Combination of H2 storage methods

Abstract

The invention relates to a system (1) for generating electrical energy, comprising a first oxygen storage device (2) for oxygen or for an oxygen-containing gas and a first hydrogen storage device (3) for fluidic hydrogen, a first fuel cell (4) which is connected to the first oxygen storage device (2) via a first oxygen line (5) and to the first hydrogen storage device (3) via a first hydrogen line (6) and has a first water outlet (7), the system (1) further comprising a second hydrogen storage device (8) for hydrogen bound to a hydrogen carrier and releasable by adding water, a second fuel cell (9) which can be supplied with oxygen via a second oxygen line (10), which is further connected to the second hydrogen storage device (8) via a second hydrogen line (11) and has a second water outlet (12), wherein the first and the second water outlet (7,12) are connected to a water inlet (15) of the second hydrogen storage device (8) via first and second water lines (13, 14). The invention further relates to a method for generating electrical energy.

Description

TECHNICAL FIELDThe invention relates to a system for generating electrical energy by means of fuel cells and to a corresponding method.BACKGROUNDIt is known from the prior art that when hydrogen (e.g. from a gaseous or liquid hydrogen storage unit) is supplied to a fuel cell together with the corresponding amount of oxygen, water is also formed in addition to electrical energy and heat.The prior art also discloses hydrogen carriers which require liquid water to release hydrogen (e.g. NaBH4). Once this process is started and the hydrogen is fed into a fuel cell together with the corresponding amount of oxygen, water is also one of the products again in addition to electrical energy and heat. It should be noted in particular that when this hydrogen is combined with oxygen in a fuel cell, the corresponding stoichiometric amount of water is again formed at least theoretically.EP 1 880 439 A1 and U.S. Pat. No. 6,864,002 B1 disclose fuel cell systems in which a hydrogen-containing fuel contained in a fuel container absorbs the by-product water and reacts therewith to generate hydrogen which is supplied to the fuel cell in order to maintain its operation without further water having to be supplied from the outside for the operation of the plant. This leads to a reduction in the weight and volume of the plant and to an internal chemical control of the hydrogen generation for maintaining the power generation and the internal water management.SUMMARY OF THE INVENTIONHowever, there are two problems. To initiate this reaction, firstly liquid water is required as starting material and secondly, since reactions generally do not have a 100% yield, additional amounts of water within the system limits are required, i.e. these additional amounts of water (for example in additional water tanks) had to be introduced into closed systems from the beginning.It is therefore an object of the invention to provide a system for generating electrical energy which solves the problems of the initial water supply and the stoichiometrically incomplete yield of water. It is also an object of the invention to specify a corresponding method.The object directed to the system is achieved by a system for generating electrical energy, comprising a first oxygen storage device for oxygen or for a gas containing oxygen and a first hydrogen storage device for fluidic hydrogen, a first fuel cell which is connected to the first oxygen storage device via a first oxygen line and for supplying hydrogen to the first hydrogen storage device via a first hydrogen line and has a first water outlet, the system further comprising a second hydrogen storage device for hydrogen bound to a hydrogen carrier and which can be released again by adding water, characterized in that the first water outlet is connected via a first water line to a water inlet of the second hydrogen storage device, and in that for supplying hydrogen in the system a second hydrogen line branches off from the second hydrogen storage device. In this case, a fuel cell is typically understood to mean a fuel cell unit or a fuel cell stack.By a combination of different hydrogen storage devices, wherein one hydrogen storage device generates water in the overall balance and another tends to consume water, the capacity can be increased in terms of the energy density (Watt hours per unit volume or Watt hours per unit mass) of a system.In an advantageous embodiment of the invention, the first hydrogen storage device is a compressed gas storage device. By compressing the hydrogen to high pressure (typically 350 to 700 bar), a higher energy density is achieved. This means that more energy can be stored in a smaller volume, which is particularly useful when space is limited or the memory needs to be transported. Furthermore, compressed gas accumulators allow a comparatively efficient storage and removal of hydrogen, since the pressure in the accumulator can be quickly adjusted if necessary. This allows rapid availability of hydrogen for various applications, such as fuel cells in the present case. Furthermore, compressed gas storage devices are generally more cost-effective than other storage technologies such as the storage of liquid hydrogen or metal hydride storage devices. And finally, compressed gas accumulators have a lower environmental impact compared to other accumulator technologies, since they do not require chemical reactions or complicated cooling methods.In an alternative embodiment of the invention, the