Green hydrogen is hydrogen made via water electrolysis using renewable energy. Hydrogen electrolysers need ultrapure water to operate efficiently. Ultrapure water can be made from a range of different water sources. Modern ultrapure water treatment systems predominantly use Reverse Osmosis (RO) to remove 98-99% of salt from the feedwater. Further polishing to achieve ASTM Type 1 water required for electrolysers is achieved with polishing either by resins or via electodeionisation.
The most common feedwater for green hydrogen ultrapure water treatment systems are:
Surface Water
Surface water sources such as lakes, rivers and dams offer a readily available source of low salinity water. Each surface water source will have different contaminants requiring removal. Surface water sources are commonly contaminated with microbiological species and suspended solids, including colloidal particles. These contaminants will determine the pre-treatment train required prior to the RO demineralization system. Most surface water sources will also have a limitation on abstraction rate due to the local watershed characteristics. If the source water is low salinity and contains sufficient volume, then a surface water treatment system will often be the most affordable.
As with other sources of water with limited volume, care needs to be taken to consider where the cooling water for the electrolyser will come from. 30% of the electricity used to produce hydrogen by electrolysis is converted to waste heat. Up to 38 L of water can be required per kg of hydrogen produced to remove this waste heat.
Groundwater
Remote facilities in arid locations will often not have access to significant surface water sources. Groundwater is an excellent alternative to surface water but requires energy to bring it to the surface. The deeper the aquifer, the more energy is required to bring groundwater to the surface. Of all feedwater sources, groundwater will often have the most contaminants. Nutrient run-off, dissolve metals, minerals, microbiological species and fuel can all be found in groundwater sources.
Globally, groundwater sources are being abstracted faster than they are being recharged. Therefore the use of groundwater for electrolysers needs to be carefully considered. 9 L of water is required to produce each kg of hydrogen. A 10 MW facility, using 41.5 kWh/kg to electrolyse hydrogen, will require about 2000 L/hr of ultrapure water. Groundwater systems will often operate at recovery rates between 70-85% (depending on contaminants). AT 70% recovery, 3000 L/hr of feedwater will be required to produce 2000 L/hr of product water. Additionally, if the water treatment system is being operated on renewable energy this can reduce the daily operating hours of the water treatment system leading to higher capacity systems being required.
Seawater
When there are no surface or groundwater sources available and the electrolyser facility is coastal, then seawater can be an option to supply the ultrapure water treatment system with feedwater. For drinking water production, seawater desalination is a treatment technology of last resort due to the high amount of energy required for the process. However, as 41.5 kWh is required to produce 1 kg of hydrogen by electrolysis, the energetic cost of seawater desalination is small by comparison. It takes around 3-4 kWh/m3 to convert seawater to ultrapure water. 1000 L is enough ultrapure water to produce 100 kg of hydrogen.
Seawater sources are an attractive option for providing cooling water as it is abundant and simple chemical dosing and material selection can make it a viable option.
Treated Wastewater
An often-overlooked source of feedwater for ultrapure water systems is treated wastewater. Treated wastewater is regularly discharged to environment instead of being reused, even in arid locations, due to health concerns of communities. Industrial users are increasingly reusing wastewater to reduce their water inventories and reduce the amount of water piped in from offsite/local water sources. There are several green hydrogen projects currently in development which use treated wastewater as the feedwater source.
Moerk Water are experts in the design and construction of ultrapure water systems for green hydrogen production. Contact Moerk Water today to find out more.
