Abstract
Cryogenic tank for storing liquefied H.sub.2 under regulated pressure, comprising an external tank (12), an internal tank (18) wherein the internal tank contains liquefied H.sub.2 and the space between tanks contains H.sub.2 in gas form, an open cell internal insulation (13) in the space between tanks and in contact with the surface of the inner tank, wherein said inner insulation layer (13) is embedded in the H.sub.2 gas atmosphere of the space between tanks, at least one outer insulation layer (14) in contact with the inside of the external tank (12) that includes closed-cell foams gasified with inert gases of low conductivity and that also includes an evaporator (16) that ensures that the passage of H.sub.2 to the space between tanks to maintain the balance of pressure is performed in gaseous state.
Claims
1. A cryogenic tank for storing liquefied H.sub.2 with regulated pressure between an inner and an outer tank without mobile elements, comprising: the outer tank; the inner tank held inside the outer tank by supports, wherein the inner tank contains liquefied H.sub.2 and a space between the tanks contains H.sub.2 in gas form; an open-cell inner insulation layer in the space between the tanks and in contact with a surface of the inner tank, wherein said inner insulation layer is embedded in the H.sub.2 gas atmosphere of the space between the tanks; at least one outer insulation layer in contact with the outer tank comprising closed-cell foams gasified with inert gases of low thermal conductivity; and an evaporator (16) inside the inner tank to ensure the exchange of H.sub.2 in gaseous state with the space between the tanks.
2. A cryogenic tank according to claim 1, wherein the evaporator is in the form of a perforated tube that extends to a bottom of the inner tank.
3. A cryogenic tank according to claim 1, wherein the evaporator comprises a drainage tube that extends to a bottom of the inner tank, a heat exchanger at an opposite end, and a connecting tube connecting the heat exchanger with the inner insulation (13).
4. A cryogenic tank according to claim 1, wherein the outer and inner tanks and insulations are concentric.
5. A cryogenic tank according to claim 1, wherein the outer insulation is in contact with an outermost surface of the outer tank.
6. A cryogenic tank according to claim 5, wherein the outer tank forms part of a fuselage of an aircraft.
7. A cryogenic tank according to claim 2, wherein the outer and inner tanks and insulations are concentric.
8. A cryogenic tank according to claim 3, wherein the outer and inner tanks and insulations are concentric.
Description
BRIEF DESCRIPTION OF THE FIGURES
[0029] In order to help a better understanding of the features of the invention and to complement this description, the following figures are attached as an integral part thereof, whose nature is illustrative and not limiting:
[0030] FIG. 1 shows a tank according to the state of the art.
[0031] FIG. 2 is a first implementation of the invention with an evaporator 16 consisting of a perforated tube.
[0032] FIG. 3A shows another implementation where the evaporator consists of a heat exchanger 21, a drainage tube 19 to the bottom of the inner tank and a connecting tube 22 that connects the exchanger to the space between tanks.
[0033] FIG. 3B illustrates the case in which the invention of FIG. 3A is in use in an inverted flight situation.
[0034] FIG. 4 is a schematic of the pressures exerted by the gas and the liquid when the temperature increases.
[0035] FIG. 5 is another diagram of the pressures when the temperature is stable.
[0036] FIG. 6 shows an implementation with the outer insulation in a position different from that of the two previous implementations.
[0037] FIG. 7 shows an implementation with different insulations and compartments with various functions, wherein the tanks are not concentric.
DETAILED DESCRIPTION
[0038] FIG. 1 shows a tank according to the state of the prior art based on a high vacuum insulation or Dewar vessel. The standard tank consists of an inner tank 3 impermeable to hydrogen and which is the one that contains the liquefied gas at a temperature between 20 and 33 Kelvin, an outer tank 1 that allows maintaining a high vacuum atmosphere with a pressure (P.sub.i)) in the space between tanks of the order of less than 100 milli-Torrance and therefore subjected to external pressure P.sub.o. In the space between tanks, a multi-layer reflective material 2 is applied in order to prevent heat transmission by radiation between the outer tank and the inner tank. The inner tank is suspended inside the outer tank by supports 4 which, in addition to supporting the loads, must minimize the transmission of heat by conduction through them. The tank has H.sub.2 refueling and extraction systems 5 and venting to the outside 6.
