compact fuel cell that will operate satisfactorily under all driving conditions

Hybrid and Alternative Fuel VehiclesBy James D Halderman and Tony Martin

© 2009 Pearson Education, Inc.Pearson Prentice Hall

Upper Saddle River, NJ 07458




OBJECTIVES

After studying Chapter 15, the reader should be able to:1. Explain how a fuel cell generates electricity.

2. Discuss the advantages and disadvantages of fuel cells.3. List the types of fuel cells.

4. Explain how ultracapacitors work.5. Discuss alternative energy sources.




FUEL-CELL TECHNOLOGY

A fuel cell is an electrochemical device in which the chemical energy of hydrogen and oxygen is converted

into electrical energy. Fuel cells are being developed to power homes and vehicles while producing low or zero

emissions.




The chemical reaction in a fuel cell is the opposite of electrolysis. Electrolysis is the process in which electrical current is passed through water in order to break it into its components, hydrogen and oxygen. It is important to note that

while hydrogen can be used as a fuel, it is not an energy source. Instead, hydrogen is only an energy carrier, as

energy must be expended to generate the hydrogen and store it so it can be used as a fuel.




A fuel cell is a hydrogen-powered battery. Hydrogen is an excellent fuel because it has a very high energy density when compared to an equivalent amount of fossil fuel. One pound (lb) of hydrogen has three times the energy content as one pound of gasoline. Hydrogen is the most abundant element on earth, but it does not exist by itself in nature. This is because its natural tendency is to react with oxygen in the

atmosphere to form water (H2O).




In order to store hydrogen for use as a fuel, processes must be undertaken to separate it from these materials.




Benefits of a Fuel Cell

A fuel cell can be used to move a vehicle by generating electricity to power electric drive motors, as well as

powering the remainder of the vehicle’s electrical system. They are powered by hydrogen and oxygen, fuel cells by themselves do not generate carbon emissions such as CO2.

A fuel cell is also much more energy-efficient than a typical internal combustion engine. While a vehicle powered by an internal combustion engine (ICE) is anywhere from 15% to 20%

efficient, a fuel-cell vehicle can achieve efficiencies upwards of 40%.




Compact fuel cell that will operate satisfactorily under all driving conditions.




Well-to-Wheel Efficiency.Well-to-wheel efficiency is calculated by multiplying a vehicle’s well-to-tank efficiency by its tank-to-wheel

efficiency. The results can be surprising, because while fuel-cell vehicles are very efficient in terms of their tank-

to-wheel measurement, they score low on the well-to-tank rating because of the high levels of energy required to produce hydrogen from natural gas. See the accompanying

chart.




Overall Efficiency (Well-to-Wheel)

Well-to-Tank % (Fuel Production Efficiency)

Tank-to-Wheel % (Vehicle Efficiency)

Overall Efficiency % (Well-to-Tank × Tank-to-Wheel)

Diesel 82 23 19

Gasoline 79 16 18

Hybrid (gasoline) Toyota Prius w/HSD

79 37 29

Hydrogen Fuel-Cell Vehicle (Compressed Hydrogen)

58 38* 22

Hydrogen Fuel-Cell Hybrid (Toyota FCHV compressed hydrogen)

58 50** 29

Source: Toyota




A fuel-cell vehicle (FCV) uses the fuel cell as its only source of power, whereas a fuel-cell hybrid

vehicle (FCHV) would also have an electrical storage device that can be used to power the vehicle. Most new designs of fuel-cell vehicles are now based on a hybrid

configuration due to the significant increase in efficiency and driveability.




Fuel-Cell Challenges

There are a number of reasons for this, including:High cost

Lack of refueling infrastructureSafety perception

Insufficient vehicle rangeLack of durability

Freeze starting problemsInsufficient power density




Fuel-Cell Types

PAFC (phosphoric acid fuel cell)

PEM (polymer electrolyte

membrane)

MCFC (molten carbonate

fuel cell)

SOFC (solid oxide fuel cell

Electrolyte Orthophosphoric acid Sulfonic acid in polymer

Li and K carbonates Yttrium-stabilized zirconia

Fuel Natural gas, hydrogen Natural gas, hydrogen, methanol

Natural gas,synthetic gas

Natural gas,synthetic gas

Operating Temp (C) (F)

360-410°F 180-210°C

176-212°F 80-100°C

1,100-1,300°F 600-700°C

1,200-3,300°F 650-1,800°C

Electric Efficiency 40% 30-40% 43-44% 50-60%

Manufacturers ONSI Corp. Avista, Ballard, Energy Partners, H-Power, International, Plug Power

Fuel Cell Energy, IHI, Hitachi, Siemens

Honeywell, Siemens-Westinghouse, Ceramic

Applications Stationary power Vehicles, portable power, small stationary power

Industrial and institutional power

Stationary power,military vehicles

The fuel-cell design that is best suited for automotive applications is the Proton Exchange Membrane (PEM).