first hydrogen storage device is a liquefied gas storage device. Hydrogen in liquid form has a higher energy density than gaseous hydrogen at ambient temperature. By converting hydrogen to liquefied gas at extremely low temperatures, more hydrogen can be stored in a given volume. This may be particularly useful when space is limited or the memory needs to be transported. Moreover, liquid hydrogen has a lower risk of leakage than gaseous hydrogen, since it can be stored at lower pressures. This reduces the risk of accidents and explosions compared to compressed gas accumulators.It is advantageous if the system comprises, in addition to the first fuel cell, a second fuel cell which can be supplied with oxygen or with oxygen-containing gas via a second oxygen line, which is furthermore connected via the first hydrogen line to the first hydrogen storage device or via the second hydrogen line to the second hydrogen storage device or to both hydrogen storage devices, and has a second water outlet which is connected via a second water line to the water inlet of the second hydrogen storage device. By having a second fuel cell, the system may continue to generate power in the event of a failure of one of the cells, which increases the reliability and availability of the system. Two fuel cells may also help increase the efficiency of the system as they may be operated in parallel to increase performance or maintain operation of the individual cells at optimal performance. With two fuel cells, the system can also respond more flexibly to changing energy demands by separately controlling and adjusting the performance of each cell. Further, a second fuel cell connected via a second hydrogen and oxygen line and a second water outlet may help optimize thermal and water management of the system. For example, the waste heat of a fuel cell can be used to heat the water in the second hydrogen storage unit and thus to increase the efficiency of the system.It is also important that fuel cells are not permanently assigned to a specific hydrogen storage unit, but can be supplied with hydrogen from both hydrogen storage units. In particular, this then also allows the operation of the fuel cells with hydrogen predominantly from, for example, the second hydrogen storage unit, which can then also be designed to be larger.This has several advantages. Hydrogen as a fluid is readily flammable and can pose a safety risk when pressurized or stored at low temperatures. In the case of a bound hydrogen storage, the hydrogen is bound to a carrier substance, which reduces the risk of inflammation or explosion.The storage density of hydrogen in bound form may be higher than in gaseous or liquid form. This enables a more compact and efficient storage of the hydrogen, which can be advantageous for energy storage and transport.While gaseous hydrogen must be stored under high pressure and liquid hydrogen at very low temperatures, bound hydrogen may in many cases be stored at ambient temperature and pressure. This reduces the requirements for the storage infrastructure and the costs associated therewith.Bound hydrogen may also be stored for extended periods of time without appreciable losses, while gaseous or liquid hydrogen may be more prone to loss due to diffusion or evaporation.In one embodiment of the invention, the second fuel cell is connected to the first oxygen accumulator via the second oxygen line. This has the advantages that, due to the sharing of an oxygen storage for both fuel cells, the system uses resources more efficiently. This may help reduce the size and weight of the system, which may be advantageous particularly in mobile applications or in situations where available space is limited. Further, a common oxygen storage for both fuel cells may help reduce system costs as fewer components are needed. This can affect both the initial costs and the maintenance costs. In addition, the use of a common oxygen storage for both fuel cells simplifies system design, as fewer components and connections are required. This can help reduce sources of error and increase the maintainability of the system.In an alternative embodiment of the invention, the second fuel cell is connected to a second oxygen accumulator via the second oxygen line. When multiple fuel cells need to operate in different operating conditions, the use of separate oxygen stores may allow better control over the individual requirements of each cell. This may help optimize the efficiency and performance of each fuel cell while allowing precise control of oxygen flow in different operating conditions. Separate oxygen storage devices may increase fail-safe and redundancy in critical applications. If an oxygen storage device fails or becomes damaged, this may only affect a single fuel cell, rather than affecting the entire system. In such cases, the failure of a fuel cell may be compensated for by other fuel cells increasing their power to meet the overall energy demand. In some applications, it may be necessary to install fuel cells spatially separated from each other, e.g., due to space or heat constraints. In such cases, the use of separate oxygen storage devices may facilitate installation and operation of the fuel cells by simplifying connections between the cells and their respective oxygen storage devices.It is advantageous if the system comprises a control unit for controlling the mass flows of oxygen or oxygen-containing gas, hydrogen and water in the system as a function of an amount of electrical energy to be generated. This is critical to the efficient operation of