Presentation on Fuel Cells

Prepared By :

Mahida Hiren R.

Mechanical Engg. Dept.

hiren.mahida@gmail.com

Outlines:

Introduction

Design Principle & operation of Fuel Cell.

Types of Fuel Cells.

Conversion Efficiency of Fuel Cell.

Application of Fuel Cell.

Introduction

It is an electrochemical device which convert hydrogen and oxygen into water producing electricity and heat in the process.

It is much like a battery that can be recharged while you are drawing power from it.

It provides a DC voltage that can be used to power motors, lights and any number of electrical appliances.

Continue……

Fuel cells differ from conventional cells in the respect that

active material (fuel & oxygen) are not contained within the cell

but are supplied from outside.

Pure or fairly pure hydrogen gas would be preferred fuel for

fuel cell.

Alternatively impure hydrogen obtained from hydrocarbon fuels,

such as natural gas , methane, LPG & liquid petroleum

products can be used in fuel cell as a fuel.

Efforts are going on to develop cells that can use carbon

monoxide as the fuel; if they are successful, it should be

possible to utilize coal as the primary energy source.

Main uses of fuel cells are in power production, automobile

vehicles and in special military use.

First demonstrated by Welsh

scientist

Sir William Robert Grove in

February 1839.

The Invention of the Fuel Cell

Sir William Grove invented the first fuel cell in 1839. Grove

knew that water could be split into hydrogen and oxygen by

sending an electric current through it (a process called

electrolysis). He hypothesized that by reversing the procedure

you could produce electricity and water.

He created a primitive fuel cell and called it a gas voltaic

battery. After experimenting with his new invention, Grove

proved his hypothesis. Fifty years later, scientists Ludwig Mond

and Charles Langer coined the term fuel cell while attempting

to build a practical model to produce electricity

What is a fuel cell?

A fuel cell is an electrochemical energy conversion device.

A fuel cell converts the chemicals hydrogen and oxygen into

water, and in the process it produces electricity.

A fuel cell is a device that generates electricity by a

chemical reaction. Every fuel cell has two electrodes, one

positive and one negative, called cathode and anode

respectively. The reactions that produce electricity take place at

the electrodes.

Why we need fuel cell?

Due to energy crisis all over the world.

Due to the issue of global warming.

Due to the unavailability of different renewable sources at each and every place due to geographic condition.

Fuel cell provides an alternate efficient non polluting power source that produces no noise and has no moving parts.

It is expected that by 2050 the global energy demand is going to rise by 2 to 3 times.

This calls for optimization of generation of energy through well-known sources, preferably renewable energy for commercial exploitation.

Classification Of Fuel Cells:

Classification of fuel cells is very difficult as several operational variable

exists.

Based on the temperature range in which they operate: low

temperature(25-100 C), medium temperature (100-500) , high

temperature(500-1000) & very high temperature(above 1000)

According to the type of electrolyte : aqueous, non aqueous, molten

or solid.

According to the physical state of the fuel: Gas(hydrogen, lower

hydrocarbons ), Liquid(alcohols, hydrazine, higher hydrocarbons),

Solid(Metals)

Primary fuel cell: Reactants are passed through the cell only once &

the products of the reaction being discarded. (H 2 – O 2 fuel cell )

Secondary fuel cell: Reactants are passed through the cell many

times because they are regenerated by different methods.( Nitric oxide

– chlorine fuel cell)

Types of fuel cell

As per the fuel used the fuel calls are classified as follows.

Hydrogen

Fossil Fuel

Hydrocarbon fuel.

Alcohol fuel.

Hydrazine fuel.

Design Principle & Operation of Fuel

Cell:

Chemistry of fuel cell

At anode:-

2H2 4H + 4e

At Cathode:-

O2 + 4H + 4e 2H2O

NET REACTION

2H2 + O2 2H2O

Hydrogen Oxygen Cell:

40% KOH solution as electrolyte (Ion exchange membrane).

The membrane is non permeable to the reactant gases, hydrogen and

oxygen, which thus prevents them from coming into contact.

The membrane is however , permeable to hydrogen ions which are the

current carriers in the electrolyte.

The desired properties of an ideal ion exchange membrane electrolyte

are:

High ionic conductivity.

Zero electronic conductivity

Low permeability of fuel and oxidant

Low degree of electro-osmosis.

High resistance to dehydration.

High resistance to its oxidation or hydrolysis and,

Mechanical stability

Continue……

Considerable amount of research has been carried out in a search for

the ideal membrane.

Interpolymers of polyflurocarbon and poltstyerene sulfonic acids have

been found to be quite satisfactory.

In order that electrolyte resistance be low as possible, a thin sheet of

this material (0.076 cm thickness) is used as the electrolyte.

An advantageous feature of this electrolyte is that it retains only a

limited quantity of water and rejects excess water produced in fuel cell.

