DE102011116679B4 - Portable fuel cell system with liquid separators and use, method for recovering a liquid and simulation model - Google Patents

Abstract

Portable fuel cell system, comprising at least two in a circuit of a fuel cell (101, 1406) arranged liquid separator (107, 108), wherein in a cathode circuit (113) between a cathode (102) and a mixer (110) of the liquid separator (108) is arranged and in an anode circuit as degasser the liquid separator (107) is arranged between an anode (103) and the mixer (110), the liquid separator (108) having at least one fluid guide channel (201, 301) with a fluid guide channel inner wall (205, 309). a fluid guide channel inlet (203, 303) and a fluid guide channel outlet (306) for guiding a multiphase mixture of at least two immiscible fluids; and at least one fluid discharge channel (204, 302, 502) extending from the fluid guide channel (201, 301) having a fluid discharge channel inner wall and a Flüssigkeitsabführkanalauslass (310) for discharging liquid t of the multiphase mixture, and wherein an inflow of fluid guided in the fluid guide channel (201, 301) into the Flüssigkeitsabführkanal (204, 302, 502) is effected by means of capillary forces and in this case in the Flüssigkeitsabführkanal (204, 302, 502) portion removed the fluid passing through the fluid guide channel (201, 301) is adjustable by changing at least one of the fluid guide channel inlet, the fluid guide channel outlet (306), or the liquid outlet channel outlet (310), and wherein a sensorless feed-forward control for liquid separation is implemented, that the temperature of the liquid separator is fixable.

Description

The present invention relates to a fuel cell system with fluid separators, preferably liquid separators and its use, in particular a portable fuel cell and a method for recovering liquid from a fuel cell cycle of a portable fuel cell and a simulation model.

Fuel cells, such as direct methanol fuel cells, abbreviated to DMFC, are capable of current lithium-ion batteries in portable systems due to their much higher energy density and the ability to replenish them instantaneously replace or at least supplement.

Many fuel cells, especially many well-known DMFC systems, use gas-liquid separators. These passive gas-liquid separators, however, usually lead to the necessity of using active sensory components, which is why a usability of the fuel cell is given only in a defined orientation. This limits the portability of such portable fuel cells.

For example, in the DE 100 40 088 A1 described a DMFC system having a water separator which is arranged downstream of the cathode output. The separated water is then fed to the anode after a subsequent rectification. The fuel cell includes, among other things, a methanol sensor. The structure of the DMFC system does not allow permissive portability.

For example, the EP 1 383 191 A1 a special embodiment of a fluid separation device for separating liquid-soluble components from gases. This fluid separation device also includes, among other things, a measuring device for determining the amount of liquid present within the fluid separation device. The system is to be arranged for operation in a special location, so it is therefore not free portable.

Out DE 10 2009 011 239 A1 a portable fuel cell system is known in which a liquid separator is integrated in a circuit of the fuel cell. The fluid guide channels have an inlet and an outlet, wherein discharge channels for removing the product water are discharged from the fluid guide channels. From a gas / water mixture can be selectively removed by capillary forces water.

A fuel cell system in which, due to insufficient capillary forces, a discharge of water by means of pressure, is out US 2011/0159399 A1 known.

Likewise is known DE 10 2009 040 786 B3 a gas distributor for the passive discharge of water from gas distribution channels of polymer electrolyte membrane fuel cells, which has a plurality of capillaries arranged continuously through the gas distributor, which serve for the passive discharge of water arising during operation of the fuel cell by capillary forces.

Previously known fuel cells, including mobile fuel cells, are functional of their orientation in space, so from their position only in a defined orientation. The location-specific behavior of fuel cells, especially of DMFC systems, can be seen as a consequence of the fact that a two-phase fluid flow is present at the exit of the cells. In conventional DMFC systems, for example, a mixture of gaseous CO ₂ and methanol solution flows out of the anode outlet, whereas a mixture of air and water droplets flows out of the cathode outlet, in which case the water must be at least partially recycled to the system. The water balance of a DMFC system is not self-stabilizing, but must be stabilized via a feedback control, for which a measurement of the amount of water in the system is necessary. The same applies analogously to other types of fuel cells with other liquids.

