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VEHICLE POWER PLANTSCh-9-10-W-15-16-17

CR and Hybrid engines%

Future Trends in AFV Engines

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Concept and requirement of low CR engines

Low CR with auxiliary inlet manifold heating Design calculations

Exhaust re-circulation Calculations for the basic design

Electric vehicles

Hybrid electric engines

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Solution to Problems of Turbocharging an NA engine

AFVs are highly boosted to achieve maximum power output; running at higher temperatures and pressure.

To overcome the above problems, following steps can be taken:-

1. Reduce Compression ratio (CR)- It reduces the peak pressure and temperature but leads to lower η th.

2. Boost now at lower P & T to get maximum power o/p.

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PRESSURE CHARGING: LOWERING THE CR

- In a hot loaded engine, lowering the CR much below normal will ensure compression ignition.

- But low CR will introduce the problem at cold start and, perhaps at sustained idle.

- To cater for the above, artificial charge warming may be necessary; by at least four methods:-

1. VCR Engine.2. Manifold heating.3. Hyper bar system.4. Exhaust recirculation.

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MANIFOLD HEATING

Fuel sprayed into the ingoingair charge and igniting it electri-cally if engine temperatures are low enough to make compression ignition uncertain.

At high load conditions, this extra fueling is automatically turned off.

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EXHAUST RECIRCULATION

- Allows some of the exhaust gases to re-circulate into the engine intake.

- Done by advancing the valve-timing of the engine during cranking. As a result the not-yet-firing engine compresses its air charge on the compression stroke more than it expands it during expansion.; so it pumps back hot air back into its own inlet.

- This can achieve light load running indefinitely, but achieving of cold start is less obvious.

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ELECTRIC VEHICLES

Low CR-Hybrid engines-modern trands Prof (Col) GC Mishra

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Early days The first electric vehicles of the 1830s used non-rechargeable batteries. Half

a century was to elapse before batteries had developed sufficiently to be used in commercial electric vehicles.

By the end of the 19th century, with mass production of rechargeable batteries, electric vehicles became fairly widely used. Private cars, though rare, were quite likely to be electric, as were other vehicles such as taxis.

An electric New York taxi from about 1901 is shown, with Lily Langtry alongside.

Indeed if performance was required, the electric cars were preferred to their internal combustion or steam powered rivals.

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Early days..Shown below is the first car to exceed the ‘mile a minute’ speed (60mph) when the Belgium racing diver Camille Jenatzy, driving the electric vehicle known as ‘La Jamais Contente’,1 set a new land speed record of 106 kph (65.7mph). This also made it the first car to exceed 100 kph.

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The relative decline of electric vehicles after 1910 The reasons for the greater success to date of IC engine vehicles are easily

understood when one compares the specific energy of petroleum fuel to that of batteries.

The specific energy of fuels for IC engines varies, but is around 9000 Whkg−1, whereas the specific energy of a lead acid battery is around 30 Whkg−1.

Once the efficiency of the IC engine, gearbox and transmission (typically around 20%) for a petrol engine is accounted for, this means that 1800 Whkg−1 of useful energy (at the gearbox shaft) can be obtained from petrol.

With an electric motor efficiency of 90% only 27Whkg−1 of useful energy (at the motor shaft) can be obtained from a lead acid battery.

To illustrate the point further, 4.5 litres (1 gallon) of petrol with a mass of around 4 kg will give a typical motor car a range of 50 km. To store the same amount of useful electric energy requires a lead acid battery with a mass of about 270 kg. To double the energy storage and hence the range of the petrol engine vehicle requires storage for a further 4.5 litres of fuel with a mass of around 4 kg only, whereas to do the same with a lead acid battery vehicle requires an additional battery mass of about 270 kg. This is illustrated in Figure

Low CR-Hybrid engines-modern trands Prof (Col) GC Mishra

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Comparison of energy from petrol

and lead acid battery

For lead acid batteries to have the effective energy capacity of 45 litres (10 gallons) of petrol, a staggering 2.7 tons of batteries would be needed!

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Another major problem that arises with batteries is the time it takes to recharge them- normally several hours, required to re-charge a lead acid battery (new batteries has been reduced to one hour), whereas 45 litres of petrol can be put into a vehicle in approximately one minute.

