Metal hydride hydrogen tank system having frosting start ability

Abstract

The present invention relates to a cold start device for an exothermic hydrogen-consuming device, such as a fuel cell, and a method for operating an exothermic hydrogen-consuming device having a metal hydride storage system. It is an object of the present invention to provide an exothermic hydrogen-consuming device, such as a fuel cell, with an efficient cold start device that is immediately operational and does not require a pressure tank. Furthermore, the cold start device can be used for an unlimited number of start-up procedures. This object is achieved by an apparatus for operating an exothermic hydrogen-consuming device, such as a fuel cell, comprising: an exothermic hydrogen-consuming device; at least one starter tank filled with a metal hydride having an equilibrium pressure for desorption of at least 100 kPa at a temperature of -40°C; and at least one operating tank filled with at least one metal hydride having an equilibrium pressure of <100 kPa at a temperature of <0°C, wherein the starter tank is integrated into the operating tank.

Description

Technical Field

[0001] The present invention relates to a metal hydride hydrogen tank system having a frost-start ability, such as a hydrogen tank system for an exothermic hydrogen-consuming device such as a fuel cell, and a method for operating this type of exothermic hydrogen-consuming device.

Background Art

[0002] In the electrolysis of water, water molecules are separated into hydrogen (H 2 ) and oxygen (O 2 ) by an electric current. In a fuel cell, this process is carried out in the reverse direction. The energy released by the electrochemical combination of hydrogen (H 2 ) and oxygen (O 2 ) to form water is efficiently converted into an electric current.

[0003] The technical implementation of the principle of fuel cells has actually led to various electrolytes and various solutions having an operating temperature between 10°C and 1000°C. Fuel cells are classified into low-temperature, medium-temperature, and high-temperature fuel cells according to their operating temperature (see, for example, Patent Document 1). Low-temperature fuel cells operate at a relatively moderate operating temperature of 60°C to 120°C, and due to the medium temperature, they are particularly suitable for mobile applications such as the operation of automobiles. In automobiles, low-temperature PEM type fuel cells are preferably used and operate at a temperature of 60°C to 90°C.

[0004] The hydrogen required as fuel is usually supplied from pressure tanks because it can be used to provide hydrogen over the entire operating temperature range. However, since these pressure tanks are relatively bulky, the scope of mobile applications such as when driving an automobile is limited due to the limited available storage space. A hydrogen storage unit based on metal hydrides, so-called metal hydride storage systems, which require far less space for the same capacity, are known. However, in principle, heat must be supplied to the metal hydride storage unit to desorb hydrogen, and heat must be dissipated to absorb hydrogen. Depending on the metal hydride, temperatures from -30 °C to 400 °C are required for hydrogen desorption. However, generally, a hydrogen pressure of less than 10 MPa is required, which is sufficient for the operation of a fuel cell and substantially simplifies the structure of these tanks compared to hydrogen pressure tanks.

[0005] Metal hydrides used for hydrogen storage are also classified into different categories depending on the desorption temperature. An overview of current metal hydrides and their properties can be found in Non-Patent Document 1, which is hereby incorporated by reference into this specification. Hydrogen is stored (absorbed) in the metal lattice or released (desorbed) from the hydride according to the following equation. Metal + Hydrogen ⇔ Metal Hydride + Heat The relationship between the pressure, temperature, and concentration of hydrogen in a metal is plotted as a concentration-pressure isotherm (CPI). At a specific temperature, hydrogen dissolves in the metal lattice, thereby increasing the pressure. This process follows Sievert's law until the saturation concentration is reached (α-phase). After that, the concentration increases in the metal without increasing the pressure, and a hydride phase (β-phase) is formed. This plateau region follows both van't Hoff's law and Gibbs' phase rule. At the end of the plateau, the pressure increases quadratically again, and hydrogen dissolves in the hydride phase according to Sievert's law. To compare different hydrides, it is standard practice to construct a van't Hoff diagram from the equilibrium value at the center of the plateau. The slope of the line gives the reaction enthalpy (ΔH abs ) of the hydrogen absorption reaction independent of temperature.

