Hydrogen can be produced from a large number of primary energy sources and by various technical processes.
Electrolysis is a promising option for hydrogen production from renewable resources. Electrolysis is the process of using electricity to split water into hydrogen and oxygen. This reaction takes place in a unit called an electrolyzer. Electrolyzers can range in size from small, appliance-size equipment that is well-suited for small-scale distributed hydrogen production to large-scale, central production facilities that could be tied directly to renewable or other non-greenhouse-gas-emitting forms of electricity production.
Hydrogen production via electrolysis may offer opportunities for synergy with variable power generation, which is characteristic of some renewable energy technologies. For example, though the cost of wind power has continued to drop, the inherent variability of wind is an impediment to the effective use of wind power. Hydrogen fuel and electric power generation could be integrated at a wind farm, allowing flexibility to shift production to best match resource availability with system operational needs and market factors. Also, in times of excess electricity production from wind farms, instead of curtailing the electricity as is commonly done, it is possible to use this excess electricity to produce hydrogen through electrolysis.
The electrolysis breaks down a feedstock, in this case water, into hydrogen and oxygen by electricity. The electrolyser consists of a DC source and two noble-metal-coated electrodes, which are separated by an electrolyte (figure y).
Electrolysers consist of individual cells and central system units (balance of plant). By combining electrolytic cells and stacks, hydrogen production can be adapted to individual needs.
Electrolysers are differentiated by the electrolyte materials and the temperature at which they are operated:
- low-temperature electrolysis (LTE), including
- alkaline electrolysis (AE),
- proton exchange membrane (PEM) electrolysis
- and anion exchange membrane (AEM) electrolysis (also known as alkaline PEM),
- and high-temperature electrolysis (HTE). The latter group most notably includes solid oxide electrolysis (SOE), but this is still at an advanced R&D stage and products are not yet commercially available. Once it reaches market maturity, its advantages are expected to include increased conversion efficiency and the possibility of producing a synthesis gas directly from steam and CO2, for use in various applications such as synthetic liquid fuels.
The efficiency of electrolysis is determined by the amount of electricity used to produce an amount of hydrogen. Depending on the method used, the efficiency of water electrolyser is currently in the region of 60 to 80 % (based on the calorific value).
While electrolys are already in operations, research continues to further improve them. Research priorities with regard to electrolysers currently include increasing the efficiency of the electrolyser system as a whole, along with its operating life, power density and stack size, reducing costs (especially material costs), introducing pressurised systems to avoid the need for subsequent compression of the H2 produced, and not least developing flexible systems adapted to intermittent and fluctuating power supply.