Gas tank

Abstract

A gas tank for storage of pressurized fuel comprises a liner 3 defining a chamber for containing pressurized gas and a composite shell 5 surrounding the liner. The liner has a cylindrical portion 14 a

Description

Technical field The present invention generally relates to gas tanks, more particularly to type III and type IV tanks for hydrogen storage. Background Art Pressure vessels are used in a wide variety of technical fields to store fluids under pressures significantly higher than atmospheric, typically between 350 and 700 bar. Many applications, such as hydrogen tanks for the automotive industry, require pressure vessels to be as light as possible whilst being strong enough to ensure reliable and durable sealing of the fluid within. Modern hydrogen tanks are typically cylindrical and can generally be categorized in five types: Type I tanks are simple metallic tanks, generally formed by a relatively thick cylinder of aluminum or steel. Type II tanks are similar to type I tanks and additionally comprise windings of glass or carbon fibers around the metallic cylinder. Type III tanks are made from a composite shell closely surrounding a relatively thin metallic liner. The metallic liner mainly ensures sealing of the fluid within the tank, whilst the composite shell bears most of the mechanical load generated by the pressure within. Type IV tanks are similar to Type III tanks but have a polymer liner instead of a metallic one. Finally Type V tanks are liner-less, fully composite tanks. The composite shell of type III and type IV tanks is typically made by filament winding. A continuous bundle / tape comprising a plurality of glass or carbon filaments is tensioned, submersed in a resin bath and wound around the liner to enclose it. Once the desired number of filaments has been wound around the liner, the resin is cured. The mechanical properties of the final composite shell largely depend on its winding configuration, which is defined by its number of windings, their winding angle (i.e. the angle between the longitudinal axis of the gas tank and the projection of the filaments onto the longitudinal plane), the width of the filament tape, the order in which windings are layered around each other, etc... Windings may be sorted in three patterns based on their winding direction. Polar windings have a winding angle comprised between about 13 and 30° and improve the mechanical strength of the composite shell in the longitudinal direction. Circumferential windings (a.k.a. hoop windings) have a winding angle between 80 and 90° and improve the mechanical strength of the composite shell in the radial direction. Finally, helical windings have a winding angle between 30 and 80°, slightly improve the mechanical strength of the composite shell in all directions, and mainly serve to maintain the polar windings in place, thereby preventing the latter from slipping. As will be further detailed below, the Inventors have found that current winding configurations could be improved to reduce the overall weight and costs of the composite shell. Technical problem It is an object of the present invention to provide a gas tank with improved winding configuration which is as light as possible whilst being strong enough to ensure reliable and durable sealing of the fluid within. This object is achieved by a gas tank as claimed in claim 1. General Description of the Invention The present invention proposes a gas tank for storage of pressurized fuel comprising a liner defining a chamber for containing pressurized gas and a composite shell surrounding the liner. The liner has a cylindrical portion and two rounded portions at bases thereof. The composite shell is formed by a plurality of resin-coated filament windings applied on the liner, whereby the composite shell has a similar shape. The plurality of filament windings include circumferential windings. As defined herein, circumferential windings are windings having a winding angle comprised between 80 and 90°. According to the invention, the plurality of filament windings further includes nongeodesic polar windings having a winding angle comprised between 2 and 10° on the cylindrical portion, and a winding angle strictly comprised between 10 and 30° on at least one of the rounded portions, preferably on both rounded portions. The inventive tank hence comprises a composite shell where non-geodesic polar windings are formed with using two different winding angles for a same winding. The second winding angle is here selected to match the geometry of the rounded portions of the gas tank and prevent fiber slipping. The first winding angle is then selected to be smaller than the second winding angle, resulting in greater mechanical strength in the longitudinal direction. The use of a polar winding as per the invention (i.e. with a winding angle comprised between 2 and 10° on the cylindrical portion and a winding angle strictly comprised between 10 and 30° on the rounded portions) as opposed to conventional winding configurations (i.e. with a single winding angle over the entire winding / loop comprised between 13 and 30°) already leads to a strength increase for a given weight of the composite shell. In fact, using more of such polar windings further improves these advantages. It is thus preferable if more polar windings, e.g. more than 50%, preferably more than 80% and even more preferably more than 90%, 95%, or up to 100% of the total number of polar windings in the composite shell, have a winding angle comprised between 2 and 10° on the cylindrical portion and a winding angle strictly comprised between 10 and 30° on the rounded portions. Any appropriate filament winding process may be used. As is known, such technique involves precisely laying down the filaments onto the liner. In the context of the invention, the filaments are preferably arranged parallel to each other as a tape, which