Electrolyzers use electricity to break down water into hydrogen and oxygen. Water electrolysis occurs through an electrochemical reaction that requires no external components or moving parts. It is very reliable and can produce ultrapure hydrogen (> 99.999%) in a non-polluting way when the electrical source is a renewable energy.
Hydrogen fuel cells.
The hydrogen produced by an electrolyser is perfect for use in hydrogen fuel cells.
The reactions that take place in an electrolyser are very similar to those in fuel cells, except that the reactions that take place at the anode and cathode are reversed.
In a fuel cell, the anode is where hydrogen gas is consumed, and in an electrolyser, hydrogen gas is produced at the cathode.
The disadvantage of electrolyzers is the need for electrical energy to complete the reaction. Ideally, the electrical energy required for the electrolysis reaction should come from renewable energy sources, such as wind, solar or hydroelectric power.
Electrolyzers are useful and ideal when incorporated into certain stationary, portable and transportation power systems.
Some examples of applications where electrolysers would be particularly advantageous are long-term use in the field, fuel cell powered vehicles, and portable electronics. A sufficient amount of hydrogen can be generated prior to use and therefore could be a beneficial addition to a system using solar and wind power.
Electrolyzer advantages.
Some of the advantages of using electrolyzers are:
- The hydrogen produced is very pure.
- It can be produced directly where and when it is to be used and does not necessarily have to be stored.
- It is a much cheaper method than gas supplied in high pressure cylinders.
There are more than enough natural solar and wind resources around the world to produce all the hydrogen needed for stationary, transportation, and portable applications. Electrolysis has the potential to meet the cost requirements specified by many governments around the world.
Types of electrolyzers.
There are many ways to build and configure an electrolyser, and different electrolytes can be used just like in fuel cells.
There are many types of electrolyzers, we can find:
- Alkaline electrolyzers.
- Polymeric membrane electrolyzers (PEM).
- Solid Oxide Electrolyzers (SOEL).
- Anion exchange membrane electrolyzers (AEM).
alkaline electrolyser.
Alkaline electrolysers are the most mature current technology. Their main characteristic is that they have a liquid electrolyte and through a diaphragm, normally potassium hydroxide (KOH) or sodium hydroxide (NaOH), we produce the separation of gases between the cathode side and the anode side.
These electrolyzers have an energy consumption of around 5 kilowatts per nominal cubic meter of hydrogen produced. They work at current densities that are considered relatively low between 200 and 600 microamps per square centimeter.
Alkaline electrolysers work well at operating temperatures between 25 – 100 °C and at pressures of 1 – 30 bar respectively.
It is currently the most established commercial solution and has the lowest manufacturing costs. Normally they are around 1,000 euros per kilowatt consumed.
The main advantage is a mature technology. The main disadvantage is that the electrolyte they use is a liquid electrolyte, which means that the designs are not compact and also do not have a quick response to power variations, which is why they are not considered optimal to be coupled with renewable energy sources. .
The general construction of an alkaline electrolyser is straightforward. It has a unipolar design consisting of two metallic electrodes suspended in an aqueous electrolyte solution. When electricity is supplied to the electrodes, hydrogen and oxygen gas are generated at each electrode. The electrolyser must be designed so that each gas is collected and removed from the electrolyser efficiently.
Polymeric membrane electrolyser.
The polymer electrolytic membrane (PEM)-based electrolyser is very popular, and many modern electrolyzers are built using PEM technology.
The PEM electrolyser uses the same type of electrolyte as a PEM fuel cell.
The electrolyte is a thin solid ion-conducting membrane, which is used instead of the aqueous solution.
These electrolysers use a bipolar design and can be operated at high differential pressures across the membrane.
PEM electrolyzers are popular because many of the typical problems with PEM fuel cells do not apply. The water supplied to the cathode can also be easily used to cool the cell, and water management is much easier as the positive electrode must be flooded with water. The hydrogen produced by this type of electrolyser is of high purity. The only problem is the presence of water vapor in the system. Water diffuses through the electrolyte as in fuel cells, so electrolyte designers use various techniques to avoid this phenomenon. A common method is to use thicker electrolytes than those used in fuel cells.
These are the electrolysers that currently have a faster response to power variations, making them optimal for integration with renewable energy sources.
