Glossary

AFC
alkaline fuel cell; with alkaline electrolyte, operating temperature 60 to 90^oC; fuel: pure hydrogen; can only be operated with pure oxygen or with air if the CO₂ has been removed; state of the art: so far used mainly in military applications and space travel; presently developed and manufactured by ZeTek Power for terrestrial applications.

biomass
all organic substances: plants, wood chips, bales of straw, liquid manure, organic wastes etc.

biomass converter
(technical) system that converts organic feedstock (biomass) into a technically usable energy carrier: e.g. steam reformer.

boil-off loss
amount of gas that vaporizes in a liquid gas storage through external heating (ambient temperature). The gas will only be vented when the operating pressure is exceeded.

catalyst
a catalyst is a material that facilitates, accelerates etc. a chemical reaction retaining its own properties and without being consumed.

catalytic combustion
in a catalytic combustion the combustion temperature is reduced by a catalyst. Lower temperatures result in near zero nitrous oxide (NO_x) emissions.

CGH₂ or CH₂
compressed gaseous hydrogen

compressed gas storage
storage device for gases (e.g. hydrogen, natural gas, nitrogen) at room temperature under high pressure (typically some 20^oMPa).

compressor
device for increasing gas pressure or gas flow rate.

cryoadsorption storage
special type of graphite storage. Carbon is able to adsorb hydrogen. Different qualities of carbon can adsorb higher quantities of hydrogen under certain temperature and pressure conditions than could be stored without the carbon under the same conditions. Temperatures are below 0^oC (cryogenic) and above boiling temperature of hydrogen (20^oK). The pressure levels are above 5^oMPa.

cryogenic
Greek kry^oos: cold, frost. Applied to gases cryogenic refers to low temperatures where the gases are in their liquid phase. For natural gas the boiling temperature (where the phase transition from liquid to gaseous occurs) is -161.5^oC (111.5^oK) and for hydrogen it is -253^oC (20^oK).

dissociation of water at high temperatures
above 2000^oK (1700^oC approximately), a temperature that can be achieved in solar furnaces without major problems, water is split into hydrogen and oxygen. Ceramic membranes permitting the permeation of hydrogen but inhibiting that of oxygen are used for the gas separation. This process is in a very early stage of development.

DMFC
direct methanol fuel cell; fuel: methanol; state of the art: basic research.

electro farming
concept that comprises the conversion of energy crops (biomass) via steam reforming and fuel cells into electricity. This way, in principle, electricity is farmed".

elektrolyzer
In an electrolyzer, an electric current splits water into hydrogen and oxygen. Reverse process of the fuel cell.

energy carrier
medium (gaseous, e.g. natural gas, hydrogen; liquid, e.g. petrol, biofuels; solid, e.g. wood, coal) in which energy is stored in chemical form; by means of energy carriers energy is storable and transportable. Non-material energy carriers are e.g. electricity and solar radiation. Within certain limits and with certain losses energy carriers can be converted into one another (e.g. solar radiation into electricity, electricity into hydrogen, hydrogen into electricity, electricity into light etc.).

energy crop
plants that are grown for the sole purpose of energy production, not for food production (e.g. rape used for the production of biofuels). The growing of energy crops is not yet very wide-spread.

fuel cell
A fuel cell is an electrochemical device in which hydrogen and oxygen combine in an controlled manner (in contrast to combustion or explosion) to directly produce an electric current and heat. Reverse process of electrolyzer.

full composite storage
storage tank produced entirely from composite materials. Presently, the market introduction of full composite compressed gas storages takes place.

GH₂
gaseous hydrogen. At room temperature (above -253^oC or 20^oK, to be exact) hydrogen is gaseous independent of the pressure.

graphite storage
carbon is able to adsorb hydrogen. The amount of adsorbed hydrogen depends on temperature, pressure and the quality/ structure of the carbon used. Carbon structures in the nanometers range (one nanometer corresponds to 10^-9 meters), e.g. balls, tubes or fibers, seem to be very promising. The developments are in a very early stage.

H₂
hydrogen

H₂/O₂ steam generator
device that produces steam via the reaction of hydrogen and oxygen. The subsequent injection of water allows a temperature control between 200 and 2000^oC. H₂/O₂ steam generators have been developed as a spinning reserve of large power plants, but have not yet been applied.

heating value
energy content of an energy carrier. Upper and lower heating value are distinguished. Upper heating value: total energy content of the energy carrier. Lower heating value: energy content reduced by the condensation energy (latent heat) of the product gas (the steam in the product gas, to be exact).

hydrogen
H is the chemical symbol for hydrogen, the lightest element of the table of elements and the most abundant element of the universe. In general, hydrogen will be found in molecular form, i.e. as a hydrogen molecule composed of two hydrogen atoms (H₂), or in other compounds (e.g. in water  H₂O, organic substances). Hydrogen as secondary energy carrier is seen as the key component of a global renewable world energy supply.

hydrogen as solar energy carrier
solar hydrogen energy economy

hydrogen energy economy
energy economy where hydrogen is used as the secondary energy carrier.

hydrogen liquefaction
liquefaction of hydrogen, which is gaseous at room temperature, by cooling it below -253^oC (20^oK).

hydrogen motor
combustion engine which uses hydrogen as a fuel.