PEM FUEL CELLS

The Proton Exchange Membrane fuel cell is also known as a Polymer Electrolyte Fuel Cell (PEFC). The PEM is a simple

design based on a membrane that is coated on both sides with a catalyst such as platinum or palladium. There are two

electrodes, one located on each side of the membrane. These are responsible for distributing hydrogen and oxygen over the membrane surface, removing waste heat, and providing a path for electrical current flow. The part of the PEM fuel cell that contains the membrane, catalyst coatings, and electrodes

is known as the Membrane Electrode Assembly (MEA).




The negative electrode (anode) has hydrogen gas directed to it, while oxygen is sent to the positive electrode (cathode). Hydrogen is sent to the negative electrode as H2 molecules, which break apart into H+ ions (protons) in the presence of the catalyst. The electrons (e-) from the hydrogen atoms are sent through the external circuit, generating electricity that can be utilized to perform work. These same electrons are then sent to the positive electrode where they rejoin the H+ ions that have passed through the membrane and have reacted

with oxygen in the presence of the catalyst. This creates H2O and waste heat, which are the only emissions from a PEM fuel

cell.




Fuel-Cell Stacks

A single fuel cell by itself is not particularly useful, as it will generate less than 1 volt of

electrical potential. It is more common for hundreds of fuel cells to be built together in a fuel- cell stack. The fuel cells are placed end-to-end in the

stack much like slices in a loaf of bread. Automotive fuel-cell stacks contain upwards of 400 cells in their

construction.




Direct Methanol Fuel Cells

High-pressure cylinders are one method of storing hydrogen onboard a vehicle for use in a fuel cell. This is a simple and lightweight storage method, but often does not provide sufficient vehicle driving

range. Another approach has been to fuel a modified PEM fuel cell with liquid methanol instead of hydrogen

gas.




Methanol has a higher energy density than gaseous hydrogen because it exists in a liquid state at normal

temperatures, and is easier to handle since no compressors or other high-pressure equipment is needed. This means that a fuel-cell vehicle can be refueled

with a liquid instead of high-pressure gas, which makes the refueling process simpler and produces a greater

vehicle driving range.




FUEL-CELL VEHICLE SYSTEMS

Humidifiers

Water management inside a PEM fuel cell is critical. Too much water can prevent oxygen from making contact with the positive electrode; too little water can allow the electrolyte to dry out and lower its conductivity. The role of the humidifier is to achieve a balance where it is providing sufficient moisture to the fuel cell by recycling water that is evaporating at the cathode. The humidifier is located in the air line leading to

the cathode of the fuel-cell stack.




Fuel-Cell Cooling Systems

One of the major challenges for engineers in this regard is the fact that the heat generated by the fuel cell is classified as low-grade heat. This means that

there is only a small difference between the temperature of the coolant and that of the ambient air. Heat transfers very slowly under these conditions, so heat exchangers with a much larger surface area must be

utilized.




In some cases, heat exchangers may be placed in other areas of the vehicle when available space at the front of the engine compartment is insufficient. An auxiliary heat exchanger is located underneath the vehicle to increase the cooling system

heat-rejection capacity.




Air Supply Pumps

Air must be supplied to the fuel-cell stack at the proper pressure and flow rate to enable proper performance under all

driving conditions.




Fuel-Cell Hybrid Vehicles

Hybridization tends to increase efficiency in vehicles with conventional drive trains, as energy that was once lost during braking and otherwise normal operation is instead stored for

later use in a high-voltage battery or ultracapacitor.




Secondary Batteries. In most FCHV designs, a high-voltage nickel-metal hydride (NiMH) battery pack is

used as a secondary battery.




Ultracapacitors. An alternative to storing electrical energy in batteries is to use ultracapacitors. A capacitor is best known as an electrical device that will block DC current, but allow AC to pass. However, a capacitor can also be used to store electrical energy, and it is able to do this without a

chemical reaction.




Ultracapacitors are built very different from conventional capacitors. Ultracapacitor cells are based on double-layer technology, in which two

activated-carbon electrodes are immersed in an organic electrolyte. The electrodes have a very large surface area and are separated by a membrane that allows ions to migrate but prevents the electrodes from touching.