In portable systems, such measurement of the amount of water in the system is not easy to realize because the available approaches to measuring amounts of water, such as the hydrostatic pressure measurement, using ultrasound or weight measurement, explicitly align the fuel cell system with the fuel cell Require direction of gravity. Therefore, the possibilities to order a portable fuel cell system can be limited.

It is therefore the object to provide a portable fuel cell system available that provides more freedom of movement in terms of use available than the previous, and in which a change in position means no restriction of operability.

The object is achieved with a portable fuel cell system having the features of claim 1 as well as with a method having the features of claim 8.

The portable fuel cell system is used in applications as disclosed in claim 9. Further advantageous embodiments will become apparent from the dependent claims as well as from the following description. One or more features of the claims, the description, as well as the figures may be linked with one or more other features thereof to further embodiments of the invention. In particular, one or more features of the independent claims may be replaced by one or more other features of the disclosure. The proposed subject matter is to be construed only as a draft to formulate the invention without, however, limiting it.

A portable fuel cell system comprising at least one fuel cell is proposed. The portable fuel cell system comprises at least two liquid separators arranged in a circuit of a fuel cell. The circuit can be a cathode circuit. But it can also be an anode circuit. Also, the portable fuel cell system may be configured so that in each case a liquid separator is provided in both cathode circuits. For example, in a liquid-conducting cathode circuit, the proposed liquid separator is arranged between a cathode of the fuel cell and a mixer or a mixing device.

The portable fuel cell system may include a single fuel cell. However, it is also possible for there to be a plurality of fuel cells which, for example, function separately from one another or are interconnected with one another.

Depending on the use, the liquid separator can either serve to enable a recovery of liquid, in particular of water, for the circulation. For this purpose, at least liquid droplets are separated out of a multiphase stream consisting mainly of gas. Such a liquid separator is connected, for example, to an air outlet of a fuel cell and can also be referred to as a condensate separator. On the other hand, the liquid separator can also serve to separate a gas from a liquid flow. For example, it can be used as a degasser in an anode circuit.

In the following, the object will be explained in more detail with reference to a first exemplary embodiment. However, individual features of this example can also be used in other embodiments of other examples. Here, as the liquid separator, a water separator which is disposed between a cathode of the fuel cell and a mixer of the fuel cell system will be described in detail. The water separator comprises at least one fluid guide channel with a fluid guide channel inner wall and at least one fluid guide channel inlet and a fluid guide channel outlet. Starting from the fluid guide channel, a Flüssigkeitsabführkanal with a Flüssigkeitsabführkanalinnenwandung and a Flüssigkeitsabführkanalauslass is arranged on the fluid guide channel. This Flüssigkeitsabführkanal preferably leads from the fluid guide channel to the mixer to supply this liquid from the fluid guide channel. The inflow of liquid guided in the fluid guide channel is effected by means of capillary forces. The proportion of the liquid in the fluid guide channel, which flows from this into the Flüssigkeitsabführkanal is adjusted by the change of at least one pressure. For example, this pressure can be changed from all pages. Preferably, the pressure at the fluid guide channel inlet changes, at the fluid guide channel outlet and at the liquid discharge channel outlet. The adjustability of the portion of the liquid present in the fluid guide channel guided into the liquid discharge channel is preferably generated by means of one or more variable pressure regulators influencing the liquid separation in a cycle of the portable fuel cell. Preferably, a controllability is even provided for this purpose instead of a control, so that the proportion of liquid flowing into the Flüssigkeitsabführkanal can be adjusted in a targeted manner.

Incidentally, the term "portable" according to the invention is defined such that the fuel cell system comprising at least the fuel cell is designed in such a compact construction as to be manually transported or moved by a user or at least several users. Thus, the term "portable" is different from the term "mobile". The latter enables transportability by using other power sources, such as cars. In contrast, "Portabel" distinguishes itself further from that by the user himself the device, or at least the fuel cell cycle can be worn as an energy source without further aids. A portable use is therefore the one that is possible with known accumulators. This includes, for example, the use of cell phones, laptops, model vehicles, function-controlled vehicles and flying objects, portable power generators, hand tools such as craft machines such as hand grinder, drill, spray guns, compressors, pumps, cordless screwdrivers or gardening tools such as hedge trimmers, lawnmowers, etc. or also forestry equipment such as in particular saws, for example chain saws, or even with mobile measuring devices. Also, a use is preferably seen as an exchangeable power, preferably drive accumulator.

