Compressible Natural Gas Flow study For Vehicle Refueling Equipment

There is considerable interest in Malaysia for the use of natural gas as the fuel
for transportation. Natural Gas Vehicles (NGV) are known to be more
environmentally friendly, safer and of lower fuel cost. To encourage the public
to use natural gas, more NGV stations would have to be built. A major
constraint in providing this facility is the high construction and operating cost
of the stations. This is mainly due to the high cost of the metering system (cost
of metering system is RM 80,000 while cost of refueling equipment is RM120,
000) and the compression power needed. The present research is undertaken to
find a solution to these problems by developing a new low cost metering system
and a new operating system approach which will lower the overall compression
power needed at the refueling station by RM 0.89 millions per year.
To understand the reasons for the high compression cost, the compressible gas
dynamics model of the refueling system is required. The flow of natural gas
between the storage and the receiver has been modeled as Fanno Flow. In the
event where the storage pressure is significantly higher than the automotive fuel
tank, Mach number increases along the dispensing hose and ultimately become
one at the tip of the hose. The Mach number cannot become higher than one
under the Second Law of Thermodynamic, causing a pressure discontinuity
between dispenser tip and automotive fuel tank. Due to the pressure
discontinuity, the gas will experience sudden expansion and the expansion
experienced by the gas at the pipeline exit was found to be significant thus
resulting in high energy loss.
To avoid this high energy loss, the study conducted indicates that by utilizing
several sources placed at different pressure levels and by gradually changing

over from the lower to the higher pressure source, the energy losses could be
reduced. From the compressible gas flow model study, a new cascaded storage
system which consists of 3 different levels of pressure banks i.e. 2MPa, 1 OMPa
and 24.8MPa as low, medium and high pressure bank respectively has been
proposed. This new cascaded storage system will help to reduce the operation
cost by 17.5% by reducing the compression energy required and also the energy
loss during refueling.
For developing a simple and cost effective measurement technique, a study on
the properties relationship of natural gas is important. From the consideration of
several equations of state (EOS) which are available in literature, it is important
to find one which can represent the natural gas system with sufficient accuracy.
For finding the most suitable equation for the system the approach used was
analyzing the relationship between pressure and mass for different types of
equations with assistance of HYSYS Software. These theoretical values were
then compared with the actual measured experimental values. For natural gas
system, the Peng Robinson equation was found to be the most accurate equation
for the system with average error of 0.19%.
Besides the high compression cost in the filling stations, high metering cost is
also an obstacle to the growth of NGV refueling facility. The flow meter used
in the metering system namely Coriolis Flow Meter represents about 33.3% of
the total typical 2 hose CNG dispenser cost. This project is believed to
contribute in reduction of at least 30% (from RM120,000 to RM80,000) of
current CNG dispenser capital expenditure (CAPEX) value. From the study of
the EOS, a new concept of metering system has been developed, which is based
only on measuring the pressure and temperature. This new metering system
eliminated the dependency on currently used Coriolis Flow Meter which is a
very expensive mass flow meter.