This cell operates at about 40-60 ̊C. The thermodynamic reversible

potentials for the reaction is 1.23 volts at 25˚C.

Fossil Fuel Cells:

The most interesting fuel cells for the near future are modified

hydrogen- oxygen cells, in which a gaseous or liquid hydrocarbon is

the source of hydrogen.

Continue…..

Coal may be serve as the primary energy source for fuel cells. Cells

based on fossil fuels have three main components.

The fuel processor which converts the fossil fuel into a hydrogen rich

gas.

The power section consisting of the actual fuel cell (or combination of

cells), and

The inverter for changing the direct current generated by the fuel cell

into alternating current to be transmitted to user.

Continue……

In phosphoric cell utilizes a concentrated aqueous solution of

phosphoric acid as the electrolyte.

The primary fuel is light hydrocarbon, such natural gas or nephtha.

The operating temperature is 150 to 200˚ C and the discharge voltage

is 0.7 to 0.8 volt.

Each cell unit is only a few millimeters thick so that a large number can

be stacked in a package of reasonable size to produce the desired

voltage and power.

Molten Carbonate Cells:

These high temperature fuel cells offer the prospect for use with

variety of fossil fuels, including coal.

A special feature of these cells is that during operation they can

oxidize CO to CO2 as well as hydrogen to water.

Continue…..

Hence gaseous mixture of H2 and CO, which are relatively

inexpensive to manufacture, can be used in the cell, the presence of

CO2 would have only a minor effect.

The electrolyte in the high temperature fuel cells under development is

a molten mixture of alkali metal (lithium, sodium, potassium)

carbonates at a temperature of 600-700 ˚C.

This is retained in an inert porous matrix sandwiched between two

porous nickel electrodes.

The mixture of H2 and CO is supplied to negative electrodes and

oxygen to the positive electrodes.

The discharge e.m.f of the cell is above 0.8 volt.

Continue…….

An important aspect of the molten carbonate fuel cell is that the

discharged gases, consisting of the steam and CO2 products and

nitrogen from the air, are at a temperature exceeding 540˚ C.

The hot gases could be used to provide industrial process heat, to

operate a gas turbine or to produce steam in a waste heat boiler to

drive a steam turbine which produce electric power.

Solid Oxide Electrolyte Cells:

Certain solid, ceramic oxides are able to connect electricity at high

temperatures and can serve as electrolyte for fuel cells.

A possible electrolyte is zirconium dioxide containing a small amount

of another oxide to stabilize the crystal structure; this material is able

to conduct oxygen ions at high temperature.

The electrode material might be porous nickel and the operating

temperature in the range 600-1000˚ C .

Electrochemical catalyst would not be required.

Other energy sources or fuels, that can be conveniently stored and

transported in liquid form such as methanol, ammonia and hydrazine

have been proposed for fuel cell.

Methanol as a fuel:

Methanol can be catalytically reformed with steam at about 200˚C to

yield a mixture of hydrogen (75 volume percent) and carbon dioxide.

This gas can be supplied to the negative electrode of a fuel cell with air

at the positive electrode.

The cell with aqueous phosphoric acid solution as the electrolyte,

might be similar to those already described.

Ammonia (NH3) as a fuel:

In this the ammonia gas obtained from the stored liquid is decomposed

catalytically into hydrogen and nitrogen.

Part of the hydrogen is burned in air to provide the heat required for

the decomposition.

Other hydrogen is supplied to the negative electrode of the hydrogen

cell in which potassium hydroxide is used as electrolyte.

The nitrogen produced due to decomposition is an inert gas and plays

no roll in the cell.

The advantage of ammonia as fuel is that it can be stored in the

liquid form.

The disadvantage include higher vapour pressure and cell

operate satisfactory at low temperature.

Hydrazine(N2H4) as a fuel:

A compact fuel cell for mobile application possibly for vehicle

propulsion utilizes the liquid hydrazine as a fuel and hydrogen

peroxide or air as the energy source.

Hydrazine is injected as required into the aqueous potassium

hydroxide potassium hydroxide electrolyte to provide the active

material at the negative electrode.

The oxygen is either supplied by decomposition or from air.

The electrode is made up of nickel (negative) or silver (positive) as the

electrochemical catalyst.

The overall cell reaction is the oxidation of hydrazine to water and

nitrogen, but the discharged e.m.f is similar to that of the hydrogen

oxygen cell.

Continue…….

Basic drawback of hydrogen oxygen cell is that hydrogen and oxygen

are gases, so there is storage difficulty.

Liquid hydrazine is highly reactive/ toxic a well as costly.

Power output is more (1.56V)as compared to hydrogen oxygen

(1.23 V).

In Aluminum oxygen cell , the aluminum forms the negative

electrode and the oxygen is the positive electrode.

