Hybrid system for energy storage using batteries and hydrogen, and methods for operating the system

Abstract

Hybrid system (100) for efficient and cost-effective energy storage using batteries and hydrogen, comprising a DC voltage source (140), at least one battery (110), at least one electrolyzer (120) for the electrochemical splitting of water into hydrogen and oxygen, and at least one pressure vessel (160), wherein the electrolyzer (120) is at least temporarily directly connected to the battery (110) via an electrical DC network (112) and a switching element (123), wherein the electrolyzer (120) consists of a stack of a number of individual electrochemical cells, characterized by that the number of individual electrochemical cells corresponds to 0.5 to 0.67 times the nominal voltage of the battery (110), comprising a power switching element (122) with which a reduced number of electrochemical cells, at least temporarily connected directly to the battery (110) via the electrical DC network (112), so that the voltage of the battery (110) is always higher than the voltage of the electrolyzer (120). so that direct operation and control of the electrolyzer (120) can take place without electronic voltage converters and the pressure vessel (160) is filled with hydrogen.

Description

[0001] The invention relates to a hybrid energy storage system comprising an electrolyzer for producing hydrogen by electrochemical splitting of water into hydrogen and oxygen, and a fuel cell stack for power generation, in combination with a battery storage system. The invention further relates to a method for operating the system. Description of the state of the art

[0002] As part of the energy transition, photovoltaic and wind power plants will be increasingly used in the future to provide climate-neutral electricity. Unlike conventional large-scale power plants, however, these plants cannot supply a continuous supply of electricity. In particular, the electricity supply from photovoltaic and wind power plants is subject to pronounced daily and seasonal fluctuations, which must be balanced to ensure an energy supply that meets demand. For this purpose, and to stabilize the grid frequency of electrical power grids, various energy storage systems will be required in the future. The complex control of power grids with renewable energy sources and energy storage systems presents a major technical and economic challenge and will lead to further increases in electricity prices.

[0003] Especially in the area of ​​residential energy systems, battery storage systems are already widely used in combination with photovoltaic systems to achieve a high degree of energy independence and reduce the need to draw expensive electricity from the public grid. During the day, when solar energy is available, the batteries are charged with inexpensive electricity from the home's own photovoltaic system, so that in the evenings and at night, when no solar energy is available, the home's electrical appliances can be powered by electricity from the battery storage. Since photovoltaic systems and batteries provide direct current (DC), this must first be converted into alternating current (AC) before it can be used or fed into the grid. This is done using electronic inverters. The system technology of such a photovoltaic system with battery storage is state-of-the-art and is installed by qualified professionals.Examples of implementation can be found, among other places, in HÄBERLE, Gregor [et al.]: Tables of Electrical Engineering: Tables - Formulas - Standard Applications. 30th revised and expanded edition. Haan: Europa Lehrmittel, 2022. pp. 281-286. - ISBN 978-3-8085-3076-4.

[0004] Storing electrical energy from renewable sources is particularly efficient when using batteries or accumulators with high storage efficiency. Lead-acid batteries, commonly known as 12V car batteries, are one example. The advantages of this battery type stem from its simple design and readily available materials, even for mass production. This results in relatively low manufacturing costs. The storage efficiency of lead-acid batteries is around 80%, and their lifespan is limited to approximately 4,000 charge cycles. Higher energy density can be achieved with modern lithium-ion batteries. This battery type can achieve a high storage efficiency of over 90%, and its lifespan is also optimized at around 5,500 charge cycles.The disadvantages of lithium-ion batteries stem from their higher purchase costs due to their complex design and high material costs. Lithium-ion batteries can experience thermal runaway, releasing large amounts of heat and gases, and potentially leading to spontaneous combustion. Therefore, the safe installation of lithium-ion batteries in residential buildings presents a significant challenge. Lithium iron phosphate batteries are another battery type that cannot spontaneously combust, but they also involve high investment costs.

[0005] Battery storage systems are generally well-suited for storing electrical energy in limited quantities. In residential energy systems, they can typically bridge periods of several hours to a few days without solar power. Battery storage systems are also suitable for electric drives in zero-emission vehicles, especially when high-energy-density lithium-ion batteries are used. The range of these battery-powered vehicles is generally between 300 and 400 km, and significantly less in winter. However, battery storage systems are only conditionally suitable for bridging longer periods or for long distances, as the high material usage would lead to disproportionately high costs and an unacceptably high vehicle weight. In STERNBERG, André; HANK, Christoph; HEBLING, Christopher: Greenhouse gas emissions for battery and fuel cell vehicles with ranges over 300 km.Freiburg: Fraunhofer ISE, 2019. pp. 1-32. URL: https: / / www.ise.fraunhofer.de / content / dam / ise / de / documents / news / 2019 / ISE Ergebnisse Studie Treibhausqasemissionen.pdf [accessed on 2023-03-29] A life cycle analysis showed that a storage capacity of around 60 kWh can be considered a sensible upper limit for battery vehicles.

[0006] Hydrogen is a suitable energy carrier for the seasonal storage of renewable energy over several months and for commercial vehicles that need to cover long distances of 500-1000 km. It can be produced primarily in summer, when solar power is plentiful, via electrolysis. In this process, water is electrochemically split using electrical energy in an electrolyzer, releasing hydrogen and oxygen gases. The oxygen is typically released into the atmosphere, while the hydrogen is compressed and stored in pressure vessels. In winter, when solar energy is scarce, the hydrogen can be converted back into electricity in fuel cells. In this process, hydrogen and atmospheric oxygen react at catalytically active electrodes separated by a membrane, producing electrical energy, heat, and water.In this way, electrical consumers can be supplied and the heating system supported in home energy technology.

