Pure water supply to an electrolyser is essential. Electrolysis splits water molecules to oxygen and hydrogen. The cost of water supply for a typical green hydrogen scheme will be insignificant compared to the cost of the renewable power and will generally be only 1% or 2% of the total operating cost.
However, the consequences of water supply issues are unacceptable. Impurities such as calcium ions in the water will rapidly damage a PEM electrolyser membrane due to the interaction with the catalyst coating. Alkaline electrolysers also have sensitivities to poisons in the water.
Failure to supply water means the electrolyser scheme must shut down. For a PEM system that will probably not be a major issue. For an alkaline system, an unplanned shutdown may result in corrosion of the electrodes and a reduction in the electrolysis efficiency during future operation.
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