Exploitation of phase change materials for temperature control during the fast filling of hydrogen cylinders

This paper explores the use of phase change materials (PCM) for the fast filling of hydrogen cylinders in order to limit the rise in the gas temperature by enhancing heat transfer. Initially, two computational models for the fast filling of hydrogen cylinders are presented and validated; the first is a 2D model and uses the commercial CFD tool FLUENT, while the second is a one dimensional model that is written in Matlab. One of the major concerns in the fast filling of hydrogen cylinders is the rise in gas temperature, which can damage the structural integrity of the cylinders. Investigations have been performed by the one dimensional model in which the PCM is part of the structure of the hydrogen cylinder. The results of the simulations have shown that only a PCM with a high thermal conductivity is capable of reducing the gas temperature during the fast fill. Hydrogen filling stations typically use a ramp profile to deliver the gas into the cylinders of vehicles, with the conventional fill time being 3-5 minutes. In addition, the gas is pre-cooled before being fed into the cylinders to lower the final gas temperature. As opposed to previous one dimensional models, the model that is presented in this document is also capable of computing the mass flow rate of the gas that enters into the cylinder and thus can be used to investigate the effect of increasing the delivery pressure ramp rate. A case study for the fast filling of a hydrogen vehicle; namely the Toyota Mirai is also performed. Hydrogen gas is stored on board of the Toyota Mirai in two cylinders having volumes of 60 L and 62.4 L. Simulations by the one dimensional model shows that the filling of the cylinders can be performed faster than the typical 3-5 minutes by delivering the gas at a faster ramp rate but requires reducing the pre-cooling temperature. The one dimensional model also shows that the presence of the high thermal conductivity PCM as part of the structure of the cylinders increases the required pre-cooling temperatures. However, while being beneficial in terms of reducing the need for lower pre-cooling temperatures, cylinders with the PCM have a higher structural mass than the original cylinders of the Toyota Mirai.