Patent Description:
Hydrogen is used in a large number of industrial applications. The largest consumers of hydrogen are companies that synthesize ammonia, oil refineries and methanol production plants. The rest is divided between the pharmaceutical, food and electronics industries.

Among the new applications of hydrogen are fuel cells and its direct use as fuel in the transport sector. The use of hydrogen as a fuel has various positive consequences for the environment:.

All these advantages make hydrogen an efficient, clean, and safe source of energy. However, its storage faces some drawbacks that require a specific approach for its use, for example in aviation, where it is of vital importance:.

Known cryogenic tanks for storing H<NUM> in liquid form (LH<NUM>) all respond to the same fundamental architecture with the following components:.

This type of tank with two tanks and a vacuum in the space between them (<FIG>) suffers from weight and volume problems (especially significant in air transport). In addition, the internal tank is subjected to pressure variations that induce tensions and deformations that cause its deterioration due to the fatigue of the material that produces the appearance of micro-cracks that in turn give rise to a loss of impermeability to hydrogen, causing the loss of the high vacuum condition necessary for the effectiveness of thermal insulation.

Therefore, it is convenient to find a solution that allows reducing the weight, volume and tensions that act on the tanks. In this sense, tanks for natural gas have been developed in which the high vacuum insulation is replaced by insulations immersed in gas coming from the fluid itself, controlling the pressure difference by means of a valve or a set of valves that limit said pressure difference. An example of these solutions is shown in patent <CIT>, which has the following drawbacks:.

The present invention solves the problems outlined above by means of a tank for the storage of liquid hydrogen provided with an inner cryogenic tank and an outer tank. A thermal insulation between both tanks allows the passage of gaseous hydrogen and a regulation of the pressure difference between the inner tank and the space between tanks by means of an evaporator without moving parts located inside the inner tank. The function of this evaporator is to ensure that, in the communication between the inner tank and the space between tanks, the flow of H<NUM> towards the latter always occurs in a gaseous state. Insulation may be permeable to hydrogen, or a small upper opening may be provided therein to allow the passage of hydrogen between the faces of the insulation facing the inner surface of the outer tank and the outermost surface of the inner tank in order to maintain a single H<NUM> pressure in the entire space between tanks.

Thanks to the maintenance of the differential pressure at predetermined or null levels, depending on how the communication between the interior tank and the space between tanks is configured through the evaporator, a weight reduction of the assembly is possible, since the exterior tank works with internal pressure of a maximum of <NUM> BAR (this pressure may be lower depending on how the safety discharge valve is configured) instead of being subjected to crushing by outside pressure as in the state of the art with high vacuum insulation. The external tank, therefore, can be a simple membrane that will work under an internal pressure that is always greater than the ambient pressure and that will be, depending on the solution applied, at temperatures similar to the ambient one or at a minimum design temperature chosen that is compatible with the outer tank material. The inner tank at a ~<NUM> cryogenic temperature is not subjected to relevant pressure cycling or loading, nor is it subject to the vapor pressure against the vacuum of the space between tanks of the state of the art. All this is achieved without depending on the reliability of any mechanical or electronic system.

In this way, the durability and reliability of the tank is improved, since it does not use devices that require maintenance or are subject to the risk of mechanical failure, and light and flexible composite materials can be used for the external tank and simply sealed solutions, without the need for high levels of impermeability or resistance in the inner tank. The need for devices outside the tank that may worsen thermal insulation is also eliminated.

Optional aspects or alternative implementations of the invention are set forth in the dependent claims.

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:.

<FIG> 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 <NUM> impermeable to hydrogen and which is the one that contains the liquefied gas at a temperature between <NUM> and <NUM> Kelvin, an outer tank <NUM> that allows maintaining a high vacuum atmosphere with a pressure (Pi)) in the space between tanks of the order of less than <NUM> milli-Torrance and therefore subjected to external pressure Po. In the space between tanks, a multi-layer reflective material <NUM> 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 <NUM> which, in addition to supporting the loads, must minimize the transmission of heat by conduction through them. The tank has H<NUM> refueling and extraction systems <NUM> and venting to the outside <NUM>.