Yet another limiting parameter with electric vehicles is that batteries are expensive and limited life (max. 5 years).

Since the 19th century ways of overcoming the limited energy storage of batteries have been used. The first is supplying the electrical energy via supply rails, the best example being the trolley bus. This has been widely used during the 20th century and allows quiet non-polluting buses to be used in towns and cities. When away from the electrical supply lines the buses can run from their own batteries.

Early on in the development of electric vehicles the concept was developed of the hybrid vehicle, in which an IC engine driving a generator is used in conjunction with one or more electric motors.

The hybrid car is one of the most promising ideas which could revolutionise the impact of electric vehicles.

The Toyota Prius (Figure) is a modern electric hybrid that, it is said, has more than doubled the number of electric cars on the roads. There is considerable potential for the development of electric hybrids and the idea of a hybrid shows considerable promise for future development.

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Uses for which battery electric vehicles have remained popular Despite the above problems there have always been uses for electric vehicles since

the early part of the 20th century. They produce no exhaust emissions in their immediate environment, They are inherently quiet (ideal for environments such as warehouses, inside

buildings and on golf courses, where pollution and noise will not be tolerated). They also retain their efficiencies in start-stop driving, when an IC engine becomes

very inefficient and polluting. One popular application of battery/electric drives is for mobility devices for the elderly

and physically handicapped.

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Of the technical developments, the battery is an area where there have been improvements, although these have not been as great as many people would have wished.

Commercially available batteries such nickel cadmium or nickel metal hydride can carry at best about double the energy of lead acid batteries, and the high temperature Sodium nickel chloride or Zebra battery nearly three times.

This is a useful improvement, but still does not allow the design of vehicles with a long range.

In practice, the available rechargeable battery with the highest specific energy is the lithium polymer battery which has a specific energy about three times that of lead acid.

This is still expensive although there are signs that the price will fall considerably in the future.

Zinc air batteries have potentially seven times the specific energy of lead acid batteries and fuel cells show considerable promise.

There have been increasing attempts to run vehicles from photovoltaic cells.

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Types of Electric Vehicle in Use Today

There are effectively six basic types of electric vehicle, which may be classed as follows.

1. Traditional battery electric vehicle, which is the type that usually springs to mind when people think of electric vehicles.

2. The hybrid electric vehicle, which combines a battery and an IC engine, is very likely to become the most common type in the years ahead.

3. Vehicles which use replaceable fuel as the source of energy using either fuel cells or metal air batteries.

4. Vehicles supplied by power lines.

5. Electric vehicles which use energy directly from solar radiation.

6. Vehicles that store energy by alternative means such as flywheels or super capacitors, which are nearly always hybrids using some other source of power as well.

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Conceptual illustration of a general EV configuration.

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Performance of EVs

Traction Motor Characteristics Variable-speed electric motor drives usually have the characteristics shown in Figure.

At the low-speed region (less than the base speed as marked in Figure), the motor has a constant torque.

In the high-speed region (higher than the base speed), the motor has a constant power. This characteristic is usually represented by a speed ratio x, defined as the ratio of its maximum speed to its base speed.

In low-speed operation, voltage supply to the motor increases with the increase of speed through the electronic converter while the flux is kept constant.

At the point of base speed, the voltage of the motor reaches the source voltage. After the base speed, the motor voltage is kept constant and the flux is weakened, dropping hyperbolically with increasing speed. Hence, its torque also drops hyperbolically with increasing speed

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Figure shows the torque–speed profiles of a 60 kW motor with different speed ratios x (x = 2, 4, and 6).

It is clear that with a long constant power region, the maximum torque of the motor can be significantly increased, and hence vehicle acceleration and gradeability performance can be improved and the transmission can be simplified.

However, each type of motor inherently has its limited maximum speed ratio. For example, a permanent magnet motor has a small x (<2) because of the difficulty of field weakening due to the presence of the permanent magnet. Switched reluctance motors may achieve x > 6 and induction motors about x = 4.

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Tractive Effort and Transmission RequirementThe tractive effort developed by a traction motor on driven wheels and the vehicle speed are expressed as

where Tm and Nm are the motor torque output in N m and speed in rpm, respectively, ig is the gear ratio of transmission, i0 is the gear ratio of final drive, ηt is the efficiency of the whole driveline from the motor to the driven wheels, and rd is the radius of the driven wheels.

