Compressed Natural Gas Fueling Stations
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.
Ensuring a Full Fill
Unlike liquid fuel, which consistently holds about the same volume of fuel across a broad range of conditions, gas can expand and contract significantly depending on the gas pressure and the temperature. For example, under industry standard conditions, a tank on a vehicle may be able to hold 20 gasoline gallon equivalents, but on a hot day the gas will expand and the tank may only fill to 75% (or less) of its potential. The goal is to get as full of a fill as possible into the vehicle's tank.
The amount of CNG that can be stored onboard varies based upon the pressure rating of the fuel system, the ambient temperature, and the fueling rate.
- Pressure ratings: The typical industry standard for CNG fueling system pressure is 3600 psi. Some systems in the U.S. and many systems overseas are rated at 3000 psi. This means that the fuel system, including the tank and safety hardware, are capable of handling these pressures safely. When fueling, the dispenser is designed to fill the tank up until it achieves these pressures.
- Ambient temperature: The outside temperature affects the temperature of the CNG. At higher temperatures, CNG is less dense, and therefore does not contain as much energy per unit volume as it would at a lower temperature. When the CNG is stored in warm ambient temperatures, it expands and becomes less dense, so when the tank reaches the rated pressure, the CNG inside does not contain as much energy as it would at lower temperatures.
- Fueling rate: As the rate of fueling increases, the temperature of the fuel also increases — dramatically. Just like with ambient temperatures, as the fuel warms up it becomes less dense and therefore contains less energy by volume when the fuel system reaches the rated pressure. For this reason, you are usually able to get more CNG into a tank with a time-fill versus a fast-fill application.
Types of Stations
There are two types of compressed natural gas (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 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 Station
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. These differences account for the large variance in the cost of installation.