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 air is supplied to the fuel cell stack (2) via a supply air path (3) and exhaust air exiting the fuel cell stack (2) is discharged via an exhaust air path (4), and wherein the air in the supply air path (3) is humidified by means of exhaust air recirculation. According to the invention, the exhaust air to be recirculated is heated with the help of the air in the supply air path (3) by passing the air and the exhaust air through an air-to-air heat exchanger (5) which is integrated into the supply air path (3) downstream of a compressor (6). 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 for supplying a fuel cell stack with air. Furthermore, the invention relates to an air system for a fuel cell system and 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. The oxygen source is typically air, generally ambient air. The air supply is typically provided via an air system that includes an air supply path for supplying air and an exhaust path for removing the exhaust air from the fuel cells.

[0003] Since the electrochemical reaction in the fuel cells requires a certain air mass flow and a certain pressure level, the air is first compressed. A compressor is integrated into the supply air path for this purpose. Since the air heats up considerably during compression, it is further cooled using a cooler integrated into the supply air path downstream of the compressor. A humidifier is also typically integrated into the supply air path to prevent the fuel cell membranes from drying out. This is because 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 from the state of the art. Humidification can be achieved, for example, with the help of a gas-to-gas membrane humidifier arranged in the supply air path, which transports water generated during operation in the form of water vapor through a membrane from the exhaust air to the supply air side. A disadvantage is that such a membrane humidifier requires a lot of space, which is particularly limited in mobile applications. A membrane humidifier is also subject to degradation, is comparatively thermally inert, and generates pressure losses.

[0005] In addition, the dry air in the supply air path can be humidified by water injection. Humidification can also be achieved by recirculating and mixing the moist exhaust air with the dry air in the supply air path.

[0006] The present invention is concerned with the task of optimizing the humidification of air in the supply air path of an air system, particularly with regard to the water absorption capacity of the air. Furthermore, the installation space requirement is to be reduced.

[0007] To achieve this objective, the method having the features of claim 1 and the air system having the features of claim 5 are proposed. Advantageous developments of the invention can be found in the respective subclaims. Furthermore, a fuel cell system with an air system according to the invention is specified. Disclosure of the invention

[0008] A method is proposed for operating an air system that supplies air to a fuel cell stack. In this method, air is supplied to the fuel cell stack via an air supply path, and exhaust air exiting the fuel cell stack is removed via an exhaust air path. The air in the air supply path is humidified by exhaust air recirculation. According to the invention, the exhaust air to be recirculated is heated with the aid of the air in the air supply path by passing the air and the exhaust air through an air-to-air heat exchanger that is integrated into the air supply path downstream of a compressor.

[0009] According to the proposed process, the exhaust air is heated before being fed into the air in the supply air path. The air in the supply air path, which is significantly warmed after compression, is used for heating. Heat transfer takes place in an air-to-air heat exchanger through which the supply air and the exhaust air paths pass. Therefore, no heat is required from outside. By heating the exhaust air, the water contained in the exhaust air evaporates, allowing it to be better absorbed by the air to be humidified when it is introduced into the supply air path.

[0010] The process thus enables effective humidification of the air in the supply air path. In particular, humidification by exhaust air recirculation according to the proposed process can replace a membrane humidifier, thus significantly saving space. Compared to water injection, the proposed process has the advantage of improving the air's water absorption capacity, especially at low load points, since the proportion of liquid water in the moist exhaust air is reduced.

[0011] The process also offers other advantages. Another advantage is that heat is extracted from the compressed air to heat the exhaust air. This means that the compressed air is cooled. A cooler downstream of the compressor for cooling the compressed air can therefore be designed smaller or even eliminated entirely.

[0012] Another advantage is that the liquid water contained in the exhaust air evaporates, thus protecting components integrated into the exhaust air path downstream of the air-to-air heat exchanger. This applies in particular to a turbine for energy recovery integrated into the exhaust air path, which is thus protected from droplet impact. At higher load points, the heated exhaust air can also significantly increase turbine efficiency.

[0013] According to a preferred embodiment of the invention, the exhaust air to be recirculated is diverted from the exhaust air path in the air-to-air heat exchanger. Since the heating of the exhaust air in the air-to-air heat exchanger primarily serves to evaporate the liquid water contained in the exhaust air, the exhaust air to be recirculated is preferably diverted at a point in the air-to-air heat exchanger where evaporation is as advanced as possible. This point can vary depending on the operating time, load point, and / or ambient temperature, so a compromise may be necessary. Alternatively, the exhaust air to be recirculated can be diverted at various points.

[0014] Alternatively or additionally, it is proposed that the exhaust air to be recirculated be introduced into the supply air path in the air-to-air heat exchanger. This saves on ductwork, as the supply air path and the exhaust air path are located close together in the air-to-air heat exchanger. Recirculation via a recirculation path integrated into the air-to-air heat exchanger is therefore particularly space-saving. It also ensures that evaporated water does not condense again before it is introduced into the supply air path.