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Tractive effort versus vehicle speed with a traction motor of x = 2 and three-gear transmission.

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Vehicle Performance Basic vehicle performance includes maximum cruising speed, gradeability, and

acceleration.

The maximum speed of a vehicle can be easily found by the intersection point of the tractive effort curve with the resistance curve (rolling resistance plus aerodynamic drag), in the tractive effort versus vehicle speed diagram shown in previous Figures.

In this case, the maximum vehicle speed is determined by the maximum speed of the traction motor as

Where, Nm max is the allowed maximum rpm of the traction motor and ig min is the minimum gear ratio of the transmission (highest gear).

Gradeability is determined by the net tractive effort of the vehicle, Ft−net (Ft−net = Ft − Fr − Fw), as shown in previous Figures.

The gradeability at mid- and high speeds is smaller than that at low speeds. The maximum grade that the vehicle can overcome at the given speed can be calculated

d = Ft − Fw/Mg is called the vehicle performance factorand fr is the tire rolling resistance coefficient.

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Concept of Hybrid Electric Drive TrainsBasically, any vehicle power train is required to (1) develop sufficient power to meet the demands of vehicle performance, (2) carry sufficient energy onboard to support the vehicle driving a sufficient range, (3) demonstrate high efficiency, and (4) emit few environmental pollutants. Broadly, a vehicle may have more than one power train. Here, the power train is

defined as the combination of the energy source and the energy converter or power source,

such as the gasoline (or diesel)–heat engine system, the hydrogen–fuel cell–electric motor system, the chemical battery–electric motor system, and so on.

A vehicle that has two or more power trains is called a hybrid vehicle. A hybrid vehicle with an electrical power train is called an HEV. The drive train of a vehicle is defined as the aggregation of all the power trains.

A hybrid vehicle drive train usually consists of no more than two power trains. More than two power trains will make the drive train very complicated.

For the purpose of recapturing braking energy that is dissipated in the form of heat in conventional IC engine vehicles, a hybrid drive train usually has a power train that allows energy to flow bidirectionally.

The other one is either bidirectional or unidirectional.

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A hybrid drive train can supply its power to the load by a selective power train. There are many available patterns of operating two power trains to meet the load requirement:

1. Power train 1 alone delivers its power to the load.

2. Power train 2 alone delivers its power to the load.

3. Both power train 1 and power train 2 deliver their power to the load simultaneously.

4. Power train 2 obtains power from the load (regenerative braking).

5. Power train 2 obtains power from power train 1.

Conceptual illustration of a hybrid electric drive train.

6. Power train 2 obtains power from power train 1 and the load simultaneously.

7. Power train 1 delivers power to the load and to power train 2 simultaneously.

8. Power train 1 delivers its power to power train 2, and power train 2delivers its power to the load.9. Power train 1 delivers its power to the load, and the load delivers thepower to power train 2.

Low CR-Hybrid engines-modern trands Prof (Col) GC Mishra

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Classifications of hybrid EVs. (a) Series (electrically coupling), (b) Parallel (mechanical coupling),(c) series–parallel (mechanical and electrical coupling), and (d) Complex (mechanical and electrical coupling).

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Two-shaft configuration.

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Pretransmission single-shaft torque combination parallel hybrid electric drive train.

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Honda has been installing 24 NiMH 12 V batteries in its electric cars for export to the US, and Toyota has been using the same type of battery in its electric vehicles. Since 1971 Toyota has developed the TownAce, an electric van, the Crown Majesta Saloon, the RAV4 and e-com, the last mentioned being a mini electric commuter vehicle.

The drive train of the RAV4 EV, Fig. 20.1, consists of the battery pack, the electric motor and the control pack. Although it is a cumbersome installation as compared with that of an internal combustion engine powered vehicle, at least it has neither an exhaust system nor a conventional transmission: the electric motor transmits its drive through a simple reduction gear to the road wheels. By virtue of optimisation of every aspect of this drive train, and the use of regenerative braking, a range of 124 miles per charge has, it is claimed, been attained.

