The purpose of the refueling station is to introduce compressed natural gas at high pressure into a motor vehicle that has been converted to run on CNG.
There are basically two fundamental concepts of refueling station;
" Fast Fill " - Station which refills vehicles in exactly the same manner as normal petrol stations. These are generally suitable for refueling cars, vans & light trucks and are public or fleet operating stations.
" Trickle Fill " - Stations which refill vehicles more slowly, for example over night. This concept is suitable for the fleet refueling of heavy vehicles such as buses and trucks. These stations are generally privately operated.
Irrespective of whether the stations are fast fill, trickle fill, mother, online, or daughter station, the same principles apply.
All stations fulfill the same purpose: to take natural gas, compress it and dispense it to specially converted motor vehicles.
There are three main types of CNG Station:
1. " On Line " or conventional stations
2. " Mother " stations
3. " Daughter " stations
Any of these stations can act as " Fast Fill " or " Trickle Fill " stations.
In some cases, " on - line " and " mother " functions may be combined in the same station. In other cases a station originally designed as a " daughter " station may be converted at a later date to an " on - line " station, or even a " mother " station. For the sake of simplicity we shall initially deal with the on - line station only. However, at this stage we shall mention the difference between all the types of station.
The " on - line " station takes gas from an existing supply line, compresses it and dispenses it to the motor vehicle.
The "Mother " station takes gas from the supply line, compresses it and delivers it to a gas Transport System. The gas Transport System then delivers the gas to a daughter station, which is located at some distance from the mother station.
Daughter stations are located in areas that are not connected to gas supply lines.
The " Daughter " station receives Gas from the Transport system and dispenses it to the customer. The gas may be used direct from the Gas transport system or it may be recompressed with a small compressor to increase the utilisation of the stored gas.
BASIC OPERATION OF A CNG STATION AND ITS MAIN COMPONENTS
There are typically 5 main components in a CNG station. Considering a conventional type of station:
The pressure regulating Station or PRS - this receives gas from the pipeline and regulates and filters it to suit the compressor inlet requirements. A gas meter may also be fitted to record the consumption of the incoming gas.
The compressor - this receives gas from the PRS typically at pipeline pressures of 0.35 to 15 bar (5 to 220psi). The gas is then compressed to a very high pressure, generally 25 MPa (3600 psi). The compressor may be electrically or gas engine driven and is usually a multi-cylinder reciprocating type machine (although in some cases hydraulic compressors have been used).
The gas storage system (or storage cascades) - receives high pressure gas from the compressor for storage until it is required at the dispenser. Storage provided enables vehicles to be filled without the compressor having to start up each time the dispenser is in used. The storage cascades can be arranged in a series of 1, 2, or 3 banks to minimise compression costs; this topic will be explained in a following section. The vehicle is filled to maximum pressure of 20 MPa for fast filling stations.
The priority Fill or Sequential System - allocates gas from the compressor to the storage and from the storage to the vehicle in the optimum sequence. It may also have compressor " top off " facility so that gas may be pumped direct from the compressor to the dispenser.
The CNG dispenser - dispenses the high pressure gas to the customer's vehicle. Dispensers can have 1 or 2 outlets (single or dual hose) and generally include electronic metering to record individual and total sales. However, manually operated and non-metering dispensers are also available.
The same equipment will also be used in the Mother and Daughter stations although arranged in a different configuration.
For example, the service station will include a PRS and compressor, but need not include gas storage or priority system. Daughter stations will incorporate means of decanting the gas from gas-to-gas transport system and should include a gas storage system and a dispenser (they may or may not include a compressor).