[0006] In the case of medium-temperature hydrides, desorption starts between 100 °C and 200 °C at a normal pressure of 10 kPa. Medium-temperature hydrides are defined by the absolute value of the reaction enthalpy (|ΔH 2 |) of the hydrogen absorption reaction between H 2 and H abs of between 30 kJ / mol and 65 kJ / mol. Generally, these have a hydrogen storage density of approximately 2.5 wt% to 5 wt% based on the base metal. Medium-temperature hydrides include, in particular, alanates such as NaAlH 4 and amides such as LiNH 2 with a hydrogen uptake capacity of up to 4.5 wt% H 2 . The optimum hydrogen uptake temperature for sodium alanate is, for example, approximately 125 °C, and the hydrogen delivery temperature is from 160 °C to 185 °C. Due to their relatively high hydrogen storage capacity and relatively low operating temperature, medium-temperature hydrides are interesting candidates for mobile applications. Medium-temperature hydrides having a hydrogen absorption temperature from 90 °C to 110 °C are described in Patent Document 2.

[0007] In the case of high-temperature hydrides, desorption starts at a normal pressure of 10 kPa above 200 °C. These are defined by the absolute value of the reaction enthalpy of the hydrogen absorption reaction of H 2 exceeding 65 kJ / mol |ΔH abs |. Generally, these have an even higher storage density for approximately 7 wt% to 15 wt% hydrogen based on the base metal. Since these are often produced from light metals (magnesium, aluminum) and / or non-metals (nitrogen, boron), they may be suitable for use in fuel cells and H 2 internal combustion engines due to their high capacity, but the high temperature is an obstacle for use as storage in fuel cells. Therefore, high-temperature hydrides are not currently used in fuel cells and H 2 internal combustion engines.

[0008] Due to the relatively low storage capacity with respect to the weight of hydrogen of less than 2% by weight, low-temperature hydrides having a desorption temperature at an ambient pressure of 10 kPa from -40 °C to less than 100 °C are only used for special cases in mobile applications, specifically prototype forklift trucks and bicycles, and for this purpose can accommodate the low storage capacity. These are defined by the absolute value of the reaction enthalpy |ΔH 2 | of the hydrogen absorption reaction of H abs less than 30 kJ / mol.

[0009] For the general use of metal hydride storage systems, an external heating system is required to heat the metal hydride storage system for its discharge. Generally, the energy required to heat the metal hydride storage system has been supplied by fuel cells or another heat source until today. However, the removal of the energy required for the discharge of the metal hydride storage system is impossible during the start-up of the fuel cell and during the first few minutes of its operation when it cannot yet provide enough heat to warm up the metal hydride storage system. Furthermore, the fuel cells known in the art can only generate current for external use above a specific start-up temperature. In the case of cold start or rapid start, the fuel cell must first be heated to a temperature higher than the start-up temperature. Due to the large heat mass of the fuel cell, this requires a significant amount of heat, especially in the case of conventional combustion engines, when the cold start must also be carried out in a similarly short period.

[0010] Patent Document 3 discloses a hydrogen storage system for a fuel cell vehicle in which both commercially available hydrogen storage alloys and hydrogen storage materials having a high hydrogen density and a low hydrogen delivery temperature are used to increase the volumetric storage density and thus the overall storage volume of hydrogen. The hydrogen storage material is present in outer and inner chambers separated by a metal filter. The reason for using the metal filter is to allow hydrogen to pass through the outer and inner chambers and to prevent the passage of metal powder. For this use of the metal hydride storage system, an external heating system is also used to heat the metal hydride storage system until discharge.

