Posted by Peter Singfield of Belize at the STOVES list: http://www.sustainable.energy.sa.gov.au/pages/advisory/renewables/type s/other/hydrogen_energy.htm:sectID=48&tempID=52
Hydrogen Energy Hydrogen is the simplest and most abundant element in the world. It is very chemically active and rarely exists in nature in its pure form. Usually it exists in combination with other elements such as oxygen in water (H2O), carbon in methane (CH4) and in numerous organic compounds. Hydrogen bound in organic matter and in water makes up about 70% of the earth's surface. Stored in liquid form, hydrogen is low weight, compact, high energy fuel. It has the highest energy content of any known fuel with a gross heating value of 142.04 MJ/kg (net heating value is 119.99 MJ/kg). Hydrogen can be regarded an energy carrier, or secondary energy source, that can be used in a number of foreseen applications such as transport, energy storage, blending with other fuels to minimise pollution and emissions, and displacement of fossil fuels for the production of electricity. Current uses of hydrogen are in industrial processes, rocket fuel and space craft propulsion. Motor vehicles and furnaces can also be converted to use hydrogen as a fuel. Since the 1950's hydrogen has also been used to power some aeroplanes, and hydrogen powered cars have been developed. Hydrogen burns 50% more efficiently than conventional gasoline and petroleum used in cars. Another use of hydrogen is in fuel cells to produce energy. Where Do we Get hydrogen from? Steam reforming - Currently, most hydrogen is produced by the steam reforming process. It involves heating fuels, such as methane and methanol, with a catalyst to separate hydrogen from the rest of the fuel. Electrolysis - Electrolysis separates the elements of water, hydrogen and oxygen, by charging the water with an electrical current. Adding an electrolyte such as salt improves the conductivity of the water and increases the efficiency of the process. Electrolysis is unlikely to become a predominant method for large scale hydrogen production. Research has been performed into usage as an energy storage mechanism in combination with photovoltaics. Steam electrolysis - This is a variation of conventional electrolysis, with part of the energy provided to split the water molecules added has heat rather than electricity. Thermal water splitting - At 2500¡C water decomposes into hydrogen and oxygen. One of the problems with this process is preventing water and oxygen from recombining at the high temperatures used. Thermochemical water splitting - Thermochemical water splitting uses chemicals such as bromine or iodine, assisted by heat to cause the water molecules to split. It takes several steps to accomplish this entire process. Photo-electrochemical processes - There are two types of photo-electrochemical processes. The first uses soluble metal complexes as catalysts. When these complexes dissolve, they absorb solar energy and produce an electrical charge that drives the water splitting reaction. This process mimics photosynthesis, however, currently there is minimal experience in this process. The second method uses semi-conducting electrodes in a photochemical cell to convert light energy into chemical energy. The semiconductor surface serves two functions, to absorb solar energy and to act as an electrode. However, light induced corrosion limits the useful life of the semiconductor. Biological and photo-biological processes - Biological and photo-biological processes us algae and bacteria to produce hydrogen. Under specific conditions, the pigments in certain types of algae absorb solar energy. The enzyme in the cell acts as a catalyst to split water molecules. Some bacteria are also capable of producing hydrogen, but unlike algae they require a surface to grow on. The organisms not only produce hydrogen but can also clean up pollution as well. Biomass decomposition and gasification/pyrolysis - Methane and ethanol can be produced by the anaerobic digestion of biomass by bacteria. Sources of such biomass include landfills, livestock wastes, and municipal sewage treatment plants. The biofuels produced can be reformed or decomposed into hydrogen and other gases via high temperature gasification processes or low temperature pyrolysis processes. Hydrogen Storage When properly stored hydrogen burns in either gaseous or liquid state. When combusted with pure oxygen, the only by-products are heat and water. However, when burned with air (which is about 68% nitrogen) some nitrogen oxides (NOx) are formed. Even then burning hydrogen produces less air pollutants than burning the same amount of fossil fuels. Liquid storage - Cooling hydrogen to below its boiling point of -252.7¡C allows storage as a cryogenic liquid without the need for pressurisation. When cooled to its liquid state, hydrogen takes up 1/700 as much room as in its gaseous state thus enabling a larger quantity to be stored and transported. However cryogenic storage is a difficult and expensive process and refrigeration to the temperature temperatures required consumes the equivalent of 25-30% of its energy content, and requires special materials and handling. Gas storage - Hydrogen may also be stored as a gas which uses less energy than converting to liquid form. The gas must be pressurised to store any appreciable amount. For large scale use, pressurised hydrogen could be stored in caverns, gas-fields and mines before being piped to individual homes in the same way as natural gas. New materials such as carbon fibre have permitted storage tanks to be fabricated that can hold hydrogen at extremely high pressures, however at present, the costs of tanks and compression are high. Thus, gas storage is not yet economically feasible for transportation. Metal hydrides - Metal hydrides are chemical compounds of hydrogen and other material such as magnesium, nickel, copper, iron and titanium. Certain metal alloys absorb hydrogen and release it when heated. Hydrogen can be stored in the form of hydrides at higher densities than by simple compression. However they still store little energy per unit weight. Gas on solid adsorption - Adsorption of hydrogen molecules on activated charcoal (carbon) can approach the storage density of liquid hydrogen. Microspheres - Very small glass spheres can hold hydrogen at high pressures, charged with gas at high temperatures where the gas can pass through the glass wall. At low temperature the glass is impervious to hydrogen and it is locked in. Customised glass spheres are currently being developed for this purpose. ------------------------ Yahoo! Groups Sponsor ---------------------~--> Free shipping on all inkjet cartridge & refill kit orders to US & Canada. Low prices up to 80% off. 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