At a Compressed Natural Gas (CNG) filling station, natural gas is stored in the high pressure reservoirs. The pressures within these reservoirs have huge effects on fast filling process of a natural gas vehicle’s (NGV) cylinder and the difficulties associated with the filling process. The accurate modeling of the fast-fill process is a complex procedure which should be thoroughly studied to optimize the filling process. Here, a theoretical analysis has been developed to study the effects of various parameters on the CNG filling process and the conditions. The analysis is based on the first and second laws of thermodynamics, conservation of mass and the AGA8 equation of state. The required properties of natural gas mixtures have been calculated making use of the AGA8 equation of state (EOS) and thermodynamics relationships. It is found that, the composition of natural gas is very effective on the CNG filling process and final in-cylinder values. For various Iranian natural gas compositions, the optimized filling stations' reservoirs pressure has been found.

Keywords: Compressed natural gas, CNG cylinder, Fast-filling process, Cascade Reservoirs, Thermodynamic Analysis

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