[0011] Patent Document 4 discloses a fuel cell for mobile use having a cold start device including a hydrogen pressure tank and a metal hydride heating device as a hydrogen storage system. Hydrogen from a pressurized gas tank passes over a desorption metal hydride storage system. This is heated while forming the corresponding metal hydride, and in this way the capacity of the fuel cell is improved. However, in the case of the cold start device known from Patent Document 4, the pressure storage system is gradually discharged each time there is a cold and rapid start and is not automatically refilled, so the number of cold start procedures and rapid start procedures is limited by the size of the pressure storage system. In this regard, this system only provides the improved function of a pressure tank-based storage system that can perform cold and rapid starts without this device.

[0012] Non-Patent Document 2 discloses a combination of complex hydride (CxH) and room temperature hydride (MeH) in a tank, and utilizes the high filling capacity of the complex hydride and the high reaction rate of the room temperature hydride to improve cold start characteristics. In the case of cold start, the room temperature hydride is filled with hydrogen and heated by a thermal reaction. As a result, the complex hydride is also heated, which then slowly returns to the operating temperature and delivers hydrogen, thus supplying hydrogen to the fuel cell. Due to this reaction cascade between the room temperature hydride and the complex hydride and the slow heating of the complex hydride, the system is relatively slow. Furthermore, this publication does not describe how the room temperature hydride is filled.

[0013] Non-Patent Document 3 considers filling and discharging a closed system formed from LaNi 4.85 Al 0.15 as an exothermic metal hydride and HYDRALLOY C5 (registered trademark) (Ti 0.95 Zr 0.05 Mn 1.46 V 0.45 Fe 0.09 ) as a hydride delivery metal hydride below 0°C.

[0014] All of the aforementioned systems are based on the fact that the fuel cell is supplied by a metal hydride that is heated by another source before it starts operating. The fuel cell can only operate after heating the metal hydride. The object of the present invention is to provide an efficient cold start device for exothermic hydrogen-consuming devices such as fuel cells that is immediately operable and does not require a disadvantageous pressure tank or an external hydrogen source due to the limited space available, for example, in a passenger vehicle. Furthermore, the cold start device must be available for an unlimited number of start procedures.

Prior Art Documents

Patent Documents

[0015]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Non-Patent Documents

[0016]

Non-Patent Document 1

Non-Patent Document 2

[0017] According to the present invention, this object is achieved by a method of operating an exothermic hydrogen-consuming device such as a fuel cell having the features defined in claim 1. Preferred embodiments of the present invention are defined in the dependent claims. [Modes for Carrying Out the Invention]

[0018] In a first embodiment, the present invention is a device for operating an exothermic hydrogen-consuming device such as a fuel cell, the device comprising an exothermic hydrogen-consuming device, at least one starter tank, and at least one operating tank, wherein at least one starter tank consists of a container that is airtight to the extent of holding pressure against hydrogen filled with a first metal hydride incorporated in the operating tank, the first metal hydride having an equilibrium pressure for desorption of at least 100 kPa of hydrogen at a temperature of -40°C, and at least one operating tank consists of a container that is airtight to the extent of holding pressure against hydrogen filled with a second metal hydride, the second metal hydride having an absolute value of the reaction enthalpy (|ΔH|) of the hydrogen absorption reaction between H 2 , preferably 20 kJ / mol of H 2 and less than 65 kJ / mol of H 2 and less than 65 kJ / mol of Habs |) and has an equilibrium pressure for desorption of hydrogen of less than 100 kPa at a temperature of -40°C. In this regard, the known generally low thermal conductivity of metal hydrides is exploited in the present invention to minimize heat transfer between pressure vessels containing hot or cold hydrides so that the vessels can be kept at separate temperature levels. In addition, since the starter tank is integrated into the operating tank, the space available for mobile applications is used to the maximum extent.

