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Hydrogen FAQs | TÜV Rheinland

Questions and answers along the hydrogen value chain.

Do you have questions about the production, storage or use of hydrogen?

Our experts have the answers. Below we answered the questions we are asked most frequently.

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Does the seawater need to be desalinated before carrying out the electrolysis process?

Yes, the seawater needs to be desalinated beforehand, as not only salts but also other minerals and impurities are contained in the water. Up to now, a technology-related water treatment is necessary for every electrolysis plant. There are no exceptions.

Is it technically possible to reuse the previously-used water?

During the electrolysis process, water (H2O) is split into its components hydrogen (H2) and oxygen (O) by applying an electrical voltage in the electrolyzer. A "reuse" can only occur, if a reconversion to electricity is carried out, e.g. via a fuel cell, and the reaction product "water" is collected on site. In this case, highly purified water could be separated after the reaction and fed back into the electrolysis process.

Is a cold start possible with alkaline electrolysis (AEL)?

Alkaline electrolysis is an industrially established process for hydrogen production which can be cold-started. However, at the beginning of the electrolysis process, a purging phase must usually be carried out to remove gas impurities. The duration varies greatly depending on the plant and it can take several minutes.

At what temperature is alkaline electrolysis operated?

The operating temperature is usually between 70 and 80 degrees Celsius.

At what load is the output of the electrolyzer the highest?

There is no blanket answer, as the production output varies depending on the plant. In general, however, it can be stated that with a plant running at operating temperature, the electrical efficiency is very high in the lower partial load range. Under full load, on the other hand, the production output is at its highest. At the same time, however, the electrical losses in the form of waste heat increase and thus reduce the electrical efficiency.

Is partial load operation more efficient?

Yes. However, good efficiency on paper is offset by the existing investment and operating costs with a lower production output.

How much water is needed to store energy? (Example calculation)

Purely stoichiometrically, 0.211 gallons (0.8 liters) of treated water are required for each standard cubic meter of produced hydrogen. Since conventional treatment plants, connected to the drinking water network, usually have an efficiency of approx. 50%, 0,423 gallons (1.6 liters) of raw water are required from the tap water interface. Depending on the initial quality of the raw water, however, the demand can also increase.

What hydrogen purity can be achieved with Polymer electrolyte membrane electrolysis (PEM) compared to alkaline electrolysis (AEL)?

Compared to an alkaline electrolysis unit, directly after the stack, the gas quality is generally higher with PEM electrolysis. However, this again depends on the specific procedural management of the PEM plant. In addition, an electrolysis plant is rarely used without downstream gas treatment, so that this difference between the generation plants hardly plays a role in practical applications.

Is it necessary to dehumidify the hydrogen after PEM electrolysis?

Whether dehumidification is necessary depends essentially on the target application and the residual moisture level in the product gas which is permissible.

What is the characteristic feature of pressure-charged electrolysis?

In contrast to atmospheric electrolysis, pressure-charged electrolysis is essentially characterized by the fact that a feed pump delivers the feed water against the system pressure and a circulation pump ensures the forced circulation of the electrolyte.

Are there already suggestions from coating technology experts about whether the service life of electrodes can be increased? And what requirements would an electrode coating have to meet in order to increase the service life?

The question of the durability of all electrolysis components has been the subject of many research and demonstration projects over several years. Due to the variety of electrolysis technologies and the variation of process parameters, this question cannot be answered without further explanations and deeper background knowledge (e.g. type of electrode material, temperature ranges, type of electrolysis technology, electrolyte concentration, etc.).

What is the maximum percentage of hydrogen that can be fed into the existing distribution gas network?

Before hydrogen is added to the existing gas network, the gas pipelines must be carefully monitored and all components checked for hydrogen compatibility. First, a survey of the current situation is conducted and a project plan is drawn up, on the basis of which the hydrogen share is determined. As of today, up to 10% by volume hydrogen may be fed into the distribution network. Depending on the gas consumers connected to the distribution network, this value may be lower (usually 2% by volume).

In various pilot projects, the suitability for blending rates of up to 20 vol% is to be demonstrated.

Does the quantity of hydrogen added to the natural gas network refer to the volume or the mass?

The specification refers to the volume.

Will the functionality of the domestic and industrial consumer equipment be affected when 20% of hydrogen is added to the existing distribution gas network?

It is very likely that the gas appliances will still function as well with 20% added hydrogen as with 100% natural gas. However, it should be noted that the devices have been designed to operate on pure natural gas and there is certainly a threshold of hydrogen concentration that may not be exceeded in some devices. We are currently testing in several pilot projects how high this threshold may be and at what point the functionality or usability of the devices is affected.

In the future, the proportion of hydrogen in the distribution gas network is to be steadily increased. Is a new test phase necessary for each additional stage or is it already possible to carry out these tests today?

After a plant-related risk assessment of the pipelines, the stations and the affected components of the gas supply, these tests would already be possible today. However, due to the low energy density of hydrogen, the pressure and flow ratio must also be observed and adjusted when feeding in a higher proportion of hydrogen while maintaining the same transport quantity of energy.

Which quantity threshold (maximum quantity) results for hydrogen from the Hazardous Incident Ordinance?

The Hazardous Incident Ordinance regulates the protection of humans and the environment against the consequences of sudden incidents at technical installations caused by the escape of hazardous substances. Hydrogen is to be classified as a hazardous substance within the meaning of this Ordinance. According to the 12th BImSchV Annex 1, the quantity thresholds are 5 tons (lower class operating areas) and 50,000 tons (upper class operating areas) of hydrogen.

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