Metal Hydrides for Hydrogen Boil-off Recovery

Kurzfassung

When Liquid Hydrogen (LH2) is heated above its boiling point, it evaporates. This phenomenon is called boil-off, and the generated Gaseous Hydrogen (gH2) is called Boil-Off Hydrogen (BOH). Normally, it is released to the environment, resulting in economical and energy-related losses. The aim of this study is to identify sources of boil-off in real applications, to determine potential use cases for a thermal compression using Metal Hydrides (MH), and to demonstrate the corresponding concept of BOHrecovery in the lab. The representative use case identified for this study is Long Term Storage (LTS). In LTS, BOH is released at 1.5 − 2 bar and room temperature. However, this is too low for further utilization. Therefore, the option to compress the boil-off up to 8 − 10 bar using a MH-reactor to enable further utilization in a fuel cell, is examined. For the experimental demonstration LaNi5 is selected as a suitable material for the given boundary conditions based on an analysis of Van’t Hoff diagrams and their corresponding Pressure-Concentration-Isotherms (PCI). Several experiments with different parameter settings show the reproducibility, possibilities, and the limits of thermal compression using metal hydrides. Pressure ratios between 2 − 6 are reached with an available temperature range of ∆T = 50 − 90 K. With ∆T = 90 K a maximum pressure ratio of 6 was reached and with ∆T = 50 K a ratio of 2 was possible. The dynamics of the reaction strongly depend on the distance of the measured points from the Van’t Hoff equilibrium graph. Based on the reactor and the determined LTS application, initial characteristic values were derived. Based on the lower heating value of hydrogen and a hypothetical fuel cell with 50 % efficiency, the potential waste heat utilization to drive the compression and desorption is calculated. The waste heat produced after one full desorption is 525 kJ, while the thermal energy required for ∆T = 70 K is 156 kJ, which is only 30 % of the available energy. This indicates that the waste heat is sufficient to provide for the required desorption energy, enabling an efficient alternative option for BOH recovery.