Liquid Organic Hydrogen Carriers (LOHCs): A Pathway to Alternative Hydrogen Storage Solutions

Liquid Organic Hydrogen Carriers (LOHCs) are emerging as a promising alternative for hydrogen storage and transport, offering improved safety, scalability, and energy density compared to conventional methods. This study provides a comprehensive overview of LOHC systems, addressing both the fundamental aspects and applied system-level modeling. Reaction kinetics for the hydrogenation and dehydrogenation processes have been developed and enhanced by incorporating innovative factors into the kinetic rate equations. These additions enable more accurate modeling of the hydrogenation reaction, particularly at elevated temperatures where traditional models often fail to capture the actual behavior. 

Thermo-physical properties of candidate LOHC molecules have been estimated using group-contribution methods, providing an alternative solution to face the lack of available data. Both small- and large-scale systems have been investigated, employing batch and plug flow reactor models under lumped-parameter and one-dimensional (1D) simulation approaches. These simulations allow for a detailed assessment of reactor performance and heat management challenges, which are critical for the efficient operation of LOHC systems. Batch reactor sizing has been optimized with the introduction of a novel dimensioneless pre-design parameter.

To address the inherent limitations of conventional reactor configurations, alternative reactor layouts have been proposed. These novel designs aim to enhance operational flexibility and reduce size. In parallel, control strategies have been implemented and characterized to ensure stable system operation under varying load conditions. Control strategies include: heat transfer mass flow rate and temperature, reactor pressure, and LOHC inlet mass flow rate. Ragone plots have been employed to support the design and sizing of energy storage systems by visualizing the trade-off between power density and energy density.
Static maps representing the steady-state behavior of LOHC subsystems have been developed. These serve as valuable tools for the integration of LOHC units into broader energy systems, offering guidance for multi-vector energy applications. 

A hybrid LOHC–Solid Oxide Fuel Cell (SOFC) system has been introduced, with a specific focus on maritime applications. The SOFC module has been fully modeled, including power generation and waste heat recovery, and its integration with the LOHC system has been evaluated through a controllability analysis.
A case study involving a small cargo ship illustrates the practical feasibility of this technology. The LOHC system’s performance has been benchmarked against alternative hydrogen storage solutions, particularly ammonia-based systems. Key metrics for feasibility include energy demand coverage, total energy consumption, and total volume and mass requirements.

Finally, different LOHC molecules have been simulated under the hypothesis of multi-step reaction mechanisms. This has allowed for a critical examination of the equivalent reaction assumption, highlighting its limitations and the importance of accurate reaction pathway representation.

Overall, this work sheds light on the technical potential of LOHC systems and their role in the broader context of the energy transition, with a focus on system-level integration and real-world application scenarios.