may comprise any kind of reinforcing fibers in a thermoset resin (for the matrix). The fibers can be made for example from carbon, glass, basalt, aramid, polyethylene or any other reinforcement fiber. The reinforcing fibers add strength and stiffness to a part along the direction of the fiber length. The fibers are preferably continuous or near continuous in length and are preferably aligned substantially parallel along the length of the tape (i.e. the longitudinal axis of the tape). In embodiments, the polar windings have a width comprised between 2 and 10 mm, preferably between 2 and 5 mm. In a surprising manner, the inventors have found that by using windings of narrower width for polar windings, it is possible to decrease their winding angle, thereby improving the mechanical strength of the composite shell whilst reducing its overall weight. In embodiments, the fibers are carbon fibers of the High-Modulus (HM) or High-Resistance (HR) type, with a tensile modulus comprised between 200 and 600 GPa and a tensile strength between 3000 and 7500 MPa. The thermoset resin may be an epoxy or unsaturated polyester or polyurethane. As with most polymer composite applications, the specific resin and fibers used and their composition may be selected by the skilled person depending on the application and properties of the composite shell to be manufactured. The filament tape can be of almost any shape, for example band shape or wave shape. Preferably, the tape is a band shape. In particular, the tape may be formed by as a continuous or nearly continuous bundle of untwisted fibers. Generally, the liner is formed as an elongate vessel, having a cylindrical middle portion closed at both ends by a rounded end portion. The liner may be metallic or plastic. An opening is provided to allow gas to flow into / from the chamber defined by the liner, typically in one of the end portions. A boss may be mounted to this opening to define a connection interface for other components of a gas delivery line. In embodiments each end portion comprises an opening fitted with a boss. Since the composite is applied directly onto the liner, it will have a similar shape. The liner can be manufactured using conventional techniques (e.g. by extrusion), where the end portion and fixed in a gas-tight manner (e.g. by wielding) to the middle cylindrical portion. Alternatively the liner may be manufactured by additive manufacturing. As used herein, the term “winding” conventionally refers to as single turn of filament, i.e. of reinforcing tape, around the liner. The term “set” of windings is used to designate the plurality of windings made with a same winding pattern / angle. The term “winding angle” defines the angle between the longitudinal axis of the gas tank and the projection of the filaments / tape onto the longitudinal plane. In practice, both sets of circumferential and polar windings comprise a multitude of windings, which are arranged in several layers or plies. A layer is typically defined as a single thickness of windings arranged next to one another so as to cover the layer underneath it. The windings may be formed by wrapping contiguous windings (adjacent windings touch each-other or overlap) or allowing some spacing between them. The entire surface of the gas tank may be covered by a plurality of layers of reinforcement tape. In particular, circumferential windings are generally applied to cover the entire surface of the cylindrical portion of the liner. The polar windings extend from one axial end to another, covering the rounded end portions. They are generally applied to cover the entire surfaces of the rounded end portions. In embodiments, the circumferential windings have a width comprised between 35 and 85 mm, preferably between 50 and 70 mm. In embodiments, the composite shell is devoid of filament windings having a winding angle comprised between 30 and 79°. In other words, the composite shell is exclusively formed from polar windings with a winding angle comprised between 2 and 10°, and circumferential windings with a winding angle comprised between 80 and 90°. It has indeed been found that by using the herein prescribed circumferential and polar windings, a composite reinforcement of sufficient strength can be obtained, without the need for windings in complimentary directions, such as conventional helicoidal windings with a winding angle between 30 and 79°. This provides a significant reduction in terms of cost and production time. For an identical liner, using the winding configuration according to the invention enables a reduction of thickness for the composite shell of about 30%, drastically reducing its weight, when compared to conventional gas tanks for a given mechanical strength. In embodiments, the polar windings define a single set of polar windings and the circumferential windings define a single set of circumferential windings, and each winding of the set of circumferential windings radially surrounds all windings of the set of the polar windings. In other words, each circumferential winding surrounds every polar windings. Hence in terms of process, all of the polar windings may be first applied as a single set, then all of the circumferential windings may be applied around the set of polar windings. In embodiments, the gas tank further comprises at least one boss configured to define a gas passage through the liner and the composite shell, said boss being arranged on a rounded portion, preferably at an apex thereof. The width of the polar windings may then be selected in accordance with the dimensions of the rounded portion and the boss. Preferably, the polar windings have a width comprised between 2 and 5 mm and the cylindrical portion of the liner has an outer diameter comprised between 150 and 300 mm, or the polar windings have a width comprised between 5 and 10 mm and the cylindrical portion of the liner has an outer diameter