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Fuel CellsFuel Cells
Larry Caretto
Mechanical Engineering 496ALT
Alternative Energy Alternative Energy
April 14, 2009
Reminder – optional campus energy facilities tour scheduled next Saturday,
April 18, 9 am to noon. Details on next chart.
Readings:
Today – Section 7.3.4 on fuel cells
Thursday and next Tuesday – Chapters 19 and 20 on energy conservation
Homework problems on biofuels due on Thursday (April 16). Download
problem assignment from web site.
2
Saturday
• Meet in PPM
conference room
at 9:00 am
• Can park in
adjoining lot on
Halstead St.
• Cannot drive
there from
campus
This Saturday we will have the campus tour of energy facilities.
We will start at 9 am in the Physical Plant Management (PPM) conference
room off Halstead Street as shown in the map.
Note that you cannot drive there from campus via Etiwanda Street; it is
closed at Halstead.
We will start with a presentation of the campus facilities for energy
conservation and alternative energy use followed by a tour to visit the
coolers that provide air conditioning to campus by using off-peak energy
storage, the solar collectors in lot E6, and the fuel cell south of the Student
Union.
You will see the actual installation of the faciltiies that we have been
discussing in class. In particular, you will see the so called balance of plant
(BOP) facilities that are required to make the individual operations, such as a
fuel cell, practicable.
Because this tour is scheduled outside of normal class hours, it is optional
and any material discussed during the day will not be covered on any
examinations. However, your attendance is strongly encouraged because it
will provide you with a first-hand view of actual equipment and the ability to
ask questions of a facilities manager responsible for the actual operation of
this equipment.
4
Electrochemistry Basics
• Oxidation and reduction
– Electrons are transferred from the reducing
agent to the oxidizing agent
– Oxidation is a loss of electrons
– Reduction is a gain of electrons
• Electrochemical cells
– Oxidation (electron loss) at the anode
– Reduction at the cathode
Reference: http://electrochem.cwru.edu/ed/dict.htm (Accessed April 17, 2007.)
An electrochemical cell device that converts chemical energy into electrical energy or vice versa when a chemical reaction is occurring in the cell. Typically, it consists of two metal electrodes immersed into an aqueous solution (electrolyte) with electrode reactions occurring at the electrode-solution surfaces.
It consists of two electronically conducting phases (e.g., solid or liquid metals, semiconductors, etc) connected by an ionically conducting phase (e.g aqueous or non- aqueous solution, molten salt, ionically conducting solid). As an electrical current passes, it must change from electronic current to ionic current and back to electronic current. These changes of conduction mode are always accompanied by oxidation and reduction reactions. An essential feature of the electrochemical cell is that the simultaneously occurring oxidation-reduction reactions are spatially separated. E.g., in a spontaneous "chemical reaction" during the oxidation of hydrogen by oxygen to water, electrons are passed directly from the hydrogen to the oxygen. In contrast, in the spontaneous electrochemical reaction in a galvanic cell the hydrogen is oxidized at the anode by transferring electrons to the anode and the oxygen is reduced at the cathode by accepting electrons from the cathode. The ions produced in the electrode reactions, in this case positive hydrogen ions and the negative hydroxyl (OH - ) ions, will recombine in the solution to form the final product of the reaction: water. During this process the electrons are conducted from the anode to the cathode through an outside electrical circuit where the electrical current can drive a motor, light a light bulb, etc. The reaction can also be reversed, water can be decomposed into hydrogen and oxygen by the application of electrical power in an electrolytic cell.
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Electrolytic Cell
• Converts electrical energy into chemical
energy
• The anode is the positive electrode and
the cathode is the negative electrode
• Current flows from the anode to the
cathode
• Electrolysis and battery recharging are
examples of electrolytic cells
Reference: http://electrochem.cwru.edu/ed/dict.htm (Accessed April 17, 2007.)
An electrochemical reaction is an oxidation/reduction reaction that occurs in an electrochemical cell. The essential feature is that the simultaneously occurring oxidation- reduction reactions are spatially separated.
An electrolytic cell is an electrochemical cell that converts electrical energy into chemical energy. The chemical reactions do not occur "spontaneously" at the electrodes when they are connected through an external circuit. The reaction must be forced by applying an external electrical current. It is used to store electrical energy in chemical form. It is also used to decompose or produce (synthesize) new chemicals by application of electrical power. This process is called electrolysis, e.g., water can be decomposed into hydrogen gas and oxygen gas. The free energy change of the overall cell reaction is positive.
The cathode is the electrode where reduction occurs in an electrochemical cell. It is the negative electrode in an electrolytic cell, while it is the positive electrode in a galvanic cell. The current on the cathode is considered a negative current according to international convention.
The anode is the electrode where oxidation occurs in an electrochemical cell. It is the positive electrode in an electrolytic cell, while it is the negative electrode in a galvanic cell. The current on the anode is considered a positive current according to international convention.