the system. A control unit enables precise adjustment of the mass flows of oxygen and hydrogen in the fuel cells to generate the desired amount of electrical energy. This can help maximize the efficiency of the system and optimize the power efficiency. A controller may dynamically adjust the mass flows of oxygen, hydrogen, and water based on the current power requirements and operating conditions. This allows for a quick response to variations in power demand and ensures that the system operates efficiently at all times. The control unit may monitor and control the consumption of hydrogen and oxygen to ensure that the resources are optimally utilized. This can help minimize the consumption of hydrogen and oxygen and reduce the operating costs of the system. By precisely controlling the mass flows, the control unit can contribute to prolonging the life of the fuel cells. Uniform and optimum supply of oxygen and hydrogen to the cells can reduce degradation of the fuel cells and increase their service life. The control unit may also help to increase the safety of the system by continuously monitoring and adjusting the mass flows of oxygen, hydrogen and water to avoid potential hazards such as leaks, overpressure or uncontrolled reactions. A control unit enables the simple integration of fuel cell systems into other electrical systems, such as the power grid or hybrid drives in vehicles. The control unit can adjust the electric power production based on the requirements of these systems, thus ensuring seamless cooperation.It is convenient if the control unit is connected to controllable valves for controlling the mass flows of oxygen or oxygen-containing gas and hydrogen. As a result, the oxygen supply and the hydrogen supply can be adapted flexibly to different operating conditions and power requirements. This allows system performance to be optimized for changing loads or environmental conditions.Furthermore, it is expedient if the control unit is connected to a controllable pump for controlling a mass flow of water. In the case of a hydrogen supply from the second hydrogen storage device, which is a hydrolysis-based hydrogen storage system, i.e. the stored hydrogen is chemically bound to a carrier material and can be released only by addition of water (or water vapor), the desorption of hydrogen is also automatically regulated by the amount of water supplied.A controllable pump for controlling a mass flow of water for desorption of hydrogen is expediently supplemented by upstream water reservoirs. This means specifically that the first water outlet of the first fuel cell opens into a first water reservoir and that the second water outlet of the second fuel cell opens into the first water reservoir or a second water reservoir. These measures can lead to more efficient use of the carrier material in the second hydrogen storage tank, because the carrier material can be supplied uniformly with water by the controlled addition of water from the buffer storage tanks. This can lead to more efficient use of the carrier material and a higher hydrogen yield. Furthermore, a water reservoir makes it possible to adapt the hydrogen production to the respective demand over a much larger range than is possible only with valves in the water lines. This is because the water supply can then not only be reduced when the hydrogen demand is lower, as a result of which the hydrogen release is slowed down, but the water supply can also be increased further when the demand is higher, in order to achieve a more rapid hydrogen production.The object directed to a method is achieved by a method for generating electrical energy in which oxygen- or oxygen-containing gas is combined electrochemically with hydrogen from a first hydrogen storage for fluidic hydrogen, electrical energy and water being formed, characterized in that the water is supplied to a second hydrogen storage for hydrogen bound to a hydrogen carrier and releasable again by addition of water, hydrogen being formed, which is in turn combined electrochemically with oxygen- or oxygen-containing gas and water formed in the process is likewise supplied to the second hydrogen storage.Excess water does not have to be discarded, but can be stored and, if required, supplied to the second hydrogen storage means.It is particularly advantageous if the method is started by electrochemically combining oxygen or oxygen-containing gas with hydrogen from the first hydrogen storage for fluidic hydrogen, wherein heat is also formed in addition to electrical energy and water, with which the second hydrogen storage is heated, and wherein the method is continued by stopping the supply of hydrogen from the first hydrogen storage and continuing with hydrogen from the second hydrogen storage.However, it can also be advantageous not to completely return the hydrogen supply from the first storage unit, but to compensate for missing quantities of water for the release of hydrogen from the second hydrogen storage unit by removing a corresponding quantity of hydrogen from the first hydrogen storage unit and electrochemically combining it with oxygen or with oxygen-containing gas.By combining different hydrogen storage methods according to the invention, the energy density of the overall system can be improved, for example by avoiding or minimizing additional water tanks.In addition, the use of the heat generated in fuel cells with hydrogen from direct hydrogen storage devices can contribute to accelerating the release of hydrogen from indirect hydrogen storage devices.In addition, fuel cell systems with integrated liquid or gaseous hydrogen storage have shorter startup times compared to hydrogen storage methods.BRIEF DESCRIPTION OF THE DRAWINGSFIG. 