Hydrogen can be produced from the electrolysis of water without any direct CO2 emissions. However, one needs to look back up the chain to the source of the electricity. If the power consumed by the electrolyser is simply taken from the grid, then it has the associated CO2 emissions from the power generation sector. The carbon intensity of the power used will potentially be greater than the average carbon intensity of the grid, since if the power demand of the electroyser is regarded as an incremental load, then the associated carbon intensity will be that of the incremental generation plant that is be added to the grid to meet this demand. At best this would be a combined cycle gas turbine (CCGT) plant; at worst a coal-fired plant, open cycle gas turbine (OCGT) plant or diesel engine set. Clearly, none of these results in the production of decarbonised or ‘green’ hydrogen, as it is called when hydrogen is produced from zero carbon sources.
To produce green hydrogen via electrolysis it is therefore necessary to have a dedicated supply of zero carbon electricity – from either a renewable source or nuclear station. The electricity does not necessarily have to be generated adjacent to the electrolysis plant – it could be connected by private wire or via the grid, provided that the renewable supply is hypothecated to the electrolyser. This allows the electrolyser to be located adjacent to the demand centre for hydrogen. If the power supply is from an intermittent renewable source then either a battery back-up would be required to maintain operation of the electrolyser, or it must be accepted that hydrogen can only be produced when the renewable power supply is available; but these options would either add capital cost or reduce output from the facility.