Method for operating an air system, air system and fuel cell system

Abstract

The invention relates to a method for operating an air system (1) for supplying a fuel cell stack (2) with air, wherein the air is supplied to the fuel cell stack (2) via an air supply path (3) of the air system (1), and wherein the air is compressed by a compressor (7) and humidified by means of water injection before entering the fuel cell stack (2). According to the invention, the water for humidifying the air in the air supply path (3) is heated before injection using the compressed air in the air supply path (3). The invention further relates to an air system (1) and a fuel cell system with an air system (1) according to the invention.

Description

[0001] The invention relates to a method for operating an air system to supply a fuel cell stack with air. Furthermore, the invention relates to an air system for a fuel cell system and to a fuel cell system with an air system according to the invention. State of the art

[0002] Fuel cells convert a fuel, such as hydrogen, and oxygen into electrical energy, heat, and water. Air, generally ambient air, typically serves as the oxygen source. The air supply is usually provided by an air system that includes an intake air path for supplying air and an exhaust air path for removing the exhaust air exiting the fuel cells.

[0003] Since the electrochemical reaction in fuel cells requires a certain airflow and pressure level, the air is compressed beforehand. A compressor is integrated into the supply air path for this purpose. Because the air heats up considerably during compression, it is also cooled by a cooler integrated downstream of the compressor in the supply air path. A humidifier is also typically integrated into the supply air path to prevent the fuel cell membranes from drying out. The proton conductivity of the membranes depends significantly on a sufficient amount of water being incorporated into their structure.

[0004] Various concepts for humidifying the air in the supply air path of an air system are known in the prior art. Humidification can be achieved, for example, using a gas-to-gas membrane humidifier installed in the supply air path. This device transports water vapor generated during operation through a membrane from the exhaust air side to the supply air side. A disadvantage of such a membrane humidifier is its large installation space, which is particularly limited in mobile applications. Furthermore, a membrane humidifier is subject to degradation, has a relatively low thermal response, and generates pressure losses.

[0005] Furthermore, humidification of the air can be achieved by recirculating and mixing the moist exhaust air with the dry air in the supply air path. Alternatively, the dry air in the supply air path can be humidified by injecting water. Such a method is known, for example, from DE 10 2017 214 312 A1.

[0006] The present invention addresses the objective of optimizing the humidification of air in the supply air path of an air system by means of water injection, particularly with regard to the air's water absorption capacity. Furthermore, the required installation space is to be reduced.

[0007] To solve the problem, the method with the features of claim 1 and the air system with the features of claim 6 are proposed. Advantageous embodiments of the invention can be found in the respective dependent claims. Furthermore, a fuel cell system with an air system according to the invention is described. Disclosure of the invention

[0008] A method for operating an air system to supply a fuel cell stack with air is proposed. The air is supplied to the fuel cell stack via an air intake path of the air system, wherein the air is compressed by a compressor and humidified by means of water injection before entering the fuel cell stack. According to the invention, the water for humidifying the air in the intake path is heated by the compressed air in the intake path before injection.

[0009] As mentioned earlier, the air heats up considerably during compression. In the proposed process, this heat is used to warm or preheat the water to be injected. The heat present in the system is therefore not lost, but put to good use. Using this heat to warm or preheat the water to be injected offers several advantages.

[0010] Firstly, preheating the injected water promotes its evaporation, thereby improving the air's capacity to absorb water. Effective humidification of the air can thus be achieved through water injection, eliminating the need for a membrane humidifier. The elimination of the membrane humidifier, along with its associated supply and exhaust air ducts, results in a gain in installation space.

[0011] Secondly, preheating the injected water with the compressed air can also cool the air. This may eliminate the need for a separate cooler downstream of the compressor.

[0012] The advantages of the proposed method are particularly evident in mobile applications, specifically during transitions from high to low load. Since humidifying the air is more challenging at low loads, the residual energy from a previous high-load phase can be used to heat the injected water during a low-load phase. This heating then promotes the evaporation of the water. Ideally, the injected water is heated to a temperature equal to or above the pressure-dependent boiling point of water. The injected water then transitions directly into a vaporous state, allowing it to be absorbed more readily by the air in the supply air path.

