Single-walled, jacketed aluminum tanks have been conceived for storing liquefied natural gas (LNG) in LNG-fueled motor vehicles. Heretofore, double- wall steel tanks with vacuum between the inner and outer walls have been used for storing LNG. In comparison with the vacuum-insulated steel tanks, the jacketed aluminum tanks weigh less and can be manufactured at lower cost. Costs of using the jacketed aluminum tanks are further reduced in that there is no need for the vacuum pumps heretofore needed to maintain vacuum in the vacuum-insulated tanks.
The single-walled, jacketed aluminum tanks are members of the class of composite overwrapped pressure vessels; that is, they comprise basically, seamless aluminum tank liners overwrapped in composite (matrix/fiber) materials. On each such tank, the composite overwrap is further encapsulated in a layer of insulating foam, which, in turn, is coated with a flexible sealant that protects the foam against abrasion, ultraviolet light, and other adverse environmental phenomena.
The innovative tank concept admits to a number of variations. For example, the aluminum tank liner can be a common, commercially available aluminum tank liner that is already certified by the United States Department of Transportation for use at pressure up to 3,000 psi (≈20.7 MPa). The composite-material overwrap can be made by winding high-strength-carbon- fiber/poly(phenylene benzobisoxazole)- fiber hybrid filaments with an epoxy matrix material. The insulating layer can be made by spraying polyurethane foam, waiting for the foam to cure to rigidity, then machining the foam to final size and shape. The protective outer layer can be formed by brush application of a ductile epoxy or spray application of a truck-bed-liner material.
Of course, if the tank liner is a pressure vessel as in the example above, then the tank can be used to store a high-pressure gaseous fuel. Moreover, in the case of storage of LNG, the high-pressure capability of the tank helps to conserve stored fuel by reducing the need to vent gas to relieve pressure as heat leaks into the tank, causing slow vaporization of the LNG.
This work was done by Tom DeLay of Marshall Space Flight Center.