Discover the types of electrolysers that currently exist, how they work and their main characteristics.
After diving into what electrolysis is in our previous publication and learning its benefits and disadvantages, this new post explains the different types of electrolysers available and their similarities, since all of them feature two electrodes where hydrogen (reduction, cathode) and oxygen (oxidation, anode) are produced.
Types of electrolysers:
Conventional alkaline electrolysis
This is the most widely developed and commercially available technology, based on the immersion of the two electrodes in an alkaline liquid electrolyte that conducts OH– anions and the electrodes are separated by a diaphragm.
It is a simple and cheap technology, easy to maintain as no noble metals are needed and it is relatively stable.
However, the current densities obtained are low, since it works within certain operating limits (lower and upper) so that there is no mixing of hydrogen and oxygen, thus losing part of the energy produced.
The electrodes eventually corrode after a certain number of stops and starts, and does not meet the requirements of accumulating renewable energy as its response time is high.
Therefore, they are generally coupled to a battery to be used as energy accumulators.
Proton-conducting polymeric membrane electrolysis (PEM)
The PEM is already commercially available on a smaller scale but is still in a developmental phase. It is a type of electrolysis where the electrolyte is a solid polymeric membrane that conducts H+. It is fed with pure water, the hydrogen produced is very pure(99.999 %) and is pressurised directly in the system.
The current densities obtained are the highest and it has a very fast and dynamic response, which are the necessary requirements to accumulate renewable energy, due to their fluctuating nature.
The disadvantages are that the electrode materials are based on noble metals and that titanium is used for the bipolar plates, both of which are expensive and have problems related to availability and supply.
Today, PEM is the ideal technology to store excess renewable energy.
Solid state electrolysis
Among the types of electrolysis, solid state electrolysis is not yet commercialised.
It is a solid system that works at high temperatures (500-1000 °C). The electrolyte is an oxygen ion conductor (O2-). The energy efficiency is virtually 100%. It does not need noble metals, can work at high pressure and can be reversed to a fuel cell.
However, due to its high temperatures there are still problems with the durability of its components and it does not meet the requirements of renewable energy due to these temperatures. This condition can be an advantage if coupled to systems in which heat is released and can be used.
It is one of the most promising electrolytic technologies.
Regarding the two types of commercial electrolysis, the alkaline electrolysers are marketed with a higher capacity, reaching up to 40 MW, whereas PEM electrolysers tend to be smaller, the largest ones on the market being approx. 6-10 MW.
The efficiency of alkaline electrolysis is somewhat higher, reaching up to 93 % under hhv (High heating value) at 1 bar or 80 % at 200 bar (NEL A series electrolysers). 150-300 Nm3/h of H2 can be obtained with these alkaline electrolysers. with an energy consumption of3.8-4.4 kWh/Nm3.
There are currently two commercial electrolysis technologies: conventional alkaline electrolysis and proton-conducting polymeric membrane electrolysis (PEM)
With PEM, efficiency can be as high as 80 % under hhv conditions at 30 bar pressure (NEL M-series electrolysers). In the case of a PEM of 1 MW, it is possible to reach 207 Nm3/h of H2(18.2 kg/h of H2) consuming an energy of 4.53 kWh/Nm3.[NEL Hyd, https://nelhydrogen.com/water-electrolysers-hydrogen-generators/].
At present, solid state electrolysers that are being developed have a capacity of approximately 10 kW.
We are clear about joining the energies of the future. We have already done a review of the existing types of electrolysers. Do not miss our next post with the Green Hydrogen projects in which IDEA is participating.
By María Retuerto, researcher at CSIC and Hydrogen expert.
Collaborator at IDEA.
IDEA is moving in a single direction: Sustainability.