Fuel Cell

Department of Electrical Engineering

MANIT, Bhopal

Fuel Cell• The issue of renewable energy is becoming significant due to

increasing power demand, instability of the rising oil prices andenvironmental problems.

• The term distribution generation means any small-scale generationis located near to the customers rather than central or remotelocations.

• Total loss over the transmission, distribution and transformers inIndia is about 32.15%.

• The major benefits of distributed generation systems (DGs) aresaving in losses over the long transmission and distribution lines,installation cost, local voltage regulation, and ability to add a smallunit instead of a larger one during peak load conditions.

• Among the various renewable energy sources, fuel cell is gainingmore popularity due to their higher efficiency, cleanliness and cost-effective supply of power demanded by the consumers.

• Applications of Fuel are residential/grid-connectedsystem, transportation, industries and commercialapplications.

• Fuel cells are static energy conversion devices thatconvert the chemical reaction of fuels directly intoelectrical energy and produces water as its byproduct

• Conventional heat engines produces electricity fromchemical energy with the use of intermediatemechanical energy conversion which results in reducedefficiency compared with fuel cells.

• The fuel cells combine the best features of engines andbatteries; like an engine they can operate for long asfuel is available without any intermediate mechanicalenergy conversion and the characteristics of fuel cellsare similar to a battery under load conditions

Comparison of Different Generation Systems

Reciprocating Engine

Diesel Turbine Generator

Photo Voltaics

Wind Turbine

Fuel cells

Capacity Range

500 kW to 5 MW

500 kW to 25MW

1 kW to 1MW

10 kW to 1 MW

200 kW to 2 MW

Efficiency 35% 29–42% 6–19% 25% 40–60%

Capital Cost ($/kW)

200–350 450–870 6600 1000 1500–3000

O&M Cost ($/kW)

0.005–0.015 0.005–0.0065

0.001–0.004

0.01 0.0019–0.0153

Working Principle

• A fuel cell is an energy conversion device that convertsthe chemical energy of a reaction directly intoelectricity with byproduct of water and heat

• The fuel cell consists of an electrolyte layer in contactwith two electrodes of porous conducting material(commonly nickel) to collect charge on either side.

• The hydrogen fuel is fed continuously to anodeelectrode and the oxidant (or) oxygen from air is fedcontinuously to the cathode electrode.

• At the anode terminal the hydrogen fuel isdecomposed into positive ions and negative ions.

• The intermediate electrolyte membrane permits onlythe positive ions to flow from anode to cathode sideand acts as an insulator for electrons.

• These electrons want to recombine on the other side ofthe membrane for the system to become stable, forwhich the free electrons moved to the cathode sidethrough an external electrical circuit.

• The recombination of the positive and negative ions withoxidant takes place at the cathode to form depletedoxidant (or) pure water.

• The chemical reactions involved in the anode andcathode and its over all reactions are given as

Anode reaction

H2 = 2H+ + 2e- (1)

Cathode reaction

1/2O2 + 2H+ + 2e- = H2O (2)

Overall reaction

H2 +1/2O2 = H2O (3)

Classification of Fuel Cell

• Proton exchange membrane fuel cell (PEMFC):

(a) Direct formic acid fuel cell (DFAFC);

(b) Direct ethanol fuel cell (DEFC).

• Alkaline fuel cell (AFC):

(a) Proton ceramic fuel cell (PCFC);

(b) Direct borohydride fuel cell (DBFC).

• Phosphoric acid fuel cell (PAFC).

• Molten carbonate fuel cell (MCFC).

• Solid oxide fuel cell (SOFC).

• Direct methanol fuel cell (DMFC).

Proton exchange membrane fuel cell (PEMFC)

• The PEM fuel cell uses a solid polymerelectrolyte (Teflon-like membrane) toexchange the ions between two porouselectrodes, which is an excellent conductor ofprotons and an insulator for electrons.

• The operating temperature of the fuel cell isas low as around 100oC. The chemicalreactions are involved in anode and cathodesides and their overall reactions are given inEqs. (1)–(3).

• The advantages of the PEM fuel cell are its higherpower density and quick start up for automotivevehicles.

• The low operating temperature makes thetechnology competitive in transportation andcommercial applications like laptop computers,bicycle, and mobile phones.

• The major drawbacks of the PEM fuel cell are itslower operating efficiency (40–45%) and use ofhigh cost platinum catalyst.

