Method for operating an electrolyte circuit and electrolyte circuit

Abstract

The invention relates to a method for operating an electrolyte circuit (1) through which, during normal operation, an electrolyte from a tank (4) is supplied to at least one electrolysis stack (2) of an electrolysis system (10) by means of a pump (3). According to the invention, the amount of electrolyte circulated via the electrolyte circuit (1) by means of the pump (3) is temporarily reduced during startup. The invention further relates to an electrolyte circuit (1) and an electrolysis system (10).

Description

The present invention relates to a method for operating an electrolyte circuit having the features of the preamble of claim 1.Preferred fields of application of the invention are electrolysis systems, in particular low-temperature electrolysis systems. Therefore, an electrolysis system with an electrolysis circuit according to the invention is also proposed.Prior ArtIn low-temperature electrolysis, the electrolyte required for electrolysis is stored in a tank and fed to an electrolysis stack with the aid of a pump. In alkaline electrolysis (AEL) and anion exchange membrane electrolysis (AEM), the electrolyte is typically potassium hydroxide solution in various concentrations. In polymer electrolyte membrane electrolysis (PEM), water is used as electrolyte.During the starting process of a low-temperature electrolysis system, the latter must first be heated to the desired temperature in order to achieve the desired current density in the permitted cell or stack voltage range. However, the process of heating delays the starting process. To enable dynamic operation of the system, a fast start is desired.The heating when starting the system can be effected with the waste heat from the electrolysis process and / or by external heating. The external heating can be realized with the aid of direct current heating or a heat transfer medium, such as water vapor, for example. An electric heater and / or heat exchanger is or are accordingly required for external heating. Correspondingly, the investment and operating costs increase with external heating, including the costs for the energy used and / or maintenance work that arises. Since the heating device must be dimensioned very large for a rapid start, but its use is only required in the case of start-up, external heating is uneconomical.The present invention is concerned with the object of increasing the rapid starting capability of an electrolysis system, so that the heating process can be shortened in the case of starting.To achieve the object, the method having the features of claim 1 and the electrolyte circuit having the features of claim 5 are proposed. Advantageous further developments of the invention can be found in the respective dependent claims. Furthermore, an electrolysis system with an electrolyte circuit according to the invention is specified.Disclosure of the InventionA method for operating an electrolyte circuit is proposed, via which an electrolyte is supplied from a tank with the aid of a pump to at least one electrolysis stack of an electrolysis system during normal operation. According to the invention, in the starting case, the amount of electrolyte circulated via the electrolyte circuit with the aid of the pump is temporarily reduced.The electrolyte represents a considerable proportion of the heat capacity of the system. If the amount of electrolyte and thus the heat capacity of the system is reduced during the starting process, less thermal energy is required in order to achieve the required operating temperature. With the aid of the proposed method, the starting process can accordingly be significantly accelerated. At the same time, the required heating energy is reduced, so that a heating device provided for heating can be dimensioned smaller or even completely omitted. Instead, the waste heat from the current electrolysis process can be used.Preferably, for temporary reduction of the electrolyte quantity, an electrolyte portion is introduced from the tank or from the electrolyte circuit into a buffer tank and temporarily stored temporarily in the buffer tank during the starting process. The electrolyte portion temporarily stored in the buffer tank decreases the fill level in the tank of the electrolyte circuit, so that less electrolyte has to be heated or the heating load decreases.In an initial step during the starting process, the buffer tank is accordingly filled with the electrolyte. Following the starting process, the buffer tank can then be emptied again, wherein the electrolyte is introduced into the tank or into the electrolyte circuit. The temporary reduction of the electrolyte quantity is thereby canceled again. In order to heat up the amount of electrolyte introduced from the buffer tank into the tank or into the electrolyte circuit, the power loss of the at least one electrolysis stack can be used. Alternatively or additionally, an electrical heating device or a heat exchanger can be used. As soon as the buffer tank is completely emptied, i.e. no more electrolyte is present in the buffer tank, the system can be operated as usual.According to a preferred embodiment of the invention, the pump of the electrolyte circuit or an additional pump is used for filling and / or emptying the buffer tank. When using the pump of the electrolyte circuit, an additional pump can be dispensed with, which helps to save installation space and costs. For this purpose, the buffer tank is connected to the electrolyte circuit on the inlet side on the pressure side of the pump. In order to prevent filling of the buffer tank with the electrolyte during normal operation, the connection is preferably made via a valve which is kept closed during normal operation and opened during start-up. When using an additional pump for filling and / or emptying the buffer tank, the latter can be connected to the electrolyte circuit or to the tank of the electrolyte circuit, so that the possibilities of connection are widened. In this case, too, the buffer tank is preferably connected via at least one valve.Advantageously, the force of gravity is used for filling and / or emptying the buffer tank with the electrolyte, so that the filling and / or emptying is driven solely by the force of gravity or is effected at least with assistance of the force of gravity. This presupposes that the buffer tank is arranged either geodetically below or geodetically above the tank of the electrolyte circuit or of the main tank. If the buffer tank is geodetically below the main tank, gravity can be used to fill the buffer tank. If the buffer tank is geodetically above the main tank, gravity can be used to empty the buffer tank.In order that the gas present in the buffer tank can be displaced by the incoming electrolyte during filling of the buffer tank, a gas outlet is preferably provided on the buffer tank. The gas can then escape via this. A valve can be integrated into the gas outlet. During the subsequent emptying of the buffer tank, air can flow into the buffer tank via the gas outlet, so that the buffer tank is filled again with air.Furthermore, an electrolyte circuit for an electrolysis system having at least one electrolysis stack is proposed. An electrolyte can be supplied to the at least one electrolysis stack via the electrolysis circuit. The electrolyte circuit comprises:a tank for storing the electrolyte,a pump for circulating the electrolyte via the electrolyte circuit,a buffer tank which can be switched on and off via at least one valve.The electrolyte circuit is suitable in particular for carrying out the