Large-Scale H2 Liquefaction: Ortho-Para Conversion in a Conceptual Brayton Cycle

Abstract

The economic viability of a hydrogen economy revolves around efficient and scalable methods for its transportation and storage. Liquefaction presents a key pathway by increasing hydrogen’s volumetric energy density, though current industrial plants are hampered by high energy consumption and costs.
While conceptual models offer pathways to higher efficiency, they often lack the detailed technical and economic analysis needed to validate their feasibility, particularly for systems based on the Brayton cycle.
This thesis presents a comprehensive framework for the process modeling, viability assessment, and techno-economic analysis of a large-scale hydrogen liquefaction plant based on a conceptual Brayton cycle. A central contribution is the detailed modeling of the critical ortho-para hydrogen conversion, which was simulated within a catalytic Plate-Fin Heat Exchanger (PFHX) using a specialized Python-based model integrated with a full-plant Aspen HYSYS simulation. The study also addresses the effective management of excess cold hydrogen gas; an initial investigation into using an ejector for recirculation was conducted, but this approach was ultimately discarded as it did not yield economic improvements. A final, optimized hybrid Brayton-Claude cycle featuring an efficient cold gas recirculation loop was developed, enabling a plant capacity of 86 tonnes per day (TPD).

The techno-economic analysis of the final design was performed using the Aspen Process Economic Analyzer (APEA). In the baseline scenario, assuming an electricity price of 0.1 €/kW h, the plant achieved a Specific Liquefaction Cost (SLC) of 1.51 €/kgLH2 and a Specific Energy Consumption (SEC) of 6.983 kWh/kgLH2 in the baseline scenario.

Moreover, sensitivity analyses show a further reduction in SLC and SEC in the scenario using power recovery from the turbines: 1.49 €/kgLH2 and 6.723 kWh/kgLH2 respectively. Additionally, electricity price is the dominant factor influencing plant economics, with the long-term cost target of below 1.00 €/kgLH2 being achievable at an electricity price of 0.035 €/kW h. Future projections, which account for reduced design allowances as the technology matures to a ”Proven Process,” suggest a potential SLC reduction of 4.64%.

In summary, this thesis establishes a robust techno-economic framework for Brayton-cycle based liquefaction, demonstrating its viability while highlighting the critical interplay between advanced process modeling, component efficiency, and energy costs in achieving a competitive large-scale liquid hydrogen supply chain.