Furthermore, the word constituent "channel" is in none of the above cases to be understood to mean that it implies a certain geometry. Rather, both the fluid guide channel and the Flüssigkeitsabführkanal can be used in any conceivable course as well as any conceivable cross-section, provided that the described functionality is ensured. Appropriately, however, the respective embodiment in a tubular shape. This promotes the production of sufficient capillary forces. The fuel cell comprises a cathode circuit. The concept of the cathode cycle refers first to the fact that it is a circuit that has its output in the cathode. However, the circuit may close at any other location, such as in the anode area. However, also possible is an output of the circuit from the anode, even in such a case, the described and claimed embodiment and operation would remain valid. In such a case, instead of the term of the cathode circuit, the term of an anode circuit or, more generally, the term of a fluid circuit circuit could be used. The term fluid circuit takes into account the fact that one or more fluids are present in each case before the fuel cell and after the fuel cell. These can be present as products or educts on the anode or cathode side.

Furthermore, the fuel cell comprises a mixer, which is preferably both part of the cathode and the anode circuit. In a fuel cell, it is the task of the mixer, for example, to mix the fuel with other liquids. For example, in the case of a direct methanol fuel cell, this means that in this case methanol originating from a methanol reservoir is mixed in the mixer with a liquid consisting predominantly of water and is subsequently conducted into the anode. In order not to have to constantly add new water to the fuel cell, it is provided to separate waste water, which originates for example from the cathode or else from the anode, and then add it to the mixer. For this purpose, a liquid separator is arranged here in the form of a water separator between the cathode and the mixer. However, this is by no means to be understood that the water separator must be arranged directly on the cathode and the mixer. In addition to lines between the cathode and the water separator and the water and the mixer further functional elements can also be arranged. For example, it is possible that a cooling device for cooling the liquid is arranged between the cathode and the water separator. With the explicit mention of the example of a direct methanol fuel cell, however, all the features described should not be limited to such. In principle, all the features described are also in other portable fuel cells such as in alkaline fuel cells, in polymer electrolyte fuel cells, in formic acid fuel cells, in phosphoric acid fuel cells, in molten carbonate fuel cells, in solid oxide fuel cells, in carbon fuel cells, in English as well Direct Carbon Fuel Cell, abbreviated DCFC, or equally usable in magnesium air fuel cells. This list is only an example, but not exhaustive.

The term water separator is to be understood to mean that this separator can also be a separator for any other liquids or liquid mixtures. The term water separator has been chosen because, when the separator is readjusted to a cathode of a direct methanol fuel cell, a liquid is separated which comprises a large amount of water. In principle, however, the separation of other liquids is possible. In an anode of a direct methanol fuel cell falls, for example, a methanol solution, which can also be deposited with a separator. However, the separation of any other liquids in other types of fuel cells is also included in the term water separator in principle.

The water separator comprises at least one fluid guide channel. This fluid guide channel can in this case have any cross-sectional shapes and cross-sectional areas and also change along its extent in cross-sectional shape and / or area. The fluid guide channel in this case runs from the fluid guide channel inlet to the fluid guide channel outlet, wherein the course between these two course not only linear must take, but any gradients are conceivable, so spiral, arcuate, serpentine, meandering or any other conceivable courses. Of the Fluid guide channel inlet is determined by the point from which the fluid guide channel going off. For example, when the fluid guide channel is directly connected to a radiator, the fluid guide channel inlet is the position at which the radiator terminates and the fluid guide channel begins to extend. The same applies to the Fluidführungskanalauslass. The latter can also open directly into the environment, for example, directly without the connection of another component, so that excess liquid can be removed from the circuit of the fuel cell together with excess gases. It is also possible that the mentioned fluid guide channel is not limited to using only one channel. It is also possible to use a plurality of fluid guide channels connected one behind the other and / or in parallel.

