical into electrical takes place isothermally itsefficiency is theoretically higher than Carnot Engine whose efficiency is limited

as it has to reject some heat to the sink while operating between a higher heat

sources and the sink.

Fuel cellfirst demonstrated (1959) by F. T. Bacin & C. Frost of Cambridge Unv


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Principle of operation of fuel cell:

Although practical fuel cell differs in design details, the essential principles

are same.

When two permeable nickel electrodes ( allow gasses to diffuse through

but not the electrolyte) embedded with catalyst are immersed in a

conducting electrolyte (eg.KOH solution for alkaline cell) with anode(

negative electrode, as per fuel cell convention) fed with hydrogen and

cathode (positive electrode)with oxygen (air), negative charges are

developed at cathode where as positive charges build up at anode thus

developing an emfbetween these two electrodes under the equilibrium

reactions between the electrolyte and the electrodes. However, when the

electrodes are connected through external circuit with electrical

resistance ( i.e. load), 2 electrons flow from cathode to anode via theelectrical circuit for every hydrogen molecules consumed.

Differences from primary( dry) and secondary ( storage battery) cells.


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At cathode the electrons on reaching there through externalcircuit react with oxygen and water from electrolyte to formhydroxyl ions ( OH-);Hydrogen ( hydrogen ion) can diffuse

through the permeable nickel electrode to move intoelectrolyte where H+ ions combines with OH- and to formwater(H2O) .The catalyst enables the hydrogen molecule to beabsorbed on the electrode surface as hydrogen reacts withhydroxyl ions in the electrolyte to form water. The net

reactions are flow of electrons through external circuit andchemical combination of hydrogen and oxygen. This processgoes on as long as reactants are fed externally to theelectrodes.

At Anode, 2H2 4 H+ + 4e- which flows through external circuit

At cathode, O2+2 H2O + 4e-

4(OH-

) + heat4 H+ +4 OH-4H2O

Net reaction, 2H2 + O2 H2O + heat


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8.3. Cell Type: hydrogen fuel cell called Hydrox are two types:

1) Low temperature Cell: Electrolyte temperature is 900C and

cell pressure up to 4 atm.

2) High Pressure Cell: Temperature and pressure are up to3000C and 45 atm.

A single hydoxyl cell produces 1.23V at 1 atmosphere and 250C.

By connecting a number of cells it is possible to have 100 to

1000V and power 1kw to 100Mw. The current depends on thephysical size of the cell. the output of the cell varies with

pressure, so to increase the cell output , gas pressure is raised.

The optimum size of the cell is about 0.27 m3 per kw.

The gases in the hydroxyl cell must be free from CO2 as thelatter reacts with KOH to form potassium carbonate(K2CO3)

which increase electrical resistance of the cell thereby lowers

output voltage.

Fuel cells are most suited for low voltage and high currentapplication .


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Classification of fuel cells: Based on

a) Temperature range of operation: low (25-100

0

C), medium(100-5000C), high temperature (5oo-10000C), very hightemperature > 10000C

b) Fuel and oxidant combination: e.g. hydrogen and oxygen,alcohol and oxygen, fossil fuel and oxygen

c) Electrolyte: e.g. aqueous, non- aqueous, H2SO4 ( acidic),

molten carbonate, KOH( alkaline)d) Direct/ indirect fuel

e) Power ratings

f) Application

g) The cell may be divided into basic categories according to (1)

whether the product of reaction must be disposed of in thecathode plenum space or in the anode plenum space and(2)whether the current flow through the electrolyte is atransfer of negative ions from the cathode to the anode or (3)a transfer of positive ions in the opposite direction fromanode through electrolyte to cathode.


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Based on electrolyte( most common):

1) Alkaline Fuel Cell(AFC)

2) Direct Methanol Fuel Cell(DMFC)3) Phosphoric Acid Fuel Cell (PAFC)

4) Proton/Polymer Exchange Membrane FuelCell(PEMFC)

5) Molten Carbonate Fuel Cell (MCFC)6) Solid Oxide Fuel Cell (SOFC)

7) Zinc air Fuel Cell (ZAFC)

8) Regenerative Fuel Cell (RFC)

8.4.1. Alkaline Fuel Cell(AFC): Electrolyte: KOH, Electrocatalyst: nickel, silver, Nobel metals: discussed earlierunder sec.8.1.