• The PEM fuel cell-based power sources are alsobeing developed for residential (3–7 kW) andbuilding (50 kW) electricity and hot waterapplications.

Direct formic acid fuel cell (DFAFC)

• In DFAFC, the inlet fuel formic acid (HCOOH) consists ofsmall organic molecules is directly fed to the anodeelectrode.

• The main advantage is formic acid does not crossover thepolymer results the efficiency of the concentrations ishigher (20–40%) compared to methanol (6%) and thepower density (17 mW/cm2) is also very low.

• The DFAFC produces an open circuit voltage of 0.55 V at60oC operating temperature which is very low compared totheoretical value of 1.45 V given by Gibbs free energy.

• It is also consider as safer fuel in case of leakage in fueltank.

• But this technology is not considered formerly due to highelectrochemical over voltage during loading conditions byusing platinum as catalyst.

• In order to improve the performance, palladium as catalystis used.

Direct ethanol fuel cell (DEFC).

• It uses ethanol as input fuel instead of hydrogen.

• The ethanol fuel can be easily extracted from biomassthrough fermentation process from renewable energysources such as sugar cane, wheat, corn or even straw.

• In this method, at the anode electrode with themixture of water, the liquid ethanol (C2H5OH) isoxidized and generating CO2, hydrogen ions andelectrons.

• The reaction involved in cathode is same as PEM fuelcell and the generated voltage at its terminal is in therange of 0.5–0.9 V.

• World’s first vehicle powered by a DEFC is presented byUniversity of Applied Sciences at Shell’s Ecomarathonin France.

Alkaline fuel cell (AFC)

• The alkaline fuel cell is one of the earlier fuel cell systememployed for NASA’s space missions. Formerly it is alsocalled as Bacon fuel cell after its British inventor.

• It operates at low temperature around 100oC like PEMfuel cell and it has the capability to reach 60–70% ofefficiency.

• It uses an aqueous solution of the potassium hydroxide(KOH) as an electrolyte and H2 and O2 as inputs.

• It generates same level of emf as PEMFC.

• It transports negative charged ions from anode tocathode and releases water as its byproduct.

• This fuel cell gives quick start, one of its advantages.

Contd…..• The major disadvantage is that it is very sensitive to

CO2. It needs a separate system to remove the CO2

from the air.

• The CO2 combines with potassium hydroxideelectrolyte and form potassium carbonate.

• Potassium carbonate increases the electricalresistance of the cell due to which available outputvoltage decreases.

• The use of a corrosive electrolyte is a disadvantagebecause it has shorter life span. Therefore it is not usedin commercial applications.

• This type of fuel cells is used in transportations (i.e. infleet vehicles and boats in Europe) and space shuttles.

Protonic ceramic fuel cell (PCFC)

• The protonic ceramic fuel cell is relatively new fuel cell type,which is developed basically with the ceramic electrolytematerial.

• The hydrogen fuel is fed continuously to anode electrode andthe oxidant (or) oxygen from air is fed continuously to thecathode electrode.

• It can be operated at high temperatures of 750oC.

• It has solid electrolyte so the membrane cannot dry out as withPAFCs or liquid cannot leak out as with AFCs.

• The open circuit voltage produced by the PCFC is almost closeto the theoretical value that is 1.45 V (Gibbs Free Energy).

• Major drawback of it is low current density that can beincreased by reducing the electrolyte thickness and optimizedelectrodes.

Direct borohydride fuel cell (DBFC)

• Sodium borohydride (NaBH4) is used as input fuelmixed with water to generate hydrogen bydecomposing into Sodium Metaborate (NaBO2)and 4H2.

• After releasing its hydrogen it gets oxidized at thecathode to produce water as byproduct.

• The DBFC fuel cell operated at low temperature of70o C.

• The major advantages are higher power density, noneed of expensive platinum catalyst and high opencircuit cell voltage (about 1.64 V).

• But the efficiency of the DBFC is low as 35%.• The cost of the sodium borohydride is too

expensive for portable power applications.

Phosphoric acid fuel cell (PAFC)

• It utilizes a liquid phosphoric acid as an electrolyte.

• Nickel is used for negative electrode and silver is used aspositive electrode.

• Pure hydrogen or hydrogen-rich gas is supplied at thenegative electrode and oxygen or air is supplied at thepositive electrode.

• Unlike the PEM and AFC, it is very tolerant to impurities.