[0015] In principle, the exhaust air to be recirculated can be introduced into the supply air path on the suction side and / or the pressure side of the compressor. If multiple recirculation paths are provided, at least one recirculation path can flow into the supply air path upstream of the compressor. At least one further recirculation path can then flow into the supply air path downstream of the compressor, for example, within the air-to-air heat exchanger.

[0016] Furthermore, it is proposed that a pressure-controlled valve or flap assembly be used for exhaust air recirculation, temporarily increasing the pressure in the exhaust air path and / or temporarily reducing the pressure in the supply air path. The pressure difference between the pressure in the supply air path and the pressure in the exhaust air path causes the valve or flap assembly to open automatically, and exhaust air from the exhaust air path is introduced into the supply air path. This means that separate actuators for operating the valve or flap assembly are not required.

[0017] Furthermore, an air system for a fuel cell system with at least one fuel cell stack is proposed. The air system comprises: - an air supply path through which air can be supplied to the fuel cell stack, - a compressor integrated into the supply air path to compress the air, - an air-to-air heat exchanger integrated into the supply air path downstream of the compressor and - an exhaust air path for removing the exhaust air escaping from the fuel cell stack.

[0018] The exhaust air path leads through the air-to-air heat exchanger and in the air-to-air heat exchanger a recirculation path branches off from the exhaust air path, via which exhaust air can be introduced into the supply air path.

[0019] The air system is particularly suitable for carrying out the method according to the invention described above or can be operated according to this method. Thus, the same advantages as with the method can be achieved with the help of the air system. In particular, the water absorption capacity of the air in the supply air path can be improved because the water to be recirculated is heated beforehand, and the heating causes liquid water contained in the exhaust air to be at least partially evaporated. This effective humidification, in turn, eliminates the need for a membrane humidifier, thus saving installation space. Furthermore, the compressed air passed through the air-to-air heat exchanger is cooled, so that a downstream cooler can be designed smaller or eliminated entirely. This further minimizes the installation space requirement.

[0020] According to a preferred embodiment of the invention, the recirculation path opens into the supply air path upstream of the compressor. This means that the exhaust air to be recirculated is introduced on the suction side of the compressor or in the low-pressure region. This embodiment is particularly advantageous when a pressure-controlled valve or flap arrangement is used for introduction, for the actuation of which the pressure in the exhaust air path must temporarily exceed the pressure in the supply air path.

[0021] According to a further preferred embodiment of the invention, the recirculation path opens into the supply air path downstream of the compressor. This means that the exhaust air to be recirculated is introduced on the pressure side and thus into the high-pressure region. The recirculation of exhaust air into the dynamic high-pressure region improves the mixing of exhaust air and supply air. Furthermore, the ingress of liquid water into the high-pressure region is less harmful. Preferably, the recirculation path opens into the supply air path within the air-to-air heat exchanger, as this allows the recirculation path to be kept very short.

[0022] Preferably, a pressure-controlled valve or flap assembly is integrated into the recirculation path. The recirculation of exhaust air can then be controlled solely by the pressure in the exhaust air path and / or the supply air path. For example, the pressure in the exhaust air path can be temporarily increased so that it exceeds the pressure in the supply air path, opening the valve or flap assembly. Alternatively or additionally, the pressure in the supply air path can be reduced to set the pressure differential required to open the valve or flap assembly.

[0023] Since the preferred application of the proposed air system is a fuel cell system, a fuel cell system comprising at least one fuel cell stack and an air system according to the invention is further proposed. With the aid of the air system, the at least one fuel cell stack can be supplied with moist air, which prevents the membranes of the fuel cells in the fuel cell stack from drying out. Since the proton conductivity of the membranes increases with the moisture content, the system efficiency of the fuel cell system can also be improved.

[0024] 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 an air system according to the invention, Fig. 2 a schematic representation of an air-to-air heat exchanger for an air system according to the invention, Fig. 3 a schematic representation of another air-to-air heat exchanger for an air system according to the invention and Fig. 4 a diagram graphically illustrating the relationship between boiling temperature and final compression pressure. Detailed description of the drawings

[0025] The Fig. The air system 1 according to the invention, shown as an example in Figure 1, serves to supply a fuel cell stack 2 with air. The air is taken from the ambient air and supplied to the fuel cell stack 2 via an air supply path 3 of the air system 1. Air or exhaust air exiting the fuel cell stack 2 is discharged via an exhaust path 4 of the air system 1.

[0026] The air taken from the environment is first freed of harmful particles using an air filter 9 integrated into the supply air path 3. The air is then compressed using a compressor 6 integrated into the supply air path 3. Downstream of the compressor 6, an air-to-air heat exchanger 5 is integrated into the supply air path 3, so that the compressed air flows through it. Since the exhaust air path 4 also runs through the air-to-air heat exchanger 5, the heat from the air, which heats up considerably during compression, is transferred to the exhaust air in the air-to-air heat exchanger 5. Heating the exhaust air helps evaporate the liquid water contained in the exhaust air before the exhaust air is recirculated. This increases the water absorption capacity of the air in the supply air path 3. Furthermore, heating the exhaust air in the exhaust air path 4 cools the air in the supply air path 3.In this way, a cooler 10 integrated into the supply air path 3 downstream of the air-to-air heat exchanger 5 is relieved. The evaporation of the liquid water contained in the exhaust air also has the advantage of protecting a turbine 11 integrated into the exhaust air path 4 from droplet impact.

