Lecture 8 2015-2016

Renewable Energy SystemsDavid Buchla | Thomas Kissell | Thomas Floyd

Copyright © 2015 by Pearson Education, Inc.All Rights Reserved

Renewable Energy

Systems12



12Fuel Cells

12-1 BASIC FUEL CELL OPERATION12-2 TYPES OF FUEL CELLS

12-3 VEHICLE APPLICATIONS

12-4 STATIONARY FUEL CELL APPLICATIONS

Chapter Outline



12-1 Basic Fuel Cell Operation

A fuel cell is a device that converts electrochemical energy from a fuel into electricity.

It uses a constant flow of fuel (usually hydrogen in some form) and an oxidizer (usually oxygen).

A chemical reaction occurs within the cell, releasing electrons in the process to produce electricity.

A fuel cell has two electrodes (電極) :

- Anode陽極 (positive)

- Cathode 陰極 (negative)

3




The basic process in which hydrogen fuel is combined with oxygen in a common type of fuel is illustrated here.

Electrons pass from the

anode to the cathode

forming current. At the

cathode side, they

combine with oxygen,

to complete the reaction.

The membrane electrode assembly



Membrane electrode assembly:

The components in a fuel cell, which includes the anode and cathode electrodes, electrolyte, and the catalyst, form an assembly, where the chemical reactions occur.

5




Every fuel cell also has an electrolyte, which carries electrically charged particles from one electrode to the other; and catalysts, which speeds the reactions at electrodes.

Hydrogen is a basic fuel, but fuel cells also require oxygen.

Fuel cell generates electricity with very little pollution. Much of the hydrogen and oxygen used in generating electricity ultimately combine to form a harmless byproduct, namely water.

6




In the photo,

hydrogen and oxygen

are fed into the fuel

cell where they react

to produce electricity.

So

urc

e: N

REL




In a battery, the reactants are stored in the battery; in a fuel cell they are not.

People frequently confuse a fuel cell with a battery.

Fuel must be constantly supplied to a fuel cell.

8

Battery Fuel cell




The type of chemical reaction in all fuel cells is an oxidation-reduction reaction (referred to as “redox”reactions).

In a redox reaction, one reactant loses one or more electrons (oxidation) and the other reactant gains the electrons (reduction).

Electrons are transferred through a circuit (electrical current).

9




The reaction for burning hydrogen is the same with the

one that occurs in many fuel cells.

The reaction is written:

2 H2(g) + O2(g) ↔ 2 H2O(l) +energy

The process involves transferring electrons from

hydrogen to oxygen; hydrogen loses electrons and

oxygen gains electrons.




2 H2(g) + O2(g) ↔ 2 H2O(l) +energy

Oxidation in Anode side:

H2 -> 2H+ + 2e-

Reduction in Cathode side:

O2 + 4H+ + 4 e- -> 2H2O

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Fuels for fuel cell:

12

Gas Species PAFC MCFC SOFC PEFC

H2 Fuel Fuel Fuel Fuel

CO Poison (>0.5%) Fuela FuelPoison

(>10ppm)

CH4 Diluent Diluentb Fuela Diluent

CO2 & H2O Diluent Diluent Diluent Diluent

S as (H2S & COS)Poison (>50

ppm)

Poison(>0.5

ppm)

Poison(>1.0

ppm)

NO studies to

date (11)

http://www.nfcrc.uci.edu/3/FUEL_CELL_INFORMATION/FCexpla

ined/Fuels.aspx




Electrolytes:

allow positively charged hydrogen ions (or protons) to move between the two sides of the fuel cell.

• Aqueous alkaline solution

• Polymer membrane

• Molten phosphoric acid (H3PO4)

• Salt water

• Molten alkaline carbonate

• …

13




Catalysts:

accelerate the chemical reaction on both the fuel and air electrodes.

14

Anode catalyst Cathode catalyst

very fine platinum powder nickel

nanomaterial-based catalyst



12-1 Fuel Cell Systems

Three parts to a fuel cell systems are:

• A group of fuel cells bound together to increase power

Fuel cell stack

• The processor that converts the fuel is into a usable form for the fuel cell

Fuel processing

unit

• Collects excess heat for other uses such as heating water

Heat recovery system




Fuel cell stack:

The cells are connected in series to increase the voltage and in parallel to increase the current they can provide.

16

http://www.deskeng.com/de/designing-fuel-cells-for-the-future/

http://www.brimosduurzameenergie.nl/en/products

This is an exploded view of a fuel

cell stack abridged to two cells for

visual clarity. The high number of

plates in the hydrogen system made

them a key subject for design for

manufacture analysis and cost

reduction.

http://www.deskeng.com/de/designing-fuel-cells-for-the-future/




Fuel processing unit:

• If hydrogen is fed into the system, a processor may not be required, or it maybe needed only to filter impurities out of the hydrogen gas.