<FIG> shows the tank with pressure regulated by an evaporator <NUM> according to the invention. The evaporator <NUM> in this implementation has the form of a perforated tube that also acts as an anti-splash element: it prevents the LH<NUM> 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<NUM> that wets it by splashing. The tank is made up of an outer tank <NUM>, an inner tank <NUM> held within the outer tank <NUM> by supports <NUM>. The inner tank contains liquid hydrogen (LH<NUM>). 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<NUM> in the inner tank or at a pressure slightly lower than that of the inner tank. The H<NUM> gas coming from the evaporator in the upper zone of the tank always fulfills this condition. However, this gas, H<NUM>, 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<NUM> gas is used to buffer the temperature up to <NUM>-<NUM> 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 <NUM> (of open-cell, fibrous or aerogel type foam) that is embedded in the H<NUM> gas atmosphere of the space between tanks and one or more outer insulation layers <NUM> that are outside the space between tanks and surrounding the external tank <NUM> on the outside, consisting of closed-cell foams made with gasified plastic materials with elements having low thermal conductivity such as N<NUM>, Kr or Xe. The closed cell insulation layers have venting holes <NUM> in order to allow hydrogen pressure to balance around the outer insulations <NUM>. The tank also comprises a safety discharge valve <NUM> and a port for filling and extracting hydrogen <NUM>. In detail <NUM> of <FIG> you can see the perforations <NUM> of the evaporator <NUM> that allow the passage of hydrogen from the tank to the interior of the evaporator <NUM>. The holes are small enough for the H<NUM> gas to pass without problems and prevent splashes and waves of liquid H<NUM> from reaching the tube <NUM> that connects the interior of the evaporator with the space between tanks.

In the implementation of <FIG>, the evaporator <NUM> is composed of a heat exchanger <NUM>, a drainage tube <NUM> and a connecting tube <NUM> connecting the exchanger with the inner insulation layer <NUM>. The evaporator connects through its ends the interior parts of the inner tank <NUM> and the outer tank <NUM>. The static pressure of the inner tank PH acts on the liquid H<NUM> in the drainage tube <NUM>. When the liquid hydrogen in the inner tank <NUM> is heated, the vapor pressure of the same PH increases and overcomes the weight of the H<NUM> in the drainage tube, causing the flow of LH<NUM> to the evaporator <NUM> and of H<NUM> gas to the space between the tanks (<FIG>).

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 <NUM>, the liquid in the tube will move, allowing cooled gas to enter the inner tank and rebalancing the pressure (<FIG>).

Referring to <FIG>, the tank of the invention has an inner tank <NUM>, an outer tank <NUM>, an inner insulation <NUM> and an outer insulation <NUM> that allow hydrogen gas to pass between both sides of the insulation through holes in the upper part <NUM> in order to maintain the same pressure throughout the volume of H<NUM> contained between the two tanks. The evaporator <NUM> 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<NUM> is the outer tank, which does not need to be under cryogenic conditions. The temperature TH of the LH<NUM> inside the cryogenic inner tank <NUM> is maintained between <NUM> and <NUM>, making it possible to use the temperature increase of all the H<NUM> in the inner tank <NUM> as a heat absorbing medium that enters the inner tank <NUM>, unlike of tanks with high vacuum insulation, in which the temperature is necessarily very close to <NUM> and all the heat must be absorbed through the evaporation of H<NUM> that is expelled outside the tank to avoid pressurization cycles under cryogenic conditions. The vapor pressure PH of the liquid hydrogen stored in the cryogenic inner tank <NUM> shall be between <NUM> BAR and <NUM> Bar. The evaporator <NUM> limits at all times the pressure difference between the inner tank <NUM> 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<NUM> flow is from the inner tank <NUM> 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 <NUM> (only in cases of rapid rise of outside temperature).

In the implementation of <FIG>, the drainage tube <NUM> starts from the bottom of the inner tank <NUM>. The pressure difference corresponding to the height H of the hydrogen column shall vary as the inner tank <NUM> of LH<NUM> is emptied. Optionally, the insulation layer <NUM> is made of open-cell, fibrous or aerogel type foam, embedded in the H<NUM> gas atmosphere of the space between tanks, a layer or layers of outer insulation <NUM> of closed-cell foam gasified with inert gases with low thermal conductivity such as N2, Kr or Xe that have venting holes <NUM> in order to allow the balance of hydrogen pressure around the outer insulation <NUM>, a safety discharge valve <NUM> and a port for the filling and extracting hydrogen <NUM>.

