The use of alternative fuel gasses as a fuel source for motor vehicle applications is gaining commercial traction. Natural gas, for example, is comprised primarily of methane (CH4) and, currently, can be combustibly consumed to power dedicated natural gas vehicles, which are fueled only by natural gas, or dual-fuel vehicles that are fueled by a combination of traditional petrol-based fuels and natural gas through separate fueling systems. Natural gas may be stored in an on-board fuel storage tank in two plausible ways: as compressed natural gas (CNG) or adsorbed natural gas (ANG). Compressed natural gas is natural gas that is contained within a tank—usually a cylindrical or spherical tank—at less than 1% of the volume it would normally occupy at standard temperature and pressure (STP). Tank pressures of 150 bar to 250 bar are typically needed to achieve this level of compression.
Adsorbed natural gas is natural gas that is adsorbed onto a natural gas storage material housed within a tank. The natural gas storage material increases the volumetric and gravimetric energy density of the gas within the available tank space such that it compares favorably to CNG but at a much lower pressure of 60 bar or less. Several different kinds of natural gas storage materials are known in the art including activated carbon and, more recently, metal-organic-frameworks (MOFs) that have an affinity for natural gas. MOFs, in general, are high surface area coordination polymers having an inorganic-organic framework, often a three-dimensional network, that includes metal ions (or clusters) bound by organic ligands. Many different types of MOFs that are able to reversibly adsorb natural gas are commercially available in the marketplace and newly-identified MOFs are constantly being researched and developed.
Another type of alternative fuel gas is hydrogen, which, like natural gas, can also be stored in a compressed state or on a hydrogen storage material. Storing hydrogen gas on a hydrogen storage material has similar thermodynamics to storing natural gas on an ANG storage material even though hydrogen uptake is chemical in nature—hydrogen is stored as a hydride—as opposed to adsorptive. Hydrogen gas, for instance, can be reversibly charged and released from a hydrogen storage material such as, for example, a complex metal hydride including various known alanates, borohydrides, and amides. Some specific complex metal hydrides include sodium alanate (NaAlH4), lithium alanate (LiAlH4), lithium borohydride (LiBH4) with or without MgH2, calcium borohydride (CaBH4) with or without MgH2, and lithium amide (LiNH2). MOFs and PPNs may also be used to store hydrogen gas. There are, of course, many other hydrogen storage materials that are commercially available.
A design consideration that factors into the commercial demand and viability of on-board fuel gas storage tanks that utilize a gas storage material—and all vehicle fuel tanks for that matter—is “conformability.” The concept of tank conformability relates to the flexibility of the tank structure and how easily it can be adapted to fit the available packing requirements across many different vehicle platforms. Cylindrical and spherical tanks, for example, which are used to store compressed fuel gas due to the high pressures involved, are generally considered to be quite non-conformable since they are typically unable to efficiently occupy the dedicated fuel tank space that vehicle manufacturers make available. And while storage tanks that include a gas storage material do not have to accommodate the pressures typically found in compressed fuel gas applications, the internal pressure range they must be equipped to handle is still high enough that provisions are often needed to provide structural integrity to the tank if tank shapes other than cylindrical and spherical are desired.
To this end, there exists a need for a fuel gas storage tank that not only stores a sufficient quantity of fuel gas to enable acceptable driving distances between fill-ups, but is also amenable to quick re-filling times, all the while being conformable to many different types of vehicle platforms. A fuel gas storage tank that possesses such attributes would simplify the integration of fuel gas such as natural gas and hydrogen gas into motor vehicles—especially passenger cars and trucks—as a source of power for operating and propelling the vehicle either alone or in combination with other power sources such as, for example, traditional petrol-based fuels (e.g., gasoline or diesel fuel) and lithium ion batteries. And, practically speaking, the flexibility and design freedom to customize the size and shape of the fuel gas storage tank to fit individual vehicle packaging requirements would also make fuel gas technologies a more economically attractive option for motor vehicle applications.