Applied Metal Hydride System Optimization to Improve Hydrogen Fuel Cell System Performance in Mobile Heavy-Duty Applications
Authors: Kordel Markus , Radler Dominik 
Published: December 2024

Hydrogen fuel cells and hydrogen internal combustion engines face challenges in competing with battery electric systems in mobile applications due to their lower overall system efficiency. Evaluating the impact of various technologies on the system level is crucial for assessing their economic and ecological potential.
Thermodynamic constraints, such as Carnot efficiency, inherently limit the efficiency of these systems. To address this, efforts to enhance the efficiency of fuel cells and internal combustion engines must explore untapped potentials, such as utilizing pressure energy between pressurized tanks and fuel cell systems, which is about 15 % of the higher heating value of the hydrogen stored in the 700 bar tanks.
One promising approach involves employing an open metal hydride refrigeration system, utilizing pressure energy through alternating exothermic hydrogen absorption and endothermic desorption in metal hydride reactors. This system provides quasi-continuous cooling power for climatizing train cabins, drivers’ cabs, and refrigerated trailers (see Fig. 1). A lab-scale prototype, currently at TRL 4, demonstrates this quasi-continuous cooling power with steady-state hydrogen mass flows and an electrochemical efficiency of 81 %.With this prototype, efficiency improvements were achieved through optimized valve switching times (Weckerle et al. 2020).
However, further optimization is warranted, particularly through simulation-based techniques leveraging realistic driving patterns of heavy-duty vehicles. Here we present the validation of a metal hydride simulation model with control strategy enhancements, evaluated under heavy-duty applications such as hydrogen regional trains and refrigeration trucks.
Our simulation model, validated by repeated experiment analyses within a 95% confidence interval, facilitates optimization of heat transfer fluid and hydrogen flow, resulting in longer desorption cycles and higher efficiency of 86 – 91 % with the lab-scale prototype as a reference. The reactors are modeled as plate heat exchangers with a heat source in Matlab Simulink, utilizing a first-order reaction approach based on isothermal pressure-temperature characterization of the material. Optimization of desorption cycles, incorporating variations in heat transfer fluid flow, absorption and desorption cycle durations, and hydrogen mass flow through bypass valves, is performed via scripted methods.
Extended desorption cycles significantly reduce switching cycles (see Fig. 2), with the main performance impact attributed to the heating or cooling requirements of the reactor's lump heat mass during transitions. The integration of this improved control strategy with existing vehicle models enables a more realistic assessment of system performance compared to current practices.
Even modest improvements, such as a 5 % points efficiency enhancement, can lead to significant energy savings, exemplified by a reduction of 370 kWh/a in the annual energy consumption of a hydrogen regional train (Kordel et al. 2024). This study emphasizes the importance of application efficiency and energy consumption as key indicators for enhancing the competitiveness of mobile hydrogen applications.
The presentation will delve into the validation process, optimization workflow and application-specific results and show the impact on the efficiency of hydrogen trains and refrigerated trailers. With this research, we aim to bridge the gap between technological advances and real-world applications in hydrogen science.

Literature:

Kordel, Markus; Heeland, Matthew Maikel; Knetsch, Kevin (2024): IRSA 2023 Proceedings: Waste Energy AC Technologies in H2-Multiple Units. In: Nils Nießen und Christian Schindler (Hg.): IRSA 2023 : Tagungsband/proceedings: Aachen, Germany 22-23 November 2023, S. 561–581. Online verfügbar unter https://elib.dlr.de/202183/.
Weckerle, C.; Nasri, M.; Hegner, R.; Bürger, I.; Linder, M. (2020): A metal hydride air-conditioning system for fuel cell vehicles – Functional demonstration. In: Applied Energy 259, S. 114187. DOI: 10.1016/j.apenergy.2019.114187.

Acknowledgement:

The author would like to acknowledge the ESCALATE project (Grant Agreement No: 101096598), which the European Union funds under the Horizon Research and Innovation Programs.

Figure 1