Natural gas vehicle fuel storage is a challenge of natural gas vehicles since the vehicles can't keep natural gas at ambient pressure and temperature. This storage has been one of the major engineering challenges of using natural gas for vehicles, but there are several solutions. Natural gas is normally a gas at 1 atm and 25°C. NGV fuel circumvents this problem in any of three different ways:
Compressed natural gas (CNG) vehicles use natural gas that has been pressurized between 3000 and 3600 psi. The storage vessel is made mostly of heavy walled steel or carbon-fiber/epoxy cylinders. Sometimes CNG vehicles have reduced cargo space because of the design and placement of the tank.
Two important modifications to CNG vehicles are:
CNG is most suited for short distance travel vehicles that don’t need a lot of space like transit, cars, vans, and pickup trucks. Retrofitting regular gasoline vehicles (do we mean CNG?) costs an additional $6,000 to $13,000. This cost is recovered from fuel savings through higher fuel efficiency.
CNG vehicles have a very high octane rating, which allows these vehicles to run smoother. CNG does not experience boil off evaporation like LNG because of it stays at ambient temperature. Additionally, the same spark plug engines used for gasoline engines works well for both CNG and LNG fuels.
LNG vehicles use natural gas that has been cooled to a cryogenic liquid, (between -160 to -128°C) and at a pressure of 50 to 150 psig. Double walled vessels having insulation and vacuum in the annular space, and is used to store LNG to help maintain the temperature. LNG tanks can hold 2 to 3 times more gas per unit volume than CNG tanks, while cutting the cost per km by almost 1.5 times.. Before the natural gas goes into the engine, LNG is converted to gas through regasifying systems.
LNG is preferred for long-haul applications like refuse trucks, buses, and trains because of energy density. An estimated fifteen North American truck and transit bus manufacturers exist currently, and they can provide LNG trucks at around $35,000.
Some LNG in the tank will boil-off due to the heat transfer from outside temperature with time. A long-haul vehicle sees the boil-off effect to a lesser degree, and a technological management system exists to reduce this problem, but this leads to an increased total weight of system. This boil-off problem makes LNG less suited to regular passenger vehicles.
Adsorbent natural gas (ANG) is a new technology that is similar to CNG, except ANG allows the storage of natural gas at a lower pressure than CNG because it uses porous adsorbent materials at relatively low pressures of 500 to 600 psig. Because of the lower pressure, we can save costs required to pump and store natural gas in ANG compared to CNG. ANG can store half to two-thirds the amount of gas at one-sixth of the pressure of CNG.
The adsorbent materials are carbon derived and highly disordered. The best carbon material is PDVC Carbon (a type of carbon structure), because it adsorbs lots of natural gas while leaving minimal void space due to narrow pores.
The biggest disadvantage is the readsorption is not complete and 15% to 30% of natural gas in adsorbent is unused. Adsorption is exothermic, which reduces the extent of adsorption during refueling. Similarly desorption has a cooling effect where temperatures drop extensively making it difficult to delivery the gas to engine.