Optimal design of the piston trajectory for the ionic liquid compressor applied in hydrogen storage

As a non-material-based compression means, the ionic liquid compressor is drawing attention in hydrogen storage, where the advantages of ionic liquid and hydraulic drive systems are combined. However, the existence and motion of ionic liquids lead to a complex gas-liquid interaction in the hydrogen cylinder, which further influences the thermodynamic behaviour of the compressor. The trajectory of the piston is a critical factor affecting the ionic liquid motion and gas compression process, thus a proper trajectory is required to improve compressor performance. In this paper, 9 different piston trajectories are proposed, under which the multi-phase flow and thermodynamic feature of the compressor were investigated by the numerical simulations combined volume of fluid (VOF) and image process methods. Liquid level stability was defined and adopted as one key performance index for the thermodynamic characteristics. Results showed that the highest value of mean liquid level stability was found as 0.739 in the case with the trajectory of T8. When the piston trajectory was T8, the maximum hydrogen product was obtained as 0.48 g with the ionic liquid discharged of 4.80 g. The optimal trajectory was identified considering the liquid level stability, the delivered mass and the energy consumption.