[0019] To simplify the approach, the reaction enthalpy of the hydrogen absorption reaction is calculated by the absolute value of the reaction enthalpy (|ΔH abs The reaction enthalpy of the hydrogen absorption reaction (ΔH abs ) is usually negative, so the absolute value is unsigned. Therefore, for H below 65 kJ / mol 2 The amount of reaction enthalpy (|ΔH abs |) includes all reaction enthalpies expressed by rational numbers from 0 to 65.

[0020] Preferably, the exothermic hydrogen consumer is a fuel cell comprising at least one cathode and at least one anode with an electrolyte therebetween. Most preferably, the exothermic hydrogen consumer is a PEM fuel cell, such as a low temperature PEM fuel cell.

[0021] Preferably, the starter tank is of spherical or cylindrical configuration. In one embodiment of the present invention, the metal hydride of the starter tank is a titanium-chromium-manganese based alloy. Furthermore, the operating tank is preferably divided into two of more modules. One or more modules of the starter tank filled with a first metal hydride having cold starting properties are incorporated into one or more of these modules. At its preferred maximum operating pressure of less than 50 bar (5 MPa), the operating tank can have almost any shape required for practically complete utilization of the available space, so that there is almost no empty space, even if the space is, for example, rectangular. Thus, volumetric efficiency can be maximized in this way.

[0022] A second embodiment of the invention is a method for operating an exothermic hydrogen consuming device, such as a fuel cell, the exothermic hydrogen consuming device comprising a first metal hydride initially supplied with hydrogen from at least one starter tank and having an equilibrium pressure for desorption of at least 100 kPa at a temperature of -40°C, after reaching an operating temperature the fuel cell produces less than 65 kJ / mol H 2 The absolute value of the reaction enthalpy of the hydrogen absorption reaction (|ΔH abs The present invention relates to a method for a hydrogen consuming device, comprising: at least one working tank having a temperature of -40°C, a starter tank, a temperature of -40°C, a pressure of at least one second metal hydride, and an equilibrium pressure for desorption of hydrogen of less than 100 kPa; the starter tank is cooled when the supply from the second working tank to the heat-generating hydrogen consuming device starts; the starter tank is refilled with hydrogen from the working tank, the starter tank being integrated into the working tank and separated therefrom by a wall that is gas-tight to the extent that it holds pressure against hydrogen, so that the first metal hydride is insulated from the environmental heat as soon as the starter tank is filled with hydrogen from the working tank; and as soon as the starter tank is completely refilled, the cooling of the starter tank can be stopped.

[0023] A wall between the starter tank and the operating tank, which is gas-tight to the extent that it holds pressure against hydrogen, is provided so that under cold start conditions the first metal hydride can accumulate a hydrogen pressure of at least 100 kPa that can be supplied to the exothermic hydrogen consumer for cold start without delivering hydrogen pressure to the second metal hydride. Obviously, the outer wall of the operating tank is gas-tight to the extent that it holds pressure to prevent hydrogen from escaping into the environment. Typically, the container of the starter tank and / or the operating tank is manufactured from steel. A container of this type, which is gas-tight to the extent that it holds pressure against hydrogen and is typically manufactured from steel, which can be used as a starter tank and / or an operating tank, is described in DE 3502311 A1, which is incorporated herein by reference.

[0024] As mentioned above, the exothermic hydrogen consuming device, for example a fuel cell such as a PEM fuel cell, for example a low temperature PEM fuel cell, is initially supplied with hydrogen from a starter tank, which preferably comprises at least one metal hydride having an equilibrium pressure for desorption of at least 300 kPa, more preferably at least 1000 kPa, in particular at least 1300 kPa, at a temperature of -40°C, which is particularly advantageous for fuel cells operating in recirculation mode.