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Direct Methanol Fuel Cell(DMFC): Polymer is an electrolyte; Fuel is liquid

methane supplied at cathode and oxygen at anode; Chemical reactions are :

At Anode: CH3OH + H2OCO2+ 6H+ + 6e+

At Cathode: 3/2 O2 + 6H+

+ 6e+

3 H2ONet Reaction: CH3OH + 3/2 O2 CO2 + H2O

Efficiency of these cells is approximately 40% at operating temperature

500C-1200C. Main disadvantage of the cell is that more active catalyst is

required for low temperature conversion of methane, hence it increases

cost and weight. Phosphoric Acid Fuel Cell (PAFC):Reactions at electrodes: (Correct the

diagrams)

At Anode : 2H2 4H+ + 4e+

At Cathode: O2 + 4H+ + 4e+ 2 H2O

Net reactions: 2H2 + O2 2 H2O

PAFC operates 1500C-2200C and efficiency over 40% which can be improved

to 85% in co-generation plant ; it offers lowest cost of electricity and has

capacity range 50 to 200kw. The waste water from PAFC has high

temperature capable of heating water and generating steam atm. Pressure.


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Proton/Polymer Exchange Membrane Fuel Cell(PEMFC): Fuel is hydrogen and

charge carriers are hydrogen ions( protons); electrolyte is solid polymer

membrane( plastic film) in the form of screens that allow protons to permeatebut not hydrogen and oxygen thereby prevent them coming in contact.

Reactions are :

At Anode : 2H2 4H+ + 4e+

At Cathode : O2 + 4H+ + 4e+ 2 H2O

Net reactions: 2H2 + O2 2 H2Othe cell operates at 40-600C.

Advantages:

a) For a given volume it generates more power ( i.e. higher power density)

b) Rapid start

c) Less expensive

d) Because of solid electrolyte , PEMFC cell has less problem with corrosion

e) High ionic conductivity; Good mechanical stability; Longer life


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Molten Carbonate Fuel Cell (MCFC):

Electrolyte is a molten mixture of lithium carbonate andsodium carbonate. The operating temperature is high,6500C; These salts are heated to this temperature andbecome conductive to (CO3)

- - ; A hydrocarbon fuelmethane or kerosene is used.

At fuel electrode (anode): (CO3)- - + H2 H2O+ CO2 + 2e+ ;At oxygen electrode (cathode): CO2 +2e

++ O2(CO3)- -

Overall reaction: H2(g)+ O2 (g)+CO2(cathode) H2O(g) +CO2 ( anode)

MCFCs are second generation fuel cells, have efficiencyaround 60%. The byproducts heat can be used to generatehigh pressure steam for commercial applications.


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Solid Oxide Fuel Cell (SOFC):

At hydrogen electrode: 2H2 + 2O--

2H2O+ 4e+

At oxygen electrode: O2 +4e+ 2O

Overall reaction: 2H2+ O2 2H2O ;

it has lower efficiency around 45% due to higheroperating temperature; However operating I co-

generation plant efficiency can be increased to 80%.

Material used in SOFC should be able to function at

high temperature . Westinghouse has developed atubular style SOFC that operate at 10000C.


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Regenerative Fuel Cell (RFC): is on in which the fuel cellproduct, e.g. water in the hydrogen-oxygen fuel cell isrecovered into its reactants i.e hydrogen and oxygen, by oneof the several processes such as thermal, RES( such as wind,solar), electrolysis, photochemical or radiochemical.

The RFC operates in closed cycle. There are two stages in aRFC: The overall efficiency of RFC is the product theefficiencies of these two stages:

1) Conversion of fuel cell reactants into products while producingelectricity

2) Reconversion of fuel cell products into reactants

The principle of photo chemically RFC( ref. next slide) isbriefly explained : The sequences of reactions are

mentioned in the slide. The nitrous oxide (NO) chlorinefuel cell in which the overall reaction is2NO+ Cl2 2NOCl. The product nitrosyl chloride is decomposed photo

chemically to chlorine and nitrous oxide. The cell electrical output is 0.21V ,current obtainable is low.


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Reversible Fuel Cell: is a cell that is designed to consume chemical A to

produce electricity and chemical B, and be reversed to consume some

electricity to produce chemical A from chemical B. For example, ahydrogen fuel cell uses hydrogen and oxygen to produce electricity and

water; a reversible hydrogen cell could also use electricity and water to

produce hydrogen and oxygen.