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Fuel Cell History
• First proposed and demonstrated by
Grove in 1839
• Small developments over next 100
years including basic theory
• Bacon (~1938 to 1958) developed
modern fuel cell
• NASA used fuel cells on Apollo
Reference: http://fuelcells.si.edu/origins/origins.htm (Accessed April 17, 2007.) William Robert Grove (1811 -1896), an English lawyer turned scientist, won renown for his development of an improved wet-cell battery in 1838. The "Grove cell," as it came to be called, used a platinum electrode immersed in nitric acid and a zinc electrode in zinc sulfate to generate about 12 amps of current at about 1.8 volts. Grove discovered that by arranging two platinum electrodes with one end of each immersed in a container of sulfuric acid and the other ends separately sealed in containers of oxygen and hydrogen, a constant current would flow between the electrodes. The sealed containers held water as well as the gases, and he noted that the water level rose in both tubes as the current flowed. Ludwig Mond (1839 -1909). In 1889, he and his assistant, Langer , described their experiments with a hydrogen-oxygen fuel cell that attained 6 amps per square foot (measuring the surface area of the electrode) at .73 volts Friedrich Wilhelm Ostwald (1853 -1932), a founder of the field of physical chemistry, provided much of the theoretical understanding of how fuel cells operate. In 1893, he experimentally determined the interconnected roles of the various components of the fuel cell: electrodes, electrolyte, oxidizing and reducing agents, anions, and cations.
Emil Baur (1873 -1944) of Switzerland (along with several students at Braunschweig and Zurich) conducted wide-ranging research into different types of fuel cells during the first half of the twentieth century. Baur's work included high temperature devices (using molten silver as an electrolyte) and a unit that used a solid electrolyte of clay and metal oxides.
Francis Thomas Bacon (1904 -1992) began researching alkali electrolyte fuel cells in the late 1930s. In 1939, he built a cell that used nickel gauze electrodes and operated under pressure as high as 3000 psi. During World War II, Bacon worked on developing a fuel cell that could be used in Royal Navy submarines, and in 1958 demonstrated an alkali cell using a stack of 10-inch diameter electrodes for Britain's National Research Development Corporation. Though expensive, Bacon's fuel cells proved reliable enough to attract the attention of Pratt & Whitney. The company licensed Bacon's work for the Apollo spacecraft fuel cells.
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Fuel Cell
• 1 – O 2 and H 2 In
• 2 – Ions move in
electrolyte
• 3 – Electrons in
external circuit
send current to
load
• 4 – Product water
leaves cell
Reference: http://www.fe.doe.gov/coal_power/fuelcells/fuelcells_howitworks.shtml
A fuel cell produces electricity by means of an electrochemical reaction much like a battery. But there is an important difference. Rather than extracting the chemical reactants from the plates inside the cells, a fuel cell uses hydrogen fuel and oxygen extracted from the air to produce electricity. As long as these substances are fed into the fuel cell, it will continue to generate electric power.
Different types of fuel cells work with different electrochemical reactions. The following is a basic description of how a phosphoric acid fuel cell generates electric power.
Hydrogen gas is extracted from natural gas or other hydrocarbon fuels and permeates the anode. Oxygen from the air permeates the cathode.
Aided by a catalyst in the anode, electrons are stripped from the hydrogen. Hydrogen ions pass into the electrolyte.
Electrons cannot enter the electrolyte. They travel through an external circuit, producing electricity.
Electrons travel back to the cathode where they combine with hydrogen ions and oxygen to form water.
A fuel cell provides DC (direct current) voltage that can be used to power motors, lights or other electrical appliances. To supply grid power (or locally generated electric power), the direct current an inverter is required to convert the DC power into AC.
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Fuel Cell Stacks
• The previous diagrams showed
individual cells
• Typical voltage of these cells is about
one volt
• Individual cells are joined together in an
assembly called a “stack”
• Separator (bipolar) plates are used to
link individual cells in the stack
The following quote is adapted from the Fuel Cell Handbook: As with
batteries, individual fuel cells must be combined to produce appreciable
voltage levels and so are joined by interconnects, which are sometimes
called bipolar plates. Because of the configuration of a flat plate cell, the
interconnect becomes a separator plate with two functions: 1) to provide an
electrical series connection between adjacent cells, specifically for flat plate
cells, and 2) to provide a gas barrier that separates the fuel and oxidant of
adjacent cells. (The interconnect of a tubular solid oxide fuel cell is a special
case.)
All interconnects must be an electrical conductor and impermeable to gases.
Other parts of the cell of importance are (1) the structure for distributing the
reactant gases across the electrode surface and which serves as
mechanical support, (2) electrolyte reservoirs for liquid electrolyte cells to
replenish electrolyte lost over life, and (3) current collectors (not shown) that
provide a path for the current between the electrodes and the separator of
flat plate cells. Other arrangements of gas flow and current flow are used in
fuel cell stack designs, and are mentioned in Sections 3 through 6 of the
Fuel Cell Handbook for the various type cells.
11
Assembling a Stack
Reference: Figure 1-4 in the Fuel Cell handbook. This shows a phosphoric
acid fuel cell. Several individual cells are placed together to form an
complete stack.
