A Sub-Atmospheric hydrogen power generation system Combining Near-Ambient-Temperature Phase-Change heat absorption and Hydrogen-Oxygen direct combustion

The production and utilization of hydrogen via water electrolysis are spatially decoupled, and the associated high-pressure storage and power-generation pathways remain limited in practice, constraining overall energy efficiency. This study proposes an innovative approach that couples near-ambient-temperature phase-change heat absorption under sub-atmospheric pressure with high-temperature hydrogen–oxygen direct combustion. A semi-closed Rankine cycle, synergistically driven by hydrogen–oxygen combustion and low-grade thermal input, is constructed accordingly. Owing to its compatibility with sub-atmospheric pressure conditions and adaptability to low-grade heat, the proposed cycle is highly suitable for integration with emerging energy storage technologies such as near-atmospheric-pressure solid-state hydrogen storage. Using low-grade solar energy as an example, the system exploits the low saturation temperature (∼60 °C) of water under sub-atmospheric pressure (0.02 MPa) to achieve efficient boiling heat transfer driven by solar energy, effectively mitigating the regeneration difficulty associated with large latent heat in subcritical Rankine cycles; combined with hydrogen–oxygen combustion, a regenerative process is employed to allocate high- and low-grade heat, thereby improving hydrogen utilization. Thermodynamic analysis and numerical simulation of sub-atmospheric water phase-change heat transfer verify the feasibility of the proposed system. At main steam conditions of 0.02 MPa and 1190 °C, thermal efficiencies of 23.51% and 83.24% (hydrogen fuel utilization efficiency excluding solar heat input) are obtained when low-grade solar heat is considered and not considered, respectively. Given the abundance and negligible cost of ∼ 60 °C solar energy, the 83.24% efficiency, equivalent to the theoretical limit of hydrogen–oxygen fuel cells, acquires realistic engineering significance. Numerical results further indicate that vapor quality is inversely correlated with flow velocity and positively correlated with solar irradiance; for an 11.7 m tube, the average outlet vapor quality reaches 83%, confirming engineering feasibility. This work provides a new technical pathway for the efficient coupling of hydrogen energy and low-grade thermal energy.