Method for humidifying a gas stream

Abstract

The invention relates to a method for humidifying a gas stream with the aid of a further gas stream having a higher moisture content using a gas-to-gas membrane humidifier (3) which has separate gas paths (1, 2) each with an inlet (1.1, 2.1) and an outlet (1.2, 2.2) for both gas streams. According to the invention, at least one inlet (1.1, 2.1) and one outlet (1.2, 2.2) are exchanged at least once during the service life of the gas-to-gas membrane humidifier (3), so that the respective gas stream flows through the gas-to-gas membrane humidifier (3) in the reverse direction.

Description

[0001] The invention relates to a method for humidifying a gas stream having the features of the preamble of claim 1. The gas stream can in particular be an air stream in an air supply path of an air system for supplying a fuel cell stack with air. 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, specifically ambient air. The required air is supplied to a fuel cell cathode via a gas or air path. Since the energy conversion process in the fuel cells requires a certain air mass flow and a certain pressure level, the air supplied on the cathode side is first compressed using an air compressor arranged in the air path. During compression, the air heats up, so it must be cooled using a cooling device before entering the fuel cells. The air is usually additionally humidified to prevent the fuel cell membranes from drying out. If the membranes are not sufficiently humidified, their proton conductivity decreases.Excessive drying of individual areas of the membranes must also be prevented, as this would otherwise lead to significant aging and thus a significant reduction in the service life of the fuel cells. The area at the cathode inlet is particularly vulnerable.

[0003] Various concepts for humidifying the air in the air path of a fuel cell system are known from the prior art. Humidification can be achieved, for example, using a gas-to-gas membrane humidifier arranged in the air path. This humidifier transports water generated during fuel cell operation from the outlet side to the inlet side of the fuel cells. Humidification can also be achieved by recirculating the moist cathode exhaust air exiting the fuel cells and mixing the recirculated exhaust air with the dry supply air. Furthermore, the dry supply air can be humidified by water injection.

[0004] The currently most widely used concept for humidifying the cathode supply air is the use of a gas-to-gas membrane humidifier. The water transport between the gases on the supply air and the exhaust air side occurs via a membrane. Similar to the membranes of fuel cells, the membrane of the membrane humidifier is also subject to aging, so that the water transport from the exhaust air to the supply air side deteriorates over time. As the membrane humidifier ages, the risk increases that the supply air supplied to the fuel cells is not sufficiently humidified, leading to a drop in fuel cell performance. In addition, the fuel cell membranes can become damaged, so that along with the reduction in performance, their service life also decreases. To prevent this, the membrane humidifier must be replaced in a timely manner. This involves effort and expense.

[0005] The present invention is concerned with the object of reducing or completely avoiding the above-mentioned disadvantages of the prior art.

[0006] To achieve this object, the method having the features of claim 1 is proposed. Advantageous further developments of the invention can be found in the subclaims. Disclosure of the invention

[0007] A method is proposed for humidifying a gas stream with the aid of another gas stream having a higher moisture content using a gas-to-gas membrane humidifier. The gas-to-gas membrane humidifier has separate gas paths, each with an inlet and an outlet for both gas streams. According to the invention, at least one inlet and one outlet are swapped at least once over the service life of the gas-to-gas membrane humidifier, so that the respective gas stream flows through the gas-to-gas membrane humidifier in the opposite direction.

[0008] The gas stream to be humidified using the gas-to-gas membrane humidifier has a very low moisture content near the inlet and a high moisture content near the outlet, as it absorbs moisture as it flows through the gas-to-gas membrane humidifier. The greatest aging stress on the membrane of the gas-to-gas membrane humidifier occurs at the point where the lowest moisture content is expected, i.e., at the inlet of the gas stream to be humidified. If, according to the proposed method, the inlet and outlet are swapped after a certain period of time, the process is reversed. This means that the greatest aging stress is now expected at the former outlet, now the inlet. The former inlet, now the outlet, is protected. The proposed method therefore contributes to homogenization and thus to a reduction in the aging of the gas-to-gas membrane humidifier over time.The function of the membrane humidifier is maintained over a longer period of time, so that its service life is increased and the membrane humidifier does not need to be replaced until much later.

[0009] To implement the proposed method, various measures can be taken, as shown in the preferred embodiments described below.