[0007] A residential energy system with an electrolyzer, hydrogen storage, and fuel cell is known from EP 3 381 102 B1. The system is designed for use in single-family and two-family homes and operates in combination with a photovoltaic system and a battery storage unit, which serves for short-term storage, primarily in summer. Excess solar energy also powers the electrolyzer, whose waste heat heats a hot water storage tank. In winter, the fuel cell supplies the house with electricity and heat. The hydrogen storage system consists of standard 50-liter gas cylinders designed for a maximum pressure of up to 300 bar, which can be stored in wire mesh containers of 12 cylinders each, for example, in the garden. With two to three of these wire mesh containers (24 steel cylinders), a single-family home can cover its electricity needs almost independently of the public power grid throughout the year. However, an additional heating system is required for heating.A disadvantage is the high investment cost, which, according to the manufacturer, ranges from the upper tens of thousands of euros to the six-figure euro range. This is partly due to a complex ventilation system for waste heat recovery, as an air-cooled fuel cell is used. Significant expenses also arise from the use of multiple electronic inverters or voltage converters. The solar array, the fuel cell, and the electrolyzer are each connected to the house's electrical system via separate DC-DC converters. Since the prices for power electronics, in particular, have risen sharply recently, and there are also supply difficulties with these components, this represents a weakness in the system technology.

[0008] In fuel cell vehicles, hydrogen gas, similar to gasoline, can be used as fuel for propulsion. At public hydrogen filling stations, the vehicle's tank is filled under high pressure of up to 700 bar. On board the vehicle, fuel cells generate electrical energy from the hydrogen, which powers electric drive motors. For optimized operation, lithium-ion batteries are combined with fuel cells in these vehicles. An overview of the essential components of such a vehicle is provided, among other sources, in patents US 7,559,389 B2 and WO 2022 / 182 332 A1. In this system, the drive motors initially draw their electrical energy from a high-current battery, which can provide high power outputs for short periods to support dynamic processes during starting and acceleration.The fuel cell, on the other hand, supplies the battery with an optimized charging current, although here too a voltage converter (DC-DC) is used, which represents a complex and expensive system component.

[0009] Hydrogen as an energy storage medium, along with its associated energy converters—electrolyzers and fuel cells—therefore represents a sensible complement to battery storage, especially when seasonal energy storage or long-distance driving is required. A disadvantage is the relatively low efficiency of energy conversion with hydrogen. While electrolyzers typically operate with an efficiency between 70 and 75%, fuel cells only achieve efficiencies between 40 and 50%. An additional 10 to 15% of the stored energy is required for hydrogen compression. Inverters operate with an efficiency of around 95%. The overall storage efficiency of hydrogen technology is therefore between 25 and 35%. This is low compared to batteries, which operate with a storage efficiency of over 80%.Hydrogen applications therefore absolutely must be optimized in terms of their efficiency.

[0010] Further disadvantages of hydrogen systems arise from their high system complexity and overall costs. Particularly in systems with acidic polymer electrolyte membranes (PEM), only the expensive precious metal catalysts platinum and iridium can be used. Electrolyzers using this technology are also subject to severe corrosion, necessitating the production of cell components from milled titanium plates, which incurs high manufacturing costs. Furthermore, hydrogen systems incorporate a multitude of components such as pumps, valves, and electronics, resulting in costly system technology. Expensive and complex mechanical compressors are required, especially for hydrogen compression. Power electronics components, particularly for voltage converters, can account for up to a quarter of the system costs.

[0011] One approach to realizing efficient and cost-effective energy storage using hydrogen is provided by alkaline electrolysis technology. While PEM electrolyzers typically achieve efficiencies of around 75%, alkaline electrolyzers achieve efficiencies of over 80%. The design and construction of conventional alkaline electrolyzers according to the previous state of the art are described, for example, in WENDT, H.; BAUER, GH: Water-splitting methods. In: Hydrogen as an energy carrier : technologies, systems, economy. Ed. By Carl-Jochen Winter et al. Berlin : Springer, 1988. pp. 166-208. - ISBN 3-540-18896-7; 978-3-642-61561-0. DOI: 10.1007 / 978-3-3642-61561-0_8. Simple nickel sheets are used as catalysts, soaked in concentrated potassium hydroxide solution and separated from each other by a separator. However, these separators cannot generate a significant pressure difference.Therefore, both the hydrogen side and the oxygen side must be built with pressure-resistant and corrosion-resistant pipes, gas separators and gas purifiers, which in turn results in high system costs.

[0012] WO 2010 / 049 214 A1 describes a pressure electrolyzer in which the entire cell stack is housed in a pressure-resistant casing. This simplifies the design of the seals and screw connections of the individual cells. However, both gases and their purification systems must still be pressure-resistant.

[0013] A further development of alkaline electrolysis is based on the use of an alkaline anion exchange membrane (AEM) instead of concentrated potassium hydroxide. This allows for the construction of compact electrolyzers, as described in EP 2 451 992 B1. Inexpensive nickel catalysts can be used, eliminating the need for expensive platinum group metals. Furthermore, the electrolyzer features a dry cathode, resulting in the production of pure hydrogen without the need for an additional electrolyte tank on the hydrogen side. This significantly simplifies the system design, enabling cost-effective production. However, the efficiency is only around 73%.

[0014] A further significant simplification of the overall system, and thus a considerable cost reduction, could also be achieved if the expensive power electronics of the voltage converters, which can account for up to 30% of the system costs, could be largely dispensed with.

[0015] US Patent 4,962,462 A describes a hybrid system in which a battery and fuel cell are directly connected without the use of an electronic voltage converter. This enables the use of a fuel cell to directly charge a battery. Essentially, a method for monitoring the battery's state of charge is described. A current sensor, used by a microcontroller, determines the charging current from the fuel cell to the battery. Depending on the battery's state of charge, a relay interrupts the connection to the fuel cell when the charging current falls below a certain threshold. Some specifications for selected charging currents are provided, and the control concept is described.Unfortunately, it is not shown how the voltages of the battery and fuel cell must be matched so that direct charging of the battery via the fuel cell is even possible.

[0016] Furthermore, US Patent 5,631,532 A describes the electric drive of a vehicle with a methanol fuel cell and a buffer battery. The battery is intended to compensate for the dynamic fluctuations in the electric current flow of the drive system. Simultaneously, the battery's state of charge is monitored, and the power output, and thus the charging current, of the fuel cell is adjusted accordingly. The patent primarily describes the control concept for adjusting the charging current, or power output, of the fuel cell in three stages. The power control of the fuel cell appears to be achieved via the methanol supply to a reformer, which generates hydrogen and ultimately supplies the fuel cell. It is questionable, however, whether power control of a fuel cell via the gas supply is even feasible.In another version, hydrogen is supplied directly to the fuel cell and the power is adjusted again via an intermediate voltage converter.