The main drawback, aside from manufacturing costs, is that they use precious metals as catalysts in both reactions.
Solid state electrolyser.
They are in a less developed state, more at the laboratory level.
The main characteristic of these electrolysers is that they work at high temperatures. They can reach efficiencies of up to 95%. As they work at high temperatures they have lower energy consumption, around three kilowatt hours. But they need an extra supply of energy to reach the temperatures necessary to produce the reactions.
The current densities at which they work are also quite high.
A disadvantage is that they cannot work at high pressures, the maximum working pressures are around 5000 bars. When working at high temperatures, there are very few materials that can work in these conditions, so both the electrolyte and the electrodes are only made up of complex ceramics.
The great advantage is that they have higher potential efficiencies than the rest of the electrolyzers.
These systems are at the development level, although there is some commercial development or some commercial provider that says they offer them, but always in low power.
Anion exchange membrane electrolyzer.
The least developed are the amniotic membrane electrolyzers.
Although the characteristics that it presents are quite promising, the current efficiencies are low, around 50%.
They can only work at maximum pressures of 30 bar.
The main advantages they present is that they do not need catalysts based on precious metals.
They can use conventional metals, which makes them quite promising in the long run.
Electrolyser efficiency.
There are many factors that influence the performance of electrolyzers. Some of them are the general design, the materials used and the operating temperature and pressure.
Operating at higher temperatures will increase efficiency, but will also increase the rate of corrosion of the electrolyser materials.
The efficiency of the electrolyser is calculated in the same way as that of a fuel cell. Losses in electrolyzers are the same as in fuel cells.
The stack efficiency should also include the energy losses due to the electricity needed for the pumps, valves, sensors, and controller, and the amount of energy that is put into the stack. Typical operating efficiencies of commercial electrolyser units are between 60 and 70%.
Opportunities for electrolysis.
The integration of electrolysers with a renewable energy system creates unique opportunities for future energy supply.
Renewable energy systems can be connected to the electrical grid through power electronics. This converts the alternating current (AC) from the mains into the direct current (DC) needed by the electrolysis cell stack.
Both photovoltaic and wind systems can be used as a source of electricity. In many of the wind/electrolyser systems used today to produce hydrogen, the electrolyser directly uses the AC from the wind turbine.
Electrolysis can help reduce the intermittent production of electricity from renewable resources. Hydrogen systems can produce hydrogen and store it for later use, which can improve the capacity factor of renewable energy systems. This would help renewable energy to be constant or used in peak periods. By allowing the co-production of hydrogen and electricity, the power company could optimize its production and storage system.
Both solar and wind systems can benefit from producing electricity alongside hydrogen. Some studies have shown that systems optimized for hydrogen and electricity generation have lower hydrogen prices, even when electricity is sold at a very low price.
How electrolyzers for hydrogen production are marketed.
Electrolyzers can be marketed in four main ways:
- Energy for mobility: Hydrogen can be used as a fuel in service stations for fuel cell electric vehicles.
- Energy for fuel: It can be used in refineries to remove sulfur from fossil fuels.
- Energy for industry: It can be used directly as industrial gas in the steel industry, flat glass plants, semiconductor industry, etc. It can also be injected directly into natural gas networks for lower carbon heating and other natural gas applications.
- Power to Gas: Can be used in the production of green chemicals like methanol, fertilizers (ammonia), and any other liquid fuel, even jet fuel!
Hydrogen fuel cells.
The hydrogen produced by an electrolyser is perfect for use in hydrogen fuel cells. Working much like a battery, fuel cells don’t run out or need to be charged and produce electricity and heat as long as fuel is supplied.
Fuel cells use hydrogen to generate electricity with zero emissions at the point of use. That means no fossil fuels and no harmful emissions from the tailpipe.
And what’s better, when the electrolyser system is powered by a renewable energy source, the hydrogen produced is considered renewable and CO2-free.
Hydrogen in the ecological transition.
Hydrogen can act as an energy storage medium to address these grid challenges, allowing renewable energy to be more easily used off the grid.
Hydrogen is a stable way to efficiently store and transport renewable electricity for long periods of time. In this way, renewable electricity generated by wind and solar energy that is not used immediately can be used at another time or in another place. Hydrogen’s potential to store and transport energy makes it a key element in the global transition to renewable energy.
