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This glossary contains terms and acronyms related to hydrogen
and fuel cell technologies.
A
- AC Generator (or Alternator)
- An electric device that produces an electric current that reverses
direction many times per second. Also called a synchronous generator.
- Adsorption
- The adhesion of the molecules of gases, dissolved substances,
or liquids to the surface of solids or liquids with which they are
in contact.
- Air
- The mixture of oxygen, nitrogen
and other gases which, with varying amounts of water
vapor, forms the atmosphere of the earth.
- Alkaline Fuel Cell (AFC)
- A type of hydrogen/oxygen fuel cell in
which the electrolyte is concentrated
potassium hydroxide (KOH), and hydroxide ions
(OH-) are transported from the cathode
to the anode.
- Alloy
- Mixture containing mostly metals. For example, brass is an alloy
of copper and zinc. Steel contains iron and other metals, but also
carbon.
- Alternating Current (AC)
- A type of current that flows from positive to negative and from
negative to positive in the same conductor.
- Alternative Fuel
- An alternative to gasoline or diesel fuel that is not produced
in a conventional way from crude oil, for example compressed
natural gas (CNG), liquefied petroleum gas(LPG),
liquefied natural gas (LNG), ethanol,
methanol and hydrogen.
- Ambient Air
- The air surrounding a given object or system.
- Ambient Temperature
- The temperature of the surrounding medium, usually used to refer
to the temperature of the air in which a structure is situated or
a device operates.
- Anion
- A negatively charged ion; an ion that is attracted
to the anode.
- Anode
- The electrode at which oxidation
(a loss of electrons) takes place. For fuel
cells and other galvanic cells, the anode is the negative terminal;
for electrolytic cells (where electrolysis
occurs), the anode is the positive terminal.
- Atmospheric Pressure
- The force exerted by the movement of air in the atmosphere, usually
measured in units of force per area. For fuel cells, atmospheric
pressure is usually used to describe a system where the only pressure
acting on the system is from the atmoshpere; no external pressure
is applied.
- Atom
- The smallest physical unit of a chemical element that can still
retain all the physical and chemical properties of that element.
Atoms combine to form molecules, and they themselves contain several
kinds of smaller particles. An atom has a dense central core (the
nucleus) consisting of positively charged particles (protons)
and uncharged particles (neutrons). Negatively charged particles
(electrons) are scattered in a relatively
large space around this nucleus and move about it in orbital patterns
at extremely high speeds. An atom contains the same number of protons
as electrons and thus is electrically neutral (uncharged) and stable
under most conditions.
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B
- Battery
- An energy storage device that produces electricity by means of
chemical action. It consists of one or more electric cells each
of which has all the chemicals and parts needed to produce an electric
current.
- Bipolar Plates
- Conductive plate in a fuel cell stack that
acts as an anode for one cell and a cathode
for the adjacent cell. The plate may be made of metal or a conductive
polymer (which may be a carbon-filled
composite). The plate usually incorporates flow channels for the
fluid feeds and may also contain conduits for heat transfer.
- British Thermal Unit (BTU)
- The mean British Thermal Unit is 1/180 of the heat required to
raise the temperature of one pound (1lb) of water
from 32°F to 212°F at a constant atmospheric
pressure. It is about equal to the quantity of heat required
to raise one pound (1 lb.) of water 1°F.
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C
- Carbon (C)
- An atom and primary constituent of hydrocarbon
fuels. Carbon is routinely left as a black deposit on engine parts,
such as pistons, rings, and valves, by the combustion
of fuel.
- Carbon Dioxide (CO2)
- A colorless, odorless, noncombustible gas that is slightly more
than 1.5 times as dense as air and becomes a solid (dry ice) below
–78.5ºC. It is present in the atmosphere as a result
of the decay of organic material and the respiration of living organisms.
It is produced by the burning of wood, coal, coke, oil, natural
gas, or other fuels containing carbon.
- Carbon Monoxide (CO)
- Carbon monoxide is a colorless, odorless, tasteless, poisonous
gas that results from incomplete combustion
of carbon with oxygen.
- Catalyst
- A chemical substance that increases the rate of a reaction without
being consumed; after the reaction it can potentially be recovered
from the reaction mixture chemically unchanged. The catalyst lowers
the activation energy required, allowing the reaction to proceed
more quickly or at a lower temperature. In a fuel cell, the catalyst
facilitates the reaction of oxygen and hydrogen.
It is usually made of platinum powder very thinly coated onto carbon
paper or cloth. The catalyst is rough and porous so that the maximum
surface area of the platinum can be exposed to the hydrogen or oxygen.