• Larger fuel cell systems frequently use methane or other hydrocarbon to produce electrical power, so the fuel processor is required to extract the hydrogen from the hydrocarbon.

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Heat Recovery System:

• Fuel cells typically operate at high temperatures (up to 600 – 900 oC), so they produce heat as a by-product.

• The heat recovery system collects excess heat for another use, which increases the overall energy efficiency of the fuel cell system.

• The excess heat can be used for hot water and/or steam generation.

• Steam can be used to drive a turbine and generator to produce electricity.

• Hot water can be used to provide heat for buildings.

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12-1 Obtaining hydrogen

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Hydrogen is the basic fuel for many fuel cells but hydrogen does not occur naturally as a element.

Rather hydrogen is found in compounds such as water and natural gas.

To obtain pure hydrogen, energy must be supplied.

In renewable energy systems, an electrical source such as solar cells can provide the electrical energy to power electrolysis. The advantage is that the energy can then be used whenever it is needed.

If the source of electricity for producing the fuel is renewable source, the hydrogen may serve as an energy storage mechanism.




You may have done an experiment like this in chemistry using electrical energy

from a battery to cause water to be

separated into hydrogen and oxygen.

This process is the electrolysis of water

and results in very pure hydrogen and

oxygen.




Hydrogen is also produced by steam reformation of

natural gas in refineries. Methane (CH4) and steam are

passed over a catalyst to produce carbon monoxide

(CO) and hydrogen (H2).

CH4(g) + H2O(g) CO(g) + 3H2(g) and

CO(g) + H2O(g) CO2(g) + H2(g)

Source: NREL




A problem with steam reformation is that the resulting hydrogen is not as pure as with electrolysis, so is not suitable for all types of fuel cells.

Using a fossil fuel such as natural gas, gasoline, etc., to obtain hydrogen is not carbon neutral except in the case of biofuels, so this may not be the most desirable way to obtain hydrogen.

22




Coal is one of the least expensive fuels and it is available in most of the world.

Coal can be gasified so that hydrogen is separated from it and used to power fuel cells, making it a cleaner way to use coal than burning it.

The problem with preparing hydrogen from fossil fuels is that the reformer creates a small amount of pollutants, and the hydrogen is not as pure and is not suitable for some types of fuel cells.

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12-2 Types of Fuel Cells

Proton Exchange Membrane Fuel Cell (PEMFC)

Direct Methanol Fuel Cell (DMFC)

Alkaline Fuel Cell (AFC)

Phosphoric Acid Fuel Cell (PAFC)

Solid Oxide Fuel Cell (SOFC)

Molten-Carbonate Fuel Cell (MCFC)

Fuel cells are described in terms of their chemistry

and electrolyte. Each type has unique operating

conditions, such as the operating temperature. The

most common types are:




Proton Exchange Membrane Fuel Cell (PEMFC)

The PEMFC is a solid polymer

cell that uses hydrogen as a

fuel.

Oxygen can be supplied from

the air. Hydrogen can be

made by a reformer, but this is an added expense.

Electrolyte: polymer membrane

Efficiency: 25%-58%Application: vehicles



• Electrodes use porous carbon that contains a platinum catalyst.

• The fuel cell uses either pure hydrogen from storage tanks or from reforming a hydrocarbon fuel with an onboard reformer.

Processes in PEMFC:

• Hydrogen gas is input into the cell, where it reacts with the anode, which in turn releases electrons as current.

• The cell needs oxygen, but it can be extracted from air and it does not require corrosive fluids.

• Oxygen from the air reacts with the remaining hydrogen ions to produce water.

26




Direct Methanol Fuel Cell (DMFC)

The DMFC uses methanol fuel

and an internal reformer to

convert it to hydrogen.

Oxygen can be supplied from

the air as in the case of the

PEMFC.

Application: vehicles



Processes in DMFC:

• Methanol is used as the fuel, hydrogen is separated by a steam reformer.

• Methanol and water are supplied to the fuel cell, and they react with the anode.

• Hydrogen is extracted from the methanol, and electrons from the hydrogen are free to flow as current out from the anode and return back through the cathode.

• Oxygen is supplied through the air and is ionized at the cathode.

• Oxygen ions combine with hydrogen ions to make water. 28




Alkaline Fuel Cell (AFC)

The AFC requires pure hydrogen

and pure oxygen to operate. It can

be poisoned by CO2 so cannot use

air as an oxygen source.

This tends to restrict applications to

cases like space flight due to

operating cost and limited lifetime.

