CNG:Compressed Natural Gas Fueling Stations
CNG stand for compressed natural gas.It is 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 the compressor and the amount of storage at the station will impact 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 the time it takes for CNG to be delivered. Most CNG stations include one of these two system types, but "combination fill" stations include both types.
Fast-Fill CNG Station
Enlarge illustration
Fast-fill: Generally, fast-fill stations are best suited for retail situations where vehicles arrive randomly and need to fill up quickly. All public CNG stations have a fast-fill option. For a station serving light-duty vehicles, the space needed to store the equipment measures about the size of a parking space. 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. CNG can also be delivered via dispensers alongside gasoline or other fuel dispensers.
Drivers fueling light-duty vehicles 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. Large truck and bus stations will fill at twice that rate. CNG at fast-fill stations is often stored in the vessels at a high service pressure (4,300 psi) so the dispenser can deliver it to a vehicle quickly. The dispenser uses sensors to calculate pressure and measure the number of gasoline gallon equivalents (GGEs) delivered to the tank, taking temperature into account.
Filling CNG Fuel Tanks
CNG Fueling Animation
CNG Fueling Animation
Use this interactive animation to learn how fill speed and temperature affect the final fill volume.
Unlike liquid fuel, which consistently maintains the same volume across a broad range of temperatures, compressed natural gas (CNG) will expand and contract significantly with a change in temperature. Heat causes CNG fuel molecules to expand, and cold causes the molecules to become denser. Therefore, under industry standard conditions (pressure and temperature), a CNG tank may typically contain 20 gasoline gallon equivalents. However, on a hot day the CNG molecules will expand, and the tank may only be able to accommodate up to 75% (or less) of its design potential under cooler temperature conditions.
Fueling equipment has been optimzed with fast-fill dispensers that are equipped with a temperature compensation feature to help maintain temperatures to industry standard conditions and assist vehicles to achieve maximum fill of their fuel tanks.
The amount of CNG that can be stored in a vehicle's tank varies due to multiple factors:
Fueling rate: As the rate of CNG fueling increases, the temperature of the fuel molecules also increases dramatically. As the fuel warms up, it expands and becomes less dense, resulting in less energy per volume when the fuel system reaches the industry-rated pressure. Therefore, it is usually more efficient to achieve a fuller fill with a slower "time-fill" method rather than a "fast-fill" process. This allows the CNG fuel molecules to remain in a dense state rather than expanding from the heat created by the rapid movement and compression of CNG into the tank.
Ambient temperature: The outside temperature also 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 in a compressed state at a lower temperature. When the CNG is stored in warm ambient temperatures, the fuel molecules expand and no longer contain as much energy as they would at lower temperatures.
Pressure rating: The typical industry standard for CNG fueling system pressure is 3,600 psi. Some systems in the United States and many systems overseas are rated at 3,000 pounds per square inch (psi) to allow for a greater safety margin. These fill pressures are based on a 70ºF ambient temperature. The CNG fuel tanks are designed to withstand up to 125% of their operating pressure. Therefore, a 3,000-psi tank can technically be filled to 3,750 psi, and a 3,600 tank can be filled to 4,500 psi. This allows a tank to be filled to a higher pressure on hot days when the gas is expanding, as well as compensate for the heat associated with the compression of the natural gas. A good rule of thumb is that for every 10ºF plus or minus 70ºF, the pressure will increase or decrease by 100 psi.
Tank type: There are four types of CNG tanks (Type 1–4). The type designation is based on the design and production method, including the tank's liner material. The materials also regulate the temperature inside of the tank and therefore affect how much CNG it can contain based on the defined limits. For example, carbon fiber tanks retain more heat compared to steel tanks and may reduce the capacity of the CNG due to the fuel molecules expanding under wamer conditions.
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