The platinum-coated side of the catalyst faces the membrane in the
fuel cell.
- Catalyst Poisoning
- The process of impurities binding to a fuel cell's catalyst, lowering
the catalyst's ability to facilitate the desired chemical reaction.
See also FUEL CELL POISONING.
- Cathode
- The electrode at which reduction (a gain
of electrons) occurs. For fuel cells and
other galvanic cells, the cathode is the positive terminal; for
electrolytic cells (where electrolysis occurs), the cathode is the
negative terminal.
- Cation
- A positively charged ion.
- Celsius
- Metric temperature scale and unit of temperature (°C). Named
for Swedish astronomer Anders Celsius (1701-1744) although the thermometer
first advocated by him in 1743 had 100° as the freezing point
of water, and 0° as the boiling point,
the reverse of the modern Celsius scale. Also called the Centigrade
scale (Latin for "hundred degrees").
- Centimeter (cm)
- A metric unit of linear measure. One centimeter equals about 0.4
inch, and one inch equals about 2.5 centimeters. One foot is equal
to approximately 30 centimeters.
- Combustion
- Burning, fire produced by the proper combination of fuel, heat,
and oxygen. In the engine, the rapid burning
of the air-fuel mixture that occurs in the combustion
chamber.
- Combustion Chamber
- In an internal combustion engine, the space between the top of
the piston and the cylinder head, in which the air-fuel mixture
is burned.
- Composite
- Material created by combining materials differing in composition
or form on a macroscale to obtain specific characteristics and properties.
The constituents retain their identity; they can be physically identified,
and they exhibit an interface between one another.
- Compressed Hydrogen Gas (CHG)
- Compressed hydrogen gas is hydrogen compressed to a high-pressure
and stored at ambient temperature.
- Compressed Natural Gas (CNG)
- Mixtures of hydrocarbon gases and vapors, consisting
principally of methane in gaseous form that
has been compressed.
- Compressor
- A device used for increasing the pressure and density
of gas. Also see Turbocharger.
- Cryogenic Liquefaction
- Process through which gases such as nitrogen, hydrogen, helium,
and natural gas are liquefied under pressure at very low temperatures.
- Current Collector
- Term used to describe the conductive material in a fuel
cell that collects electrons (on the
anode side) or disburses electrons (on the
cathode side). Current collectors are microporous
(to allow for fluid flow through them) and lie in between the catalyst/electrolyte
surfaces and the bipolar plates.
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D
- Density
- Density is defined as the amount of mass in a unit volume. Density
varies with temperature and pressure.
- Direct Methanol Fuel Cell (DMFC)
- A type of fuel cell in which the fuel
is methanol (CH3OH), in gaseous
or liquid form. The methanol is oxidized directly at the anode
instead of first being reformed to produce hydrogen. The electrolyte
is typically a PEM.
- Dispersion
- The spatial property of being scattered about over an area or
volume
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E
- Electrode
- A conductor through which electrons enter or leave an electrolyte.
Batteries and fuel cells
have a negative electrode (the anode) and a
positive electrode (the cathode).
- Electrolysis
- A process that uses electricity, passing through an electrolytic
solution or other appropriate medium, to cause a reaction that breaks
chemical bonds, e.g., electrolysis of water to produce hydrogen
and oxygen.
- Electrolyte
- A substance that conducts charged ions from one electrode to the
other in a fuel cell, battery, or electrolyzer.
- Electron
- A stable atomic particle that has a negative charge; the flow
of electrons through a substance constitutes electricity.
- Emission Standards
- Regulatory standards that govern the amount of a given pollutant
that can be discharged into the air from a given source.
- Endothermic
- Describes a chemical reaction that absorbs or requires energy
(usually in the form of heat).
- Energy
- The quantity of work a system or substance is capable of doing,
usually measured in British thermal units (Btu)
or Joules (J).
- Energy Content
- Amount of energy for a given weight of fuel.
- Energy Density
- Amount of potential energy in a given measurement of fuel. See
Gravimetric Energy Density and Volumetric
Energy Density.
- Engine
- A machine that converts heat energy into mechanical energy.
- Ethanol (CH3CH2OH)
- An alcohol containing two carbon atoms.
It is a clear, colorless liquid and is the same alcohol found in
beer, wine, and whiskey. Ethanol can be produced from cellulosic
materials or by fermenting a sugar solution with yeast.
