The influence of additionality and time-matching requirements on the emissions from grid-connected hydrogen production

The literature provides conflicting guidance about the appropriate time-matching requirement between electricity consumption by electrolysers and contracted variable renewable energy (VRE) for qualifying hydrogen (H2) as 'low carbon'. Here we show that these findings are highly influenced by different interpretations of additionality. Substantially lower consequential emissions are achievable under annual time matching when presuming that VRE for non-H2 electricity demand does not compete with VRE contracted for H2, as opposed to when assuming that all VRE resources are in direct competition. Further analysis considering four energy system-relevant policies suggests that the latter interpretation of additionality is likely to overestimate the emissions impacts of annual matching and underestimate those of hourly matching. We argue for starting with annual time matching in the near term for the attribution of the H2 US production tax credits, where conditions resemble the 'non-compete' framework, followed by phase-in and subsequent phase-out of hourly time-matching requirements as the grid is deeply decarbonized. There is debate about when electrolytic hydrogen produced from grid-connected renewables should qualify as 'low carbon'. Here the authors explore how additionality and the degree of time matching between electrolysers' electricity consumption and contracted renewable energy generation impacts emissions and costs.