[0013] Another advantage of the proposed method is that, in the event of a freeze start of the system, frozen water can be thawed more quickly before injection, and the air system can be ready for operation more quickly.

[0014] To ensure heat transfer from the compressed air to the water being injected, a water pipe carrying the water to be injected can be arranged, at least in sections, in or alongside the supply air path. The heated water can then be fed via the water pipe to a metering valve, which is used to inject the water.

[0015] Alternatively or additionally, it is proposed that the injected water be heated by passing through a heat exchanger integrated into the supply air path downstream of the compressor. The heat exchanger is thus subjected to flow by both the compressed air and the injected water. The heat exchanger ensures efficient heat transfer.

[0016] Preferably, the heated water is injected downstream of the compressor integrated into the supply air path to humidify the air. Further cooling of the compressed air can then be achieved by means of evaporative cooling. For this purpose, the metering valve is preferably integrated into the supply air path or, if present, into the heat exchanger. This further minimizes the required installation space.

[0017] In a multi-stage compressor, the proposed method can also be used to achieve intercooling if the heated water is injected between two compression stages or compressors.

[0018] Water obtained through condensation of moist exhaust air, which is discharged via an exhaust air path of the air system, can be used to humidify the air. In this way, product water generated during the operation of the fuel cell stack is put to use.

[0019] Alternatively or additionally, it is proposed that water obtained from an excess of water on the cathode and / or anode side of the fuel cell stack be used for humidification.

[0020] Furthermore, an air system for a fuel cell system with at least one fuel cell stack is proposed. The air system includes: - an air supply path through which air can be supplied to the fuel cell stack, - a compressor integrated into the supply air path for compressing the air, - a metering valve for injecting water into the supply air path and - a water pipe to supply the dosing valve with water.

[0021] The water pipe is at least partially located downstream of the compressor in the supply air path and / or in a heat exchanger integrated downstream of the compressor in the supply air path.

[0022] The proposed air system is particularly suitable for carrying out the previously described method according to the invention, or can be operated according to this method. Consequently, the same advantages can be achieved with the aid of the air system. In particular, effective humidification can be achieved by injecting pre-heated water, making a membrane humidifier unnecessary. The elimination of the membrane humidifier, in turn, helps to save installation space. Furthermore, cooling of the air compressed by the compressor can be achieved.

[0023] Because the water pipe is at least partially integrated into the supply air path and / or the heat exchanger, rapid thawing of the water can be achieved during a freeze-start. This improves the system's freeze-start capability.

[0024] The water line can be formed by a separate pipe, which is then at least partially contained within the supply air path and / or the heat exchanger. Alternatively, the water line can be formed by a cavity, for example, in a wall bordering the supply path and / or in the heat exchanger.

[0025] Preferably, the metering valve is connected to a water reservoir via the water line, which can in particular be a water reservoir for collecting product water. In this way, the product water generated during the operation of the fuel cell stack can be put to use.

[0026] Furthermore, the metering valve is preferably integrated into the supply air path downstream of the compressor. Since the air heats up considerably during compression, injecting water downstream of the compressor can humidify and simultaneously cool the compressed air via evaporative cooling.

[0027] In a multi-stage compressor, the metering valve for injecting water can be used – alternatively or additionally – to implement intercooling. In a multi-stage compressor, at least one metering valve can then be located downstream of a compressor in the first compression stage and / or downstream of a compressor in the second compression stage.

[0028] Furthermore, a fuel cell system comprising at least one fuel cell stack and an air system according to the invention is proposed. The air system allows the fuel cell stack to be supplied with humid air, which prevents the membranes of the fuel cells in the stack from drying out. Since the proton conductivity of the membranes increases with the moisture content, the system efficiency can also be improved.

[0029] Preferred embodiments of the invention are explained in more detail below with reference to the accompanying drawings. These show: Fig. 1 a schematic representation of a first air system according to the invention, Fig. 2 a schematic representation of a second air system according to the invention and Fig. 3 a diagram to graphically represent the relationship between boiling temperature and final compression pressure. Detailed description of the drawings

[0030] The one in Fig. An exemplary air system 1 according to the invention serves to supply a fuel cell stack 2 with air. The air is taken from the environment and supplied to the fuel cell stack 2 via an air supply path 3 of the air system 1. Air exiting the fuel cell stack 2, or exhaust air, is discharged via an exhaust air path 8 of the air system 1.