comprised between 300 and 500 mm. In embodiments, the overall length of the gas tank is comprised between 0.5 and 3 m, and / or its outer diameter is comprised between 150 to 500 mm. Brief Description of the Drawings A preferred embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings in which: Fig. 1 is a schematic diagram representing a cross section of a typical type III or type IV gas tank; Fig. 2 is a schematic diagram representing a side view of the type III or type IV gas tank of fig 1; Fig. 3 is a schematic diagram representing a side view of a type III or type IV gas tank according to the invention; Fig. 4 is a flowchart of the method for manufacturing a type III or type IV gas tank according to the invention. Description of Preferred Embodiments The schematic diagram of figure 1 illustrates a type III or type IV hydrogen tank 1 having a spherocylindrical shape and comprising a spherocylindrical liner 3 surrounded by a spherocylindrical composite shell 5. The composite shell is applied directly on the liner 3. The liner 3 is hollow and defines a chamber 7 in its inner volume. The liner 3 is relatively thin with a thickness t1 and can be made from any appropriate metal for a type III tank, e.g. aluminum, aluminum alloys, steel, stainless steel, titanium and / or titanium alloys, nickel-based alloys, etc., or from any appropriate polymer for a type IV tank. The liner is typically formed as a spherocylinder, i.e. a cylinder with domes sealingly attached at the ends thereof. At least one of the domes is provided with an opening in which a boss is fixed, here both domes as shown on figure 2. Such a spherocylinder can be manufactured by deep extrusion. Alternately the liner may be manufactured in one piece (cylinder + end domes) by additive manufacturing. These are only examples of liner construction and shall not be construed as limiting. The function of the liner 3 is mainly to fluidly seal the volume of the chamber 7 from the atmosphere, i.e. it defines a fluid-tight volume. The liner 3, due to its low thickness t1, is by itself not meant to bear the load generated by the high pressures within the chamber 7. Instead, the liner 3 is closely surrounded by the composite shell 5 of comparatively larger thickness t2, which bears the vast majority of the mechanical load generated by the pressure inside the chamber 7. The composite shell is best seen on the schematic diagram of figure 2, which illustrates a side view of the tank of figure 1. As it can be seen, the gas tank comprises a cylindrical portion 12 and two rounded portions 14 at the bases thereof. Here the rounded portions 14 have a quasi-hemispherical shape (e.g. elliptical or isotensoid), at least for the outer surface. The overall shape of the gas tank is thus similar to that of a capsule. The composite shell 5 is typically formed by winding / wrapping continuous resin coated filaments around the liner. The filaments may be made of e.g. glass (type E, S or other), carbon, polymeric material (polyamide such as aramid, polyethylene, or other), natural materials etc. In embodiments, the fibers are carbon fibers of the High-Modulus (HM) or High-Resistance (HR) type, with a tensile modulus comprised between 200 and 600 GPa and a tensile strength between 3000 and 7500 MPA. Filaments are typically arranged parallel to each other as a continuous, untwisted bundle / tape. The resin forms the matrix of the composite and may be typically made of polymeric material, in particular thermosetting polymer such as e.g. epoxy or polyester. As previously mentioned, the mechanical strength of the composite shell 5 largely depends on the configuration of its windings, in particular on the winding angle (i.e. the angle between the longitudinal axis of the gas tank and the projection of the filaments onto the longitudinal plane) of each winding. A winding is generally defined as a single turn of a wound material (here the filament tape) around an object (here the liner). According to typical prior art designs, the shell includes three types of windings (shown on figure 2): polar windings 18 with winding angle ap and width wp, circumferential windings 20 with winding angle ac and width wc, and helical windings 22 with winding angle oh and width wh. For the sake of clarity, only a single winding of each type is shown on figure 2, but composite shells normally comprise a multitude of such windings. Polar windings 18 conventionally follow a geodesic path having a single winding angle ap around the liner. In other words, the winding angle ap of polar windings is typically constant along their length. Generally, in conventional processes, the winding process follows a sequence, whereby a set of polar windings is wound around the liner, followed by a set of helicoidal windings which maintain the polar windings in place, itself followed by a set of radial windings. This sequence is then repeated until the desired number of filaments has been wound around the liner, at which point the resin is cured. Currently available gas tanks are typically made using a single type of filament tape of constant width. As such, the width of each winding is equal, i.e. wp = wc = wh. This approach is certainly attractive as it reduces the complexity of the gas tank manufacturing process, and increases its manufacturing speed. Indeed, using larger tape / winding width requires fewer windings to be performed, thereby saving time. As represented in the drawings, the width (e.g. wp) of a winding designates the transversal dimension perpendicular to the length axis of the individual winding / tape. <lnvention> The inventive gas tank 10 is show on Fig.3, comprising a composite shell 5 with polar and circumferential windings 18, 20. The liner design is similar to that described in reference to Figs.1 and 2, comprising a cylindrical portion 12 closed by two rounded portions 14. As shown on figure 3, in the design according