hydrogen propulsion
mobile propulsion system that uses hydrogen as fuel. The propulsion energy is produced in a fuel cell and an electric motor, in a combustion engine (hydrogen motor) or a gas turbine.

hydrogen storage
compressed gas storage, cryoadsorption storage, graphite storage, iron sponge storage, liquid hydrogen storage, metal hydride storage.

hydrogen jet engine
hydrogen fueled jet engine for aviation use

iron sponge storage
iron sponge can be used as a hydrogen storage material. Hydrogen and rust" (Fe₃O₄) are converted into pure iron ( "iron sponge" ) which is transported to the hydrogen consumption site. In the reverse reaction (oxidation) rust" is produced liberating the hydrogen. The iron sponge storage can also be filled/ loaded with synthesis gas (mixture of hydrogen and carbon monoxide) also liberating pure hydrogen in the reverse reaction. Iron sponge storage is in an early stage of development.

LH₂
liquid hydrogen

LH₂ storage
liquid hydrogen storage

liquid gas storage
tanks for the storage of liquids that are gaseous under normal conditions (room temperature, atmospheric pressure). The substances are kept in the liquid phase either by applying a slight over-pressure (e.g. LPG - liquefied petroleum gas; 0.5 - 1.5^oMPa) or by storing it at low temperatures in superinsulated devices (e.g. hydrogen at -253^oC).

liquid hydrogen
below -253^oC or 20^oK hydrogen is in its liquid phase.

liquid hydrogen storage
liquid gas storage for cryogenic hydrogen at atmospheric pressure and cryogenic temperatures.

MCFC
molten carbonate fuel cell; with molten alkaline carbonate electrolyte; operating temperature 600 to 650^oC; fuel: carbon containing gases (e.g. natural gas, synthesis gas); state of the art: prototypes are being manufactured, demonstration planned for the period 1997 to 2000, first small series production starting after 2000.

metal hydride storage
device that can store hydrogen by use of a metal alloy. The hydrogen is soaked into the alloy like into a sponge and fills the spaces in the crystal lattice of the alloy. The storage is filled applying a modest over-pressure and is usually operated in the temperature range of 20 - 80^oC.

MPa
mega Pascals (SI pressure unit); one MPa corresponds to a pressure of 10 atmospheres (10 bar_abs).

PAFC
phosphoric acid fuel cell; with phosphorous electrolyte; operating temperature 160 up to 220^oC; fuel: pure hydrogen; state of the art: 200 kW_e systems commercially available.

partial oxidation
conversion of hydrocarbons (diesel, residual oil etc.) into a synthesis gas that consists of hydrogen, carbon monoxide (CO) and carbon dioxide (CO₂). The necessary energy is supplied by the combustion ( "oxidation" ) of parts ( "partial" ) of the feedstock in the process itself. Partial oxidation is a common process for the production of hydrogen (the synthesis gas is converted into pure hydrogen by converting the carbon monoxide and water into carbon dioxide and hydrogen and by subsequently separating the carbon dioxide).

PEFC
PEMFC

PEMFC
proton exchange membrane fuel cell; with proton conducting membrane as electrolyte; operating temperature 60 to 80^oC; fuel: pure hydrogen; state of the art: in 1997 first systems in commercial operation in the very small power range (>50^oW), larger units in series production for mobile and stationary applications before the turn of the century.

photobiological water splitting
there are different biological processes that liberate hydrogen or where hydrogen is produced as an intermediate product. Photobiological processes as e.g. photosynthesis use the solar radiation as source of energy, while fermentation processes that take place in the absence of light take advantage of the energy stored in the feedstock (e.g. glucose). There are several first efforts to use photobiological water splitting for the technical production of hydrogen.

primary energy
energy carrier to be found in nature (e.g. solar energy, wood, coal, petroleum, natural gas).

primary energy carrier
primary energy, energy carrier

renewable energy
form of energy which is never exhausted because it is renewed by nature (within short time scales; e.g. wind, solar radiation, hydro power).

renewable raw material
biomass that is only harvested to an extent that allows a (natural) regeneration. It is used for energetic or other purposes (e.g. as a construction material).

secondary energy
energy carrier which has been produced from primary energy in a conversion process (e.g. electricity, hydrogen, petrol).

secondary energy carrier
secondary energy, energy carrier

SOFC
solid oxide fuel cell; with oxygen ion conducting ceramic electrolyte; operating temperature 800 to 1000^oC; fuel: pure hydrogen, carbon containing gases (e.g. natural gas, synthesis gas); state of the art: first demonstration projects are presently being carried out, commercialization planned after 1998.

solar energy
solar radiation reaching the earth and its use for the production of electricity and heat.

solar hydrogen energy economy
energy economy where solar energy is the primary energy and hydrogen is used as secondary energy carrier.

SPFC
solid polymer fuel cell = PEMFC

steam reformer
device for steam reforming

steam reforming
catalytic conversion of light hydrocarbons (biomass, fossil energy carriers e.g. natural gas) producing a synthesis gas that consists of hydrogen (H₂), carbon monoxide (CO) and methane (CH₄). The process is heat consuming. Steam reforming of natural gas is a common process for the production of hydrogen (the synthesis gas is converted into pure hydrogen by converting the carbon monoxide and water into carbon dioxide and hydrogen and by subsequently separating the carbon dioxide).