Ultracapacitors can charge and discharge quickly and efficiently, making them especially suited for electric assist

applications in fuel-cell hybrid vehicles.




Ultracapacitors that are used in fuel-cell hybrid vehicles are made up of multiple cylindrical cells

connected in parallel.




Fuel-Cell Traction Motors. The electric traction motors used in fuel-cell hybrid vehicles are very

similar to those being used in current hybrid electric vehicles. The typical drive motor is based on an AC

synchronous design, which is sometimes referred to as a DC brushless motor. This design is very reliable as it does not use a commutator or brushes, but instead has a

three-phase stator and a permanent magnet rotor.




Some fuel cell hybrid vehicles use a single electric drive motor and a transaxle to direct power to the vehicle’s wheels. It is also possible to use wheel

motors to drive individual wheels.




Transaxles. Fuel-cell hybrid vehicles are effectively pure electric vehicles in that their drive train is electrically

driven.Fuel-cell hybrid vehicles use electric drive motors that require only a simple reduction in their final drive and a differential to send power to the drive wheels. No gear

shifting is required and mechanisms such as torque converters and clutches are done away with completely. The transaxles used in fuel cell hybrid vehicles are extremely simple with few moving parts, making them extremely durable, quiet, and

reliable.




Power Control Units. The drive train of a fuel-cell hybrid vehicle is controlled by a power control unit (PCU), which controls fuel-cell output and directs the flow of electricity between the various components.




Power to and from the secondary battery is directed through the power control unit, which is also

responsible for maintaining the battery pack’s state of charge and for controlling and directing the output of

the fuel-cell stack.




Hydrogen Storage

Modern drivers have grown accustomed to having a minimum of 300 miles between refueling stops, a goal

that is extremely difficult to achieve when fueling the vehicle with hydrogen. Hydrogen has a very high energy

content on a pound-for-pound basis, but its energy density is less than that of conventional liquid fuels.

This is because gaseous hydrogen, even at high pressure, has a very low physical density (mass per

unit volume).




A number of methods of hydrogen storage are being considered for use in fuel-cell hybrid vehicles. These include high-

pressure compressed gas, liquefied hydrogen, and solid storage in metal hydrides.




High-Pressure Compressed Gas. It is common for a pressure of 5,000 psi (350 bar) to be used, but

technology is available to store hydrogen at up to 10,000 psi (700 bar).




In order to refuel the compressed hydrogen storage tanks, a special high-pressure fitting is installed in

place of the filler neck used for conventional vehicles.




There is also a special electrical connector that is used to enable communication between the vehicle and the filling station during the refueling process.




Liquid Hydrogen. Hydrogen can be liquefied in an effort to increase its energy density, but this

requires that it be stored in cryogenic tanks at -423°F (-253°C).

One liter of liquid hydrogen only has one-fourth the energy content of 1 liter of gasoline.




Solid Storage of Hydrogen. One method discovered to store hydrogen in solid form is as a metal hydride, similar to how a

nickel-metal hydride (NiMH) battery works.A metal hydride is formed when gaseous hydrogen molecules

disassociate into individual hydrogen atoms and bond with the metal atoms in the storage tank. This process uses powdered metallic alloys capable of rapidly absorbing hydrogen to make

this occur.




Hydrogen Fuel = No Carbon

During combustion, the first element that is burned is the hydrogen. If combustion is not complete, carbon monoxide is formed, plus leaving some unburned carbon to accumulate in the

combustion chamber.




HYDRAULIC HYBRID STORAGE SYSTEM

Ford Motor Co. is experimenting with a system it calls Hydraulic Power Assist (HPA). This system converts kinetic energy to hydraulic pressure, and then uses that pressure to help accelerate the vehicle. It is currently being tested on a four-wheel-drive (4WD) Lincoln Navigator with a 4.0-L V-8

engine in place of the standard 5.4-L engine.




While the concept is simple, the system itself is very complicated. Additional components include:

Pulse suppressors Filters

An electric circulator pump for cooling the main pump/motor




HCCI

Homogeneous-Charge Compression Ignition (HCCI) is a combustion process. HCCI is the combustion of a very lean gasoline air-fuel mixture without the use of a spark ignition. It is a low-temperature, chemically

controlled (flameless) combustion process.




While the challenges of HCCI are difficult, the advantages include having a gasoline engine being able to deliver 80% of diesel efficiency (a 20% increase in fuel economy) for 50% of the cost. A diesel engine using HCCI can deliver gasoline-

like emissions.




