Compressed Natural Gas Fueling Stations
Unlike gasoline or diesel stations, compressed natural gas stations (CNG) 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 will make a big difference in the cost of fuel and range for vehicles.
Types of Stations
There are two types of CNG infrastructure: time-fill and fast-fill. 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 Station
Fast-fill: Generally, fast-fill stations are best suited for retail situations where vehicles arrive randomly and need to fill up quickly. The space needed to store the equipment measures about the size of a parking space. CNG can also be delivered via dispensers alongside gasoline or other alternative fuels dispensers. 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 available for a quick fill-up.
Drivers filling up at a fast-fill station experience similar fill times to a conventional gasoline fueling station—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 (4,300 psi), so it can deliver fuel to a vehicle faster than the fuel coming directly from the compressor, which delivers fuel at a lower volume. Drivers use a dispenser to transfer CNG into the tank. The dispenser uses sensors to calculate pressure and measure the number of GGEs delivered to the tank, taking temperature into account.
Learn more about filling CNG tanks.
Time-Fill CNG Station
Time-fill: Time-fill stations are used primarily by fleets and work best for vehicles with large tanks that refuel at a central location every night. Time-fill stations can also work well for small applications, such as a fueling appliance 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, vehicles at time-fill stations are generally filled directly from the compressor, not from fuel stored in tanks. The size of the compressor needed depends on the size of the fleet. 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—wasting electricity and causing undue wear and tear on the compressor. The storage tanks are sometimes used to "top off" vehicle tanks during the day.
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 several minutes to many hours to fill. The advantage of using a time-fill station is that the heat of recompression is less, so you usually get a fuller fill then with a fast-fill station. Also, with a time-fill station you can control when you fill the vehicles. This means you can choose to run the compressor during off-peak hours (like at night) to achieve lower electricity rates.
Time-fill stations are carefully architected based on the application they will be used for. For example, a transit bus company may need a larger compressor that can deliver 8 to 9 gallons per minute, while a refuse truck company can make due filling trucks at 3 gallons per minute using a smaller compressor. A consumer application may require far less—such as less than half of a gallon an hour. These differences account for the large variance in the cost of installation.
Learn more about filling CNG tanks.