Compressed natural gas (CNG) stations contain important features that can be designed to meet the needs of various fueling applications. A small fleet of CNG cars needing fast refueling will have different requirements than a large fleet of CNG school buses or refuse trucks that can be refueled on a time fill system. These too will be different from a commercial CNG station requiring larger equipment, greater redundancy, greater fuel storage, and multiple dispensers. To properly size a CNG fueling station it is important to know as much as possible about the vehicle fueling rates and arrival patterns. In particular, the designer will need to know the peak hourly fueling rate, not just the daily or weekly usage as is the case with traditional gasoline and diesel stations.
CNG Station Configurations
CNG station designers are encouraged to consider redundancy. This is accomplished by installing more than one compressor and insures that a station can continue to operate even if one compressor fails. While the density of CNG stations is relatively low, and an alternate refueling station may not be easily located, redundancy improves system reliability and customer satisfaction. With an increase in CNG station density, the need for redundancy may be diminished.
There are four predominant configurations of CNG stations in North America:
- Cascade Fast-Fill
- Buffer Fast-Fill
- Combination-Fill, which combines two of the three configurations
Cascade fast-fill stations primarily fill from storage tanks and are typically used for retail applications or for vehicles that require refueling at varying times. Some fleet operations and most public CNG stations are examples of a cascade fast-fill configuration.
Cascade fast-fill systems include:
1) Dryer—removes water or water vapor from the natural gas supply prior to compression. Dryers work via a desiccant material and some dryers require the desiccant periodically be replaced, while others will regenerate the desiccant.
2) Compressor—compresses natural gas to the appropriate pressure required to deliver a fully temperature compensated fill to the vehicle. Compressors come in multiple sizes and are often “ganged” to provide redundancy and consistent pressurized operation.
3) Priority-Sequential Panels/Buffer/Time-fill valve—determines the priority and sequence of flow of CNG from the compressor into storage or directly to the dispenser. These valve systems are often custom built to station requirements.
4) Storage—American Society of Mechanical Engineers (ASME) storage tanks are used as the current acceptable standard to store compressed natural gas. Tanks are configured as banks of cylinders or as spheres.
5) Dispenser—dispenses natural gas into vehicles. There are many types of dispensers available on the market today and many offer similar features of metering and charging of conventional fuel dispensers.
a. Temperature compensation system—uses an algorithm to adjust for ambient temperature and temperature of compression into the vehicle fuel tank to ensure that vehicles receive a complete fill. Most CNG fuel dispensers perform this function.
Buffer fast-fill stations are ideal for high fuel use vehicles that require immediate refueling, one after another. Taxis and transit buses frequently utilize this configuration due to their need to sequentially refuel and also because of the overall fuel demand of these types of vehicles.
Buffer systems primarily fuel directly from the compressor into the vehicle and therefore require a smaller quantity of storage. These stations typically serve a captive fleet and are designed and sized for the need and fueling patterns of the specific fleet. They are often located onsite and allow for large quantities of fuel to be dispensed in a relatively short period of time. Typical components of a buffer fast fill CNG system include those for a fast-fill with the priority panel and sequencing valves replaced by a buffer control panel that routes fuel directly from the compressor(s) to the dispensers using stored fuel only if compressor capacity is exceeded.
Fleets of vehicles that return to central locations for periods of time frequently utilize the time-fill configuration. This is a lower cost option and is a preferred method of refueling refuse trucks, school buses, and other fleets with similar refueling requirements. Time-fill stations have significantly lower equipment and installation costs because they do not require storage, priority, or sequential refueling components.
Time-fill applications provide fuel to the vehicle directly from the compressor and are ideal for fleets that return to a central location for an extended period of time. This station configuration is easily modified to also accommodate vehicles requiring a fast-fill with the addition of a small amount of storage and fast-fill dispensing equipment.