Electrolyte: aqueous electrolyte

solution of potassium hydroxide (KOH)

in a porous stabilized matrix

Efficiency: 60-70%



Processes in AFC:

• Pure hydrogen is supplied to the fuel cell at the anode, and pure oxygen is supplied at the cathode. The anode and cathode are made of lower cost, nonprecious metals such as nickels.

• Hydrogen gas is oxidized to hydrogen ions and combines with the hydroxide ions, which produces water and releases two electrons.

• The electrons flow through the external circuit and return to the cathode, where they reduce oxygen to form more hydroixde ions and water.

30




Phosphoric Acid Fuel Cell (PAFC)

The PAFC has a structure like the PEMFC but uses liquid

phosphoric acid for the electrolyte. PAFCs operate with

relatively high temperatures (150 oC to 300 oC), so are generally

best used in larger buses or permanent installations. but can be

used in cars.

The PAFC in the photo is at

the Anchorage mail

processing center in

Alaska. It has five 200 kW

PAFCs that include a

unique software control

package to run the units.

Sourc

e:

NR

EL



Efficiency: 40-50%

When energy produced by the waste heat is considered, the efficiency rises to about 80%.

The structure is equivalent to proton exchange membrane fuel cell (PEMFC), but the electrolyte is different.

32




Solid Oxide Fuel Cell (SOFC)

The SOFC uses a solid oxide

electrolyte. It can use hydrogen or

other fuels such as methane or

natural gas and obtains oxygen from air. The SOFC runs very hot

(800 oC to 1000 oC). For this reason,

it has not been used in cars but has been tested in trucks.

An SOFC unit was tested by the

National Energy Technology Laboratory (NETL) and provided

800 W for 10 hours, providing rest time for drivers without idling the

engine, thus saving fuel and lowering emissions.



Processes in SOFC:

• Hydrogen fuel is applied to anode, which is oxidized to hydrogen ions.

• The electrons are given off to an external circuit as electrical current.

• The oxygen from air is reduced to oxygen ions that diffuse through the solid electrolyte to the porous anode.

• The electrolyte is a hard, nonporous solid oxide that allows only the oxygen ions to pass and blocks electrons.

34




Molten-Carbonate Fuel Cell (MCFC)

The MCFC is similar to the SOFC

but with carbonate ions as the

charge carrier.

It runs hot (800 oC to 1000 oC) but

this is a lower temperature than

the SOFC.

Like the SOFC, it can convert

fuels to hydrogen without the

need for a reformer.

Electrolyte: sodium carbonate,

lithium, sodium



Processes:

• Hydrogen is ionized at the anode by the catalyst.

• After carbonate ions pass through the electrolyte, they combine with the hydrogen ions to form water and carbon dioxide, which is returned to the input.

• The carbon dioxide and electrons from the external circuit combine with oxygen to supply more carbonate ions, and the process continues.

• The net reaction is hydrogen plus oxygen, which produces water as in other fuel cells.

36



37



12-3 Vehicle Applications

Fuel cells are used to provide power for automobiles, trucks, and buses.

Hydrogen fuel or a compound fuel such as gasoline can be used to generate electrical power to operate electric drive motors.

The applications include:

• Fuel cell electric vehicle (FCEV)

• Buses

• Refueling stations for hydrogen fuel cell vehicles

• Fuel cell-powered working vehicles

• Others

38




A fuel cell electric vehicle (FCEV) uses electricity from

a fuel cell to power an electric motor drive.

The PEMFC and

the PAFC have

shown the most

promise as fuel

cells for cars and

smaller vehicles.




Issues of concern on FCEV:

Cost: the current cost to build a production model, fuel cell vehicle is still much more that a comparable gasoline car. Government programs have enhanced the development of hydrogen for automotive applications, the infrastructure for fuel stations, and advanced technology fuel cells.

Difficult to start: if the ambient temperature is below -10oC, the automobiles using fuel cell are difficult to start and slow to reach the operating temperature.

40




Fuel cell electric buses offer advantages for air

pollution in cities and congested areas and have

accumulated thousands of hours of operating time

and many generations of buses have been

constructed.

At this time, fuel cell buses are

generally considered to be

experimental because they are

not cost competitive with

traditional buses, but development work continues.

Sourc

e:

NR

EL




Working vehicles (mining locomotives, forklifts, etc.)

can take advantage of pollution free operation,

particularly in enclosed spaces. One advantage over

battery power vehicles is that turn around time for fuel

cell power is fast.

In another application, Airbus is

developing a multifunctional fuel cell for the auxiliary power unit on

an A320. It has the potential to

reduce total aircraft fuel use by up

to 15% because a byproduct is clean water for aircraft use and

nitrogen for fire suppression.

So

urc

e: N

REL



12-4 Stationary Fuel Cell Applications

Large fuel cells can provide power for buildings or small

cities. The largest stationary fuel cell power park in North

America is in Bridgeport, CT where a 14.9 MW installation

supplies power to the electric grid adequate to power

15,000 homes.

