Unlike gasoline or diesel stations, compressed natural gas (CNG) 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 3,600 psi. Some systems in the U.S. and many systems overseas are rated at 3,000 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 CNG fueling stations: 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.
Time-Fill CNG Fueling Stations
Time-fill CNG 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 CNG stations can also work for small applications, such as a small station at a driver’s home. At a time-fill CNG station, a fuel line from a utility delivers fuel at a low pressure to a compressor on site. Unlike fast-fill CNG stations, time-fill CNG stations may have smaller 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 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 than 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 CNG 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.
Benefits of a Time-Fill CNG Fueling Station
- Time filling allows a fleet to fill overnight while trucks are not in service.
- Reduce cost associated with drivers waiting each day to fill their vehicles. On average, saves 60 hours per year - per truck.
- Use smaller compressors and related equipment verses fast-filling to achieve fuel production. Typical time-fill CNG systems cost 30% less than fast-fill CNG systems.
- Reduce electric costs to operate compressors by filling trucks during off-peak hours.
- Compressors start and stop only one time per day. Reduces maintenance and extends the life of the equipment.
- Complete 100% fill achieved with every fill when compared to fast-fill.
- Requires less hourly gas flow than a fast-fill CNG system. Utility extensions are typically minor.
- Production costs are lower than fast-fill CNG systems.
- Filling the vehicles will happen automatically when the vehicle is connected to the hose and will shut off when the vehicle is full. Timers can turn the station on and off each day automatically.
Fast-Fill CNG Fueling Stations
Fast-fill CNG stations are generally best suited for retail environments where light-duty vehicles, such as vans, pickups, and sedans, arrive randomly and need to fill up quickly. These stations receive fuel from a local utility line at a low pressure and then use a compressor to increase the gas to a high pressure. Once compressed the CNG moves to storage vessels so the fuel is ready to dispense so drivers experience similar fill times to gasoline fueling stations—less than 5 minutes for a 20 gallon equivalent tank.
Benefits of a Fast-Fill CNG Fueling Station
- CNG is metered and dispensed similar to ordinary liquid fuel dispensers.
- Fast filling requires about the same time as fueling with conventional fuel.
- Fast-fill CNG systems can incorporate a credit card reader for authorizing, dispensing, and billing the transaction.
- Public-access CNG stations can be unmanned and open to the public 24/7.
- Multiple customers can fuel their vehicles simultaneously.
- Requires less space for construction and dispensing.
- Can be used in conjunction with a time-fill CNG system or added to an existing time-fill CNG system at a later date.
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