The fluid guide channel is preferably provided to guide fluids that have been introduced into the fuel cell and / or caused by reaction processes, especially in the cathode or in the anode lead. For example, flows from the output of the direct methanol fuel cell, a mixture of gaseous CO ₂ and methanol solution, while flowing from the outlet of the cathode of the direct methanol fuel cell, a biphasic mixture of air and water droplets. In principle, however, the described system is by no means limited to the example of a direct methanol fuel cell and can also be used in the presence of other multiphase mixtures in which at least two, but possibly also more, phases may be present separately. This also applies in all conceivable cases of existing fluid mixtures having at least two phases, such as in particular the plug flow, in which clearly separate gas and liquid segments are present, the bubble flow, present in the gas bubbles in the liquid flow or liquid bubbles in the gas stream, or the ring flow, the is characterized by a gas core and a liquid film on the wall or vice versa.

As a component of the described water separator, at least one liquid discharge channel arranged on the at least one fluid guide channel is provided. This Flüssigkeitsabführkanal can be arranged at any point of the fluid guide channel, wherein preferably the Flüssigkeitsabführkanal leads away from the sheath of the fluid guide channel perpendicular. But even an arrangement at a different angle can be advantageous under given conditions. The described arrangement of at least one Füssigkeitsabführkanal to the fluid guide channel is to be understood that a plurality of Flüssigkeitsabführkanälen may be disposed on the fluid guide channel, in particular, depending on the requirement, a plurality of Flüssigkeitsabführkanälen can also be arranged on more than one fluid guide channel. The fluid guide channels can hereby differ from each other in cross-sectional area and geometry and their length. The same applies to the Flüssigkeitsabführkanäle. In particular, the Flüssigkeitsabführkanäle a smaller flow area than the or the fluid guide channel. For example, several Flüssigkeitsabführungskanäle can be arranged offset from one another on the circumference of the fluid guide channel, not only along the fluid guide channel, but also around the circumference. For example, the liquid guide channels can be arranged offset by 20 ° to 45 ° around the circumference and longitudinally offset from one another on the fluid guide channel.

In the proposed fuel cell, for example, it is provided that the inflow of liquid from the fluid guide channel into the Flüssigkeitsabführkanal is effected by means of capillary forces. This means that the Flüssigkeitsabführkanal at least partially, but preferably over its entire extent, a sufficiently small inner cross-sectional area, so that the resulting capillary forces as a result of the surface tension of the liquid and the interfacial tension between the liquid and the inner wall of the Flüssigkeitsabführkanals and possibly the opposite direction of gravity Press liquid in the Flüssigkeitsabführkanal and can move in the direction away from the fluid guide channel direction. A stationary pressure acting on the liquid from the side of the fluid guide channel inlet and the fluid guide channel outlet on the liquid still support this effect. On the other hand, a pressure acting from the side of the liquid discharge channel outlet reduces the effect of the capillary forces again to the greater extent as this pressure is stronger. This means that a variation of these pressures affects the effect of the capillary forces in the sense that they can be strengthened, weakened or even completely disappear as well as negative, for example, if pressures acting from the side of the liquid discharge channel outlet become sufficiently strong. By changing one or more of these pressures, it is thus possible to adjust the proportion of the liquid present in the fluid guide channel at any given time, which flows into the liquid discharge channel. When there is a controlled change in these pressures, either by direct change or as indirect change, for example, in response to the change in certain process parameters, the flow of fluid from the fluid guide passage through the fluid discharge passage can thus be controlled, preferably regulated.

In a preferred embodiment, it is provided that a dimensioning and design of the liquid separator, in particular of the water separator takes place in such a way that the acting capillary forces allow neglecting a gravitational force. This means, for example, that gravitational forces are not necessary to ensure the guidance of liquid from the fluid guide channel through the Flüssigkeitsabführkanal. This is ensured, for example, if, with otherwise unchanged parameters, the proportion of fluid present in the fluid guide channel which flows into the fluid discharge channel differs by 10% or less when moving against gravity and when moving without the influence of gravity. A use for which the gravitational force can be neglected can be particularly advantageous in cases in which the orientation of the fuel cell during use is not determined from the outset or even changes during use. Furthermore, a use in weightless space may be provided, so in quasi-stationary space stations, in probes, in equipment, in particular measuring instruments at z. As freefall tests, as used in parabolic flights or in towers, or in spaceships.

