Hydrogen Conversion Pathways for Zero-Emission Residential Heating Using Air-to-Water Heat Pumps

The use of hydrogen for heating is one of the pathways toward zero-emission buildings. Two possibilities for using hydrogen in combination with heat-pump systems were analyzed: an electrically driven heat pump powered by a fuel cell and a mechanically driven heat pump powered by a hydrogen internal combustion engine (ICE). Heat-pump operation was adapted to local climatic conditions using typical meteorological year datasets for 30 representative locations across Bulgaria. The seasonal coefficient of performance (SCOP) values were further adjusted toward actual operating conditions using a field-performance correction factor. The results reveal a thermodynamic transition between the two pathways, defined by a critical seasonal coefficient of performance, SCOPcrit. For the reference ICE heat-recovery fraction of 0.45 and without direct fuel-cell heat recovery, SCOPcrit is 2.53. Above this threshold, the fuel-cell-driven pathway requires less hydrogen, whereas below it, the mechanically driven pathway requires less hydrogen. A strong correlation was identified between pathway dominance and the mean outdoor temperature during the heating period, with a transition temperature of about 2.4 °C. The sensitivity analysis shows that SCOPcrit ranges between 1.40 and 3.52. Overall system efficiencies ranged from approximately 1.2 to 1.8 on a lower heating value (LHV) basis, exceeding typical direct hydrogen-combustion efficiencies.