1 shows a system for generating electrical energy according to the invention, FIG. 2 shows an alternative system for generating electrical energy according to the invention; and Fig. 3 shows another electric power generation system according to the invention.DESCRIPTION OF THE EMBODIMENTSFIG. 1 shows an embodiment of a system 1 for generating electrical energy 22 according to the invention. Furthermore, FIG. 1 indicates that not only electric energy 22, but also water (first water reservoir 20, second water reservoir 21) is generated and that a certain amount of heat 23 is likewise produced.The system 1 comprises a first oxygen storage 2 for oxygen or for an oxygen-containing gas and a first hydrogen storage 3 for fluidic hydrogen. The hydrogen storage device 3 can be designed as a compressed gas storage device or as a liquefied gas storage device. The system 1 further comprises a first fuel cell 4, which is connected to the first oxygen storage 2 via a first oxygen line 5 and to the first hydrogen storage 3 via a first hydrogen line 6 and has a first water outlet 7.The system 1 further comprises a second hydrogen storage unit 8 for hydrogen bound to a hydrogen carrier and re-releasable by addition of water, a second fuel cell 9 which can be supplied with oxygen via a second oxygen line 10, which is furthermore connected to the second hydrogen storage unit 8 via a second hydrogen line 11 and has a second water outlet 12.According to the invention, the first and the second water outlet 7, 12 are connected via first and second water lines 13, 14 to a water inlet 15 of the second hydrogen storage 8.In the embodiment of FIG. 1, the second fuel cell 9 is connected to a second oxygen storage device 16.Efficient operation of the system 1 is ensured by a control unit 17 for controlling the mass flows of oxygen, hydrogen and water as a function of an amount of electrical energy to be generated. For example, the amounts of oxygen to be supplied to the first and second fuel cells 4, 9 are controlled by controllable valves 18 arranged in the first and second oxygen lines 5, 10.Analogously, controllable valves 18 for regulating the hydrogen supply to the fuel cells 4, 9 are arranged in the first and second hydrogen lines 6, 11. In addition, for the regulation of the hydrogen supply to the second fuel cell 9, adjustable power pumps 19 are arranged in the first and second water lines 13, 14 and are connected to the control unit 17.For more flexibility in the operation of the system 1, the first water outlet 7 opens into a first water reservoir 20, and the second water outlet 12 can open into the first water reservoir 20 or, as shown in FIG. 1, into the second water reservoir 21.FIG. 2 shows a greatly simplified variant of the system 1 for generating electrical energy according to the invention compared to the exemplary embodiment of FIG. 1. In this case, the system 1 for generating electrical energy comprises only a first oxygen storage device 2 for oxygen or for an oxygen-containing gas and a first hydrogen storage device 3 for fluidic hydrogen, a first fuel cell 4, which is connected to the first oxygen storage device 2 via a first oxygen line 5 and for supplying hydrogen to the first hydrogen storage device 3 via a first hydrogen line 6 and has a first water outlet 7. According to the invention, the system 1 further comprises a second hydrogen storage unit 8 for hydrogen bound to a hydrogen carrier and releasable again by addition of water, the first water outlet 7 is connected via a first water line 13 to a water inlet 15 of the second hydrogen storage unit 8, and a second hydrogen line 11 branches off from the second hydrogen storage unit 8 and opens into the first fuel cell 4.In the exemplary embodiment of FIG. 2, a controller 17 with the associated controllable valves 18 and pump 19 is also provided for the efficient operation of the system 1.The embodiment of FIG. 3 shows the system 1 for generating electrical energy according to the invention, comprising a first fuel cell 4 and a second fuel cell 9. In addition to the embodiment of FIG. 1, the first fuel cell 4 is not only connected to the first hydrogen storage 3 but can also be supplied with hydrogen from the second hydrogen storage 8, as is already shown in FIG. 2. For reasons of clarity, FIG. 3 does not show that such a hydrogen supply from both hydrogen storage units 3, 8 is of course also expedient and possible for the second fuel cell 9.LIST OF REFERENCE CHARACTERS1 System for generating electrical energy 2 First oxygen storage device 3 First hydrogen storage device (for fluidic hydrogen) 4 First fuel cell 5 First oxygen line 6 First hydrogen line 7 First water outlet 8 Second hydrogen storage device (with hydrogen carrier) 9 Second fuel cell 10 Second oxygen line 11 Second hydrogen line 12 Second water outlet 13 First water line 14 Second water line 15 Water inlet 16 Second oxygen storage device 17 Control unit 18 Controllable valve 19 Controllable pump 20 First water storage device 21 Second water storage device 22 Electrical energy 23 HeatReferences included in the specificationThis list of documents cited by the applicant has been produced in an automated manner and is only included for the better information of the reader. The list is not part of the German patent application or utility model application. The DPMA does not take any adhesion for any faults or omissions.Patent Literature citedEP 1 880 439 A1