Nissan’s Altra represents a major advance. It is powered by a watercooled, permanent magnet, synchronous electric motor developing 62 kW and 159 Nm torque at 13 000 rev/min. A key feature of this motor is the use of the highly efficient nodimium-iron-boron (Nd-Fe-B) magnet. The outcome is a motor weighing 39 kg giving a power weight ratio of 1.6 W/kg. Incidentally, its speed is considerably higher than the average, which is 8000 to 9000 rev/min.

The lithium-iron battery pack was developed by Sony. Its nominal output is 345 V from 12 modules each comprising 8 cells. The output from each cell is 36 V fully charged and 20 V discharged. With a gross weight of 350 kg, the energy density is 90 Wh/kg and power density 300 W/kg. A nominal life of 1200 cycles, based on 5% reduction in energy density, is claimed, but lives in excess of 2000 cycles without significant further loss of efficiency have been obtained during tests.

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Hydrogen fuel cells:the power of tomorrowBY GC Mishra

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Hydrogen Fuel CellsClean tomorrow

BY

Prof (Col) Girish Chandra MishraDefence University College of Engineering

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Fuel Cell Basics

Electrolysis“What does this have to do with fuel cells?”

By providing energy from a battery, water (H2O) can be dissociated into the

diatomic molecules of hydrogen (H2) and oxygen (O2). 33

Fuel Cell Basics“Put electrolysis in reverse.”

fuelcell

H2OO2

H2

heat

work

The familiar process of electrolysis requires work to proceed, if the process is put in reverse, it should be able to do work for us spontaneously.

The most basic “black box” representation of a fuel cell in action is shown below:

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(a) The electrolysis of water. The water is separated into hydrogen and oxygen by the passage of an electric current-Electrolysis.

(b) A small current flows. The oxygen and hydrogen are recombining-Reverse.

Electrolysis and the Reverse

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Another way of looking at the fuel cell is to say that the hydrogen fuel is being ‘burnt’ or combusted in the simple reaction

2H2 + O2 → 2H2O

However, instead of heat energy being liberated, electrical energy is produced.

The experiment shown in Figures makes a reasonable demonstration ofthe basic principle of the fuel cell, but the currents produced are very small.

The main reasons for the small current are:-

1. The low ‘contact area’ between the gas, the electrode, and the electrolyte – basically just a small ring where the electrode emerges from the electrolyte.

2. The large distance between the electrodes – the electrolyte resists the flow of electric current.

Fuel Cell Basics

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To overcome these problems, the electrodes are usually made flat, with a thin layer of electrolyte as in Figure.

The structure of the electrode is porous so that both the electrolyte from one side and the gas from the other can penetrate it. This is to give the maximum possible contact between the electrode, the electrolyte, and the gas.

Fuel Cell Basics

Basic cathode–electrolyte–anode construction of a fuel cell

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At the anode of an acid electrolyte fuel cell, the hydrogen gas ionizes, releasing electrons and creating H+ ions (or protons).

2H2 → 4H+ + 4e−

This reaction releases energy. At the cathode, oxygen reacts with electrons taken from the electrode, and H+ ions from the electrolyte, to form water.

O2 + 4e− + 4H+ → 2H2O

Fuel Cell Basics-Acid Electrolyte Fuel Cell (as used by Grove)

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Clearly, for both these reactions to proceed continuously, electrons produced at the anode must pass through an electrical circuit to the cathode.

Also, H+ ions must pass through the electrolyte. An acid is a fluid with free H+ ions, and so serves this purpose very well.

Certain polymers can also be made to contain mobile H+ ions. These materials are called proton exchange membranes, as an H+ ion is also a proton.

It should be noted that the electrolyte must only allow H+ ions to pass through it, and not electrons. Otherwise, the electrons would go through the electrolyte, not a round the external circuit, and all would be lost.

Fuel Cell Basics-Acid Electrolyte Fuel Cell (as used by Grove)..

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In an alkaline electrolyte fuel cell the overall reaction is the same, but the reactions at each electrode are different.

In an alkali, hydroxyl (OH−) ions are available and mobile. At the anode, these react with hydrogen, releasing energy and electrons, and producing water. At the cathode, oxygen reacts with electrons taken from the electrode, and water in the electrolyte, forming new OH− ions.

Fuel Cell Basics-Alkaline Electrolyte Fuel Cell

Fuel Cell BasicsThermodynamics H2(g) + ½O2(g) H2O(l)

Other gases in the fuel and air inputs (such as N2 and CO2) may be present, but as they are not involved in the electrochemical reaction, they do not need to be considered in the energy calculations.

