CNG stations

CNG stations can have a direct national grid connection or have CNG delivered by tanker (a so-called Mother and Daughter arrangement). Grid connected stations allow an efficient and cost effective supply of gas to be compressed into storage tanks. The suitability of the grid connection depends on the pressure and distance to the national gas network; low pressure connections provide uneconomic station performance. Mother and Daughter station arrangements can also be used where a Mother-grid connected CNG station supplies a local Daughter station via a tube trailer.

A fast fill grid-connected CNG station maintains sufficient site storage to allow vehicles to be filled in minutes. Where vehicle use is less intensive (RCVs for example), slow fill stations can be utilised. Here gas is compressed directly into vehicle tanks over several hours, saving the cost of site pressurised gas storage. CNG stations can dispense biomethane, and are sometimes referred to as CBM (compressed biomethane) stations.

LNG stations

LNG stations consist of a cryogenic tank and a fuel dispenser. They deliver fuel at 30 to 120 PSI. LNG stations may suffer from very slow fuel loss as heat ‘boils off’ the stored LNG and increases pressure in the storage tank, although modern stations have vent capture systems to prevent leakage. Stations are designed to ensure an optimal storage capacity that allows for regular LNG deliveries to maintain a low temperature in the storage tank. The availability of LNG refuelling stations allows fleets to convert to LNG without the complexity of managing onsite refuelling facilities.

LNG is not available via a grid connection. LNG stations are also able to dispense biomethane. In this case the stations are sometimes referred to as LBM (liquefied biomethane) stations.

Grid-connected stations

Gas from grid-connected stations is compressed on-site for dispensing into vehicles. It’s therefore important to understand the specification of the local gas main; it must have enough pressure and capacity to supply the required demand.

• Connecting to a relatively high pressure part of the gas network reduces the amount of compression needed and therefore lowers running costs. However, installation costs increase with distance from the network to the dispensing unit.
• A lower pressure connection will require additional compression and supply fewer vehicles. However, it may be suitable if vehicles are stationary for several hours, allowing gas to be compressed into tanks gradually.

Daughter stations refuelled by tanker can be used if a grid connection is not feasible. These stations offer flexibility as the location and capacity can easily be adapted.