Hydrogen Energy Efficiency Index to Compare Low-Carbon Hydrogen Production and Assess Potential Carbon-Negative Technologies

Abstract The hydrogen energy efficiency index (HEEI) is used to compare H2 production from electrified methane cracking with conventional (SMR, including with the option to CO2 capture and storage) or electrified steam methane reforming (e-SMR), and with electrolysis (using 100% or 70% renewable electricity). This novel metric also enables analysis of the conditions under which electrified methane cracking technology could become carbon-negative. We consider methane cracking with molten metals as the reference, with comments on alternative possibilities. The results demonstrate the superior energy efficiency of this technology compared to SMR, e-SMR, and electrolysis, as well as its reduced dependence on renewable electricity, thanks to energy credits from carbon utilization, expressed as a percentage of graphite yield of the carbon produced. Other aspects should be included to assess feasibility, but this HEEI index provides the basis for a longer-term vision towards carbon-negative H2 production technologies. Sensitivity analysis indicates that, above a graphitic content of around 30% (used as a simple index to lump carbon characteristics and uses), the technology becomes carbon-negative, even if this index should be used for comparative and trend analysis rather than as an absolute metric. The impact of HEEI on grid-renewable electricity and biogas use, as well as other parameters, is also analyzed. The results from this novel metric approach highlight strong interest in this technology, which, although known for decades, may still be improved to become a competitive means of producing carbon-negative H2.