The electrolyte is an aqueous solution of sodium hydroxide.

Regenerative Systems:

A regenerative fuel cell is one in

which the fuel cell product

(water) is recovered into the

reactants (hydrogen and

oxygen) by one of the several

possible methods – thermal,

chemical, photochemical,

electrical and radio chemical.

There are two stages in a

regenerative fuel cell:

Conversion of fuel cell

reactants into products while

producing electrical energy, and

Reconversion of fuel cell

products into reactants.

Conversion efficiency of Fuel Cell:

The electrical energy generated by fuel cell depends on what is called

the “free” energy rather than on heat energy of the overall cell reaction.

The free energy of formation of 1 mole (18 grams) of liquid water from

hydrogen and oxygen gases at atmospheric pressure is 56.67 Kcal or

237 KJ at 25˚C while the heat energy of the reaction under the same

conditions is 68.26 Kcal (286 KJ)

Thus theoretical efficiency of the conversion of the heat energy into

mechanical energy in a hydrogen oxygen fuel cell is

(56.67/68.26)*100 = 83 %

Efficiencies as high as 70% have been observed, but the practical

cells using pure fuel have conversion efficiency of 50-60%.

The efficiency is lower when we use air as the source of oxygen and

when hydrogen is derived from hydrocarbon sources.

However this value is higher than the efficiency that is obtain by using

these sources in the conventional plants.

Continue…..

The discharged voltages observed in actual cells are always below the

theoretical value, the difference increasing with increasing strength of

the current drawn from the cell.

For the hydrogen oxygen cell at 25˚C with the gases at atmospheric

pressure , the ideal e.m.f is 1.23 Volts & for the moderate currents at

which fuel cells normally operate the e.m.f is 0.7 to 0.8 volts.

This deviation from the theoretical e.m.f accounts for the conversion

efficiency of a fuel cell being below the ideal maximum value.

The departure of a fuel cell from ideal behavior arises from several

factors one is the Inherent slowness of the electrode reactions.

This is dominant at low current drains.

It can be reduced by an effective electrochemical catalyst and by

increasing the operating temperature.

Continue…….

At larger currents there is an additional contribution from the electrical

resistance of the electrolyte.

A low resistance (i.e. high conductivity) electrolyte is therefore

desirable.

In ideal hydrogen oxygen cell, (100-83) = 17 % of the chemical

reaction energy would be liberated as heat & this proportion increases

in an actual cell.

In order to avoid this temperature rise, heat is removed from the fuel

cell during operation.

In some cell design it is proposed to utilize the heat to provide space

heating of a building and to supply hot water.

The heat released in high temperature cells might be used for

industrial purposes (i.e. process heat) or to generate steam.

Polarization in Fuel Cells:

The difference between the

theoretical voltage and the

actual voltage is known as

the polarization. This is also

called as overvoltage.

The effect of polarization is to

reduce the efficiency of the cell

from the theoretical maximum.

Significant drop in voltage and

hence energy loss takes place

as the current density is

increased.

Continue…..

There are mainly three types of polarization:

Activation Polarization.(Chemical polarization)

Resistance or ohmic Polarization and

Concentration Polarization.

Activation Polarization:

This is related to the activation energy barrier for the electron transfer

process at the electrode.

In fuel cells electrons are liberated and reaction is chemisorption

reaction.

At low current densities significant number of electrons are not

emitted, which results in such a potential loss.

This process requires that certain minimum activation energy supplied

so that sufficient number of electrons are emitted, this energy is

supplied by the output of the cell.

This loss is known as activation or chemical polarization.

This polarization may be reduced by using better electrode

catalysts, increasing surface area, and by raising operating

temperature.

Resistance Polarization:

The voltage drop in linearly related to the current flow according to the

ohm’s law.

The internal resistance is composed of the electrode resistance, the

bulk electrolyte resistance and interface contact resistance between

electrode and electrolyte.

The loss due to resistance polarization is significant when current

density is quite large.

The reduction in the internal resistance is the main design criteria for

low resistance polarization losses.

The electrolyte resistance can be decreased usually by using

more concentrated electrolyte by closer spacing of electrodes

and by increased temperature.

Continue…..

Loss due to resistance polarization can be reduced by:

Selecting proper shape of the electrode to have minimum contact

between electrode and electrolyte.

Reducing the gap between electrodes.

By using concentrated electrolyte.

Concentrated Polarization:

This type of polarization tends to limit the current drawn.

It is generally divided into two categories

Electrolyte side polarization: This is due to the slow diffusion in the

electrolyte causing a change in concentration at the electrodes.

This effect can be minimized by increasing the electrolyte

concentration or by stirring or circulating the electrolyte.

Gas side polarization: This is caused from slow diffusion of reactants

through the porous electrode to the reaction site or of slow diffusion of

products away from the reaction site.