[0031] The ambient air is first purified of harmful particles by means of a filter 9 integrated into the supply air path 3. The air is then compressed by a compressor 7 integrated into the supply air path 3. Downstream of the compressor 7, a heat exchanger 4 is integrated into the supply air path 3, through which the compressed air flows. Water from a water reservoir 10 flows through the heat exchanger 4, which is supplied to the heat exchanger 4 via a water line 6 by a pump 11. In the heat exchanger 4, the water absorbs heat from the compressed air. The water is thus heated, while the air is simultaneously cooled. The heated water is then supplied via the water line 6 to a metering valve 5, which is located downstream of the heat exchanger 4 in the supply air path 3. The heated water can then be injected into the supply air path 3 using the metering valve 5.Evaporative cooling can then be used to cool the air further.

[0032] The Fig. Figure 2 shows a further preferred embodiment of an air system 1 according to the invention. This differs from that of the Fig. 1. This is achieved in particular by the fact that the metering valve 5 is located upstream of the heat exchanger 4 in the supply air path 3. To preheat the water to be injected, the water flows through the heat exchanger 4 in the opposite direction. This means that the flow direction of the water is opposite to the flow direction of the air in the supply air path 3.

[0033] The optimal position of the metering valve 5 depends – especially in mobile applications – on the specific operating strategy. Ideally, the water to be injected is heated at least to its boiling point, which in turn is pressure-dependent. In the case of highly fluctuating pressure conditions, which are common in mobile applications, a compromise solution may be advantageous.

[0034] In the Fig. Figure 3 shows the relationship between boiling point and pressure or final compression pressure. Depending on the pressures and final compression temperatures achieved, different arrangements of the metering valve 5 may prove advantageous. The pressure-dependent boiling point (Graph A) and the pressure-dependent final compression temperature (Graph B) were calculated approximately based on the ideal gas law at an initial pressure of 1 bar. QUOTES INCLUDED IN THE DESCRIPTION

[0000] This list of documents cited by the applicant was automatically generated and is included solely for the reader's convenience. The list is not part of the German patent or utility model application. The DPMA accepts no liability for any errors or omissions. Cited patent literature

[0000] DE 10 2017 214 312 A1

[0005]

Claims

[1] Method for operating an air system (1) for supplying a fuel cell stack (2) with air, wherein the air is supplied to the fuel cell stack (2) via an air supply path (3) of the air system (1), and wherein the air is compressed by means of a compressor (7) and humidified by means of water injection before entering the fuel cell stack (2), characterized by that the water for humidifying the air in the supply air path (3) is heated before injection with the aid of the compressed air in the supply air path (3). [2] Method according to claim 1, characterized by that the water to be injected is passed through a heat exchanger (4) for heating, which is integrated into the supply air path (3) downstream of the compressor (7). [3] Method according to claim 1 or 2, characterized by that the heated water for humidifying the air in the supply air path (3) is injected downstream of the compressor (7) integrated in the supply air path (3). [4] Method according to one of the preceding claims, characterized by that water is used for humidification, which is obtained by condensation from moist exhaust air which is discharged via an exhaust air path (8) of the air system (1). [5] Method according to one of the preceding claims, characterized by that water obtained from a cathode- and / or anode-side water surplus of the fuel cell stack (2) is used for humidification. [6] Air system (1) for a fuel cell system with at least one fuel cell stack (2), comprising - an air supply path (3) via which air can be supplied to the fuel cell stack (2), - a compressor (7) integrated into the supply air path (3) for compressing the air, - a dosing valve (5) for injecting water into the supply air path (3) and - a water line (6) for supplying the metering valve (5) with water, wherein the water line (6) is accommodated or formed at least in sections downstream of the compressor (7) in the supply air path (3) and / or in a heat exchanger (4) integrated into the supply air path (3) downstream of the compressor (7). [7] Air system (1) according to claim 6, characterized by that the metering valve (5) is integrated into the supply air path (3) downstream of the compressor (7). [8] Fuel cell system with at least one fuel cell stack (2) and an air system (1) according to claim 6 or 7.

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