• Chemical reaction involved in this fuel cell is same asPEM fuel cell.

• The phosphoric acid fuel cell operates at about 175–200oC. This operating temperature is almost double ascompared to that of PEM fuel cell.

Contd• At atmospheric pressure, it produces 1.23 V at

25oC which reduces to 1.15 V at 200oC• The process is very slow and a catalyst is required

in the electrode to accelerate the reaction whichis very costly.

• The cogeneration is possible due to its highoperating temperature and the potential is alsoavailable for hot water supply as well aselectricity depending on the heat and electricityload profile.

• The 100, 200 and 500 kW size plants are availablefor stationary and heat applications.

• PAFC have been installed at more 100 sites inEurope, USA and Japan.

Molten carbonate fuel cell (MCFC)

• It consists of two porous (Nickel) electrodes with goodconductivity are in contact with a molten carbonate of alkalimetals (Na, K or Li) cell.

• Gaseous mixer of hydrogen and carbon monoxide is used as afuel which is less expensive .

• Special feature of these cell is that during operation, they

oxidize hydrogen to water and CO to CO2.

H2 + CO3-- = H2O +CO2 + 2e-

CO + CO3-- = 2 CO2 + 2e-

These electrons circulate through external resistance, forming

load current and reach the oxidant electrode.

O2 + 2 CO2 + 4 e- = 2 CO3--

The produced CO3-- ions are responsible for transportation of

charge from positive to negative electrode within the

electrolyte. The over all reaction may be written as:

H2 +CO + O2 = H2 O + CO2

Contd….• The molten carbonate fuel cell operates at high

temperature, which is about 600–700 oC.

• The discharge consisting of steam, carbon dioxide &nitrogen and the operating temperature is 540oC.These hot gases could be used to provide industrialprocess.

• Theoretical value of emf is approximately 1 volt at noload and at loaded condition voltage is 0.8 V.

• The major advantages of MCFC are higher efficiencyas 50–60%, no need of metal catalyst and separatereformer due to its high operating temperature.

• Slow start up is one of its drawbacks.

• It is mainly used for medium and large powerapplications.

Solid oxide fuel cell (SOFC)

• The SOFC’s are basically high temperature fuel cells.

• They use dense zirconia oxide containing a smallamount of other oxide to stabilize the crystalstructure, which is a solid ceramic material as itselectrolyte.

• The negative electrode is made of porous nickel andthe positive electrode employs a indium oxide.

• The SOFC produce electricity at a high operatingtemperature of about 1000 oC.

• Due to high temperature catalyst is not required.

• This cell use the same fuels (H2 and CO) as used inMCFC.

Contd…..• The output voltage at full load is about 0.63 V.

• The main advantages of the SOFC is that they areoperated at high efficiency of 50–60%.

• Separate reformer is not required to extract hydrogenfrom the fuel due to its internal reforming capability.

• Waste heat can be recycled to make additionalelectricity by cogeneration operation.

• The slow start up, high cost and intolerant to sulfurcontent of the fuel cell are some of its drawbacks.

• It is not suitable for larger fluctuations in loaddemand.

• The SOFC is used for medium and large powerapplications.

Direct methanol fuel cell (DMFC)

• The DMFC technology is relatively new.• Like PEM fuel cell, the DMFC uses polymer

electrolyte (like Polymer).• But DMFC uses liquid methanol or alcohol as fuel

instead of reformed hydrogen fuel.• During chemical reactions, the negative electrode

draws hydrogen by dissolving liquid methanol(CH3OH) in water in order to eliminate the need ofexternal reformer.

• At the positive electrode, the recombination of thepositive ions and negative ions takes place, whichare supplied through external circuit and it iscombined with oxidized air to produces water as abyproduct.

Contd…….• Normally a single DMFC can supply only 0.3–0.5 V

under loaded conditions.

• It is mainly used to replace the batteries for cameras,notebook computers and other portable electronicapplications in the range of 1 W to 1 kW capacity.

• The one of the main advantages is that negativeelectrode catalyst draws the hydrogen from themethanol and reduces overall cost due to the absenceof reformer.

• Its characteristics are similar to the PEM fuel cell.

• Performance is limited by two important factors like,crossover of methanol lowers the system efficiency(40%) and the slow kinetics of the electrochemicaloxidation of methanol.

