Patent Application
20080026167
The invention includes a tank for storing a fluid, such as a gas, under pressure. The tank has an inner core of an open-celled foam that is characterized by voids that are at least partially interconnected by passages and are surrounded by a fibrous or ligament structure/network (collectively, “ligaments”). Attached to the inner core is an outer skin.
The outer skin 13 can be of the same material as the core and thus can be created at the same time as the interior open-celled foam region. Or, the outer skin can be of different material from the core and be added to an open-celled foam structure.
If the materials are the same, localized welding (a “weldment”) occurs between the molten skin material (upon dipping or plasma spraying, for instance) and the cells in the outer layer(s) cell of the foam. As depicted in
The open-celled nature of the invention allows a fluid, such as the gas to be stored under pressure to diffuse throughout the inner space. However, since the cells can be small in size, 25 micrometers for instance, there is some point at which the gas flow through the open-celled structure is regulated due to the resulting flow restriction. Flow may be restricted by the small passageways from cell to cell. This increases the drag for a given flow velocity, noting that flow drag increases also with the square of flow velocity. As such, the entire tank can be self-regulating to some degree.
Alternatively, (
In one embodiment, the gas-bubbled foam with applied outer skin (
In some embodiments, the void density of the core structural foam exceeds 75%. Correspondingly, the foam mass density of the bulk material is less than about 25%. In some cases, the void density may decrease closer to the outer skin of the tank.
If desired, the structural foam core is made by passing gas bubbles through or creating gas bubbles in a molten polymer or metal. Alternatively, the structural foam is made by dissolving gas from a molten polymer or metal by changing ambient pressure and/or temperature during solidification.
Where the core is made of structural foam that is of a syntactic type, the foam is made by injecting a molten polymer or metal to fill spaces between hollow spheres. In such cases, the outer skin may be attached to the structural foam core by a weldment during solidification of the foam core. Optionally, hollow spheres can be broken to allow fluid to access their inner space.
As noted earlier, the structural foam core can be made from a material that is selected from the group consisting of polymers, metals, metal alloys, and ceramics.
Thus, the present invention makes use of open-celled foam to provide increased pressure capacity for a given tank mass or external tank volume. The invention employs an impermeable outer skin having a shape that may be symmetrical or asymmetrical (i.e., shape is not necessarily cylindrical or spherical as are typical pressure vessels/tanks). The pressurized fluid is stored under pressure inside the voids of the foam structure. The outer skin is supported in many places by the ligament structure of the foam.
An internal pressure creates tensile loading of the ligament network that is relatively equal in all directions, if there are substantial uniformities of cell size. This yields greater overall pressure-to-mass capability than, for example, a cylindrical (shell-only) or spherical tank. The tank shape is constrained only by the combination of cells, i.e., it can be any shape with geometric features larger than the cell size.
Since the ligament structure impedes fluid flow, as it does when metallic foams are used for constriction, the invention can provide some degree of self-regulation for controlled delivery of the fluid. For example, it is anticipated that upon compromising the outer skin, less violent exhaust occurs of the pressurized fluid to the environment, and explosive forces are substantially reduced.
An increased pressure capability results for a given type of tank material (e.g., aluminum foam versus an aluminum and/or carbon-fiber-wound cylindrical tank), since the ligament network within the structural foam acts as miniature cables that hold all the outer surfaces to each other.
It may be possible to create an “interior cabling” structure, such as a matrix of fibers (like ceramic fibers) with an outer shell. However, geometrical assessment of the volume that individual fibers would consume indicates it would be excessive since the fibers must run in all (three, for instance) directions, and where they cross, they must overlap rather than intersect. In contrast, the ligament structure of the open-celled structural foam is like many intersecting fibers, not overlapping fibers. Hence, it can provide better strength per material/fiber volume. Furthermore, using structural foam provides a natural way of creating the fibrous matrix structure.
Arbitrary shape of the pressure vessel is possible since the pressure acting on a small element of the outer skin, the area corresponding to a single void in the structural foam, is carried as a tensile stress in the ligaments surrounding that void. That load is then transferred to other ligaments, and eventually balanced by the pressure acting on the outer skin on an opposite side of the tank. Since the ligament network has a spatial resolution that is very small, it can accommodate any shape that is made up of “cells” of that size. This has a similar effect to the pixel size on a computer screen and its relation to the shapes one can draw in a specified envelope. In fact, it is similar to the idealized case of many nested (in a close-packed structure) spheres with solid ligaments filling in around them, each sphere acting like a microscopic spherical pressure tank that is linked to and overlaps with its adjacent spherical tanks.
Traditional spherical or cylindrical tanks, on the other hand, even if made smaller and nested together, would have to become micro-sized to achieve the similar effect. Of course, they would not be integrated or “glued” together as in this structural foam approach. This would mean that their effective ligaments (micro-cylinder walls) would be doubled and less weight-effective.
There are various ways of manufacturing the tanks of the present invention. They can be manufactured, for example, from polymers, ceramics, and metal alloys. The material to use depends on the particular application and its relative prioritization of tank mass versus total external volume.
Structural foams, both polymeric and metallic, can be made to void densities greater than 90%. As void density increases, there is more internal volume (relative to a given external tank volume) for gas to consume, though maximum gas pressure would decrease. Thus, a larger external volume may be needed to hold the same mass of gas. Since the density of gas increases disproportionately (at a slower rate) as the pressure increases, increased void density at lower gas pressure can contain a greater mass of hydrogen per mass of the tank, though possibly at some sacrifice of total external volume. There is a variety of optimum void densities depending on the relative importance placed on mass of the tank versus total external volume of the tank.
One way in which polymer foams can be made is by gas bubbles forming during solidification. Bubbles form due to change in pressure and temperature. The gas that is in solution in the molten polymer may come out of solution, in the same way carbonation comes out of solution from a carbonated beverage when the pressure changes upon opening the container. When the bubbles form, a void is created. Depending on the concentration of gas in solution and other factors, the void density can be controlled. The quick cooling of the outer layer at the die surface can freeze the outer layer without bubble formation, potentially yielding the desired outer skin. This may also be extendible to metallic foam manufacture.
However, since the spheres (e.g., hollow glass spheres or cenospheres, byproducts of fly ash formed in coal-fired power generation) are brittle, slight deformation in multiple directions causes the spheres to fracture. This creates pathways between them along which the small gas molecules may travel. By encapsulating the matrix of spheres with a layer of material (e.g., a binder) that will burn away when the molten metal hits it, an outer skin would be formed around the cellular interior. This approach could enhance the self regulating nature, though it could be over regulated when the pressure becomes lower as the tank becomes partially emptied.
One material of choice is often high strength aluminum. However, if weight is not a concern, e.g. for SCUBA applications, steel can be used. Titanium may also be a suitable material, depending on the application. However, its cost is high compared to aluminum, so it is likely to be effective only for special applications, perhaps for hydrogen (and even oxygen) needs on the battle field.
While embodiments of the invention have been illustrated and described, it is not intended that these embodiments illustrate and describe all possible forms of the invention. Rather, the words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the invention.
