CNG COMPRESSION AND MULTI LINE REFUELLING
Natural gas in its uncompressed form is not a very efficient storage of energy for transport. Gas at a typical pipeline pressure of 0.15 MPa would require a volume of around 5 cubic metres of storage to provide sufficient energy to run a small vehicle 150 km.
If however, the gas is compressed to 20 MPa (2950 psi), sufficient energy to run the same car 150 km can be contained in a cylinder of only 50 litres water capacity small enough to fit in the back of the vehicle.
Hence the requirements for a compressor at each station. Typically, compressors vary in capacity from around 50 m3 per hour to 1000m3 per hour or more, with power requirements ranging from 15 to 250 kilowatts. A typical CNG station would have 200 - 300 cubic metres per hour compressor, fitted with a 56 kilowatt drive motor.
The compressor compresses the gas from inlet (pipeline) pressure to 248 bar.
Once the gas is compressed it stores energy in a very condensed form. The compressed gas is either piped from the compressor to the storage cascades or, if a compressor top off system is fitted, the gas flows directly to the high pressure forecourt line. Cascades consist of, for example, 20 high pressure gas storage cylinders each of 40 - 50 litres water capacity. Each cylinder will store around 12 cubic meters of gas at 248 bar.
A cascade of 20 cylinders are piped in banks of 10, 5 and 5. These are referred to as the low pressure, medium pressure and high pressure banks respectively.
Other cylinder configurations may occur in cascade, for example cascade systems of 40, 60, or 80 cylinders.
A typical CNG station will have two or three storage cascades, each of 20 cylinders.
When the compressor first starts and the storage system is empty, the priority fill system diverts the compressor discharge gas to the high pressure bank of the cascade. It is then immediately available to fill a vehicle from the station forecourt. When the high pressure bank is up to its full pressure of 210 bar (3100 psi) the priority system diverts the compressor discharge to the medium pressure bank of the cascade. This is also filled to a final pressure of 210 bar and the priority system then diverts the compressor discharge to the low pressure bank. When the low pressure bank is completely filled, also to 210 bar, the priority panel diverts the compressor discharge back to the high pressure bank charging it to its final fill pressure of 248 bar (3600 psi).
This sequence is repeated until all storage banks are charged to 248 bar. The compressor then shuts down automatically.
When a vehicle is connected to the dispenser for refueling, the dispenser auto sequencing system allocates gas from the cascade to the vehicle cylinder in the reverse sequence to which it is filled. The dispenser sequencing system is instigated by flow rate drop off which indicates equalisation between each bank of the cascade and the vehicle cylinder.
In this manner the gas from the largest storage volume, which may will have already been drawn off to a pressure less than its initial fill pressure, fills the vehicle cylinder from its low pressure. Power is not wasted in compressing gas to high storage pressure and then reducing it again to match an empty or near empty vehicle cylinder, which is already at a low pressure.
Likewise the medium and high pressure storage banks only top off the vehicle cylinder above certain pressures; the stored gas does not have to be unnecessarily reduced in pressure.
This system is known as multi refueling.
To minimise the station power consumption, multi line systems are nearly always used. Multi line can refer to 2, 3, 4, or even more lines and therefore cascade banks between the compressor and the dispenser. In practice it has been found that a 3 line system is the best comprise between high efficiency and lack of complexity.
For example the following efficiencies in gas utilisation will result from the use of the multi line system:
1 LINE - 25 %
2 LINE - 45 %
3 LINE - 60 %
4 LINE - 65 %
5 LINE - 68 %
From this above it is seen that the 3 line system is the best compromise.
The priority fill system is a system which allocates gas from the compressor to the cascade storage in the correct sequence.
The dispenser takes gas from either the compressor or the storage, regulates it down to vehicle fill pressure, normally 20 bar (300 psi), and fills the vehicle gas cylinder, recording the amount of gas dispensed.
