A thermodynamic analysis of refueling a Natural Gas Vehicle cylinder from a cascade reservoir using chilled natural gas

Abstract

The aim of this study was to simulate the refueling of a Natural Gas Vehicle (NGV) Cylinder from a Cascade Reservoir using chilled Natural Gas. The model was developed using the First Law of Thermodynamics, Mass balances and Gas Dynamics laws. Three distinct Cascade Reservoir temperatures were selected for simulations for this study; 288 K, 291 K and 293 K respectively. Simulations were conducted employing various initial NGV Cylinder parameters (Temperature and Pressure). The empirical findings from the simulations showed that the use of chilled Natural Gas to refuel an NGV Cylinder resulted in a reduced temperature increase during refueling in the order of 50 K-60 K as opposed to 70 K-80 K when refueling at ambient temperature. It was also observed that when using chilled natural gas to refuel, the initial cylinder temperature had no effect on the overall refueling process. However, varying initial cylinder pressures had a significant effect.Fill ratios obtained using the chilled gas were greater than refueling at ambient, however it was expected that using chilled gas fill ratios would be greater than 80%, this was not observed. The highest fill ratio obtained was 70.13%. Further analysis on the performance of the reservoir temperatures selected showed that the lower reservoir temperatures mitigated the effects of the Heat of Compression during refueling far better than refueling from ambient. This manifested in reduced in-cylinder temperature rise during refueling and higher fill ratios at completion of fill. Therefore, it was concluded that the Reservoir Temperature (Tres) has the greatest effect on the refueling process and lower Reservoir Temperatures are key to obtaining fill ratios >80%. Further study is definitely required into this novel method of obtaining a quality fill with CNG.