Electrolysis
For the generation of hydrogen from water now and in the foreseeable future electrolysis is the only process of practical significance among the possible alternatives.
The conventional process is the alkaline electrolysis which has been in commercial use for more then 80 years.
Because hydrogen generated by electrolysis uses electric power (also a secondary energy carrier) this is economic only in those regions of the world where electric power can be generated very cheaply. This is the case almost exclusively in big hydroelectric plants (e.g. in Egypt, Iceland, Norway..)
General Description
The water decomposition via electrolysis takes place in two partial reactions at both electrodes, which are separated by an ion conducting electrolyte.
At the negative electrode (cathode) hydrogen is produced and on the positive electrode (anode) oxygen is produced. The necessary charge exchange works via ion conduction. To keep the product gases separated the two reaction compartments are separated by an ion separator (diaphragm).
The energy for the splitting of the water is provided by electric power. The following types of electrolysis exist:
Conventional Water Electrolysis
This process works with alkaline, aqueous electrolytes. Anode compartment and cathode compartment are separated by a microporous diaphragm to avoid the blending of the product gases.
Good dynamic performance is a feature of the latest developments which allows for fluctuating operation. Therefore they are perfectly suited for applications with renewable energy generating plants.
High-Pressure Water Electrolysis
With high-pressure electrolyzers hydrogen pressures up to and even over 5 MPa (50 bar) are possible. This is feasible because of a specific choice and optimisation of materials.
Some technologies which are in the development stage at the moment shall make feasible an unproblematic operation of an electrolyzer powered by a fluctuating electric power unit (e.g. wind or PV power) thus enabling the building of stand-alone plants.
High-Temperature Electrolysis
High-temperature electrolysis has been discussed as an interesting alternative some years ago. It would be an advantage to put part of the energy needed for dissociation as high-temperature heat at around 800-1000°C into the process and then to be able to run the electrolysis with reduced electric power. These considerations were directed at using in this way the heat set free in a solar-concentrator or waste heat from power plants. But in the last years the interest in this type of electrolysis decreased and therefore we will not go into more detail.
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