In
thermolysis, water molecules split into
hydrogen and
oxygen. For example, at about three percent of all H2O are dissociated into various combinations of hydrogen and oxygen atoms, mostly H, H2, O, O2, and OH. Other reaction products like H2O2 or HO2 remain minor. At the very high temperature of more than half of the water molecules are decomposed. At ambient temperatures only one molecule in 100 trillion dissociates by the effect of heat. The high temperature requirements and material constraints have limited the applications of the thermal decomposition approach. Other research includes
thermolysis on defective
carbon substrates, thus making hydrogen production possible at temperatures just under . One side benefit of a nuclear reactor that produces both
electricity and hydrogen is that it can shift production between the two. For instance, a nuclear plant might produce electricity during the day and hydrogen at night, matching its electrical generation profile to the daily variation in demand. If the hydrogen can be produced economically, this scheme would compete favorably with existing
grid energy storage schemes. As of 2005, there was sufficient hydrogen demand in the
United States that all daily peak generation could be handled by such plants. The hybrid thermoelectric
copper–chlorine cycle is a
cogeneration system using the
waste heat from nuclear reactors, specifically the
CANDU supercritical water reactor.
Solar-thermal Concentrated solar power can achieve the high temperatures necessary to split water. Hydrosol-2 is a 100-kilowatt pilot plant at the
Plataforma Solar de Almería in
Spain which uses sunlight to obtain the required to split water. Hydrosol II has been in operation since 2008. The design of this 100-kilowatt pilot plant is based on a modular concept. As a result, it may be possible that this technology could be readily scaled up to megawatt range by multiplying the available reactor units and by connecting the plant to
heliostat fields (fields of sun-tracking mirrors) of a suitable size. Material constraints due to the required high temperatures are reduced by the design of a membrane reactor with simultaneous extraction of hydrogen and oxygen that exploits a defined thermal gradient and the fast diffusion of hydrogen. With concentrated sunlight as heat source and only water in the reaction chamber, the produced gases are very clean with the only possible contaminant being water. A "Solar Water Cracker" with a concentrator of about 100 m can produce almost one kilogram of hydrogen per sunshine hour. The
sulfur–iodine cycle (S–I cycle) is a series of
thermochemical processes used to
produce hydrogen. The S–I cycle consists of three
chemical reactions whose net reactant is water and whose net products are
hydrogen and
oxygen. All other chemicals are recycled. The S–I process requires an efficient source of heat. More than 352
thermochemical cycles have been described for water splitting by
thermolysis. These cycles promise to produce hydrogen and oxygen from water and heat without using electricity. Since all the input energy for such processes is heat, they can be more efficient than high-temperature electrolysis. This is because the efficiency of electricity production is inherently limited. Thermochemical production of hydrogen using chemical energy from coal or natural gas is generally not considered, because the direct chemical path is more efficient. For all the thermochemical processes, the summary reaction is that of the decomposition of water: 2H2O [\ce{Heat}] 2H2{} + O2 ==References==