Hydrogen production offshore a problem or not?

First of all back to the basics:

H?O → H? + O? (unbalanced)

2H?O → 2H? + O? (balanced)

To produce 1 kg H? it requires 8,92 l H?O

How much pure water is needed in practice?

9 liter of pure water is required to produce 1 kg of hydrogen in theory; in reality the pure water consumption needed depends on how much energy the electrolyser needs to convert this theoretical 9 liter water to 1kg of hydrogen.

On average we see that electrolysers these days require 44 -55 kWh per 1 kg hydrogen production.

This means that 9/ 44 or 0,2 liters of pure water is required per kWh.

If we talk hydrogen production offshore than we talk MW or GW scale:

1 MW → 200 liter/h

10 MW → 2 m3/h

1 GW → 200 m3/ h

Offshore pure water production:

Seawater contains on average 35 grams of salt per liter where salinity varies depending the area:

• Dead Sea: 270 grams/l
• Baltic Sea: 7 grams/l

Salt consists of 86% sodium chloride and 13% magnesium, calcium carbonate and sulphate.

We can not use seawater in the hydrogen electrolyser so we need to convert seawater into pure water.

In theory from 1 m3 seawater (35 gram salt content) we can produce 0,5 m3 pure water and create 0,5 m3 concentrated seawater or brine (70 gram salt concentration)

There is nothing new here as every ship and offshore installation produces potable water on daily basis, and many shore based desalination installations exist.

What desalination methods could be installed offshore where footprint and weight are the biggest factors?

On ships we see usually distillation plants, this works fine as they use the waste heat from the engines. To produce pure water by distillation is not really feasible as we do not have a sufficiently big source of waste heat available.

Reverse Osmosis is another method, proven technology on offshore oil & gas installations, where 300 m3/ day pure water production or more is possible.

Reverse Osmosis has however the concern of energy consumption as the principle works at high pressure. Fouling of the membranes is another issue which has an impact in the operational cost.

Similarities with oil & gas effluent discharges?

Globally it is understood that the salinity in the seawater does not change due to the water cycle, all desalinated water comes back to the sea.

However, what about local impacts?

As discussed previously, the seawater is partially returned to sea as brine with a higher salt concentration. In theory, if we take the average 35 gram/ liter we return brine with 70 gram/ liter salt concentration back to the sea.

In practice we can mix the brine with the seawater cooling water overboard line to reduce salt concentration to an average value of 40 gram per liter. This is a proven method performed on offshore oil & gas installations. The brine will be dispersed in the sea and dispersion analysis is the method to identify the impact.

In Norway the overboard criteria are mentioned in Norsok S-003 "Environmental Care".

In many places in the world the criterium is set as:

The increase in salinity < 1 gram per liter at 75 meter from the discharge point.

What is the relevance with offshore hydrogen production ?

To make hydrogen production offshore economically feasible it means that the production to be scaled up.

Currently we note that 500 MW hydrogen facilities become feasible which means that we go back to the 200 liter / 1 MW pure water needed for the electrolyser, for the 500MW hydrogen plant this translates into 100 m3/h of pure water and theoretically at least of 200 m3/h of seawater and more practically we come closer to 260 m3/h. This means that from the 260 m3/ h seawater we produce 100 m3/h of pure water for the electrolyser and the 160 m3/h goes overboard as brine with a higher concentration of salt.

From the offshore oil & gas operations we know that the effluent requirements are very strict (e.g. Norsok S-003) and if we use the same methodology we could estimate the impact. It is already easy to say that offshore hydrogen production in the Baltic Sea has a different local impact (due to the low salinity of the seawater) than elsewhere.

Offshore oil & gas operations give good lessons- learned as production of potable or process water involve the use of chemicals. Especially pre- treatment chemicals, chlorine and biocides are concerned as they end up with the brine in the sea.

What is the impact of hydrogen production offshore?

This to be evaluated locally. Methodologies and lessons-learned from oil & gas and desalination operations are available.

Conclusion:

The point to make is that by scaling- up offshore hydrogen production we have to consider the local impact from the brine and cooling water going overboard. The impact from the higher salinity, higher temperature and the effect of chemicals used to produce pure water to be analysed. We also need cooling water to operate the electrolyser(s), this has not been discussed in this document as I only wanted to show how much seawater will be needed to produce pure water for the hydrogen production.

The local impact of the brine (and cooling water) which is higher in salinity and temperature compared to the ambient seawater; plus the effect of the chemicals going back to sea is an area which require more investigation because we are scaling up the offshore hydrogen production substantially.