69.91 J/mol·K205.14 J/mol·K130.68 J/mol·KEntropy (S)

-285.83 kJ/mol00Enthalpy (H)

H2O (l)O2H2

Table 1 Thermodynamic properties at 1Atm and 298K

Enthalpy is defined as the energy of a system plus the work needed to make room for it in an environment with constant pressure.

Entropy can be considered as the measure of disorganization of a system, or as a measure of the amount of energy that is unavailable to do work.

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Fuel Cell Basics: Thermodynamics

Enthalpy of the chemical reaction using Hess’ Law:

ΔH = ΔHreaction = ΣHproducts – ΣHreactants

= (1mol)(-285.83 kJ/mol) – (0)

= -285.83 kJEntropy of chemical reaction:

ΔS = ΔSreaction = ΣSproducts – ΣSreactants

= [(1mol)(69.91 J/mol·K)] – [(1mol)(130.68 J/mol·K) + (½mol)(205.14 J/mol·K)]

= -163.34 J/K

Heat gained by the system:

ΔQ = TΔS= (298K)(-163.34 J/K)= -48.7 kJ 42

Fuel Cell Basics: Thermodynamics

The Gibbs free energy is then calculated by:

ΔG = ΔH – TΔS = (-285.83 kJ) – (-48.7 kJ) = -237 kJ

The external work done on the reaction, assuming reversibility and constant T

W = ΔG

The work done on the reaction by the environment is:

The heat transferred to the reaction by the environment is:

W = ΔG = -237 kJ

ΔQ = TΔS = -48.7 kJ

More simply stated:

The chemical reaction can do 237 kJ of work and produces 48.7 kJ of heat to the environment.

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Fuel Cell BasicsComponents

1. Anode: Where the fuel reacts or "oxidizes", and releases electrons.

2. Cathode: Where oxygen (usually from the air) "reduction" occurs.

3. Electrolyte: A chemical compound that conducts ions from one electrode to the other inside a fuel cell.

4. Catalyst: A substance that causes or speeds a chemical reaction without itself being affected.

5. Cogeneration: The use of waste heat to generate electricity. Harnessing otherwise wasted heat boosts the efficiency of power-generating systems.

6. Reformer: A device that extracts pure hydrogen from hydrocarbons.

7. Direct Fuel Cell: A type of fuel cell in which a hydrocarbon fuel is fed directly to the fuel cell stack, without requiring an external "reformer" to generate hydrogen.

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Fuel Cell BasicsPutting it together.

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Fuel Cell BasicsPutting it together.

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Low CR-Hybrid engines-modern trands Prof (Col) GC Mishra

Fuel Cell BasicsPutting it together.

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Fuel cells come in a variety of sizes. Individual fuel cells produce very small amounts of electricity, about 0.7 volts, so cells are "stacked", or placed in series or parallel circuits, to increase the voltage and current output to meet an application’s power generation requirements.

In addition to electricity, fuel cells produce

water,

heat and, depending on the fuel source,

very small amounts of nitrogen dioxide and

other emissions.

The energy efficiency of a fuel cell is generally between 40-60%, or up to 85% efficient if waste heat is captured for use.

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What Limits the Current?At the anode, hydrogen reacts, releasing energy. However, just because energy is released, it does not mean that the reaction proceeds at an unlimited rate. The reaction has the ‘classical’ energy form shown in Figure.

Although energy is released, the ‘activation energy’ must be supplied to get over the ‘energy hill’. If the probability of a molecule having enough energy is low, then the reaction will only proceed slowly. Except at very high temperatures, this is indeed the case for fuel cell reactions.

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The three main ways of dealing with the slow reaction rates are:-

1. The use of catalysts,

2. Raising the temperature,

3. Increasing the electrode area.

The first two can be applied to any chemical reaction. However, the third is special to fuel cells and is very important.

If we take a reaction such as that of Alkaline fuel cell, we see that fuel gas and OH− ions from the electrolyte are needed, as well as the necessaryactivation energy. Furthermore, this ‘coming together’ of H2 fuel and OH− ions must take place on the surface of the electrode, as the electrons produced must be removed.