13
Reference: http://216.51.18.233/library/ADLCostModel.pdf
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http://www.gmfuelcell.com/w_shoop/pdf/Andrew%20Bosco%20FC(E).pdf
Latest Fuel Cell Stack Developments
Andrew Bosco, Manager - Architecture, Design & Modeling
Global Alternative Propulsion Center (GAPC)
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Fuel Cell Systems
Reference: http://www.dodfuelcell.com/fcdescriptions.html
Fuel cells are typically grouped into three sections: (1) the fuel processor, (2)
the power section (fuel cell stack), and (3) the power conditioning.
In the fuel processor, a fuel such as natural gas is reformed to boost the
concentration of hydrogen. The hydrogen rich fuel and oxygen (air) then
feeds into the power section to produce DC electricity and reusable heat.
The power section includes a fuel cell stack which is a series of electrode
plates interconnected to produce a set quantity of electrical power. The
output DC electricity is then converted to AC electricity in the power
conditioning section where it also reduces voltage spikes and harmonic
distortions.
The notion of a fuel processor is an important one because fuel cells
generally work best on hydrogen. We will see that some types of fuel cells
can reform the fuel internally.
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Reference: http://www.lanl.gov/mst/fuelcells/adiabatic.shtml Adiabatic fuel cell stacks have been developed and patented by Los Alamos National Laboratory.
The simplicity of adiabatic stacks is their most attractive feature and is accomplished primarily through two technological elements. First is the direct humidification of the fuel cell membrane electrode assemblies (MEAs) with liquid water, and the second is operation of the fuel cell stack at very-near-ambient pressure. Direct MEA humidification is made possible through the introduction of an anode-wicking backing that conveys liquid water from the anode flow-field plenum through the nominally hydrophobic gas diffusion layer directly to the membrane throughout the active area.
Because even modest pressure can result in high compression power requirements, near- ambient pressure operation is critical to the stack’s efficiency. In conventional systems humidification modules, internal manifolding, and two-phase flows in the cathode channels create high-pressure drops that necessitate air inlet pressurization, but the direct humidification system avoids these pressure drops and allows the inlet pressure to be kept to about six inches of water. During the normal operation of this well-humidified fuel cell stack with a dry, ambient temperature cathode air inlet, the ai rstream becomes heated and saturated with water vapor as it passes through the cells. This effect provides in situ evaporative cooling of the stack, eliminating the need for separate cooling systems or in- stack cooling plates. The non-isothermal stack operation and evaporative cooling result in an “adiabatic” stack.
The simplicity of the adiabatic system is easy to appreciate when compared to the conventional system, with its extensive flow and control elements. A simple plastic condenser is used to recover surplus water and works effectively even in Los Alamos’ high desert climate. The single-step heat exchange process allows higher temperature differentials in the condenser than could be attained in a radiator, and may prove to be a general improvement over more conventional approaches using radiators and coolants.
19
Reference: http://fuelcells.si.edu/basics.htm
Alkali or Alkaline fuel cells operate on compressed hydrogen and oxygen.
They generally use a solution of potassium hydroxide (chemically, KOH) in
water as their electrolyte. Efficiency is about 70 percent, and operating
temperature is 150 to 200 degrees C, (about 300 to 400 degrees F). Cell
output ranges from 300 watts (W) to 5 kilowatts (kW). Alkali cells were used
in Apollo spacecraft to provide both electricity and drinking water. They
require pure hydrogen fuel, however, and their platinum electrode catalysts
are expensive. And like any container filled with liquid, they can leak.
At the cathode, the oxygen reacts with the electrolyte, typically potassium
hydroxide to form two hydroxyl ions, OH–^. These hydroxyl ions then migrate
to the anode where they react with hydrogen to produce water. The reaction
with hydrogen liberates electrons that then go through the external circuit.
Although these cells are quite effective in space applications, operating on
H 2 and pure O 2 , the deteriorate rapidly when used with air as the oxidizer
due to the CO 2 in the air. The CO 2 goes into solution forming carbonate ions
that reduce the performance of the cell much more rapidly than the normal
deterioration when operating with pure oxygen. Further research on alkaline
cells for terrestrial applications is essentially zero.
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Fuel Cell Types
Efficiency 40 – 50% 50 – 60% 45 – 55% 40 – 50%
Oxidant O 2 /Air O 2 /Air O 2 /Air O 2 /Air
Reforming External Internal Internal External
Fuels H 2 H 2 /CO H 2 /CH 4 H 2
Temperature 375 F 1200 F 1830 F 175 F
Molten car- Ceramic Polymer
bonate salt
Phosphoric
Acid
Electrolyte
PAFC MCFC SOFC PEMFC
Reference: http://www.dodfuelcell.com/fcdescriptions.html
These four fuel cell types are the ones under most intensive investigation by
the DOE for distributed power generation. Note that the MCFC and the
SOFC fuel cells are high temperature cells. The use of these fuel cells is
typically done in conjunction with some application for the heat rejected by
them.
Specific details about each of these four are presented on the next set of
charts.