Performance analysis of a fuel cell:

The emf(E) that will drive the electrons through external load, isproportional to Gibbs free energy change;

E=-G/(ne. F) volts, where, G= change in Gibbs free energy(g/mol) ; ne=no. of electrons per mole of fuel= 2 for hydrogen; F= Faradays constant=96487 coulomb per mole. Also, -G= T ln(a2/a1), where , a1 and a2 are

the activities at electrode 1 and 2. E= T ln(a2/a1).. Nernst equation.G = H - TS kcal /mole ( by definition: G= H-TS and as temperaturesof flow stream at entrance and exist are taken same), where H=change ofenthalpy for reaction known as heat of reaction, S = change of enthalpy;TS =Q: isothermal heat transfer;


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Thermal efficiency, T = Free energy / heat of reaction= G / H=1-T(S/ H);

For, reversible emf of the cell, the efficiency , i= nFE/H = -( ItE/ H), where = current and t time for which current flows; For maximumefficiency, the process is should be reversible.Overall efficiency of a reversible of fuel cell, o= T loss factor

Power output of a reversible fuel cell, Prev = gmax / (molar mass ofhydrogen);

Molar mass of hydrogen = 2.016 kg/molActual electrical power output, P= Prev oThe rate of heat release, Q = Prev P

Losses from fuel cell: In fuel cell, all the available energy cannot be converted into electricity; some are lost in the form of

heat energy. The losses occur as follows:1) Activation losses: ability o the cell to dissociate and drive

in chemical reactions. These depend on the temperatureand , the type and the amount of the catalyst.


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2) Fuel cross-over losses are caused by leakages and diffusionof fuels.

3) Ohmic and resistance losses are the result of the electricalresistance of the cell to current

4) Mass transfer losses ( concentration losses) occur whenthe ability to maintain adequate concentration of hydrogen and oxygen in the fuel cell, is limited by highdemand.

Conversion efficiency: the electrical energy generated by afuel cell depends on what oxygen at atm. pressure is 56.67kcal ( 237 kJ) at 250C . The heat energy of reaction( enthalpy)under the same condition is 68.626 kcal. The theoreticalefficiency of conversion is 56.67/ 68.626 i.e. 83%. Thedischarge voltages observed in actual cells are always below

the theoretical value which can be calculated from thereaction free energy at 56.67/ 68.626 kcal 56.67/ 68.626 kcal250C and at 1atm (1.23V); he difference increases withincreasing strength of the current drawn from the cell. Thedifference between the theoretical voltage and the actualvoltage is known as polarization or overvoltage.


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1) Efficiency of fuel cells are high compared to other powersources.

2) Fuel cells are simple, safe, compact and noiseless.

3) Fuel cells dont have moving parts.4) Fuel cells are pollution free. Only, if fuel source is

hydrocarbon, some CO2 is produced. Whatever heat is isproduced as by-product can be utilized or dissipated in theenvironment without causing thermal pollution.

5) The unit is lighter, smaller and requires little maintenance as

no moving parts are there.6) No overhead lines are required for transmission, as it can be

installed near the user point.

7) A fuel cell gives a few times more electrical energy per unitweight as compared to a turbo-generator or storage battery.

8) A variety of fuel cell with fuels such as methane, ammonia,LPG, biogas or coal gas can be used.

9) Long life span


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10) Quick start, even in sub-freezing temperature downto -400C.

11) It is odorless and quiet for application.

12) The space requirement for fuel cell power plant isconsiderably less as compared to conventional powerplants.

8.10 Limitations of fuel cells:a) The developments or initial costs are high. It isnecessary to work at high temperatures andpressure or use costly catalysts for the reactions totake place at high speed to give high current

densities required for the economic plant.

b) Low voltage and low service life.


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Applications of Fuel cells:

I. Domestic use : Generally up to 5kw capacity-e.g. homeappliances, street lights; up to 25w for laptop, cell phone,toys, hearing aids, digital camera, watches, calculators etc.

II. Commercial, industrial and Central power stations: 300kw-5Mw

III. Automobile vehicles( cars, buses, trucks) locomotives(commuter trains, mining trains), submarines: 5-300kw

IV. Special applications e.g. military, space craft

Recent Development:

Hydrogen-oxygen and hydrocarbonoxygen cells will beused to an increasing extent in special military and spaceprojects.

DNES has funded the import of a 200kw PAFC system

from ONSI to evaluate its operation. DMFC is underway in IISC. SPIC-SC is working on

PEMFC.

First electric car with a fuel cell REVA-EV is atdevelopmental stage.

IISC is working on DMFC and also SOFC.

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