<P> When the energy supplied is in the form of heat (originating from solar thermal, or nuclear), the best path to the production of hydrogen is through high - temperature electrolysis (HTE). In contrast with low - temperature electrolysis, HTE of water converts more of the initial heat energy into chemical energy (hydrogen), potentially doubling efficiency to about 50% . Because some of the energy in HTE is supplied in the form of heat, less of the energy must be converted twice (from heat to electricity, and then to chemical form), and so the process is more efficient . </P> <P> HTE processes are generally only considered in combination with a nuclear heat source, because the other non-chemical form of high - temperature heat (concentrating solar thermal) is not consistent enough to bring down the capital costs of the HTE equipment . Research into HTE and high - temperature nuclear reactors may eventually lead to a hydrogen supply that is cost - competitive with natural gas steam reforming . HTE has been demonstrated in a laboratory, but not at a commercial scale . </P> <P> Using electricity produced by photovoltaic systems potentially offers the cleanest way to produce hydrogen . Again, water is broken down into hydrogen and oxygen by electrolysis, but the electrical energy is obtained by a photoelectrochemical cell (PEC) process . The system is also named artificial photosynthesis . </P> <P> The conversion of solar energy to hydrogen by means of water splitting process is one of the most interesting ways to achieve clean and renewable energy . However, if this process is assisted by photocatalysts suspended directly in water rather than a photovoltaic or an electrolytic system, the reaction takes place in one step, it therefore can be more efficient . </P>

How much energy does it take to separate hydrogen and oxygen from h2o