- Exhaust Emissions
- Materials emitted into the atmosphere through any opening downstream
of the exhaust ports of an engine, including water, particulates,
and pollutants.
- Exothermic
- A chemical reaction that gives off heat.
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F
- Fahrenheit
- Temperature scale and unit of temperature (°F). Named for
German physicist Gabriel Daniel Fahrenheit (1686-1736) who was the
first to use mercury as a thermometric fluid in 1714.
- Flammability Limits
- The flammability range of a gas is defined in terms of its lower
flammability limit (LFL) and its upper flammability limit (UFL).
Between the two limits is the flammable range in which the gas and
air are in the right proportions to burn when ignited. Below the
lower flammability limit there is not enough fuel to burn. Above
the higher flammability limit there is not enough air to support
combustion.
- Flashpoint
- The lowest temperature under very specific conditions at which
a substance will begin to burn.
- Flexible Fuel Vehicle
- A vehicle that can operate on a wide range of fuels blends (e.g.,
blends of gasoline and alcohol) that can be put in the same fuel
tank.
- Fuel
- A material used to create heat or power through conversion in
such processes as combustion or electrochemistry.
- Fuel Cell
- A device that produces electricity through an electrochemical
process, usually from hydrogen and oxygen.
- Fuel Cell Poisoning
- The lowering of a fuel cell's efficiency due to impurities in
the fuel binding to the catalyst.
- Fuel Cell Stack
- Individual fuel cells connected in series.
Fuel cells are stacked to increase voltage.
- Fuel Processor
- Device used to generate hydrogen from fuels such as natural
gas, propane, gasoline,
methanol, and ethanol,
for use in fuel cells.
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G
- Gas
- Fuel gas, such as natural gas, undiluted liquefied
petroleum gases (vapor phase only), liquefied petroleum gas-air
mixtures, or mixtures of these gases.
Liquefied Petroleum Gases (LPG) as
used in this standard, shall mean and include any material which
is composed predominantly of any of the following hydrocarbons,
or mixtures of them: propane, propylene, butanes (normal butane
or isobutane) and butylenes.
LP Gas-Air Mixture - Liquefied petroleum gases
distributed at relatively low pressures and normal atmospheric
temperatures which have been diluted with air to produce desired
heating value and utilization characteristics.
Natural Gas - Mixtures of hydrocarbon gases and
vapors consisting principally of methane
(CH4) in gaseous form.
- Gas Diffusion
- Mixing of two gases caused by random molecular motions. Gases
diffuse very quickly; liquids diffuse much more slowly, and solids
diffuse at very slow (but often measurable) rates. Molecular collisions
make diffusion slower in liquids and solids.
- Graphite
- Mineral consisting of a form of carbon that
is soft, black, and lustrous and has a greasy feeling; used in pencils,
crucibles, lubricants, paints, and polishes.
- Gravimetric Energy Density
- Potential energy in a given weight of fuel.
- Greenhouse Effect
- Warming of the Earth's atmosphere due to gases in the atmosphere
that allow solar radiation (visible, ultraviolet) to reach the Earth's
atmosphere but do not allow the emitted infrared radiation to pass
back out of the Earth's atmosphere.
- Greenhouse Gas (GHG)
- Gases in the Earth's atmosphere that contribute to the greenhouse
effect.
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H
- Heat Exchanger
- Device (e.g., a radiator) that is designed to transfer heat from
the hot coolant that flows through it to the air blown through it
by the fan.
- Heating Value (TOTAL)
- The number of British Thermal Units (BTUs)
produced by the combustion of one cubic
foot of gas at constant pressure, when the products of combustion
are cooled to the initial temperature of the gas and air, when the
water vapor formed during combustion is condensed,
and when all the necessary corrections have been applied.
Lower (LHV) - The value of the heat
of combustion of a fuel as measured by allowing all products of
combustion to remain in the gaseous
state. This method of measure does not take into account the heat
energy put into the vaporization of water (heat of vaporization).
Higher (HHV) - The value of the heat of combustion
of a fuel as measured by reducing all of the products of combustion
back to their original temperature and condensing all water vapor
formed by combustion. This value takes into account the heat of
vaporization of water.
- Higher Heating Value (HHV)
- See Heating Value.
- Hybrid Electric Vehicle (HEV)
- A vehicle combining a battery-powered electric motor with a traditional
internal combustion engine. The vehicle can run on either the battery
or the engine or both simultaneously, depending on the performance
objectives for the vehicle.
- Hydrides
- Chemical compounds formed when hydrogen gas reacts with metals.
Used for storing hydrogen gas.