method according to the invention described above, so that the same advantages can be achieved. In particular, the quick startability of the electrolysis system having the electrolyte circuit can be increased. This is because in the starting case, by connecting the buffer tank, an electrolyte portion can be introduced into the buffer tank and temporarily stored temporarily, so that less electrolyte has to be heated. At the same time, less energy is required for heating.According to a preferred embodiment of the invention, the buffer tank has a gas outlet. The gas present in the buffer tank can be displaced via the gas outlet when filling the buffer tank with the electrolyte.For filling and / or emptying the buffer tank, the pump of the electrolyte circuit or an additional pump can be used. The additional pump extends the possibilities of connecting the buffer tank to the electrolyte circuit. For example, the connection can then also take place indirectly via the tank of the electrolyte circuit. The use of the already present pump of the electrolyte circuit reduces the installation space requirement and the costs.Advantageously, the buffer tank is arranged geodetically below the tank, so that the force of gravity can be used to fill the buffer tank with the electrolyte. In this case, the buffer tank is filled by gravity or at least with the assistance of gravity.Alternatively, it is proposed that the buffer tank is arranged geodetically above the tank, so that the force of gravity can be used to empty the buffer tank. The emptying of the buffer tank takes place in this case by means of gravity or at least with the assistance of gravity.The buffer tank can also be structurally integrated into the tank of the electrolyte circuit. This measure allows the installation space requirement to be further reduced. Furthermore, line lengths can be reduced.In addition, an electrolysis system with an electrolyte circuit according to the invention is proposed, in which at least one electrolysis stack is integrated. The at least one electrolysis stack can be supplied with an electrolyte via the electrolyte circuit, wherein this can be in particular a potassium hydroxide solution or water. By way of example, the electrolyte is supplied via the electrolyte circuit to an anode of the at least one electrolysis stack. The proposed concept can, however, also be transferred to a cathode-side circuit.Preferred embodiments of the invention are explained in more detail below with reference to the attached drawings. These show: FIG. 1 shows a schematic illustration of an electrolysis system having a first electrolyte circuit according to the invention, FIG. 2 shows a schematic illustration of an electrolysis system having a second electrolyte circuit according to the invention, and FIG. 3 shows a schematic illustration of an electrolysis system having a third electrolyte circuit according to the invention.DETAILED DESCRIPTION OF THE DRAWINGSFIG. 1 shows an electrolysis system 10 with an electrolysis stack 2. The electrolysis stack 2 has an anode "A" and a cathode "C". During operation, an electrolyte is supplied to the anode via an electrolyte circuit 1. In the case of a PEM electrolysis system, the electrolyte is water, in particular deionized water or DI water.A tank 4 for holding the electrolyte is integrated into the electrolyte circuit 1 of FIG. 1. The tank 4 can be filled with the electrolyte via a line 13. Between the tank 4 and the electrolysis stack 2 a pump 3 is arranged, with the aid of which the electrolyte is circulated via the electrolyte circuit 1. Downstream of the pump 3, a heat exchanger 11 is additionally integrated into the electrolyte circuit 1, by means of which the temperature is regulated during operation, in particular the excess heat is dissipated.In the starting case, the system does not have to be cooled, but rather heated. The heating can be effected, for example, with the aid of an electric heating device 12 accommodated in the tank 4. In order to relieve the load on the heating device 12, a buffer tank 5 is integrated into the electrolyte circuit 1, which buffer tank can be switched on and off via valves 7, 8. In the starting case, the valve 7 is opened and a pump 6 is activated, so that electrolyte flows from the tank 4 into the buffer tank 5 and the fill level in the tank 4 falls. This also reduces the amount of electrolyte to be heated, which is supplied to the electrolysis stack 2 via the electrolyte circuit 1 with the aid of the pump 3. The heating and thus the starting process can thereby be shortened.When filling the buffer tank 5, the gas present in the buffer tank 5 is displaced by the inflowing electrolyte. This leaves the buffer tank 5 via a gas outlet 9, The gas outlet 9, as shown by way of example in FIG. 1, can open into a line 14, via which the product gas is discharged from the tank 4. Alternatively, the gas outlet 9 can be discharged in another way, as long as the different pressure levels of tank 4 and buffer tank 5 do not prevent the emptying of buffer tank 5 in electrolysis operation.To empty the buffer tank 5, the valve 7 is closed and a valve 8 is opened, so that the electrolyte discharged from the buffer tank 5 reaches back into the tank 4 again. Emptying can be effected by means of gravity. However, this presupposes that the buffer tank 5 is arranged geodetically above the tank 4.A further preferred embodiment of an electrolyte circuit 1 according to the invention can be seen from FIG. 2. Here, the buffer tank 5 is connected on the inlet side via the valve 7 to the electrolyte circuit 1, namely downstream of the pump 3. To fill the buffer tank 5 with the electrolyte, only the valve 7 needs to be opened and the pump 3 operated. The buffer tank 5 is emptied again directly into the tank 4 via the valve 8, Ideally, the buffer tank 5 is arranged geodetically above the tank 4, so that when the valve 8 is open, the force of gravity can be used for emptying.A further variant of an electrolyte circuit 1 according to the invention is illustrated in FIG. 3. The basic principle is the same as the embodiment shown in FIG. 2, so that an additional pump 6 is also unnecessary here. However, the buffer tank 5 is arranged geodetically below the tank 4 in the present case, so that the latter fills with electrolyte when the valve 7 is open because of the height difference, that is to say driven by gravity. The required pressure equalization takes place via the gas outlet 9. By closing the two valves 7, 8, the filling process is stopped. To empty the buffer tank 5, the valve 7 is opened, so that the return quantity exiting from the electrolysis stack 2 is fed to the buffer tank 5. The oxygen carried along by the return flow volume displaces the amount of electrolyte present in the buffer tank 5, so that the latter reaches back into the electrolyte circuit 1. It should be noted that the return line of the electrolyte circuit is located geodetically below the buffer tank 5 and the connection of the buffer tank 5 takes place via a continuously rising line. In this way, it is ensured that, by opening the valve 7, the oxygen rises into the buffer tank 5 and can displace the electrolyte there in the opposite direction of flow into the electrolyte circuit 1. The rate of emptying can be regulated by way of the position of the valve 7. When the buffer tank 5 has been emptied, the valve 7 is closed again.In contrast to the embodiments shown in FIGS. 1, 2 to 3, a buffer tank 5 can serve the electrolyte circuit 1 of a plurality of electrolysis stacks 2. In addition, the buffer tank 5 can be structurally integrated into the tank 4.