[0010] According to a first preferred embodiment, to replace at least one inlet and one outlet, the gas-to-gas membrane humidifier is at least partially removed and reinstalled in a correspondingly modified installation position. In this case, nothing needs to be changed in the installation situation; only the membrane humidifier is rotated—as a whole or in parts—so that the gas stream to be humidified is supplied via the previous outlet and discharged from the membrane humidifier via the previous inlet. For this purpose, the membrane humidifier is preferably designed symmetrically so that it can be rotated easily or with minimal workshop effort.

[0011] Gas-to-gas membrane humidifiers are known from the prior art, comprising multiple membrane fiber bundles or membrane stacks. These can form one large functional unit or several individual functional units. In this case, instead of the entire membrane humidifier, only one or more functional units can be rotated so that the respective gas flow flows through them in the opposite direction.

[0012] Ideally, at least one part of the membrane humidifier, particularly a functional unit, through which the gas stream to be humidified flows is rotated, since the aging stress is highest in the inlet area. However, since the other gas stream also releases moisture as it flows through the membrane humidifier, thus reducing its moisture content, the flow direction of the gas stream can also be reversed here by swapping the inlet and outlet.

[0013] According to a further preferred embodiment, to swap at least one inlet and one outlet, at least two line sections of a gas line connected to the gas-to-gas membrane humidifier are swapped. In this case, the installation position of the membrane humidifier does not need to be changed, only the connection situation. This means that the removal and reinstallation of the membrane humidifier is not necessary. Instead, the line sections of a gas line connected to the membrane humidifier are simply swapped. After the swap, the line section that was previously connected to the inlet for the gas flow is connected to the outlet, and the line section that was previously connected to the outlet is connected to the inlet. As a result, this measure also reverses the flow direction of the respective gas flow through the membrane humidifier.

[0014] The connection configuration can be changed for one or both gas streams. If only one connection configuration is to be changed, then preferably the one for the gas stream to be humidified, since the aging stress is highest in the inlet area.

[0015] The previously described measures for swapping at least one inlet and one outlet for a gas stream can also be combined. This can prove particularly advantageous if the gas-to-gas membrane humidifier is not symmetrically constructed. In this case, the flow direction for a first gas stream can be reversed by removing, rotating, and reinstalling the membrane humidifier.

[0016] To reverse the flow direction of the further gas flow, the connection situation of the gas lines can then be changed.

[0017] Furthermore, the at least one inlet and the at least one outlet are preferably swapped over time and / or depending on the load. For example, swapping can occur after a defined period of time, preferably operating time. The operating time, in particular, allows conclusions to be drawn about the aging state of the membrane humidifier.

[0018] Alternatively or additionally, at least one state variable can be used to determine the optimal time for swapping at least one inlet and one outlet for a gas flow. The at least one state variable can be measured using sensors and / or a model. The degree of stress can be determined using the at least one state variable, and this can be used to determine the aging status of the membrane humidifier.

[0019] For example, the amount of water transferred via the membrane humidifier can be measured by sensors. Alternatively or additionally, the period over which a specified temperature limit was exceeded at the inlet of the gas stream to be humidified can be determined or measured by sensors.

[0020] Advantageously, when implementing the proposed process, the two gas streams are passed along the gas paths in countercurrent or crosscurrent. This means that the flow directions of the two gas streams differ. In countercurrent, they are passed parallel to each other. In crosscurrent, their gas paths intersect. This can promote the transport of water from one side to the other.

[0021] In a further development of the invention, it is proposed that the gas-to-gas membrane humidifier is integrated into an air supply path of an air system for supplying a fuel cell stack with air, and that an air supply stream is passed through the gas-to-gas membrane humidifier via the first gas path and an exhaust air stream is passed through the gas-to-gas membrane humidifier via the further gas path. In this method, the moist exhaust air on the cathode side of a fuel cell stack is used to humidify the dry air supply. Since the membrane of the gas-to-gas membrane humidifier is subject to high aging stress in the inlet area of ​​the dry air supply, the aging stress can be distributed more evenly and thus reduced by exchanging at least one inlet and one outlet of a gas stream, preferably the gas stream to be humidified.