Fuel cell plants are decentralized,

which can help provide power in

the event of outages of the main

grid to the area served. Here a

food processing plant uses a

2 MW fuel cell; the gases from

food processing can help supply fuel for the fuel cells.

Co

urt

esy

of

Fue

lCe

llEn

erg

y, In

c.




Some advantages to larger fuel cell power plants for

electrical production are:

Clean

•Virtually no NOx, SOx or particulate matter is produced.

Efficient

•For their size, fuel cells are efficient.

Scalable

•Power can be added incrementally as it is needed.

Distributed generation

•Power is generated near the end user, avoiding transmission cost and losses and need for distribution grids.

Small footprint

•The small requirement for land and the quiet operation simplify siting requirements.




The excess heat from a large plant can be captured

and used to drive a low-temperature Organic

Rankine Cycle (ORC) generator or the heat can be

used for another purpose. Turbine/

Generator

Condenser

Pump

Boiler

In Bridgeport, excess heat is

used to drive an ORC

generator. The cycle is like

that of any stream

generator but the working fluid has a lower boiling

point.



Selected Key Terms

Alkaline fuel cell

Fuel Cell

Fuel processing unit

Heat recovery

system

A very efficient fuel cell that requires pure

hydrogen fuel and pure oxygen. It uses an

aqueous (water-based) electrolyte solution of

potassium hydroxide (KOH) in a porous stabilized

matrix.

A portion of a fuel cell system that converts the

input fuel into a form useable by the fuel cell.

A part of the fuel cell that processes the excess

heat for another use such as hot water or steam.

A device that converts electrochemical energy

into dc directly by using a constant flow of fuel

(usually hydrogen) from an outside source.



Selected Key Terms

Membrane

electrode assembly

Proton exchange

membrane fuel

cell (PEMFC)

Reformer

Solid oxide fuel

cells (SOFC)

The components in a fuel cell that include the

anode and cathode electrodes, the electrolyte

and the catalyst.

A type of fuel cell that uses hydrogen fuel and

oxygen (obtained from air) input and produces

pure water and electricity. It uses porous carbon

electrodes that contain a platinum catalyst.

A device that extracts hydrogen from another

fuel such as methane.

A fuel cell named for the solid oxide electrolyte

that is used. The fuel is hydrogen which is supplied

as a gas along with oxygen from the air. The

electrolyte is a hard, non-porous solid oxide or

ceramic compound made of Yttria-stabilized

zirconia.



true/false quiz

1. A fuel cell is the same as a battery.



true/false quiz

2. All fuel cells require an oxidizer,

which is supplied from outside the

fuel cell.



true/false quiz

3. When hydrogen and oxygen react,

an unwanted by product is carbon

dioxide.



true/false quiz

4. The membrane electrode assembly

includes the load.



true/false quiz

5. The fuel processing unit converts

the fuel into a usable form for the

fuel cell.



true/false quiz

6. The process

illustrated here is

called electrolysis.



true/false quiz

7. The proton exchange membrane

fuel cell (PEMFC) uses ordinary

battery acid for the electrolyte.



true/false quiz

8. The solid oxide fuel cell shows great

promise to be used in cars because

it runs cooler than other fuel cells.



true/false quiz

9. A good application for fuel cells is in

vehicles that work in enclosed

spaces like warehouses.



true/false quiz

10. An advantage of larger fuel cell

plants for electrical production is

they do not require an inverter.



true/false quiz

Answers:

1.F

2.T

3.F

4.F

5.T

6.T

7.F

8.F

9.T

10. F



Multiple choice quiz

1. The two fuel cells that are most likely to be used in automotive applications are the

A. PEMFC and the PAFC

B. PEMFC and the SOFC

C. PAFC and the SOFC

D. PAFC and the MCFC

59



2. The electrolyte in a proton-exchange membrane fuel cell passes

A. Electrons but not hydrogen ions

B. Both electrons and hydrogen ions

C. Hydrogen ions but not electrons

D. Hydrogen molecules and electrons

60



3. An advantage to fuel cell-powered vehicles over gasoline-powered vehicles is that they

A. Have a much greater range

B. Emit only water vapor for exhaust

C. Do not have a cooling system

D. Feature all of these

61



4. The reforming process

A. Separates hydrogen from hydrocarbon fuels

B. Separate water from hydrocarbon fuels

C. Combines water and hydrogen into a new fuel

D. Combines oxygen molecules with water to make a new fuel

62



1. A

2. C

3. B

4. A

63



Calculation

Question:

How many 25 watt fuel cell stacks are needed to produce 5kW?

Solution:

5kW/0.025kW = 200 fuel cell stacks

64

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