[0025] In the method according to the invention, under cold start conditions, an exothermic hydrogen consumer, such as a fuel cell, is supplied with enough hydrogen from the starter tank so that the hydrogen consumer, for example a fuel cell, can start and heat up to its operating temperature. The waste heat from the exothermic hydrogen consumer can then also heat the working tank, as the main hydrogen storage system, to its operating temperature and take over the supply of the hydrogen consumer. When the hydrogen consumer is supplied by the working tank, the starter tank is then refilled with hydrogen from the working tank. In this respect, it is necessary to thermally separate the starter tank from the working tank. In this respect, the invention makes use of the fact that metal hydrides have low thermal conductivity and can act as insulators.

[0026] In contrast to previously known solutions for refilling a starter tank during the operation of an exothermic hydrogen consumer, the described method allows to perform substantially more cold start procedures, which are not limited by the volume of the starter tank and thus is an advantage of the invention, in addition to a simplified structure of the device for operating, for example, a fuel cell. In addition, the solution according to the invention provides a very cheap storage solution for the main hydrogen storage system, without the need to have its own cold start properties. Thus, more efficient metal hydrides and / or cheaper metal hydrides can be used in the operating tank. With the device according to the invention and the method according to the invention, a fast supply of hydrogen for the operation of, for example, a fuel cell, especially at low temperatures, for example for automotive applications, is ensured. This is also the case with pressure tanks, but with a higher volume density and the ability to choose almost any shape of the operating tank due to the lower operating pressure.

[0027] According to a further embodiment of the invention, the starter tank has a shell for the metal hydride contained therein, capable of withstanding hydrogen pressures up to the maximum equilibrium pressure of the hydride at the filling and operating temperatures of the operating tank. Duplex stainless steel is preferred for the shell of the starter tank. The lower the equilibrium pressure at the operating temperature of the operating tank, and consequently the maximum possible temperature in the system, the thinner and lighter the shell of the starter tank can be.

[0028] According to yet another embodiment of the invention, the device for operating a fuel cell comprises a cooling system that cools the starter tank when it is being filled as soon as the operating tank takes over the supply to the hydrogen consuming device. Peltier elements are advantageous here since they are very compact and allow direct cooling of the shell by thermal conductivity or by coupling with a heat exchanger to cool the tank system via internal cooling channels. In reverse mode, they can also function as heating elements. Alternatively, conventional compressor-based cooling may be employed.

[0029] Preferably, the metal hydride of the starter tank is selected such that hydrogen is desorbed under frosting start conditions, such as at temperatures of -40°C or similarly low. If a metal hydride with a higher desorption temperature is selected, heat transfer from the environment may be provided, if necessary, such that hydrogen desorption is still possible, for example at extremely low temperatures, for example by a Peltier element acting as a heating element, by a conventional separate heating system or by a cooling medium for an exothermic hydrogen consuming device.

[0030] It is advantageous that heat transfer is derived from low supply pressure for the exothermic hydrogen consuming device and high fill pressure during operation so that additional heat is not required when the system is already capable of delivering this heat.

[0031] Once the normal operating temperature is reached after the operation of the exothermic hydrogen consumers enabled by the starter tank (approximately 60°C to 80°C for low-temperature PEM fuel cells), the operating tank as well as any other consumers present in the vehicle, such as the in-vehicle heating, can be supplied with thermal energy from the waste heat from the fuel cell. In the case of normal power, a further portion (e.g. less than 5%) of the power from the exothermic hydrogen consumers is used after reaching the operating state to cool the starter tank by means of its cooling system.

[0032] The construction of the operating tank is preferably such that at its own operating temperature, which is not necessarily the same as that of the exothermic hydrogen consumer, it produces a hydrogen pressure higher than the equilibrium pressure of the cold starter tank, in this way allowing the starter tank to be refilled from the operating tank.

[0033] The heat generated when refilling the starter tank is preferably dissipated in order to keep its equilibrium pressure lower than that of the operating tank and allow refilling of the starter tank. As soon as the starter tank is completely filled, cooling can be stopped.