[0017] The publication PASCUZZI, Simone [et al.]: Electrolyzer performance analysis of an integrated hydrogen power system for greenhouse heating. A case study. In: Sustainability, Vol. 10.3390 / su8070629. URL: https: / / www.mdpi.com / 2071-1050 / 8 / 7 / 629 / pdf?Version=1467697169 [accessed on 2023-03-29] presents an investigation of a hybrid system for supplying energy to a greenhouse. Here, electrical energy is generated by a large photovoltaic system, which is fed into the electrical grid via a DC-AC inverter and drives a heat pump. A further AC-DC inverter powers an alkaline pressure electrolyzer, which can store hydrogen in steel cylinders at a pressure of up to 30 bar. The hydrogen stored in the tank then feeds a fuel cell, which charges a battery when there is no sunlight.A further DC-AC inverter allows the heat pump to be operated with battery power even at night. Tests on real-world systems revealed that the electrolyzer operates inefficiently when powered directly by the solar array. To improve operation, the electrolyzer was connected directly in parallel with the battery, resulting in significantly more stable operation and higher efficiency. The fuel cell was also connected directly to the battery. The study authors have thus demonstrated that it is fundamentally possible to operate an electrolyzer and a fuel cell directly in parallel with a battery. However, the overall system described still incorporates three inverters, resulting in high costs.Furthermore, the component descriptions are superficial, presented as a black-box model without delving into the internal behavior of the main components. For example, it is merely mentioned that the fuel cell has an output voltage of 48 V. However, this is insufficient to actually charge a battery with a nominal voltage of 48 V, as the fuel cell's charging voltage must always be significantly higher than the battery voltage. It is stated, at least, that the electrolyzer used is composed of 33 cells. Upon closer inspection, however, it becomes clear that this is an excessive number of cells, meaning that even at maximum battery voltage, the electrolyzer can only operate at a cell voltage of approximately 1.7 V, which corresponds to a low partial load operation.

[0018] In summary, several approaches exist, based on the current state of the art, for constructing an efficient and cost-effective hybrid energy storage system using batteries in combination with an electrolyzer and a fuel cell. In particular, direct electrical interconnection of these main components appears promising, as this eliminates the need for voltage converters and thus a significant cost factor. Unfortunately, there is currently no well-designed system in which the components are optimally matched. A generally applicable guideline would be particularly desirable for determining the number of cells during the design of the fuel cell and electrolysis units. Summary of the invention

[0019] The object of the invention is to provide a hybrid system for efficient and cost-effective energy storage using batteries and hydrogen, which does not require electronic voltage converters. Furthermore, a method for operating such a system is to be provided.

[0020] This problem is solved according to the invention by a hybrid system with the features of the independent claim.

[0021] According to one aspect of the invention, a hybrid system for efficient and cost-effective energy storage using batteries and hydrogen is created, comprising a DC voltage source, at least one battery, at least one electrolyzer for the electrochemical splitting of water into hydrogen and oxygen, and at least one pressure vessel, wherein the electrolyzer is at least temporarily directly connected to the battery via an electrical DC voltage network and a switching element, wherein the electrolyzer consists of a stack of a number of individual electrochemical cells.The hybrid system is characterized by the fact that a switching element connects a number of individual electrochemical cells in efficient normal operation, corresponding to 0.5 to 0.67 times the nominal voltage of the battery, and by a power switching element, which connects a reduced number of electrochemical cells, corresponding to 0.5 to 0.6 times the nominal voltage of the battery in power operation, directly to the battery, at least temporarily, and that direct operation and control of the electrolyzer takes place without electronic voltage converters, so that the pressure vessel is filled with hydrogen.

[0022] Accordingly, the invention provides a new hybrid system for energy storage with batteries and hydrogen, comprising a battery, an electrolyzer and / or a fuel cell, wherein the number of electrochemical cells in the electrolyzer and in the fuel cell are matched to the battery in such a way that they can be directly electrically connected to the battery, at least temporarily, for highly efficient operation without intermediate voltage converters.

[0023] The invention enables an efficient and cost-effective design, as expensive electronic voltage converters can be eliminated.

[0024] The invention therefore provides a method for the design of hybrid systems, with the help of which the number of electrochemical cells N can be determined. Zellen in the main components electrolyzer and fuel cell. For this purpose, a characteristic value k is used. hybintroduced for the hybrid system, which is defined as follows: khyb=NZallenUBatt

[0025] Where U Batt corresponds to the nominal voltage of the battery pack used.

[0026] The following area has proven to be preferred for the design of the electrolyzer: khyb,EL=0.5…0.67

[0027] According to the invention, the electrolyzer provides that the electrochemical cells connected in series are at least temporarily connected directly to the battery via the DC electrical network by means of a switching element, thus enabling normal operation of the electrolyzer. It is advantageous that the efficiency of the electrolyzer during normal operation can be preset by adjusting the number of cells.

[0028] In a favorable further development, it is provided that in the electrolyzer the reduced number of electrochemical cells corresponds to 0.5 to 0.6 times the nominal voltage of the battery and is connected at least temporarily to the battery via the electrical DC network by means of a power switching element, so that in this way a power operation (boost) of the electrolyzer is realized.

[0029] This significantly reduces the manufacturing costs of the system, as only a power switching element is required instead of a complex electronic power controller. This element achieves the electrolyzer's power output by temporarily connecting a reduced number of cells, allowing them to operate at increased cell voltage and maximum power. A low k value is generally used in the design. hyb,EL to select for performance operation and a high value k hyb,ELfor normal operation.

[0030] In a preferred embodiment, a fuel cell is provided which is connected, at least temporarily, directly to the battery via the DC electrical network by means of a fuel cell switching element. The fuel cell consists of a stack of individual electrochemical cells, the integer number of which corresponds to 1.4 to 1.8 times the nominal battery voltage, so that the voltage of the fuel cell is always higher than the voltage of the battery and the battery is charged directly without electronic voltage converters. This results in a significant cost advantage, since only a fuel cell switching element is required instead of an expensive electronic power controller. For an efficient design of the fuel cell, the following range is preferred: k hyb,BZ = 1.4 ... 1.6.

