RO permeate: the key to sustainable water use in hydrogen production?

When it comes to the energy transition, hydrogen is a much discussed topic. But how sustainable is large-scale hydrogen production in reality? While attention is mainly focused on electricity and efficiency, there is one raw material that remains frequently overlooked: water. In the production of hydrogen, water has two crucial functions: it is used as both raw material and for cooling. In practice, cooling requires large volumes of water. At the same time, there is increasing pressure on available water sources. Iv investigated whether (pre-purified water) from a reverse osmosis (RO) installation could also be used as cooling water, resulting in surprising water savings.

Green hydrogen is produced via electrolysis. During this process, water is split into hydrogen and oxygen using electricity. Various technologies exist for electrolysis. At the present time, alkaline and proton-exchange membranes are most commonly used commercially. Other technologies are still undergoing development and being tested in pilot projects, for example, solid oxide and anion exchange systems. In all cases, however, water of the required purity level (ultrapure water) is necessary to ensure the process runs in a stable manner.

The importance of water in the plant

It remains the case that, in economic analyses of hydrogen production, water receives little attention. At first glance, this seems logical: electricity accounts for 75 to 80 percent of the total cost of hydrogen, and other investments a further 15 to 20 percent. Water – including both treatment and purification – accounts for less than 1 percent of total costs.

“That small share says very little about the importance of water in the plant, however,” says Amir Haidari, senior water technology specialist at Iv. “In theory, around 9 litres of water are required for each kilogramme of hydrogen produced. In practice, as a result of losses and purging flows, the required volume is higher. Assuming energy consumption of approximately 50 kilowatt-hours per kilogramme of hydrogen, a 100 megawatt installation produces in the region of 2.4 gigawatt-hours of energy per day. This amounts to approximately 48,000 kilogrammes of hydrogen per day. If we include purging and process losses, this requires between 400 and 450 m³ of ultrapure water each day. And that’s not all.”

Hydrogen production requires water, not only in the electrolyser itself, but also for cooling systems, gas drying, as well as various safety and auxiliary systems. “By far the most water goes to the cooling water system, which removes heat from the electrolyser stacks, transformers, compressors, and gas dryers,” says Amir. Water is, of course, also necessary for fire extinguishers and sprinkler systems, instrumentation and gas treatment, during drying and purification, for example. In some cases, it is also used as sealing or scrubber water.”

How much water, then, is actually required to produce hydrogen? During research Iv conducted together with TU Delft, something quickly caught Amir’s attention. “Much more water is used for cooling than for the production of hydrogen,” he explains.

“If we include all applications, it transpires that a hydrogen plant uses five to ten times more water than is required for electrolysis alone. That is a lot. And that raised an important question for Iv: can this be done more efficiently? And, if so, how?”

Purer water, higher cycles of concentration

The quality of the water largely determines the reliability and lifespan of an electrolysis installation. The electrolyser requires highly pure water, free from salts, organic substances, silica and microorganisms. This prevents corrosion of membranes and electrodes. Therefore, a combination of RO and electrodeionisation (EDI) is usually applied. For other applications, such as cooling, less stringent requirements apply. “Hydrogen plants, therefore, often have multiple water lines: one for ultrapure water and one, or more, for process and cooling water,” says Amir. “We investigated whether or not this is actually efficient.”

Amir says that the permeate from an RO installation – the pre-purified water that is usually used in the EDI stage – can also be used as make-up water for the cooling tower system. Make-up water is the fresh water that is added to a cooling system to replenish water losses resulting from evaporation, drift and blowdown. “Because RO permeate is virtually free of salts and microorganisms, the cooling water system can operate with a higher cycle of concentration (COC). Whereas conventional cooling towers usually have a COC of 2 to 3, with purer water this can rise to 7 or 8. As a result, significantly less fresh water is required; in some cases 60 to 70 percent less.”

In Amir’s opinion, these supporting systems present important opportunities. “For hydrogen production, you have to significantly improve water quality in any case,” he says. “So why not utilise that for other parts of the installation as well?” The result is a more stable cooling system with less blowdown and a lower environmental impact.

A more sustainable hydrogen chain

Hydrogen must become a pillar of the future energy supply. How sustainable that chain truly is depends on smart choices in the design of installations, however. The research conducted by Iv and TU Delft shows that there is still much to be gained in water management: reduced water usage, fewer chemicals and a more efficient system. “By using pure water in a more intelligent way, we can make the hydrogen chain more sustainable,” says Amir. “With that, we can contribute to the energy transition that will shape our industry in the coming decades.”