Claims

1. 1. An apparatus for operating an exothermic hydrogen consuming device, the apparatus comprising: the exothermic hydrogen consuming device; at least one starter tank; and at least one operating tank, the at least one starter tank being comprised of a container that is gas-tight to hold a pressure against a hydrogen filled with a first metal hydride incorporated in the operating tank, the first metal hydride having an equilibrium pressure for desorption of hydrogen of at least 100 kPa at a temperature of −40° C.; and the at least one operating tank being comprised of a container that is gas-tight to hold a pressure against a hydrogen filled with a second metal hydride, the second metal hydride having an equilibrium pressure for desorption of hydrogen of less than 65 kJ / mol. ２ The absolute value of the reaction enthalpy of the hydrogen absorption reaction (|ΔH ａｂｓ |) and having an equilibrium pressure for desorption of hydrogen of less than 100 kPa at a temperature of -40°C.

2. The exothermic hydrogen-consuming device is a fuel cell, the device according to claim 1.

3. The exothermic hydrogen-consuming device is a PEM fuel cell, the device according to claim 1.

4. The starter tank is completely accommodated by the operation tank, the device according to any one of claims 1 to 3.

5. The starter tank is characterized by comprising a metal hydride having an equilibrium pressure for desorption of at least 300 kPa at a temperature of -40 °C, the device according to any one of claims 1 to 4.

6. The starter tank is characterized by comprising a metal hydride having an equilibrium pressure for desorption of at least 1000 kPa at a temperature of -40 °C, the device according to claim 5.

7. The starter tank is characterized by comprising a metal hydride having an equilibrium pressure for desorption of at least 1300 kPa at a temperature of -40 °C, the device according to claim 6.

8. The metal hydride of the starter tank is a titanium-chromium-manganese-based alloy, the device according to any one of claims 1 to 7.

9. The second metal hydride of the operation tank has an H of 20 kJ / mol ２ and an H of less than 65 kJ / mol ２ and has an absolute value (|ΔH ａｂｓ |) of the reaction enthalpy of the hydrogen absorption reaction between them, and the device according to any one of claims 1 to 8.

10. The cooling of the starter tank is performed by a Peltier element or by compressor-based cooling, the device according to any one of claims 1 to 8.

11. 1. A method for operating an exothermic hydrogen consumer, the exothermic hydrogen consumer comprising a first metal hydride initially supplied with hydrogen from at least one starter tank and having an equilibrium pressure for desorption of at least 100 kPa at a temperature of −40° C., wherein after reaching an operating temperature, the fuel cell produces less than 65 kJ / mol H ２ The absolute value of the reaction enthalpy of the hydrogen absorption reaction (|ΔH ａｂｓ |), hydrogen is supplied from at least one operating tank comprising at least one second metal hydride having an equilibrium pressure for desorption of hydrogen of less than 100 kPa at a temperature of -40°C, said starter tank is cooled when supply from the operating tank to said exothermic hydrogen consuming device begins, said starter tank is refilled with hydrogen from said operating tank, said starter tank is integrated into said operating tank and separated therefrom by a wall that is gas-tight to hold pressure against hydrogen, so that said first metal hydride is insulated from environmental heat as soon as said starter tank is filled with hydrogen from said operating tank.

12. The exothermic hydrogen-consuming device is a fuel cell, the method according to claim 11.

13. The exothermic hydrogen-consuming device is a PEM fuel cell, the method according to claim 11.

14. The starter tank is characterized by comprising a metal hydride having an equilibrium pressure for desorption of at least 300 kPa at a temperature of -40 °C, the method according to any one of claims 11 to 13.

15. The metal hydride of the starter tank is a titanium-chromium-manganese-based alloy, the method according to any one of claims 11 to 14.

16. The cooling of the first metal hydride storage system is performed by a Peltier element or by compressor-based cooling, the method according to any one of claims 11 to 15.

17. When supplying to the fuel cell by the operation tank, the waste heat from the exothermic hydrogen-consuming device is used to maintain the operation tank at the desorption temperature, the method according to any one of claims 11 to 16.