- Hydrocarbon (HC)
- An organic compound containing carbon and hydrogen, usually derived
from fossil fuels, such as petroleum, natural gas,
and coal.
- Hydrogen (H2)
- Hydrogen (H) is the most abundant element in the universe but
it is generally bonded to another element. Hydrogen gas (H2) is
a diatomic gas composed of hydrogen atoms and is colorless and odorless.
Hydrogen is flammable when mixed with oxygen
over a wide range of concentrations.
- Hydrogen-rich Fuel
- A fuel that contains a significant amount of hydrogen,
such as gasoline, diesel fuel, methanol
(CH3OH), ethanol (CH3CH2OH),
natural gas, and coal.
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I
- Impurities
- Undesirable foreign material(s) in a pure substance or mixture.
- Internal Combustion Engine (ICE)
- An engine that converts the energy contained in a fuel inside
the engine into motion by combusting the fuel. Combustion
engines use the pressure created by the expansion of combustion
product gases to do mechanical work.
- Ion
- Atom or molecule that carries a positive or
negative charge because of the loss or gain of electrons.
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K
- Kilogram (kg)
- Metric unit of weight or mass, equal to approximately 2.2 lb.
Related units are the milligram (mg) at 1000 per kg, and the metric
tonne at 1000 kg.
- Kilowatt (kW)
- A unit of power equal to about 1.34 horsepower or 1,000 watts.
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L
- LH2
- See Liquefied Hydrogen.
- Liquefied Hydrogen (LH2)
- Hydrogen in liquid form. Hydrogen can exist in a liquid state,
but only at extremely cold temperatures. Liquid hydrogen typically
has to be stored at -253°C (-423°F). The temperature requirements
for liquid hydrogen storage necessitate expending energy to compress
and chill the hydrogen into its liquid state.
- Liquefied Natural Gas (LNG)
- Natural gas in liquid form. Natural gas is a liquid at -162°C
(-259°F) at ambient pressure.
- Liquefied Petroleum Gas (LPG)
- Any material that consists predominantly of any of the following
hydrocarbons or mixtures of hydrocarbons: propane,
propylene, normal butane, isobutylene, and butylenes. LPG is usually
stored under pressure to maintain the mixture in the liquid state.
- Liquid
- A substance that, unlike a solid, flows readily but, unlike a
gas, does not tend to expand indefinitely.
- Lower Heating Value
- See Heating Value.
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M
- Mechanical Energy
- Energy in a mechanical form.
- Megawatt (MW)
- A unit of power equal to one million watts or 1,000 kilowatts.
- Membrane
- The separating layer in a fuel cell that
acts as electrolyte (a ion-exchanger)
as well as a barrier film separating the gases in the anode
and cathode compartments of the fuel cell.
- Meter (m)
- Basic metric unit of length equal to 3.28 feet, 1.09 yards or
39.37 inches. Related units are the decimeter (dm) at 10 per meter,
the centimeter (cm) at 100 per meter, the millimeter (mm) at 1000
per meter and the kilometer (km) at 1000 meters.
- Methane (CH4)
- See Natural Gas.
- Methanol (CH3OH)
- An alcohol containing one carbon atom. It has been used, together
with some of the higher alcohols, as a high-octane gasoline component
and is a useful automotive fuel in its own right.
- Miles Per Gallon Equivalent (MPGE)
- Energy content equivalent to that of a gallon of gasoline (114,32
Btu).
- Millimeter (mm)
- Metric unit of length, equal to 0.04 inch (there are 25 mm in
an inch). There are 1000 millimeters in a meter.
- Milliwatt (mW)
- A unit of power equal to one-thousandth of a watt.
- Molten Carbonate Fuel Cell (MCFC)
- A type of fuel cell that contains a molten
carbonate electrolyte. Carbonate ions
(CO3-2) are transported from the cathode
to the anode. Operating temperatures are typically
near 650°C.
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N
- Nafion®
- Sulfonic acid in a solid polymer form that is usually the electrolyte
of PEM fuel cells.
- Natural Gas
- A naturally occurring gaseous mixture of simple hydrocarbon
components (primarily methane) used as a
fuel.
- Nitrogen (N2)
- A diatomic colorless, tasteless, odorless gas that constitutes
78% of the atmosphere by volume.
- Nitrogen Oxides (NOx)
- Any chemical compound of nitrogen and oxygen.
Nitrogen oxides result from high temperature and pressure in the
combustion chambers of automobile
engines and other power plants during the combustion process. When
combined with hydrocarbons in the presence of
sunlight, nitrogen oxides form smog. A basic air pollutant; automotive
exhaust emission levels of nitrogen oxides are regulated by law.