The loss in voltage due to gas side polarization is reduced by using

electrodes of smaller pore size and by increasing temperature.

We have observed that all the three loses in a fuel cell are

decreased by increasing temperature , due to this a given cell is

usually operated in practice at the higher end of its temperature

range.

PEM

Requirement for fuel cells

Hydrocarbons

Oxygen

Water

Hydrogen

Carbon dioxide

Carbon monoxide

And hardware elements are

Fuel processor or reformer

Fuel cell

Convertor

Alkali Fuel Cell

Advantages & Disadvantages:

Fuel cell system are environmentally.

High conversion efficiency .

Extremely low emission.

Noise less operations so readily accepted in residential areas.

Has no moving parts.

Availability to use at any location. So less transmission & distribution

losses.

No requirements of cooling tower as conventional plants.

Less space require as compared to conventional plants.

The main disadvantages of fuel cells are their high initial costs

and low service life.

Application:

The application of the fuel cell may be discussed in the following areas.

Domestic Power

Central Power Station.

Automotive Vehicles.

Special Application

Some useful points regarding application:

The e.m.f. or voltage of a fuel cell depends to some extend on the

discharge current strength. The average voltage per cell is 0.75 volt.

By joining a number of cells in series and parallel can provide any

reasonable voltage and current.

Types of current that are generated by fuel cell.

If the fuel cells of reasonably low cost and long life can be produced , a

major use might be by electrical utilities for load leveling

A long term possibility is a central station power plant in which coal is

gasified and the gas is used to generate electricity directly by means of

fuel cells.

Such an installation is expected to have a higher efficiency for fuel

utilization than a conventional steam- electrical plant.

Continue……

Portable generating sets seem to be a favorable field for fuel cells.

Here already fuel cells appear to be competitive as compared with

conventional sources. Low temperature fuel cells have a favorable

position for operating times of 3000 to 4000 hours per year, using

methanol as a fuel.

As we know that demand for power is variable. when the demand is

less than the rated output, the excess would be used to generate

hydrogen by electrolysis of water.

At the times when the load is greater than the rated power, the

hydrogen would be used in fuel cells to satisfy the additional demands.

By using fuel cell at the site of power, the transmission and distribution

cost would be reduced.

For new load centers different fuel might be utilized more economically

in fuel cells located near the new load centers.

Continue……

Some fuel cells have been proposed for remote or rural areas or

unattended location , for mobile and emergency power sources, and

for vehicle propulsion.

The aluminum- air cell is of special interest for electrical vehicle

propulsion because of the high specific energy that is possible. The

weight of these batteries are same as conventional batteries.

More than this methanol –air and hydrogen – oxygen cell are also

used for vehicle propulsion.

Many of the fuel cells currently under development are for special

application where convenience is of importance, cost is secondary.

For these application hydrogen is the superior fuel from the view point

of the reactivity and availability of invariant electrolyte, although it is

relatively costly.

It seems likely that hydrogen- oxygen and hydrogen carbon- oxygen cells

will be used to an increasing extent in special military & space application.

Applications

Fuel cells powered cars will start to replace gas and diesel engine cars in about 2055.

Fuel cell powered buses are already running in several cities..

This promising application will one day even power our houses.

Applications

Fuel cells also make sense for

portable power like laptop

computers and cellular phones.

Telecommunications - With the use of computers, the Internet, and

communication networks steadily increasing, there comes a need for

more reliable power than is available on the current electrical grid, and

fuel cells have proven to be up to 99.999% (five nines) reliable. Fuel

cells can replace batteries to provide power for 1kW to 5kW telecom

sites without noise or emissions, and are durable, providing power in

sites that are either hard to access or are subject to inclement

weather. Such systems would be used to provide primary or backup

power for telecom switch nodes, cell towers, and other electronic

systems that would benefit from on-site, direct DC power supply.

Nokia mobile with fuel cell battery

Wheel chair powered by fuel cell

Toshiba mp3 with a fuel cell

Major organizations working in the field:

Ministry of NCES

IITs

CSIR labs

BHEL

GAIL

BARC

MIT

Disadvantages

Initial cost of installation is higher.

Comparative cost of energy storage of fuel cells is around twice that of

conventional sources of energy.

Energy produced by one fuel cell is around 0.7 volts.

Recent development:

LLC, Latham, NY has successfully developed 50 KW power plant.

Fuel cell of capacity 1.5KW is powering houses in Australia.

GAIL is actively involved in establishing fuel infrastructure for fuel cell vehicles in India.

CECRI, Karaikudi has developed and tested MCFC stack.

Conclusion

The above discussion prove the existence of huge market for fuel

cells.

The commercial power units are technically feasible .

Research and development should be aimed at reducing cost and

increasing life.

Future cities can be planned on fuel cell systems for their power and

energy requirements.

Thank you