<FIG> 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 <NUM>. To ensure that the liquid contained in the drainage tube <NUM> 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 <NUM> below the intake of the connecting tube <NUM>, which joins the heat exchanger <NUM> with the space between tanks, is greater than the volume (Vt) of LH<NUM> contained in the drainage tube <NUM>, so that it remains contained in the bottom of the heat exchanger <NUM> without reaching the mouth of the connecting tube <NUM>.

<FIG> shows the operation of the tank pressure self-regulation faced to a rapid increase in ambient temperature that expands the H<NUM> between tanks, which pushes the hotter H<NUM> gas to the heat exchanger <NUM>, moving the H<NUM> liquid through the drainage tube. If the pressure of the space between tanks exceeds the pressure of the inner tank <NUM> by a value greater than the height H of the LH<NUM> column, then the H<NUM> gas would exit through the drainage tube <NUM> towards the inner tank <NUM>. In this way, the pressure of the space between tanks shall never exceed the pressure of the inner tank <NUM> by a value greater than that of said LH<NUM> column of value H.

<FIG> shows the operation of the self-regulation of the pressure system of <FIG> during a stable situation in which the LH<NUM> absorbs heat and progressively increases its temperature. In this situation, the heat flow is practically constant, producing a slow evaporation of the LH<NUM> stored in the inner tank <NUM> and increasing the pressure of the gas inside the inner tank <NUM> until it exceeds that of the space between tanks by the value corresponding to the height H of the LH<NUM> column which will move the cooler liquid from the bottom of the inner tank <NUM> through the drainage tube <NUM> to the heat exchanger <NUM>, wherein it shall absorb heat from the hotter gas from the top of the inner tank <NUM> attenuating the pressure exerted on the LH<NUM> and evaporating the liquid that has entered the heat exchanger <NUM> slightly increasing the pressure in the space between tanks.

<FIG> shows a tank similar to the one in <FIG> with the following particularities:.

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<NUM> inside the foam are avoided. In exchange for these advantages, the structural outer tank, responsible for supporting the vapor pressure of the LH<NUM>, shall be at temperatures below ambient temperature which will depend on the thickness of the open cell inner insulation <NUM> within the space between tanks.

The solution of <FIG> allows the adaptation of the tank to store H<NUM> in the form of cryogenic gas simply by increasing the resistance of the external tank <NUM> to the typical values of this storage format (between <NUM> and <NUM> BAR).

<FIG> schematically shows a solution for integrating the tank of <FIG> into the fuselage of an aircraft, in such a way that the external tank <NUM> is formed by the fuselage skin <NUM> and two pressure bulkheads <NUM> and <NUM>. In the tank a pump <NUM> has been added for the extraction of liquid hydrogen towards the aircraft systems, two possible units (<NUM> and <NUM>) fed by LH<NUM>, one <NUM> located inside the H2 atmosphere of the external tank <NUM> and another <NUM> located outside the tank and a regulating valve <NUM> for the supply of H<NUM> gas to equipment <NUM> outside the tank. The inner tank <NUM> is equipped with two (depending on the length of the tank the number of inner tank bulkheads may vary) separation bulkheads <NUM> which limit the formation of waves and excessive movements of the center of gravity. The bulkheads have flapper valves <NUM> to allow the passage of LH<NUM> to the evaporator compartment <NUM> and vent tubes <NUM> to keep the pressure balanced between the three compartments of the inner tank.

Claim 1:
Cryogenic tank for storing liquefied H<NUM> with regulated pressure between the inner and outer tank without mobile elements, comprising an outer tank (<NUM>), an inner tank (<NUM>) held inside the outer tank (<NUM>) by means of supports (<NUM>), wherein the inner tank contains liquefied H<NUM> and the space between tanks contains H<NUM> in gas form, characterized in that it comprises an open-cell inner insulation layer (<NUM>) in the space between tanks and in contact with the surface of the inner tank, wherein said inner insulation layer (<NUM>) is embedded in the H<NUM> gas atmosphere of the space between tanks, at least one outer insulation layer (<NUM>) in contact with the outer tank (<NUM>) comprising closed-cell foams gasified with inert gases of low thermal conductivity and an evaporator (<NUM>) inside the inner tank to ensure the exchange of H<NUM> in gaseous state with the space between tanks.