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O
- Oxidant"
- A chemical, such as oxygen, that consumes electrons in an electrochemical
reaction.
- Oxidation
- Loss of one or more electrons by an atom,
molecule, or ion.
- Oxygen (O2)
- A diatomic colorless, tasteless, odorless, gas that makes up about
21% of air.
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P
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R
- Reactant
- A chemical substance that is present at the start of a chemical
reaction.
- Reactor
- Device or process vessel in which chemical reactions (e.g., catalysis
in fuel cells) take place.
- Reformate
- Hydrocarbon fuel that has been processed into
hydrogen and other products for use in fuel
cells.
- Reformer
- Device used to generate hydrogen from fuels such as natural
gas, propane, gasoline,
methanol, and ethanol
for use in fuel cells.
- Reforming
- A chemical process in which hydrogen containing fuels react with
steam, oxygen, or both to produce a hydrogen-rich
gas stream.
- Reformulated Gasoline
- Gasoline that is blended so that, on average, it significantly
reduces volatile organic compounds and air toxics emissions relative
to conventional gasolines.
- Regenerative Fuel Cell
- A fuel cell that produces electricity
from hydrogen and oxygen and can use electricity from solar power
or some other source to divide the excess water into oxygen and
hydrogen fuel to be re-used by the fuel cell.
- Renewable Energy
- A form of energy which is never exhausted because it is renewed
by nature (within short time scales; e.g. wind, solar radiation,
hydro power).
- Reversible Fuel Cell
- See Regenerative Fuel Cell.
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S
- Solid Oxide Fuel Cell (SOFC)
- A type of fuel cell in which the electrolyte
is a solid, nonporous metal oxide, typically zirconium oxide (ZrO2)
treated with Y2O3, and O-2 is transported
from the cathode to the anode.
Any CO in the reformate gas is oxidized to CO2 at the
anode. Temperatures of operation are typically 800-1,000°C.
- Sorbent
- Material that sorbs another; i.e., has the capacity or tendency
to take it up either by adsorption or absorption.
- Sorption
- Process by which one substance takes up or holds another.
- Stack
- See Fuel Cell Stack.
- Steam Reforming
- The process for reacting a hydrocarbon fuel,
such as natural gas, with steam to produce hydrogen as a product.
This is a common method for bulk hydrogen generation.
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T
- Technology Validation
- Confirming that technical targets for a given technology have
been met.
- Temperature
- A measure of thermal content. See also Ambient
Temperature.
- Turbine
- Machine for generating rotary mechanical power from the energy
in a stream of fluid. The energy, originally in the form of head
or pressure energy, is converted to velocity energy by passing through
a system of stationary and moving blades in the turbine.
- Turbocharger
- A device used for increasing the pressure and density
of a fluid entering a fuel cell power plant
using a compressor driven by a turbine that extracts energy from
the exhaust gas.
- Turbocompressor
- Machine for compressing air or other fluid (reactant
if supplied to a fuel cell system) in order
to increase the reactant pressure and concentration.
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V
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W
- Water (H2O)
- A colorless, transparent, odorless, tasteless liquid compound
of hydrogen and oxygen.
The liquid form of steam and ice. Fresh water at atmospheric
pressure is used as a standard for describing the relative density
of liquids, the standard for liquid capacity, and the standard for
fluid flow. The melting and boiling points of water are the basis
for the Celsius temperature system. Water
is the only byproduct of the combination of hydrogen and oxygen,
and is produced during the burning of any hydrocarbon.
Water is the only substance that expands on freezing as well as
by heating, and has a maximum density at 4°C.
- Watt (W)
- A unit of power equal to one joule of work performed per second;
746 watts are the equivalent of one horsepower. The watt is named
for James Watt, Scottish engineer (1736-1819) and pioneer in steam
engine design.
- Wt.%
- The term wt.% (abbreviation for weight percent) is widely used
in hydrogen storage research to denote the amount of hydrogen stored
on a weight basis (the term mass % is also occasionally used). The
term can be used for materials that store hydrogen or for the entire
storage system (e.g., material or compressed/liquid hydrogen as
well as the tank and other equipment required to contain the hydrogen
such as insulation, valves, regulators, etc.). For example, 6 wt.%
on a system-basis means that 6% of the entire system by weight is
hydrogen. On a material basis, the wt.% is the mass of hydrogen
divided by the mass of material plus hydrogen.
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