[0027] In the Fig. Figure 1 shows various locations where exhaust air can be diverted from the exhaust air path 4 and fed into the supply air path 3 via a recirculation path 8. The ideal location depends on various factors, particularly the operating time, the load point, and / or the ambient temperature. If a compromise solution cannot be found, the exhaust air to be recirculated can also be diverted at multiple locations.

[0028] The Fig. Figure 2 shows an example of an air-to-air heat exchanger 5 for an air system 1 according to the invention. To improve heat transfer, it has a plurality of fins 12. The supply air path 3 is in the Fig. 2 is arranged above the exhaust air path 4. This arrangement is chosen merely as an example. The exhaust air path 4 can also be arranged above the supply air path 3. Furthermore, the supply air path 3 and the exhaust air path 4 can also be arranged side by side.

[0029] In the Fig. Figure 3 shows an example of another air-to-air heat exchanger 5 for an air system 1 according to the invention. This has a pressure-controlled valve or flap arrangement 7, through which exhaust air from the exhaust air path 4 can be introduced into the supply air path 3. To open the pressure-controlled valve or flap arrangement 7, the pressure in the exhaust air path 4 is briefly increased so that it is higher than the pressure in the supply air path 3. The phases in which the pressure in the exhaust air path 4 is higher than the pressure in the supply air path 3 then define the recirculation phases.

[0030] In the diagram of the Fig. Figure 4 illustrates the recirculation phases based on the pressure curves on the supply air and extract air sides. While the pressure on the supply air side remains constant (solid line), the pressure on the extract air side is cyclically increased (dashed line) so that it temporarily exceeds the pressure in supply air path 3. During these phases, marked with the letter a, the extract air is recirculated because the valve or flap arrangement 7 is open due to the pressure difference.

[0031] As exemplified in the Fig.As shown in Figure 3, a recirculation path 8 can branch off from the exhaust air path 4—alternatively or in addition to the valve or flap arrangement 7 integrated into the air-to-air heat exchanger 5—which does not open into the supply air path 3 within the air-to-air heat exchanger 5. The recirculation path 8 can, for example, open into the supply air path 3 upstream of the compressor 6. A pressure-controlled valve or flap arrangement 7 can also be integrated into this recirculation path 8.

Claims

[1] Method for operating an air system (1) for supplying a fuel cell stack (2) with air, wherein air is supplied to the fuel cell stack (2) via an air supply path (3) and exhaust air emerging from the fuel cell stack (2) is discharged via an exhaust air path (4), and wherein the air in the air supply path (3) is humidified by means of exhaust air recirculation, characterized by that the exhaust air to be recirculated is heated with the aid of the air in the supply air path (3) by passing the air and the exhaust air through an air-to-air heat exchanger (5) which is integrated into the supply air path (3) downstream of a compressor (6). [2] Method according to claim 1, characterized by that the exhaust air to be recirculated is branched off from the exhaust air path (4) in the air-to-air heat exchanger (5) and / or introduced into the supply air path (3) in the air-to-air heat exchanger (5). [3] Method according to claim 1 or 2, characterized bythat the exhaust air to be recirculated is introduced into the supply air path (3) on the suction side and / or on the pressure side of the compressor (6). [4] Method according to one of the preceding claims, characterized by that a pressure-controlled valve or flap arrangement (7) is used for the exhaust air recirculation and the pressure in the exhaust air path (4) is temporarily raised and / or the pressure in the supply air path (3) is temporarily lowered. [5] 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 (6) integrated into the supply air path (3) for compressing the air, - an air-to-air heat exchanger (5) integrated into the supply air path (3) downstream of the compressor (6) and - an exhaust air path (4) for discharging the exhaust air emerging from the fuel cell stack (2), wherein the exhaust air path (4) leads through the air-to-air heat exchanger (5) and wherein in the air-to-air heat exchanger (5) a recirculation path (8) branches off from the exhaust air path (4), via which exhaust air can be introduced into the supply air path (3). [6] Air system (1) according to claim 5, characterized by that the recirculation path (8) opens into the supply air path (3) upstream of the compressor (6). [7] Air system (1) according to claim 5, characterized by that the recirculation path (8) opens into the supply air path (3) downstream of the compressor (6), preferably within the air-to-air heat exchanger (5). [8] Air system (1) according to one of claims 5 to 7, characterized by that a pressure-controlled valve or flap arrangement (7) is integrated into the recirculation path (8). [9] Fuel cell system with at least one fuel cell stack (2) and an air system (1) according to one of claims 5 to 8.

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