[0022] The invention and its advantages are explained in more detail below with reference to the accompanying drawings. These show: Fig. 1 a schematic representation of a gas-to-gas membrane humidifier through which a first and a second gas stream are passed in cross flow, Fig. 2 a schematic representation of the gas-to-gas membrane humidifier of the Fig. 1 after changing the connection situation for the gas lines for the gas flow to be humidified and Fig. 3 a schematic representation of the gas-to-gas membrane humidifier of the Fig. 1 after turning a functional unit of the membrane humidifier. Detailed description of the drawings

[0023] The Fig. 1 shows a gas-to-gas membrane humidifier 3, through which a first gas path 1 for a gas stream to be humidified and a second gas path 2 for a humid gas stream lead. Each gas path 1, 2 has an inlet 1.1, 2.1 and an outlet 1.2, 2.2. The inlet 1.1 and the outlet 1.2 of the first gas path 1 are each connected to line sections 4.1, 4.2 of a gas line 4. The gas line 4 can, in particular, be an air supply path 6 of an air system for supplying a fuel cell stack with air. The inlet 2.1 and the outlet 2.2 of the second gas path 2 are each connected to line sections 5.1, 5.2 of a further gas line 5. The further gas line 5 can, in particular, be an exhaust air path 7 of an air system for supplying a fuel cell stack with air.

[0024] The gas paths 1, 2 are arranged so that they intersect. The gas streams are thus guided past each other in crossflow. The area where the gas stream to be humidified is introduced and faces the other, already dehumidified, gas stream in the membrane humidifier 3 is the driest area 9 and thus the area where aging stress is highest.

[0025] To slow down the aging process, as exemplified in the Fig. 2 - the line sections 4.1 and 4.2 are swapped, so that the former outlet 1.2 now represents the inlet 1.1, and the former inlet 1.1 now represents the outlet 1.2. The driest area 9 is now on the opposite side. The aging stress in this area was previously not very high, so changing the connection situation can achieve more even aging of the membrane humidifier 3.

[0026] As shown by way of example in the figures, the membrane humidifier 3 can have a housing 10 with a functional unit 8 accommodated therein. Within the functional unit 8, the gas paths 1, 2 can be divided into many parallel gas paths.

[0027] Fig. Figure 3 shows another way to slow down the aging process of the membrane humidifier 3 by not changing the connection situation, but by rotating the functional unit. This swaps the positions of the inlet 1.1 and the outlet 1.2, so that the gas flow through path 1 is reversed. The driest area 9, or the area with the highest aging stress, is located in the Fig. 3 now not in the upper left corner, but in the lower right corner (reference number 9').

[0028] The invention is not limited to cross-flow arrangements, but can be applied to counter-flow arrangements. The driest area 9 is then located not in a corner, but on one side of the membrane humidifier 3.

Claims

[1] Method for humidifying a gas stream with the aid of a further gas stream having a higher moisture content using a gas-to-gas membrane humidifier (3) which has separate gas paths (1, 2) each with an inlet (1.1, 2.1) and an outlet (1.2, 2.2) for both gas streams, characterized by that over the service life of the gas-to-gas membrane humidifier (3) at least one inlet (1.1, 2.1) and one outlet (1.2, 2.2) are exchanged at least once, so that the respective gas flow flows through the gas-to-gas membrane humidifier (3) in the opposite direction. [2] Method according to claim 1, characterized by that in order to replace at least one inlet (1.1, 2.1) and one outlet (1.2, 2.2) the gas-to-gas membrane humidifier (3) is at least partially removed and reinstalled in a correspondingly changed installation position. [3] Method according to claim 1 or 2, characterized bythat in order to exchange at least one inlet (1.1, 2.1) and one outlet (1.2, 2.2), at least two line sections (4.1, 4.2, 5.1, 5.2) of a gas line (4, 5) connected to the gas-to-gas membrane humidifier (3) are exchanged. [4] Method according to one of the preceding claims, characterized by that the at least one inlet (1.1, 2.2) and the at least one outlet (2.1, 2.2) are exchanged depending on time and / or load. [5] Method according to one of the preceding claims, characterized by that the two gas streams are passed by each other via the gas paths (1, 2) in countercurrent or crosscurrent. [6] Method according to one of the preceding claims, characterized bythat the gas-to-gas membrane humidifier (3) is integrated into a supply air path (6, 7) of an air system for supplying a fuel cell stack with air and a supply air flow is guided through the gas-to-gas membrane humidifier (3) via the first gas path (1) and an exhaust air flow is guided through the gas-to-gas membrane humidifier (3) via the further gas path (2).

Images