[0039] FIG. 2 shows the tank with pressure regulated by an evaporator 16 according to the invention. The evaporator 16 in this implementation has the form of a perforated tube that also acts as an anti-splash element: it prevents the LH.sub.2 from passing into the space between tanks due to agitation, while favoring the cooling of the gas phase inside the interior tank by absorbing its heat and transferring it to the liquid phase or using it to evaporate the LH.sub.2 that wets it by splashing. The tank is made up of an outer tank 12, an inner tank 18 held within the outer tank 12 by supports 15. The inner tank contains liquid hydrogen (LH.sub.2). Due to the extremely low temperatures of the inner tank, in the layers next to it, there are only two elements that can be found in a gaseous state: He and H2 that is at a temperature slightly higher than that of the LH.sub.2 in the inner tank or at a pressure slightly lower than that of the inner tank. The H.sub.2 gas coming from the evaporator in the upper zone of the tank always fulfills this condition. However, this gas, H.sub.2, is not a good insulation, so it is preferred to replace it with other gases with less heat transmission as soon as the temperature allows it; the insulation layer in H.sub.2 gas is used to buffer the temperature up to 78-93K from which it is possible to use inert gases that are less heat conductors as gasifying agents for closed cell foam insulation. For this reason, the tank insulation comprises a permeable inner insulation layer 13 (of open-cell, fibrous or aerogel type foam) that is embedded in the H.sub.2 gas atmosphere of the space between tanks and one or more outer insulation layers 14 that are outside the space between tanks and surrounding the external tank 12 on the outside, consisting of closed-cell foams made with gasified plastic materials with elements having low thermal conductivity such as N.sub.2, Kr or Xe. The closed cell insulation layers have venting holes 17 in order to allow hydrogen pressure to balance around the outer insulations 14. The tank also comprises a safety discharge valve 6 and a port for filling and extracting hydrogen 5. In detail 1 of FIG. 2 you can see the perforations 20 of the evaporator 16 that allow the passage of hydrogen from the tank to the interior of the evaporator 16. The holes are small enough for the H.sub.2 gas to pass without problems and prevent splashes and waves of liquid H.sub.2 from reaching the tube 22 that connects the interior of the evaporator with the space between tanks.
[0040] In the implementation of FIG. 3A, the evaporator 16 is composed of a heat exchanger 21, a drainage tube 19 and a connecting tube 22 connecting the exchanger with the inner insulation layer 13. The evaporator connects through its ends the interior parts of the inner tank 18 and the outer tank 12. The static pressure of the inner tank PH acts on the liquid H.sub.2 in the drainage tube 19. When the liquid hydrogen in the inner tank 18 is heated, the vapor pressure of the same PH increases and overcomes the weight of the H.sub.2 in the drainage tube, causing the flow of LH.sub.2 to the evaporator 16 and of H.sub.2 gas to the space between the tanks (FIG. 4). The same happens if hydrogen is extracted from the space between tanks by decreasing the value of Pi. If the gas pressure in the space between the tanks Pi exceeds the pressure Ph plus the pressure of the weight of the hydrogen column inside the drainage tube 19, the liquid in the tube will move, allowing cooled gas to enter the inner tank and rebalancing the pressure (FIG. 5).
[0041] Referring to FIG. 3A, the tank of the invention has an inner tank 18, an outer tank 12, an inner insulation 13 and an outer insulation 14 that allow hydrogen gas to pass between both sides of the insulation through holes in the upper part 17 in order to maintain the same pressure throughout the volume of H.sub.2 contained between the two tanks. The evaporator 16 maintains at all times a limited differential pressure between the space between tanks and the inner tank so that the tank subjected to the expansive pressure associated with the vapor pressure of LH.sub.2 is the outer tank, which does not need to be under cryogenic conditions. The temperature T.sub.H of the LH.sub.2 inside the cryogenic inner tank 18 is maintained between 20K and 33K, making it possible to use the temperature increase of all the H.sub.2 in the inner tank 18 as a heat absorbing medium that enters the inner tank 18, unlike of tanks with high vacuum insulation, in which the temperature is necessarily very close to 20K and all the heat must be absorbed through the evaporation of H.sub.2 that is expelled outside the tank to avoid pressurization cycles under cryogenic conditions. The vapor pressure P.sub.H of the liquid hydrogen stored in the cryogenic inner tank 18 shall be between 1 BAR and 12.8 Bar. The evaporator 16 limits at all times the pressure difference between the inner tank 18 and the space between tanks to a value ΔP dependent on of the tank fill level. The transient outside temperature conditions determine whether the H.sub.2 flow is from the inner tank 18 to the space between tanks (predominant in stable situations or when the outside temperature drops) or from the space between tanks to the inner tank 18 (only in cases of rapid rise of outside temperature).