[0031] Advantageously, the DC voltage source is an AC grid with a power converter, or preferably a photovoltaic system or a wind turbine with a power converter, or, most preferably, a directly connected photovoltaic system or a wind turbine with a rectifier. A hybrid system designed with these components can store electrical energy from renewable sources cost-effectively, both in the short and long term, and thus provide climate-neutral electricity year-round, independent of public or private electricity suppliers.

[0032] In a favorable implementation, the pressure vessel is connected to an additional hydrogen extraction port, allowing hydrogen to be transferred from the pressure vessel to a pressure storage tank or the vehicle tank of a fuel cell vehicle by opening a valve. This offers further advantages, as excess hydrogen can be stored in pressure vessels and sold, or a fuel cell vehicle can be refueled cost-effectively with self-produced hydrogen.

[0033] In a favorable design, a temperature control unit for the electrolyzer and / or a temperature control unit for a fuel cell can be connected to a heating system or a hot water system, so that the waste heat generated during electrochemical energy conversion can be used for a thermal process. In this way, the overall efficiency of the hybrid system can be increased to up to 90 percent when both electrical energy and heat are utilized.

[0034] Preferably, an electronic control unit with inputs and outputs is provided, with which, according to predefined parameters, measurement data acquired via the inputs from the measuring devices assigned to the hybrid system are controlled via the outputs in such a way that electrical energy is directed from the DC voltage source to the electrolyzer and / or to the battery, or that electrical energy is directed from the fuel cell to the battery and / or to the consumer connection.

[0035] According to one aspect of the invention, a method for operating a hybrid system as described above is specified, comprising the following steps: (a) Measuring a battery voltage using a voltage measuring device; (b) Check whether a DC voltage source is connected by measuring an electric current with a current measuring device; (c) Check whether an electrolyzer is connected to the battery by measuring the electric current with a current measuring device; (d) Check that a fuel cell is connected to the battery by measuring the electric current with a current measuring device; (e) Generating hydrogen in the electrolyzer during normal operation by establishing an electrical connection with the switching element when a DC voltage source is connected and when no fuel cell is connected and when the battery voltage is above a limit for normal operation of the electrolyzer; (f) Generating hydrogen in the electrolyzer during power operation by establishing an electrical connection with the power switching element when a DC voltage source is connected and when no fuel cell is connected, and when the battery voltage is above a limit for the power operation of the electrolyzer; (g) Stopping the production of hydrogen in the electrolyzer by disconnecting the electrical connections with the switching element and the power switching element when the battery voltage falls below a limit for normal operation less a hysteresis, the hysteresis being able to be increased or decreased by an external parameter, for example weather data, so that the electrolyzer can be operated with battery power for a longer period in the evening. (h) Generating electrical energy and heat from hydrogen in the fuel cell by establishing an electrical connection with the fuel cell switching element when the battery voltage is below a limit for fuel cell operation and no DC voltage source and electrolyzer is connected. (i) Stopping the generation of electrical energy and heat from hydrogen in the fuel cell by disconnecting the electrical connection with the fuel cell switching element when the battery voltage is above a limit value for fuel cell operation plus a hysteresis, or when an external voltage source is connected.

[0036] The described method is advantageous because it enables simple, efficient, and cost-effective control of the hybrid system. The method can be easily implemented in a control program and can also be transferred to cost-effective control units. This results in particular cost advantages compared to complex controllers for controlling sophisticated power electronics, as both development costs and component costs are lower.

[0037] It goes without saying that the aforementioned steps do not necessarily have to be carried out in the specified order. In particular, it is possible to perform individual steps multiple times.

[0038] The invention thus comprises a method for optimal operation of the hybrid system. For this purpose, the battery voltage U is first measured. Batt and the electrical currents / from the voltage source (PV), electrolyzer (EL), and fuel cell (FC). The control is based on the following logic: UBatt>UEL,on & IFC=0 & IPV>0 => EL=on (Battery full, charging with PV) UBatt>UEL,boost & IFC=0 & IPV>0 => EL=off, ELboost=on (Battery full, charging with PV) UBatt<UEL,aus & IFC=0 & IPV=0 => EL=off (battery partially full, no PV)UBatt<UBZ,ein & IEL=0 & IPV=0 => BZ=on (Battery empty, no PV)UBatt<UBZ,aus & IEL=0 & IPV=0 => BZ=off (battery full, no PV)

[0039] The control logic is to be supplemented by selecting suitable hysteresis parameters, in which different switch-on and switch-off values ​​are defined as parameters for the electrolyzer and for the fuel cell, so that a pulsed operation can be set up near limit values.

[0040] By dynamically adjusting the hysteresis parameters, for example with upstream control programs that process weather forecasts, the efficiency of the hybrid system can be advantageously readjusted dynamically. If, for instance, the weather forecast indicates that the following day will be overcast with low PV yield, the hysteresis is adjusted so that the electrolyzer switches off when the battery voltage is higher and the battery is full, since as much electrical energy as possible will be needed from the battery storage the next day. Conversely, if sunshine and high PV yield are expected the following day, the electrolyzer will operate on battery power for longer in the evening hours and only switch off when the battery voltage is low and the battery is partially discharged. This allows more hydrogen to be produced and improves the overall energy balance of the hybrid system. Brief summary of the characters In Fig. Figure 1 shows a preferred embodiment of the hybrid system. In Fig. 2 shows the characteristic curves of battery charging and fuel cell. Detailed description of a preferred embodiment

[0041] In Fig. Figure 1 shows a preferred embodiment of the invention in combination with a photovoltaic system. The simplest embodiment shown here, with a 12 V battery and a 100 W photovoltaic module, could be used, for example, in outdoor and camping applications, for the self-sufficient power supply of off-grid mountain huts, or in similar areas. However, the invention is fundamentally designed to be modular, allowing systems of any size to be constructed by connecting photovoltaic modules and batteries in series. For example, connecting four large 12 V battery blocks (with a capacity of 100 Ah) in series results in a typical nominal voltage of 48 V and a storage capacity of 4.8 kWh, which is well-suited for single-family homes with photovoltaic systems. However, it is also possible to design much larger systems by connecting batteries and photovoltaic modules in series and parallel.