[0004] U.S. Pat. No. 6,864,002 B1

[0004]

Claims

System (1) for generating electrical energy, comprising a first oxygen storage unit (2) for oxygen or for an oxygen-containing gas and a first hydrogen storage unit (3) for fluidic hydrogen, a first fuel cell (4) which is connected to the first oxygen storage unit (2) via a first oxygen line (5) and for supplying hydrogen to the first hydrogen storage unit (3) via a first hydrogen line (6) and has a first water outlet (7), the system (1) further comprising a second hydrogen storage unit (8) for hydrogen which is bound to a hydrogen carrier and can be released again by addition of water, characterized in that the first water outlet (7) is connected via a first water line (13) to a water inlet (15) of the second hydrogen storage unit (8), and in that, for supplying hydrogen in the system (1), a second hydrogen line (11) branches off from the second hydrogen storage unit (8).The system (1) according to claim 1, wherein the first hydrogen storage (3) is a compressed gas storage.The system (1) according to claim 1, wherein the first hydrogen storage (3) is a liquefied gas storage.The system (1) according to any one of the preceding claims, further comprising a second fuel cell (9) which is supplyable with oxygen or with oxygen-containing gas via a second oxygen line (10), which is further connected via the first hydrogen line (6) to the first hydrogen storage device (3) or via the second hydrogen line (11) to the second hydrogen storage device (8) or to both hydrogen storage devices (6, 11) and has a second water outlet (12) which is connected via a second water line (14) to the water inlet (15) of the second hydrogen storage device (8).The system (1) according to claim 4, wherein the second fuel cell (9) is connected to the first oxygen storage device (2) via the second oxygen line (10).The system (1) according to claim 4, wherein the second fuel cell (9) is connected to a second oxygen storage device (16) via the second oxygen line (10).The system (1) according to any of the preceding claims, further comprising a control unit (17) for controlling the mass flows of oxygen or oxygen-containing gas, hydrogen and water in the system (1) as a function of an amount of electrical energy to be generated.The system (1) according to claim 7, wherein the control unit (17) is connected to controllable valves (18) for controlling the mass flows of oxygen or oxygen-containing gas and hydrogen.The system (1) according to any of claims 7 or 8, wherein the control unit (17) is connected to a controllable pump (19) for controlling a mass flow of water.The system (1) according to any one of the preceding claims, wherein the first water outlet (7) opens into a first water reservoir (20).The system (1) according to claims 4 and 10, wherein the second water outlet (12) opens into the first water reservoir (20) or a second water reservoir (21).Method for generating electrical energy, in which oxygen or oxygen-containing gas is combined electrochemically with hydrogen from a first hydrogen storage means (3) for fluidic hydrogen, electrical energy and water being formed, characterized in that the water is fed to a second hydrogen storage means (8) for hydrogen bonded to a hydrogen carrier and releasable again by addition of water, hydrogen being formed, which in turn is combined electrochemically with oxygen or oxygen-containing gas and water formed in the process is likewise fed to the second hydrogen storage means (8).The method of claim 12, wherein excess water is stored.The method according to one of claims 12 or 13, wherein the method is started by electrochemically combining oxygen or oxygen-containing gas with hydrogen from the first hydrogen storage (3) for fluidic hydrogen, wherein heat is also generated in addition to electrical energy and water, with which the second hydrogen storage (8) is heated, and wherein the method is continued by stopping the supply of hydrogen from the first hydrogen storage (3) and continuing it with hydrogen from the second hydrogen storage (8).The method according to any one of claims 12 or 13, wherein missing amounts of water for releasing hydrogen from the second hydrogen storage (8) are compensated by taking a corresponding amount of hydrogen from the first hydrogen storage (3) and electrochemically combining it with oxygen or with oxygen-containing gas.

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