[0042] In the implementation of FIG. 3A, the drainage tube 19 starts from the bottom of the inner tank 18. The pressure difference corresponding to the height H of the hydrogen column shall vary as the inner tank 18 of LH.sub.2 is emptied. Optionally, the insulation layer 13 is made of open-cell, fibrous or aerogel type foam, embedded in the H.sub.2 gas atmosphere of the space between tanks, a layer or layers of outer insulation 14 of closed-cell foam gasified with inert gases with low thermal conductivity such as N2, Kr or Xe that have venting holes 17 in order to allow the balance of hydrogen pressure around the outer insulation 14, a safety discharge valve 6 and a port for the filling and extracting hydrogen 5.
[0043] FIG. 3B shows the compatibility of this system with the use of the tank in an inverted position for a short period of time, wherein it is seen that the liquid does not pass into the space between tanks through the evaporator 16. To ensure that the liquid contained in the drainage tube 19 at the beginning of the turning does not pass into the space between tanks, it is necessary that the volume (Vc) inside the heat exchanger 21 below the intake of the connecting tube 22, which joins the heat exchanger 21 with the space between tanks, is greater than the volume (Vt) of LH.sub.2 contained in the drainage tube 19, so that it remains contained in the bottom of the heat exchanger 21 without reaching the mouth of the connecting tube 22.
[0044] FIG. 4 shows the operation of the tank pressure self-regulation faced to a rapid increase in ambient temperature that expands the H.sub.2 between tanks, which pushes the hotter H.sub.2 gas to the heat exchanger 21, moving the H.sub.2 liquid through the drainage tube. 19. If the pressure of the space between tanks exceeds the pressure of the inner tank 18 by a value greater than the height H of the LH.sub.2 column, then the H.sub.2 gas would exit through the drainage tube 19 towards the inner tank 18. In this way, the pressure of the space between tanks shall never exceed the pressure of the inner tank 18 by a value greater than that of said LH.sub.2 column of value H.
[0045] FIG. 5 shows the operation of the self-regulation of the pressure system of FIG. 3A during a stable situation in which the LH.sub.2 absorbs heat and progressively increases its temperature. In this situation, the heat flow is practically constant, producing a slow evaporation of the LH.sub.2 stored in the inner tank 18 and increasing the pressure of the gas inside the inner tank 18 until it exceeds that of the space between tanks by the value corresponding to the height H of the LH.sub.2 column which will move the cooler liquid from the bottom of the inner tank 18 through the drainage tube 19 to the heat exchanger 21, wherein it shall absorb heat from the hotter gas from the top of the inner tank 18 attenuating the pressure exerted on the LH.sub.2 and evaporating the liquid that has entered the heat exchanger 21 slightly increasing the pressure in the space between tanks.
[0046] FIG. 6 shows a tank similar to the one in FIG. 3A with the following particularities: [0047] The outer insulation layers 14 are located in contact with the surface of the outer tank, but in this case outside it, that is, in contact with the outer surface of the tank 12. [0048] Holes 17 in this solution are not necessary.
[0049] With this solution, the requirements of resisting the pressures of the space between tanks for the external insulation made up of closed-cell foams and of being impermeable to the infiltration of H.sub.2 inside the foam are avoided. In exchange for these advantages, the structural outer tank, responsible for supporting the vapor pressure of the LH.sub.2, shall be at temperatures below ambient temperature which will depend on the thickness of the open cell inner insulation 13 within the space between tanks.
[0050] The solution of FIG. 6 allows the adaptation of the tank to store H.sub.2 in the form of cryogenic gas simply by increasing the resistance of the external tank 12 to the typical values of this storage format (between 250 and 300 BAR).
[0051] FIG. 7 schematically shows a solution for integrating the tank of FIG. 6 into the fuselage of an aircraft, in such a way that the external tank 12 is formed by the fuselage skin 23 and two pressure bulkheads 24 and 25. In the tank a pump 26 has been added for the extraction of liquid hydrogen towards the aircraft systems, two possible units (27 and 28) fed by LH.sub.2, one 27 located inside the H2 atmosphere of the external tank 12 and another 28 located outside the tank and a regulating valve 32 for the supply of H.sub.2 gas to equipment 33 outside the tank. The inner tank 18 is equipped with two (depending on the length of the tank the number of inner tank bulkheads may vary) separation bulkheads 29 which limit the formation of waves and excessive movements of the center of gravity. The bulkheads have flapper valves 31 to allow the passage of LH.sub.2 to the evaporator compartment 16 and vent tubes 30 to keep the pressure balanced between the three compartments of the inner tank.
[0052] In view of this description and figures, the person skilled in the art will understand that the invention has been described according to some preferred embodiments thereof, but that multiple variations can be introduced in said preferred embodiments, without exceeding the object of the invention as claimed.
