Natural gas is a significant energy source for heating, electricity generation, and transportation. Because the demand for this fuel fluctuates significantly, especially between summer and winter, storage is necessary to maintain a steady and reliable supply to consumers. Storing this gas in a tank is possible, but it requires changing its physical state to reduce its volume. This transformation allows for the efficient containment and transport of natural gas in manufactured vessels.
The Two Primary Storage States
Storing large quantities of natural gas requires changing its state to maximize the amount held within a given space. The industry uses two distinct methods to achieve this volume reduction, each relying on a different physical principle. The first method involves compressing the gas at ambient temperatures, while the second uses extreme cooling to liquefy it.
Compressed Natural Gas (CNG) is created by subjecting the gas to very high pressure. Storage vessels typically hold the gas at pressures ranging from 2,900 to 3,600 pounds per square inch (psi) at normal temperatures. This compression reduces the gas volume to less than one percent of its original state, allowing a greater quantity of energy to be packed into a tank. The primary concern with this method is maintaining the structural integrity of the container against the immense internal force.
The second method produces Liquefied Natural Gas (LNG) by chilling the gas to approximately -260°F (-162°C). This cryogenic process converts the gas into a liquid, resulting in a volume reduction of about 600 times compared to its gaseous state. This volume reduction makes it economical to transport the fuel across vast distances without pipelines. LNG must be kept in specialized, heavily insulated tanks to maintain the required temperature.
Small-Scale Applications and Transport Tanks
The choice between CNG and LNG determines the design and materials of the tanks used for smaller-scale applications and transport. Compressed Natural Gas is stored in robust cylinders designed to withstand the persistent high internal pressure. These tanks are constructed from materials like high-strength steel, aluminum, or lightweight composite materials such as carbon fiber-reinforced plastics. Composite tanks are increasingly popular for vehicles because they offer a better weight-to-storage ratio, improving fuel efficiency.
LNG storage relies on double-walled, vacuum-insulated cryogenic tanks to minimize heat transfer from the environment. These specialized vessels are used for transporting the fuel via truck or rail in what is often called a “virtual pipeline.” Smaller LNG tanks can also be used on-site by industrial users or for temporary power generation in areas without pipeline access. Insulation is paramount, as heat gain causes the liquid to vaporize into “boil-off gas,” which must be managed to prevent pressure buildup.
Utility-Scale Storage Facilities
Utility-scale storage requires infrastructure capable of holding large volumes to balance regional energy demand, unlike manufactured tanks used for transport and localized supply. Most large-scale storage is accomplished underground in geological formations that act as natural reservoirs. This method is essential for managing seasonal fluctuations in natural gas consumption, primarily storing gas during periods of low demand for use during high-demand winter months.
The three types of underground storage are depleted natural gas or oil reservoirs, aquifers, and salt cavern formations. Depleted reservoirs are the most common, as they already possess the necessary geological structure and confinement properties. Salt caverns, created by dissolving salt beds with water, allow for very rapid injection and withdrawal of gas.
Handling and Safety Requirements
Storing natural gas in its condensed forms introduces unique safety considerations that require specific handling protocols. Compressed Natural Gas systems focus on managing the inherent risk of high pressure, which can exceed 3,000 psi. CNG tanks must be regularly inspected to ensure their structural integrity and are equipped with pressure relief devices to safely vent gas if the temperature or pressure becomes dangerously high, such as in the event of a fire.
Liquefied Natural Gas storage involves risks associated with extreme cold. Direct contact with the cryogenic liquid can cause severe freeze burns, necessitating specialized personal protective equipment for handling. If an LNG leak occurs, the liquid rapidly vaporizes and expands 600 times, potentially creating a large, localized cloud of flammable gas near the ground before it dissipates. Specialized containment and ventilation systems are required to manage these hazards.