Hydrogen storage by adsorption on activated carbon: experimental and numerical study

Abstract

Session The use of hydrogen for fuel cell systems requires an adequate hydrogen storage medium. Solid state storage, as a safe and efficient handling of hydrogen, attains commercial interest if the amount of reversible hydrogen is more than 6.5 wt.. The storage in a solid-state matrix fails for most materials with respect to the total weight of the tank system. Therefore carbon with its low atomic weight could help to overcome these disadvantages. According to the American Department of Energy standard (DOE), the tank must be able to store 63 kg of hydrogen in a 1 m3 tank and to satisfy a ratio of 6.5% between the mass of stored hydrogen and the total mass of the system. Moreover, the viability of this storage method requires very short filling times, typically less than 5 minutes. Due to the exothermic nature of the adsorption process, these low filling times cause an increase in the adsorbing-bed temperature, which in turns limits the tank-gas density to a level lower than which could be expected in the absence of adsorbing-bed heating. The main objective of this work was the investigation of thermal effects during high-pressure charge in the adsorbent packed bed for the storage reservoir. The adsorption column is a laboratory-scale (2L) stainless-steel cylinder. Its geometric shape is similar to that of the commercial gas cylinder. The column is packed with activated IRH3 carbon, which has an average surface area of 2800 m2/g. The IRH3 activated carbon is produced from coconut coal by Institut de Recherche sur l'Hydrogene. Six type J thermocouples were positioned along the column to obtain radial and axial temperature profiles. The pressure was measured by a Heise digital pressure transducer (model ATS 2000, precision: 0.02% of the full scale). The hydrogen flow is measured by a HTDS turbine volumetric flowmeter (model FTO-1NIR3-PHC-5). The precision represents 0.02% of the full-scale the range of which is included between 6.9.10-7 et 9.5.10-6 m3.s-1. This range permits to obtain filling times ranging between 210 s et 2810 s. An upstream micrometer valve allows to regulate the hydrogen flow with a reproducibility of + - 0.3%. The data were acquired by means of the Labview (Labview 7.0) software platform. This interface allows us to follow the inlet flow, the pressure increase and the temperature profiles. Several tank-charging experiment were carried out with and without the adsorbent present in the tank and using either hydrogen or helium. These experiments allowed us to get qualitative information on the interplay between flow dynamic and adsorption processes in the observed heating of the bed. Typical average temperature increase measured during hydrogen charging experiments with IRH3 activated carbon was about 50K at 10 Mpa. The simulation of this charge conditions has been carried out with the commercial code Fluent. The results from simulations agreed reasonably with experiments. An undergoing work is carried out with a higher performances new adsorbent bed. A particular effort will be devoted to increase the heat transfer from the centre of the tank toward the outside walls. This can be done by increasing the thermal conductivity of the adsorbent in the radial direction or by inserting supplementary interbed heat exchangers, such as fins, to the system