Types Of CNG Fueling Station
There are 2 major types of CNG fueling stations: time-fill and fast-fill. Unlike gasoline or diesel stations, compressed natural gas stations are not “one size fits all.” Building a CNG station for a retail application or a fleet requires calculating the right combination of pressure and storage needed for the types of vehicles being fueled. Making the right choices about the size of compressor and the amount of storage at the station makes a big difference in the cost of fuel and range for vehicles.
The main structural differences between the two systems are the amount of storage capacity available and the size of the compressor. These factors determine the amount of fuel dispensed and time it takes for CNG to be delivered.
Fast-fill CNG fueling station
Fast-fill stations are generally best suited for retail situations where light-duty vehicles, such as vans, pickups, and sedans, arrive randomly and need to fill up quickly. The equipment needed takes up about the size of a parking space. CNG can also be served up in dispensers alongside gasoline or other alternative fuels.
Fast-fill stations receive fuel from a local utility line at a low pressure and then use a compressor on site to compress the gas to a high pressure. Once compressed the CNG moves to a series of storage vessels so the fuel is ready to go for a quick fill-up. Drivers filling up at a fast fill station experience similar fill times to gasoline fueling stations—less than 5 minutes for a 20 gallon equivalent tank.
CNG at fast-fill stations is often stored in the vessels at a high service pressure (4300 psi), so it can deliver fuel to a vehicle faster than fuel coming directly from the compressor, which delivers fuel at a lower volume.
Drivers use a dispenser to transfer CNG gasoline gallon equivalents (GGEs) into the tank. The dispenser displays the pressure and temperature at which the tank is being filled then calculates and shows how many GGEs are delivered into the vehicle.
Time-fill CNG Fueling Station (Slow Fill)
Time-fill stations are used primarily by fleets. This type of setup works great for vehicles with large tanks that refuel at a central location every night. Time-fill stations can also work for small applications, such as a small station at a driver’s home. At a time-fill station, a fuel line from a utility delivers fuel at a low pressure to a compressor on site. Unlike fast-fill stations, time-fill stations may have larger compressors and the vehicles are generally filled directly from the compressor, not from fuel stored in tanks. Although there is a small buffer storage tank, its purpose is not to fill vehicles, but to keep the compressor from turning off and on unnecessarily and wasting electricity.
The time it takes to fuel a vehicle depends on the number of vehicles, compressor size, and the amount of buffer storage. Vehicles may take a several minutes to many hours to fill. The advantage of time-fill is that the heat of recompression is less so you usually get a fuller fill then with fast-fill. And you can control when you fill the vehicles, and thus, get better electricity rates needed to run the compressor, such as off-peak hours at night.
Time-fill stations are carefully architected based on the application. A transit bus company may need a larger compressor that can deliver 8 to 9 gallons per minute, while a refuse truck company may be fine filling trucks at 3 gallons per minute using a smaller compressor. A consumer application may need far less—such as, less than half of a gallon an hour.
The cost difference between those types of CNG fueling stations can be very significant.