What Limits the Current?..

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This reaction, involving fuel or oxygen (usually a gas), with the electrolyte (solid or liquid) and the electrode, is sometimes called the three phase contact.

The bringing together of these three things is a very important issue in fuel cell design.

The rate at which the reaction happens will be proportional to the area of the electrode. Electrode area is such a vital issue that the performance of a fuel cell design is often quoted in terms of the current per cm2.

However, the straightforward area (l × b) is not the only issue. The electrode is made highly porous. This has the effect of greatly increasing the effective surface area. Modern fuel cell electrodes have a microstructure that gives them surface areas that can be hundreds or even thousands of times their straightforward ‘l × b’ (See Figure on next slide.).

In addition to these surface area considerations, the electrodes may have to incorporate a catalyst and endure high temperatures in a corrosive environment.

What Limits the Current?...

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TEM image of fuel cell catalyst. The black specks are the catalyst particles finely

divided over a carbon support. The structure clearly has a large surface area. (Reproduced by kind permission of Johnson Matthey Plc.)

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Connecting Cells in

Series The Bipolar Plate

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Connecting Cells in Series – the Bipolar PlateThe voltage of a fuel cell is quite small, about 0.7V when drawing a useful current. This means that to produce a useful voltage many cells have to be connected in series. Such a collection of fuel cells in series is known as a ‘stack’.

The most obvious way to do this is by simply connecting the edge of each anode to the cathode of the next cell, all along the line, as in Figure (For simplicity, this diagram ignores the problem of supplying gas to the electrodes.)

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Connecting Cells in Series – the Bipolar Plate..

The problem with this method is that the electrons have to flow across the face of the electrode to the current collection point at the edge.

The electrodes might be quite good conductors, but if each cell is only operating at about 0.7V, even a small voltage drop is important.

Therefore, unless the current flows are very low, and the electrode is a particularly good conductor, or very small, this method is not used.

A much better method of cell interconnection is to use a ‘bipolar plate’.

This makes connections all over the surface of one cathode and the anode of the next cell (hence ‘bipolar’);

At the same time, the bipolar plate serves as a means of feeding oxygen to the cathode and fuel gas to the anode.

Although a good electrical connection must be made between the two electrodes, the two gas supplies must be strictly separated.

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Single cell, with end plates for taking current from all over the face of the

electrodes,and also supplying gas to the whole

electrode.

The grooved plates are made of a good conductor such as graphite, or stainless steel.

Connecting Cells in Series – the Bipolar Plate…

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Two bipolar plates of very simple design. There are horizontal grooves

on one sideand vertical grooves on the other.

Bipolar plates’ (or, cell interconnects) have channels cut in them so that the gases can flow over the face of the electrodes. At the same time, they are made in such a way that they make a good electrical contact with the surface of each alternate electrode

Connecting Cells in Series – The Bipolar Plate….

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A three-cell stack showing how bipolar plates connect the anode of one cell to

the cathode of its neighbour.

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The design of the bipolar plate is not simple.

If the electrical contact is to be optimised, the contact points should be as large as possible, but this would mitigate the good gas flow over the electrodes.

If the contact points have to be small, at least they should be frequent. However, this makes the plate more complex, difficult, and expensive to manufacture, as well as fragile.

Ideally the bipolar plate should be as thin as possible, to minimise electrical resistance and to make the fuel cells stack small.

However, this makes the channels for the gas flow narrow, which means it is more difficult to pump the gas round the cell. This sometimes has to be done at a high rate, especially when using air instead of pure oxygen on the cathode.

In the case of low-temperature fuel cells, the circulating air has to evaporate and carry away the product water. In addition, there usually have to be further channels through the bipolar plate to carry a cooling fluid.

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Gas Supply and CoolingThe arrangement shown in previous Figure has been simplified to show the basic principle of the bipolar plate. However, the problem of gas supply and of preventing leaks means that in reality the design is somewhat more complex.

Because the electrodes must be porous (to allow the gas in), they would allow the gas to leak out of their edges. The result is that the edges of the electrodes must be sealed. Sometimes this is done by making the electrolyte somewhat larger than one or both of the electrodes and fitting a sealing gasket around each electrode, as shown in below.

The construction of anode/electrolyte/cathode

assemblies with edge seals. These prevent the gases leaking in or out

through the edges of the porous electrodes.