I–V characteristic of fuel cells

• The fuel cell voltage is usually very small, around 1.2

V at the no-load condition.

• Under loading condition, voltage and efficiency drop

significantly.

• A typical fuel cell polarization characteristic with

electrical voltage against current density is shown.

• Departure of output voltage from ideal condition is

mainly due to

– Activation Polarization: This is due to activation energy

barrier for the electrons transfer process at the electrode.

– Some energy is required so that sufficient number of

electrons are emitted.

– This energy is supplied by the cell resulting drop in

voltage.

– Resistance Polarization: At large current, voltage

drops due to internal resistance.

– Internal resistance is composed of resistance of bulk

electrolyte and interface contact resistance between

electrode and electrolyte.

– However internal resistance can be reduced by

• More concentrated (highly conductivity) electrolyte

• Increasing operating temperature

– Concentration Polarization: This type of polarization

tends to limit the current. Due to slow diffusion in

electrolyte causing change in concentration at the

electrode. This effect can be reduced by increasing the

electrolyte concentration or by stirring the electrolyte.

Power-conditioning units (PCUs)

• Due drooping characteristics of fuel cell the development ofpower-conditioning units (PCUs) plays an important role tointerface the fuel cell system with standalone/ grid-connected system.

• The available fuel cell in the market is only in the range of25–50 V due to its higher production cost.

• The generated fuel cell voltage is converted into directly acsupply by using single stage dc/ac inverter topologies or bya combination of a dc/dc converter in series with dc/acinverter forming multistage conversion.

• The selection of power-conditioning unit is based on somesignificant factors like lower cost, higher efficiency, electricalisolation, ripple free and reliable operation.

Contd……..• The efficiency of the power-conditioning unit depends

upon the conduction and switching losses.

• The conduction losses can be effectively reduced byreducing the usage of components and their operatingranges.

• The switching losses can be reduced by soft switchingtechniques either by zero voltage crossing (ZVS) or zerocurrent crossing (ZCS) techniques.

• The major advantages of soft switching technique overhard switching conditions are to reduce the losses overthe device by about 20–30%.

• An electrical isolation is required to protect the fuel cellstacks under overload conditions.

Comparison of different dc/dc convertersTopologies Electrical

isolation

Voltage

stress

η Advantages Disadvantages

Boost No Less 98% Simple design and control; minimum

component count

Low power applications

Push pull Yes Less 92% Medium power applications; not more

than one switch in series conducts at any

instant of time; the voltage drop across

more than one switch in series would

results in a significant reduction in

energy losses

Center tap saturation problem at high

power; high transformer leakage

inductance

Half bridge Yes Less 92-94% The input capacitors are act as dc

blocking capacitors; the transformer

leakage inductance energy does not

present a problem to the switches

Requires twice current rating of

switches compared to full bridge; large

capacitance is required

Full Bridge Yes Less 95% Soft switching possible

Voltage

doubler

Yes Less Soft switching possible;

current ripples is low

High cost; complex control

Current Fed Yes High 94% High conversion ratio (contains voltage

doubler); minimization of conduction

loss; lower transformer turns ratio;

simplicity of construction

It suffers from severe voltage

overshoots at turn off due to storage

energy in the leakage inductor of the

transformer; large inductor is required

Series

resonant H

bridge

Yes It is inherited short circuit protection

(snubber circuit); high boost capabilities

High step up 96% Coupled inductor prevents voltage drifts

at the output, it gives larger gain

It needs a snubber circuit has been

added to provide protection to the circuit

components; component count is two

times higher than conventional

dc/dc converters

Three-phase

transformer

isolated (V6

Yes 95-97% Low leakage reactance, It reduces high

frequency current ripples; modular;

reduced filter size

Highly Complex

Comparison of different DC/AC Inverter

Topologies Electrical

isolation

η Advantages Disadvantages

Single bus inverter with

two paralleled half

bridge

No Minimum component count Large dc filter components

Dual bus inverter with

two split half bridge

No Reliability and flexibility High component count

Single phase 3 wire

inverter

Yes Small

passive

component

Complex control; for non-isolated

circuit, it requires additional passive

components

Z source inverter No 98% Boosting capability; additional

dc/dc converter is not necessary; it

saves component cost

Complex control; current stress is

high

LLCC resonant inverter No 95% Lower current ripples; soft

switching techniques

Low power density; it needs large

volume and heavy weight of the

resonant filter magnetic components