Claims

Method for operating an electrolyte circuit (1), via which an electrolyte is supplied from a tank (4) with the aid of a pump (3) to at least one electrolysis stack (2) of an electrolysis system (10) during normal operation, characterized in that, in the starting case, the amount of electrolyte circulated via the electrolyte circuit (1) with the aid of the pump (3) is temporarily reduced.Method according to Claim 1, characterized in that, in order to temporarily reduce the amount of electrolyte, a partial amount of electrolyte is introduced from the tank (4) or from the electrolyte circuit (1) into a buffer tank (5) and temporarily stored temporarily in the buffer tank (5) during the starting operation.Method according to claim 2, characterised in that the pump (3) or an additional pump (6) is used for filling and / or emptying the buffer tank (5).Method according to claim 2 or 3, characterised in that the force of gravity is used for filling and / or emptying the buffer tank (5) with the electrolyte.Electrolyte circuit (1) for an electrolysis system (10) having at least one electrolysis stack (2) to which an electrolyte can be fed via the electrolyte circuit (1), comprising - a tank (4) for storing the electrolyte, - a pump (3) for circulating the electrolyte via the electrolyte circuit (1), - a buffer tank (5) which can be connected and disconnected via at least one valve (7, 8).Electrolyte circuit (1) according to Claim 5, characterized in that the buffer tank (5) has a gas outlet (9).Electrolyte circuit (1) according to Claim 5 or 6, characterized in that the buffer tank (5) is arranged geodetically below the tank (4), with the result that the force of gravity can be used to fill the buffer tank (5) with the electrolyte.Electrolyte circuit (1) according to claim 5 or 6, characterised in that the buffer tank (5) is arranged geodetically above the tank (4) so that the force of gravity can be used for emptying the buffer tank (5).Electrolyte circuit (1) according to one of Claims 5 to 8, characterized in that the buffer tank (5) is structurally integrated into the tank (4).Electrolysis system (10) having an electrolyte circuit (1) according to one of Claims 5 to 9, in which at least one electrolysis stack (2) is integrated.

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