[0042] The exact workings of the in Fig. The hybrid system 100 shown in section 1 results as follows: In this application, a photovoltaic module 144, with a peak power of, for example, 110 W and a maximum voltage of 19.25 V, represents a DC voltage source 140, which is directly connected to a battery 110 via electrical lines 143. The battery 110 has a nominal voltage of 12 V and can be charged directly by the solar module 144 without the need for a voltage converter. To prevent overcharging of the battery 110, the battery voltage is continuously monitored by a voltage measuring device 111 via an electronic control unit 150. If a defined voltage U is exceeded, Batt,maxThe connection to the solar module 144 can be interrupted by a source switching element 142. Furthermore, the solar module 144 is typically fully charged by switching it on and off in a timed manner using the source switching element 142. With this arrangement, a stable DC network 112 (DC bus) can be established, to which electrical loads are connected via a load connection 170. In addition to loads with a 12 V operating voltage, an inverter is usually connected here, which generates a sinusoidal AC voltage of 230 V, so that all typical household appliances can be supplied. Thus, with this basic arrangement based on photovoltaics and battery storage, a home energy supply can be easily established in accordance with the state of the art.The disadvantage here is that, especially in winter when there is a lack of solar power, the battery 110 is empty after 1 to 2 days and cannot be recharged under unfavorable weather conditions.

[0043] The arrangement is now extended according to the invention such that an electrolyzer 120, for the production of hydrogen by electrochemical splitting of water into hydrogen and oxygen, is connected to the DC power supply 112. The electrolyzer 120 is designed such that the number of electrochemical cells connected in series corresponds to 0.5 to 0.67 times the nominal battery voltage, and the electrolyzer 120 can thus be connected directly to the DC power supply 112, at least temporarily, via a switching element 123, without the need for an additional voltage converter. For the design of the system, the determination of the number of electrochemical cells in the electrolyzer 120 is crucial for the efficiency of operation. According to the invention, this determination is made via a characteristic number k of the hybrid system introduced herein. hyb , which is defined as follows: khyb=N cells UBatt

[0044] Here NZellen the number of electrochemical cells connected in series in the electrolyzer 120 and U Batt the nominal voltage of the battery used. According to the invention, the following range of the characteristic number of the hybrid system is to be assumed for the design of the electrolyzer 120: khyb,EL=0.5…0.67

[0045] The design of the electrolyzer 120 and thus the characterization of the hybrid system 100 according to the invention will be illustrated below using examples.

[0046] For efficient normal operation of the electrolyzer 120, which is designed for optimized efficiency, the maximum value of the characteristic value of 0.67 should be used. For a battery 110 with a nominal voltage of 12 V, this results in a number of 8 electrochemical cells (rounded to the nearest whole number) in the electrolyzer 120. Therefore, in a system with 4 battery blocks connected in series with a total voltage of 48 V, an electrolyzer 120 with 32 electrochemical cells would be required. When operating the hybrid system 100, the electrolyzer 120's task is to produce hydrogen using excess solar energy. This is most effectively done when the battery 110 is already partially charged, which, in the case of a 12 V lead-acid battery, can be indicated by a rise in the charging voltage above 13.2 V. The electrolyzer 120 is then switched on via the switching element 123.The operating voltage of the electrolyzer 120 is then in a voltage range of up to 14.0 V, resulting in a cell voltage of up to 1.75 V. The cells thus operate (with respect to the thermoneutral voltage of 1.48 V) at an efficiency of over 85%, which corresponds to a very efficient operating mode. On the other hand, the electrolyzer 120 therefore only operates at a low partial load, so that only a relatively small proportion of the maximum possible hydrogen flow is produced.

[0047] If additional solar energy is available and the charging voltage of battery 110 rises above a defined value, for example 13.8 V, a further object of the invention is to operate the electrolyzer 120 at a higher power. This object is achieved according to the invention by realizing the power operation of the electrolyzer 120 (boost) by opening the switching element 123 and closing a power switching element 122, thereby electrically connecting a smaller number of electrochemical cells of the electrolyzer 120 to the DC power grid 112. The characteristic value of the hybrid system is also to be used for the design of the power operation of the electrolyzer 120, but in this case, a lower value is to be used.For example, if a value of 0.58 is chosen for the design of the power stage, this results in a (rounded) number of 7 electrochemical cells to be electrically connected for a 12 V battery (110). For a hybrid system with a nominal battery voltage of 48 V, this number is 28 cells. If, in the case of the 12 V hybrid system, the battery charging voltage is above 14 V, the electrochemical cells of the electrolyzer (120) are operated at slightly over 2.0 V per cell, which corresponds to the maximum power output. Consequently, the maximum hydrogen flow is also produced. Design calculations have shown that operating at maximum power output can achieve approximately three times the power output of efficient normal operation, even though fewer cells are in operation. The significantly higher current density of the cells in the power stage more than compensates for the effect of the reduced number of cells.

[0048] For the system technology of the electrolyzer 120 in the hybrid system 100 and for the storage of the produced hydrogen, further components are required. An aqueous electrolyte solution must be supplied to at least one side of the electrochemical cells. For alkaline electrolyzers, concentrated or diluted potassium hydroxide (KOH) is generally used, while for PEM electrolyzers, deionized water is used. An electrolyte tank 126 is required for this purpose, in which the electrolyte solution is stored and maintained at the optimal operating temperature by means of a temperature control unit 127 of the electrolyzer 120. Advantageously, the temperature control unit 127 of the electrolyzer 120 is coupled to a hot water tank or to the building's heating system, so that the waste heat from the electrolyzer 120 can be used for heating.Furthermore, a supply of pure water must be provided for the electrolyte container 126, as water is constantly consumed during the electrochemical splitting process in the electrolyzer 120. The electrolyte fluid is pumped by a pump 124 through a filter 125 into the electrolyzer 120, whereby a large portion of the electrolyte fluid exits the electrolyzer 120 and flows back into the electrolyte container 126. The oxygen produced in the electrolyzer 120 is also carried away in this process and can be separated and discharged from the electrolyte container 126. Utilizing the produced oxygen is optional if corresponding applications or storage are economically viable. The hydrogen produced on the other side of the electrochemical cells first flows into a separator 162, where residual electrolyte fluid is separated.In PEM electrolyzers, this usually involves small amounts of water that permeate the membrane and can be returned to the electrolyte reservoir 126 for further use. In conventional alkaline electrolyzers, there is also an electrolyte circuit on the hydrogen side, which necessitates the separation of the hydrogen gas and subsequent purification in a gas scrubber. Depending on the requirements for the use of the produced hydrogen, further purification steps are required. For example, residual oxygen in the hydrogen can be reacted with a catalyst to form water. Since the hydrogen is generally saturated with water vapor, drying should also be provided. Large amounts of water can be removed by condensation using a condenser. Further drying is usually carried out using adsorption dryers, for example, with molecular sieves or other drying agents.