to the invention, polar windings18 follow a non-geodesic path, having a first winding angle api on the cylindrical portion 12, and a second winding angle ap2 strictly on the rounded portions 14. Hence, the path is non-geodesic over the entire surface of the gas tank, but it exhibits geodesic behavior over the rounded portions 14 and the cylindrical section 12 of the gas tank when considered separately. The second winding angle ap2 is selected to match the geometry of the rounded portions of the gas tank and of the boss 16, and prevent / limit fiber slipping. The first winding angle ocpi is smaller than the second winding angle ap2 resulting in greater mechanical strength in the longitudinal direction. In other words, a given polar winding 18 (i.e. a single turn of the filament tape around the liner) is laid with angle ap2 on the rounded portions 14 and with angle api on the cylindrical portion 12. In this context, the inventors have found that by using a comparatively narrower tape / winding width for polar windings 18, it is possible to further decrease the first winding angle api, thereby improving the mechanical strength of the composite shell whilst reducing its overall weight. Indeed, the inventors have found that when a polar winding follows a non-geodesic path, a crease is likely to form around the location the winding angle changes (represented by area 24 on figure 3). Said crease defines a line along which stress is concentrated, thereby lowering the overall mechanical strength of the composite shell. However, the inventors have found that, by decreasing the width of the winding in combination with its winding angle, it is possible to orient the crease such that its effects become negligible. In particular, the inventors have found that using polar windings with the prescribed winding angles and a width (wp) comprised between 2 and 10 mm, it is possible to achieve a given mechanical strength with a composite shell thickness reduction of 30% when compared to the traditional winding configuration of figure 2. Figure 4 is a flowchart representing an embodiment of the method for manufacturing a gas tank according to the invention. In an initial step, a thin liner defining a chamber for pressurized hydrogen is provided. A plurality of polar windings 18 is then performed by winding a continuous length of filament tape around said liner, thereby forming a single set of polar windings, each polar winding having two different winding angles api and ap2 as specified above and a width comprised between 2 and 10 mm. Subsequently, a plurality of circumferential windings 20 is performed by winding a continuous length of filament tape around said single set of polar windings, thereby forming a single set of circumferential windings, each circumferential winding having a winding angle comprised between 80 and 90° and a width comprised between 50 and 70 mm. Once all windings performed, the resin is cured. Advantageously, the polar windings 18 may be performed by providing a filament tape pre-impregnated with resin, pre-heating said resin, and subsequently winding said filament tape around the liner. Pre-heating the resin increases its viscosity, which in turn increases the friction coefficient between the fibers. Increases in friction coefficient are particularly relevant in the context of the invention due to the low winding angle of the polar windings, which may otherwise be prone to slipping, in particular on the rounded ends. The resin may e.g. be pre-heated by a laser head arranged below the filament tape. The filament tape is subsequently wound around the liner such that its pre-heated surface faces the liner. The step of preimpregnating the filament tape with resin may be conventionally performed using known technologies such as TowPreg and will not be further detailed here. Hence, towpreg-like filaments may advantageously be used for the polar windings due to their improved adherence to the liner (or underlying layers); but the towpreg-like filaments can also be used for the circumferential windings, if desirable. The skilled person will adapt the towpreg process (resin composition / heating temperature) to obtain the desired stickiness (adhesion / friction) for a given application (i.e. in function of liner geometry &winding angles). As previously mentioned, traditional composite shell manufacturing typically repeats an ordered sequence of polar, helicoidal, and radial windings. Such methods require to reorient the filament tape about the liner each time a different winding pattern is performed. In contrast, the inventive method provides a manufacturing process for a gas tank, whereby all polar windings are consecutively performed before all circumferential windings, and whereby no helicoidal winding is performed. The inventive method therefore requires far fewer reorientation of the filament tape about the liner, thereby reducing manufacturing time. Furthermore, as the polar windings and the circumferential windings are performed using different filament tapes (of width comprised between 2 and 5 mm and between 50 and 70 mm, respectively), performing all polar windings consecutively before all circumferential windings further streamlines the manufacturing process by requiring a single change of filament tape. <Exemplary application> A conventional type III or type IV tank with a nominal working pressure of 700 bar may have a liner with an outer diameter of 121.5 mm. The tank may have a length of 1.8 m and external diameter of 148.5 mm (with composite). The end domes have an elliptical shape from inner diameter 44.5 mm and length of 60 mm. Its composite 5 shell would conventionally require a thickness of about 13.5 mm to prevent leaks. However, for an identical liner, using the winding configuration according to the invention enables a reduction of thickness for the composite shell of about 30%, drastically reducing its weight.

