CNG

NGVs are refuelled directly by a compressor, and several vehicles can be refuelled simultaneously. Slow-fill refuelling usually takes several hours; it is performed when the vehicle is not in operation – overnight or while standing idle for a longer period of time.

In practice, we can come across many names of the slow-fill equipment. The official international designation is VRA – Vehicle Refuelling Appliance. The name FuelMaker is often used, derived from the Canadian manufacturer holding a dominant position in the market. In Czech, the designation “home gas filler” best describes the use of this equipment.

The relevant standard defines a VRA as an appliance, the main part of which is a natural gas compressor but which does not include a gas storage tank. The operation of this equipment is limited by a maximum capacity of 20 cu m/hour, a maximum filling pressure of 26 MPa, and a maximum gas storage capacity of 0.5 cu m.

Advantages and disadvantages of slow-fill stations

Advantages

Installation

• Easy: the VRA can be installed in all places supplied with gas and power;
• It is easy to relocate in case of need;
• The erection of VRA is fast.

Easy operation

• Prior to refuelling, the hose is connected to the vehicle’s filling valve, then the START button is pushed;
• After refuelling, the hose is disconnected;
• There are only minimum requirements for maintenance.

Fully automated operation

• Vehicle refuelling is fully automated; once the maximum permissible pressure is reached, the appliance is turned off automatically;
• An electronic system diagnoses the operation of the appliance – input and output pressure, ambient temperature, operating hours …
• Compensation of the maximum filling pressure depending on the ambient temperature.

Economics

• Lower price of the fuel – the price depends on the price of the natural gas and electricity at the point of refuelling.

Safety

• Automatic interruption of refuelling when the gas leaks or the hose is damaged;

Low noise level

Independence

• Independent of the infrastructure of public natural gas filling stations, and of the filling stations for conventional fuels; “I go out with a full tank every day.”

Disadvantages

Initial cost

VRAs are suitable primarily for passenger cars and light-duty vans and lorries that park at the same place all the time and are not used in continuous operation. In Canada and the US, they are also used for some special vehicles – forklift trucks or ice surfacers at winter stadiums.

One small slow-fill station will normally refuel one to two vehicles; if the timetable of refuelling is optimised it may refuel up to four to six vehicles. The optimum number of vehicles depends on their specific driving rates and in turn the requirements for the frequency of refuelling.

Companies operating large fleets of NGVs, for example postal services, supply companies, and gas companies, use up to dozens of small natural gas refuelling stations in their compounds.

A FuelMaker can be extended to include a pressurised gas storage tank and a dispenser, which makes fast-fill possible. With this version, the number of vehicles is limited by the capacity of the gas storage tank and the time needed to refill it. This option is therefore suitable at the time when the conversion to gas-based transport is only beginning and there is only a small number of NGVs, and the building of fast-fill stations would not yet pay off.

In some cases small slow-fill stations are a more appropriate approach than large fast-fill stations. Their advantage is that they can be procured and erected quickly; they can be installed wherever there is a supply of natural gas, and their size can be adjusted with a view to optimising the economics of their operation. For many smaller and larger businesses they may constitute an economically interesting project as they convert their vehicle fleets to natural gas.