[0049] The hydrogen is advantageously stored in a 160-liter pressure tank. Often, several such pressure tanks, typically designed for pressures up to 200 or 300 bar, are connected in bundles housed in wire mesh containers. Larger systems utilize mechanical compressors to fill the pressure tanks. However, since these are heavy machines, sometimes associated with noise emissions, maintenance requirements, and high investment costs, they are only conditionally suitable for use in residential areas. Especially in smaller residential energy systems, it is therefore advantageous to produce the hydrogen directly at a high pressure level using pressure electrolyzers. While alkaline electrolyzers are typically designed for a hydrogen pressure of up to 30 bar, modern PEM electrolyzers can provide hydrogen at pressures of up to 100 bar.Here, the hydrogen from the electrolyzer 120 is routed through the separator 162 and, optionally, a dryer, via a check valve 163 directly into the pressure tank 160. The check valve 163 is necessary for safety reasons to prevent all the hydrogen from escaping from the pressure tank 160 in the event of a leak in the electrolyzer 120. In an extended embodiment, a hydrogen extraction port 166 is connected to the pressure tank 160, for example, via a flexible pressure line, so that by opening an additional valve 165, hydrogen can be transferred from the pressure tank 160 to another pressure tank. This can be done, for example, if the hydrogen is to be filled into commercially available gas cylinders and sold. Advantageously, a compressor is provided downstream of the pressure tank 160 to fill the second pressure tank at an increased pressure.Another application example arises when the hydrogen extraction nozzle 166 is designed to allow refueling of a fuel cell vehicle or filling of hydrogen into steel cylinders for sale. In this case, too, it is advantageous to provide a compressor to fill the vehicle tank or the steel cylinders at increased pressure.

[0050] In the event of a lack of solar energy, especially in winter with short days and cloudy skies, the battery 110 can no longer be sufficiently recharged via the photovoltaic module 144 and is significantly discharged by the consumers. In this case, a fuel cell 130 is connected, which is operated with hydrogen from the pressure tank 160 and with atmospheric oxygen, and generates electrical energy, water, and heat. According to the invention, the fuel cell 130 is designed such that the number of electrochemical cells connected in series corresponds to 1.4 to 1.6 times the nominal battery voltage, and the electrolyzer 120 can thus be connected directly to the DC power grid 112, at least temporarily, via a switching element 123, without the need for an additional voltage converter.

[0051] According to the invention, the following range of the characteristic number of the hybrid system is to be assumed for the design of the fuel cell 130: khyb,BZ=1.4…1.8

[0052] The design of the fuel cell 130 and thus the characterization of the hybrid system 100 according to the invention will be illustrated by the following examples.

[0053] For optimal performance of the fuel cell 130, a high value for the key figure, for example 1.8, should be used. This results in approximately 22 electrochemical cells for the construction of the fuel cell 130. With an optimal battery charging voltage of around 14 V, this results in a cell voltage of 0.63 V, which already corresponds to the upper power range and an efficiency of 42%. At the beginning of charging, when the state of charge of the battery 110 is still low, the terminal voltage is 13 V and the cell voltage is therefore 0.59 V. This corresponds to an even higher power range, although the efficiency of the fuel cell 130 is already below 40%. On the other hand, high currents already flow at this operating point, so the battery 110 can be charged correspondingly quickly.

[0054] If, however, a lower coefficient of performance (COP) of, for example, 1.4 is chosen, only 17 electrochemical cells are required for the construction of the fuel cell 130. With an empty battery 110 and a terminal voltage of 13 V, the cell voltage is already at 0.76 V, which corresponds to an efficiency of over 51%. The power output is in the medium range here, so only moderate charging currents are applied, and the battery 110 requires a correspondingly long charging time. With a nearly full battery 110 and a charging voltage of 14 V, the electrochemical cells operate at a cell voltage of 0.82 V and an efficiency of over 55%.

[0055] Accordingly, the application of the hybrid system's key performance indicator (KPI) provides an effective methodology for the efficient design and construction of a hybrid system 100 that is optimally tailored to the requirements. If the fuel cell 130 is to provide the highest possible charging power because the system has to cover high consumption, a higher KPI is chosen at the expense of efficiency. Conversely, if high efficiency is the primary focus, a lower KPI is chosen, although this necessitates accepting a longer charging time.

[0056] For the system technology of the fuel cell 130, several additional components and parts must also be integrated into the hybrid system 100. Hydrogen is supplied to the fuel cell 130 from the pressure accumulator 160 via a pressure reducer 164, ensuring a maximum pressure of 1 bar to prevent damage to the internal membranes. While most of the hydrogen is consumed in the fuel cell 130, foreign gases and water can be purged from the cell by temporarily opening a valve 135 at the outlet. At the other end of the cell, atmospheric oxygen is supplied via a blower or compressor. Most of the air flows through the fuel cell 130, allowing the reaction water to be carried away along with the airflow. In a cell design with open cathodes, the cells are also cooled by the airflow from the blower.If the system is a closed-loop construction, an additional coolant circuit must be provided. In this circuit, the coolant is pumped through the fuel cell 130 by a pump 133 and maintained at an optimal operating temperature by the fuel cell 130's temperature control unit 134. Advantageously, the fuel cell 130's temperature control unit 134 is also connected to a hot water tank or the house's heating system, so that the waste heat from the fuel cell 130 can be used for heating.

[0057] The hybrid system 100 is controlled by an electronic control unit 150, which includes inputs 151 for recording system states via sensors and outputs 152 for controlling system components via actuators.