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Three-cell stack, with external manifolds. The electrodes now have edge seals.

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The arrangement of previous Figures and here is used in some systems. It is called external manifolding. It has the advantage of simplicity. However, it has two major disadvantages:-

1. It is difficult to cool the system. In practice, this type of cell has to be cooled by the reactant air passing over the cathodes. This means air has to be supplied at a higher rate than demanded by the cell chemistry; sometimes this is sufficient to cool the cell, but it is a waste of energy.

2. Increased probability of leakage of the reactant gases-the gasket round the edge of the electrodes is not evenly pressed down – at the point where there is a channel, the gasket is not pressed firmly onto the electrode.

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Types of Fuel Cells

There are many types of fuel cells, but they all consist of

1. Anode (negative side),

2. Cathode (positive side) and

3. Electrolyte that allows charges to move

between the two sides of the fuel cell.

Electrons are drawn from the anode to the cathode through an external circuit,producing direct current electricity.

As the main difference among fuel cell types is the electrolyte, fuel cells are classified by the type of electrolyte they use.

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Types of Fuel Cells

The five most common types:

Alkali

Molten Carbonate

Phosphoric Acid

Proton Exchange Membrane

Solid Oxide

Types of Fuel Cells

SOFC

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Alkali Fuel Cell

Compressed hydrogen and oxygen fuel

Potassium hydroxide (KOH) electrolyte

~70% efficiency

150˚C - 200˚C operating temp.

300W to 5kW output

Requires pure hydrogen fuel and platinum catylist → ($$) Liquid filled container → corrosive leaks

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Molten Carbonate Fuel Cell (MCFC)

carbonate salt electrolyte

60 – 80% efficiency

~650˚C operating temp.

cheap nickel electrode catalyst

up to 2 MW constructed, up to 100 MW designs exist

Figure 5

The operating temperature is too hot for many applications.

carbonate ions are consumed in the reaction → inject CO2 to compensate

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Phosphoric Acid Fuel Cell (PAFC)

phosphoric acid electrolyte

40 – 80% efficiency

150˚C - 200˚C operating temp

11 MW units have been tested

sulphur free gasoline can be used as a fuel

Figure 6

The electrolyte is very corrosive

Platinum catalyst is very expensive

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Proton Exchange Membrane (PEM)

thin permeable polymer sheet electrolyte

40 – 50% efficiency

50 – 250 kW

80˚C operating temperature

Electrolyte will not leak or crack

Temperature good for home or vehicle use

Platinum catalyst on both sides of membrane → $$

Figure 7

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Solid Oxide Fuel Cell (SOFC)

hard ceramic oxide electrolyte

~60% efficient

~1000˚C operating temperature

cells output up to 100 kW

High temp / catalyst can extract the hydrogen from the fuel at the electrode

High temp allows for power generation using the heat, but limits use

SOFC units are very large

Solid electrolyte won’t leak, but can crack

Figure 8

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Data for different types of fuel cell

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Chart to summarize the applications and main advantages of fuel cells of different types, and in different applications.

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Comparison of Fuel Cell Technologies

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Benefits1. Efficient: In theory and in practice

2. Portable: Modular units

3. Reliable: Few moving parts to wear out or break

4. Fuel Flexible: With a fuel reformer, fuels such as natural gas, ethanol, methanol, propane, gasoline, diesel,

landfill gas, wastewater, treatment digester gas, or even ammonia can be used

5. Environmental: produces heat and water (less than combustion in both cases), near zero emission of CO and NOx

reduced emission of CO2 (zero emission if pure H2 fuel)

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the future of fuel cells

Used to power personal electronic devices: cell phones, iPods, laptops

Enough energy to run for days, or weeks (instead of hours)

Potentially power all cars, airplanes, ships, etc. 60 million tons of carbon dioxide could be

eliminated from yearly greenhouse gas production Development of cheaper and more reliable catalysts Higher demand = cheaper

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The future of fuel cells

• Economic crisis has greatly slowed technological advancements

• Past predictions for 2010 seem unlikely• Hydrogen cannot be the only alternative fuel

source to solve the energy crisis• Many more years of research before mass

production will be possible

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THANK YOU

The End………

QUESTIONS ? ?CONCLUSION