PLUG-IN HYBRID ELECTRIC VEHICLES

A plug-in hybrid electrical vehicle (PHEV) is a hybrid electric vehicle that is designed to be plugged into an electrical outlet at night to charge the batteries. By

charging the batteries in the vehicle, it can operate using electric power alone (stealth mode) for a longer time thereby reducing the use of the internal combustion engine (ICE).




FUTURE FOR ELECTRIC VEHICLES

The future of electric vehicles depends on many factors including:

1. The legislative and environmental incentives to overcome the cost and research efforts to bring a usable electric

vehicle to the market.2. The cost of alternative energy.

3. Advancement in battery technology that would allow the use of lighter-weight and high-energy batteries.




Cold-Weather Concerns

Cold weather is a major disadvantage to the use of electric vehicles for the following reasons:

Cold temperatures reduce battery efficiency.Additional electrical power from the batteries is need to heat

the batteries themselves to be able to achieve reasonable performance.

Passenger compartment heating is a concern for an electric vehicle because it would require the use of resistance units

or other technology that would reduce the range of the vehicle.




Hot-Weather Concerns

Batteries do not function well at high temperatures, and therefore some type of battery cooling must be added to the

vehicle to allow for maximum battery performance.




Recharging Methods and Concerns

How far an electric vehicle can travel on a full battery charge is called its range. The range of an electric vehicle

depends on many factors, including:Battery energy storage capacity

Vehicle weightOutside temperature

Terrain (driving in hilly or mountainous areas requires more energy from the battery)

Use of air conditioning and other electrical devices




When the state of California mandated the use of zero emission vehicles (ZEV), charging stations were setup in many areas, usually in parking lots of businesses

and schools.




Charging Methods

When the state of California set up electric vehicle charging stations, both a conductive- and inductive-type charger was made available at each station.Conductive charging. Conductive charging uses an electrical plug that makes physical contact with

terminals in the vehicle. The charger can be powered by 110-V AC or most commonly 220-V AC.




Inductive charging. Inductive charging is achieved by inserting a paddle-like probe into a charging

receptacle (opening) in the vehicle. The charger is powered by 220-volt AC and does not make physical

contact with the vehicle.




NEDRA is the National Electric Drag Racing Association that holds drag races for electric- powered vehicles throughout the

United States. The association does the following:1. Coordinates a standard rule set for electric vehicle drag

racing, to balance the needs and interests of all those involved in the sport.

2. Sanctions electric vehicle drag racing events.3. Promotes electric vehicle drag racing.




WIND POWER

Wind power is used to help supplement electric power generation in many parts of the country. Because AC

electricity cannot be stored, this energy source is best used to reduce the use of natural gas and coal to help reduce CO2 emissions. Wind power is most economical if the windmills are located where the wind blows consistently above 8 miles per hour (13 km/h). Windmills are usually grouped together to form wind farms where the product is electrical energy.

Energy from wind farms can be used to charge plug-in hybrid vehicles, as well as for domestic lighting and power needs.




HYDROELECTRIC POWER

Hydroelectric power is limited to locations where there are dammed rivers and hydroelectric plants. However, electricity can and is transmitted long distances so that electricity generated at the Hoover Dam can be used in California and

other remote locations. Hydroelectric plants are limited as to the amount of power they can produce, and constructing new

plants is extremely expensive.




Information concerning hazardous materials is available on Material Safety Data Sheets (MSDS). Employers must have the Material Safety Data Sheets available

for employees.




SUMMARY

1. The chemical reaction inside a fuel cell is the opposite of electrolysis in that electricity is created when hydrogen

and oxygen are created from gases.2. A fuel cell produces electricity and releases heat and

water as the only by-products.3. The major disadvantages of fuel cells include:

High costLack of hydrogen refueling stations

Short rangeFreezing-temperature starting problems

4. Types of fuel cells include PEM (the most commonly used), PAFC, MCFC, and SOFC.




SUMMARY

5. Ultracapacitors are an alternative to batteries for the storage of electrical energy.

6. A gasoline-powered engine can be more efficient if it uses a homogeneous-charged compression ignition (HCCI) combustion

process.7. Plug-in hybrid electric vehicles could expand the range of

hybrid vehicles by operating on battery power alone.8. Wind power and hydroelectric power are being used to

recharge plug-in hybrids and provide electrical power for all uses, without harmful emissions.




REVIEW QUESTIONS

1. How does a fuel cell work?2. What are the advantages and disadvantages of fuel cells?3. What are the uses of the various types of fuel cells?

4. How does an ultracapacitor work?5. What are the advantages and disadvantages of using

hydrogen?6. What alternative power sources could be used for vehicles

use?

compact fuel cell that will operate satisfactorily under all driving conditions

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