[0058] The battery voltage U is used as the main input variable for controlling the hybrid system 100. BattThe voltage is measured by a voltage measuring device 111, which is connected to the two rails of the DC power network 112. Furthermore, the electrical current flow of the DC power source 140 (photovoltaic module) I is also measured. in The current flow I is detected by a current measuring device 141, which is attached to the supply line of the DC voltage source 140. EL The current of the electrolyzer 120 is detected by a current measuring device 121, which is arranged on the supply line to the electrolyzer 120. The current flow I FC The current of the fuel cell (FC) 130 is measured by a current measuring device 131, which is arranged on the supply line to the fuel cell 130. Furthermore, a pressure gauge 161 is attached to the pressure accumulator 160 to determine the gas pressure in the pressure accumulator 160.

[0059] For controlling the system components, a source switching element 142 is arranged to interrupt the current flow in the supply line to the photovoltaic module 140. A switching element 123 serves to interrupt the current flow in the supply line to the electrolyzer 120, and a power switching element 122 serves to interrupt the current flow to a reduced number of cells in the electrolyzer 120 to enable power operation. A fuel cell switching element 132 also serves to interrupt the current supply in the supply line to the fuel cell 130. The switching elements for controlling the system components are electromechanical relays or contactors, or semiconductor relays such as transistors, MOSFETs, or IGBTs.

[0060] In the electronic control unit 150, a control program runs with which, according to the invention, a method for operating the system is implemented.

[0061] The operation of the plant is now to serve as an example. Fig. 2 will be explained. The charging characteristic of battery 210 describes the course of the battery voltage with increasing charging time t. The charging time depends on the charging current, which is set according to the battery capacity or is determined automatically. Fig. Figure 2 shows that the battery voltage U_Batt increases continuously as the battery's state of charge increases. From a voltage of approximately 13.5 V, the battery voltage rises exponentially with increasing charging time. Here, the charging voltage should be limited to prevent damage to the battery. This limitation is usually achieved using electronic inverters. However, these will be omitted here to reduce system costs. According to the invention, the charging voltage is limited by the following method: For electrolysis operation: • Regularly measure the battery voltage U_Batt, preferably every second or minute. • Compare the battery voltage U_Batt with the threshold values ​​for normal operation of electrolyzer 120 and for power operation of electrolyzer 120 • Switching on the normal operation of the electrolyzer 120 via a switching element 123 when the battery voltage (U_Batt) is greater than the switch-on voltage (U_EL_ein) of the normal operation of the electrolyzer 120. • Switching off the normal operation of the electrolyzer 120 via the switching element 123 when the battery voltage U_Batt is less than the switch-off voltage (U_EL_off) of the normal operation of the electrolyzer 120, or when the power operation of the electrolyzer 120 is switched on. • Switching on the power operation (boost) of the electrolyzer 120 via power switching element 122 when the battery voltage (U_Batt) is greater than the switch-on voltage (U_EL_boost_ein) of the power operation of the electrolyzer. • Switching off the power operation of the electrolyzer 120 via the power switching element 122 when the battery voltage (U_Batt) is lower than the switch-off voltage (U_EL_boost_off) of the power operation of the electrolyzer 120

[0062] In Fig. Figure 2 shows how the charging characteristic of battery 210 interacts with the characteristic curve of normal operation of electrolyzer 220 and with the characteristic curve of power operation of electrolyzer 230. Since the battery has a large capacity, it will dictate the voltage, which can be read as U_Batt, even when the electrolyzer is directly electrically connected. The electrolyzer will also adjust itself at this voltage. Fig.Figure 2 shows, for example, a horizontal arrow indicating a battery voltage of 13.2 V, which points to the characteristic curve of the efficient normal operation of electrolyzer 220. From this characteristic curve 220, a downward-pointing vertical arrow indicates the current that will be set in the electrolyzer, in this case approximately 6 A. If, after a certain charging time t (here 6.2 h), the battery voltage U_Batt exceeds the switch-on value for the power operation of the electrolyzer U_EL_boost_in (here 13.8 V), the power switching element 122 activates the power operation of the electrolyzer with a reduced number of cells, and simultaneously, switching element 123 deactivates the normal operation of the electrolyzer. The characteristic curve for the power operation of electrolyzer 230 can then be read from another horizontal arrow. The corresponding electric current of the electrolyzer I_EL will now assume a significantly higher value of 12 A.In this way, simple control can be achieved for the voltage range of normal operation (240 V) and for the voltage range of power operation (250 V) of the electrolyzer, without the need for complex control systems and electronically controlled inverters.

[0063] Furthermore, the method includes a procedure for controlling the fuel cell 130. If, especially in winter, a significant discharge of the battery 110 occurs due to a lack of solar energy, a fuel cell 130 is to be switched on according to the invention. The number of cells in this fuel cell corresponds to 1.4 to 1.8 times the nominal battery voltage, so that the fuel cell 130 can be directly connected to the battery 110 via the DC power supply 112 and a fuel cell switching element 132, without the need for an additional voltage converter. The method for controlling the fuel cell 130 comprises the following steps: • Regular measurement of the battery voltage U_Batt, as well as the electrical currents I_PV and I_EL, preferably every second or minute. • Comparing the battery voltage with the threshold values ​​for operating the fuel cell 130 • Switching on the fuel cell 130 via the fuel cell switching element 132 when the battery voltage U_Batt is less than the switch-on voltage U_BZ_ein of the fuel cell 130 and when the current I_PV = 0, as well as the current I_EL = 0, i.e. only when the voltage source AND the electrolyzer 120 are switched off. • Switch off the fuel cell 130 if the battery voltage U_Batt is greater than the fuel cell switch-off voltage U_BZ_off, OR if another voltage source, for example a photovoltaic system, feeds an electric current into the system in the meantime, i.e. when I_PV becomes greater than 0.

