Decarbonisation Technology - May 2023 Issue

The main quality parameter associated with the pure water supply to an electrolyser is conductivity. As a rule of thumb, a conductivity of less than 2 μS/cm (0.2 mS/m) should be the target. Ions, such as calcium or sodium, that are dissolved in the water will increase conductivity. So, measurement of this parameter will confirm that damaging dissolved salts are not present. Electrolyser manufacturers will provide a more detailed specification for the feed water, and most will provide the necessary deionisation equipment as part of a complete package. Purity standards for electrolyser feed water Two internationally recognised standards refer to demineralised water purity for electrolysis. The US-based ASTM D1193-06(2018) Standard Specification for Reagent Water identifies three grades of purity. Many electrolyser producers will request supply of Type 2 water as a minimum purity. It has a maximum permissible conductivity of 1 μS/cm (0.1 mS/m). ISO 3696:1987 is an alternative to the ASTM document. It is titled ‘Specifications for Water for Analytical Laboratory Use’. As with the ASTM document, the ISO Standard also includes three grades of purity, and the typical feed for an electrolyser would be Grade 2 with a organic content and total silica are important parameters for electrolysis feedwater. Maximum concentrations of these impurities are also specified in the above standards. Other contaminants to be avoided include carbonate and sulphate ions, as well as silicon and aluminium oxides. Water desalination and purification The main source of renewable power generation globally at present is hydropower. Where electrolysers are used in proximity to a hydro dam, there will always be access to fresh water to create hydrogen. However, the main ramp- up in renewable power generation is from wind and solar power. The optimum location for wind power generation is often offshore, in salt water. The best places to generate low-cost solar maximum conductivity of 0.1 mS/m. In addition to conductivity, the total power are generally in arid desert locations with limited access to fresh water. Hence there will often be the need to desalinate water.

The technologies that will bring fresh water to electrolysis schemes are exactly those that are relied on today to make potable water available in arid locations. For example, thermal desalination is used extensively in the Middle East to make water available for the emerging vertical farming sector and coastal cities. Water tankers deliver fresh water to many villages in South Asia. Inner Mongolia in China has vast solar and wind resources. The province’s green hydrogen production target for 2025 is 500,000 tonnes. However, in this land-locked location, seawater desalination would be expensive and long pipelines would be required. The most cost- effective option here will be to drill deep for groundwater. For use with electrolysis for green hydrogen, the default technology for seawater desalination and brackish water purification is reverse osmosis. This can be combined with some In addition to conductivity, the total organic content and total silica are important parameters for electrolysis feedwater thermal heat from solar radiation to avoid the use of fossil fuels. When used with freshwater, the reverse osmosis plant will operate at around 15 bar to generate water of electrolyser feedstock purity. For seawater desalination, a higher operating pressure of around 80 bar is used. Pumping water to these pressures requires heavy-duty pumps with powerful motors. However, in the case of freshwater purification, the power required to operate the RO plant will be only 1 or 2% of the electrolyser scheme total. For seawater treatment, this may rise to 5%. If a solar-powered thermal desalination process is used upstream of the reverse osmosis plant, the water purification power requirement can be reduced. To achieve the low levels of permissible conductivity of 0.1 mS/m, it may be required to use a polishing stage after the reverse osmosis plant. The most suitable technology for this is electro deionisation (EDI). The water treatment plant can represent around 5% of the capital cost of an electrolyser


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