OPTIMIZATION OF THE REFUELLING SYSTEM OF COMPRESSED NATURAL GAS VEHICLES

Compressed Natural Gas (CNG) is increasingly becoming an important fuel for
automobiles due to the considerable reduction of air pollution compared with
gasoline and diesel, and reduction of oil dependence. However, its application has
not become widespread in Malaysia yet mainly due to the high operating cost of
developing the infrastructure for filling stations. The objective of the present studyis
to model the compression and flow mechanism in the storageand deliverysystemof
natural gas in the filling stations, so that an improved understanding on the elements
leading the high operating cost of the process can be established, and then solutions
for future optimization could then be made.
This paper is based on theoretical modelling of the compressible gas flow using
Fanno Flow concept, taking place in a dispenser unit i.e. from the cascaded CNG
storage tanks (sources) at a filling station to the CNGstorage tank of an automobile
(receiver). The cascaded CNG storage tanks are held at a constant pressure of 24.8
MPa (3600psig) and the maximum allowable pressure set for the car storage tank is
21 MPa (3000 psig). The mathematical model developed by using MATLAB
software is able to simulate the pressure distribution along the dispenser unit, the
Mach number at the pipe entrance and exit, and differential changes in the vehicle
storage tank in terms of pressure, temperature and gas content. From this model, the
energy required for compression of gas, which is necessary to produce high
pressurized source, is calculated.
The results have shown that a new filling scheme, as a series of three sources, with
increasing pressures of 2 MPa, 10 MPa and 24.8 MPa, is able to reduce 17.5%
energy requirement for gas compression compared with the current compression
system. This is based onthe assumption of empty vehicle tankas initial condition. In
short, all the objectives were achieved, which successfully makes the project the
basis for the future optimization study on Natural Gas Vehicles.
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