[0064] The hybrid system is advantageously suited for use wherever electrical energy, preferably from renewable sources, needs to be stored for extended periods. This is typically the case when large amounts of energy, such as surplus photovoltaic power in the summer or surplus wind power in the autumn, need to be stored and then preferably converted back into electricity in the winter when solar energy is scarce. This sometimes requires a large storage capacity that is no longer economically viable with battery storage alone. A hybrid system, consisting of a battery storage unit and at least one electrolyzer, can produce large quantities of hydrogen and store it in large pressure vessels.Therefore, to increase capacity, it is not necessary to enlarge the actual energy converter (electrolyzer or fuel cell), but only the storage container. Besides bundles of any number of standard pressurized gas cylinders, underground storage facilities made of large steel containers or created by pressure-resistant cavities in salt caverns are also suitable for seasonal storage.

Claims

[1] Hybrid system (100) for efficient and cost-effective energy storage using batteries and hydrogen, comprising a DC voltage source (140), at least one battery (110), at least one electrolyzer (120) for the electrochemical splitting of water into hydrogen and oxygen, and at least one pressure vessel (160), wherein the electrolyzer (120) is at least temporarily directly connected to the battery (110) via an electrical DC network (112) and a switching element (123), wherein the electrolyzer (120) consists of a stack of a number of individual electrochemical cells, characterized by , that the number of individual electrochemical cells corresponds to 0.5 to 0.67 times the nominal voltage of the battery (110), comprising a power switching element (122) with which a reduced number of electrochemical cells, at least temporarily connected directly to the battery (110) via the electrical DC network (112), so that the voltage of the battery (110) is always higher than the voltage of the electrolyzer (120). so that direct operation and control of the electrolyzer (120) can take place without electronic voltage converters and the pressure vessel (160) is filled with hydrogen. [2] Hybrid system (100) according to claim 1 , characterized by , that through the power switching element (122) in the electrolyzer (120) a reduced number of electrochemical cells, corresponding to 0.5 to 0.6 times the nominal voltage of the battery (110), is at least temporarily connected directly to the battery (110) via the electrical DC voltage network (112), so that in this way a power operation of the electrolyzer (120) is realized. [3] Hybrid system (100) according to one of claims 1 to 2, characterized by, that a fuel cell (130) is provided which is connected at least temporarily directly to the battery (110) via the electrical DC network (112) by means of a fuel cell switching element (132), wherein the fuel cell (130) consists of a stack of individual electrochemical cells, the integer number of which corresponds to 1.4 to 1.8 times the nominal voltage of the battery, so that the voltage of the fuel cell (130) is always higher than the voltage of the battery (110) and direct charging of the battery (110) takes place without electronic voltage converters. [4] Hybrid system (100) according to one of claims 1 to 3, characterized by that the DC voltage source (140) is an AC network with a power converter, or preferably a photovoltaic system (144) or a wind power plant with a power converter, or particularly preferably a directly connected photovoltaic system (144) or a wind power plant with a rectifier. [5] Hybrid system (100) according to any one of claims 1 to 4, characterized by , that the pressure vessel (160) is connected to an additional extraction port (166) for hydrogen, with which hydrogen can be directed from the pressure vessel (160) into a pressure storage device or into a vehicle tank of a fuel cell vehicle by opening a valve (165). [6] Hybrid system (100) according to any one of claims 1 to 5, characterized by , that a temperature control device (127) of the electrolyzer (120) and / or a temperature control device (134) of a fuel cell (130) can be connected to a heating system or a hot water system, so that the waste heat generated during the electrochemical energy conversion can be used for a thermal process, in particular for heat utilization in domestic energy technology. [7] Hybrid system (100) according to any one of claims 1 to 6, characterized by, that an electronic control unit (150) with inputs (151) and outputs (152) is provided, with which measurement data acquired via the inputs (151) from the hybrid system (100) are controlled via the outputs (152) by measuring devices (111, 141, 121, 131, 161) assigned to the hybrid system (100) according to specified parameters, such that electrical energy from the DC voltage source (140) is directed to the electrolyzer (120) and / or to the battery (110), or that electrical energy from the fuel cell (130) is directed to the battery (110) and / or to the consumer connection (170). [8] Method for operating a hybrid plant according to any one of claims 1 to 7, comprising the following steps: (a) Measuring a battery voltage using a voltage measuring device (111); (b) Checking whether a DC voltage source (140) is connected by measuring an electric current with a current measuring device (141); (c) Check whether an electrolyzer (120) is connected to the battery (110) by measuring the electric current with a current measuring device (121); (d) Checking whether a fuel cell (130) is connected to the battery (110) by measuring the electric current with a current measuring arrangement (131); (e) Generating hydrogen in the electrolyzer (120) in efficient normal operation by establishing an electrical connection with the switching element (123) when a DC voltage source (140) is connected and when no fuel cell (130) is connected and when the battery voltage is above a limit value for the normal operation of the electrolyzer (120); (f) Generating hydrogen in the electrolyzer (120) during power operation by establishing an electrical connection with the power switching element (122) when a DC voltage source (140) is connected and when no fuel cell (130) is connected, and when the battery voltage is above a limit for the power operation of the electrolyzer; (g) Stopping the production of hydrogen in the electrolyzer (120) by disconnecting the electrical connections with the switching element (123) and the power switching element (122) when the battery voltage falls below a limit for normal operation less a hysteresis, the hysteresis being able to be increased or decreased by an external parameter, for example weather data, so that the electrolyzer (120) can be operated with battery power for a longer period in the evening; (h) Generating electrical energy and heat from hydrogen in the fuel cell (130) by establishing an electrical connection with the fuel cell switching element (132) when the battery voltage is below a limit for fuel cell operation and no DC voltage source (140) and no electrolyzer (120) is connected; (i) Stopping the generation of electrical energy and heat from hydrogen in the fuel cell (130) by disconnecting the electrical connection with the fuel cell switching element (132) when the battery voltage is above a limit value for fuel cell operation plus a hysteresis, or when an external voltage source (140) is connected. [9] Method according to claim 8, characterized by, that an electronic control unit (150) with inputs (151) and outputs (152) is provided, with which measurement data acquired via the inputs (151) from the hybrid system (100) are controlled via the outputs (152) by measuring devices (111, 141, 121, 131, 161) assigned to the hybrid system (100) according to specified parameters, such that electrical energy from the DC voltage source (140) is directed to the electrolyzer (120) and / or to the battery (110), or that electrical energy from the fuel cell (130) is directed to the battery (110) and / or to the consumer connection (170).

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