Claims

1. A gas tank for storage of pressurized fuel, comprising a liner (3) defining a chamber for containing pressurized gas and a composite shell (5) surrounding said liner;wherein the liner has a cylindrical portion (14) and two rounded portions at bases thereof;wherein the composite shell is formed by a plurality of resin-coated filament windings, the plurality of filament windings including circumferential windings (20) having a winding angle comprised between 80 and 90°;characterized in that the plurality of filament windings further includes nongeodesic polar windings (18) having a winding angle (api) comprised between 2 and 10° on the cylindrical portion (12), and a winding angle (ap?) strictly comprised between 10 and 30° on at least one of the rounded portions (14).

2. The gas tank according to claim 1, wherein the plurality of filament windings further includes non-geodesic polar windings having a winding angle comprised between 2 and 10° on the cylindrical portion, and a winding angle strictly comprised between 10 and 30° on both rounded portions.

3. The gas tank according to any of the preceding claims, wherein the polar windings (18) have a width (wp) comprised between 2 and 10 mm, preferably between 2 and 5 mm.

4. The gas tank according to any of the preceding claims, wherein the circumferential windings (20) have a width (wc) comprised between 35 and 85 mm, preferably between 50 and 70 mm.

5. The gas tank according to any of the preceding claims, wherein the composite shell is devoid of filament windings having a winding angle comprised between 30 and 79°.

6. The gas tank according to any of the preceding claims, wherein the polar windings (18) define a single set of polar windings and the circumferentialwindings define a single set of circumferential windings, and wherein each winding of the set of circumferential windings (20) radially surrounds all windings of the set of the polar windings.

7. The gas tank according to any of the preceding claims, wherein the composite shell is formed, at least for the polar windings, from a filament bundle pre-impregnated with resin, by pre-heating said resin and subsequently winding said filament bundle around the liner.

8. The gas tank according to any of the preceding claims, further comprising at least one boss (16) configured to define a gas passage through the liner and the composite shell, said boss being arranged on a rounded portion, preferably an apex thereof.

9. The gas tank according to the preceding claim, whereinthe polar windings have a width comprised between 2 and 5 mm and the cylindrical portion of the liner has an outer diameter comprised between 150 and 300 mm; orthe polar windings have a width comprised between 5 and 10 mm and the cylindrical portion of the liner has an outer diameter comprised between 300 and 500 mm.

10. Method for manufacturing a gas tank according to any of the preceding claims, comprising the ordered steps of:- providing a liner defining a chamber, said liner having a cylindrical portion and two rounded portions at bases thereof;- forming a composite shell by:o applying a plurality of non-geodesic polar windings having a winding angle comprised between 2 and 10° on the cylindrical portion, and a winding angle strictly comprised between 10 and 30° on at least one of the rounded portions ;o applying a plurality of circumferential windings around the liner, said circumferential windings having a winding angle comprised between 80 and 90° and a width comprised between 35 and 85 mm, preferably between 50 and 70 mm;- curing the composite shell.

11. Method for manufacturing a gas tank according to claim 10, wherein the step of applying a plurality of polar windings around the liner is achieved by providing a filament bundle pre-impregnated with resin, pre-heating said resin, and subsequently winding said filament bundle around the liner.

12. Method for manufacturing a gas tank according to claim 11, wherein the step of pre-heating the resin involves irradiating a surface of the pre-impregnated filament bundle with a laser.

13. Method for manufacturing a gas tank according to claim 13, wherein the filament bundle is wound around the liner such that the surface of the pre-impregnated filament bundle irradiated by the laser faces the liner.

14. Method for manufacturing a gas tank according to any of claims 10 to 13, wherein the polar windings have a width comprised between 2 and 10 mm, preferably 2 and 5 mm.

15. Method for manufacturing a gas tank according to any of claims 10 to 14, wherein the plurality of polar windings are applied as a single set of polar windings around the liner; and the plurality of circumferential winding are applied as a single set of circumferential windings around the set of polar windings; andpreferably wherein the composite shell includes no filament windings having a winding angle comprised between 30 and 79°.

Images