According to one embodiment, it is provided that a sensorless feed-forward control is provided for the fuel cell system, preferably implemented in a fuel cell cycle. For example, such a feed-forward control regulates essential process parameters without the need for an active sensory measurement of other process parameters. According to one embodiment, an increase in the liquid level in a component, preferably the mixer of the fuel cell, lead to an increase in the pressure therein, whereupon, due to a suitable design of the water separation, the proportion of the water present in the fluid guide channel which is deposited decreases again. In addition to the mixer, however, the pressure increase can also take place in other components, such as in a line system itself, in a specially constructed for this purpose bypass channel and / or a specially designed for regulating a back pressure collecting container, which, for example, by the selection of its material Ensures sufficient sensitivity to a pressure change. The material is preferably selected according to its expansion properties, in particular according to the modulus of elasticity. This concept is not only usable for water, but also for any other liquids and gases. Due to the knowledge of the relationship between the water output from the cathode and the condenser temperature, the feed-forward control can be limited, for example, to simply fixing the temperature of the condenser or of the liquid separator. In particular, the design of the feed-forward control allows a self-adjusting and regulating liquid recirculation system that works independently of location.

In a further embodiment, it is provided that the fluid guide channel inner wall is hydrophobic. This first relates to the property of the fluid guide channel inner wall that a fluid drop applied thereto, in particular water drops, forms a contact angle of more than 90 ° with the surface of the wall. Such a design exerts an additional force on the water, which promotes an inflow of water into the Flüssigkeitsabführkanal. In this case, only a partial embodiment of the fluid guide channel inner wall as a hydrophobic surface is already helpful for achieving such an effect, in particular if hydrophobicity is present, preferably at least in the area around the branching off of the liquid discharge channel. The hydrophobicity can be achieved by a variety of measures. A first possible measure is a suitable material selection of the fluid guide channel inner wall or, for example, only its coating with suitable hydrophobic materials such as waxes, paraffins, polymers such as in particular polytetrafluoroethylene. Another possible measure is the suitable structuring of the surface of the fluid guide channel inner wall or the selection of suitably structured surfaces. For this purpose, for example, the highest possible surface roughness is provided, preferably with R _q = 50 nm or more, preferably in combination with a microstructured or nanostructured texturing. However, the term "hydrophobic" should not be limited to the repulsion of water, although this is a preferred example considering the example of direct methanol fuel cells. Already also in direct methanol fuel cells, however, as already explained above, the separation of methanol from a fluid mixture of gaseous CO ₂ and liquid methanol is desired. For such a case, the term "hydrophobic" is to be understood to mean that the contact angle with a drop of methanol is 90 ° or greater. Similarly, in embodiments of fuel cells of other types, the surface property would be chosen according to the fluid flowing through the fluid guide channel. Furthermore, the term "hydrophobic" can be understood even further, namely to the effect that the surface of the fluid guide channel inner wall is less hydrophilic than the surface of the Flüssigkeitsabführkanalinnenwandung.

In a further embodiment, it is provided that the Flüssigkeitsabführkanalinnenwandung is hydrophilic. This refers first to the property of Flüssigkeitsabführkanalinnenwandung that a forms on this applied water droplets with the surface of the wall a contact angle of less than 90 °. Such a design exerts an additional force on water, which promotes a flow of water into the Flüssigkeitsabführkanal and favors a flow along the Flüssigkeitsabführkanals in a direction away from the fluid guide channel direction. Here, only a partial embodiment of the Flüssigkeitsabführkanalinnenwandung as a hydrophilic surface for the achievement of such an effect is helpful, especially if there is preferably at least in the area around the branch of Flüssigkeitsabführkanals hydrophilicity. The hydrophilicity can be achieved for example by a variety of measures. A first possible measure is a suitable material selection of the Flüssigkeitsabführkanalinnenwandung or for example their coating with one or more hydrophilic materials such as glasses, metals, polar substances or salts. Another possible measure is a structuring of the surface of the Flüssigkeitsabführkanalinnenwandung or a selection of suitably structured surfaces. For this purpose, for example, it is advantageous to provide as high a surface roughness as possible with R _q = 50 nm or more, in particular in combination with a microstructured or nanostructured texturing. However, the term "hydrophilic" is not intended to be limited to wettability with water, although this is an advantageous example considering the example of direct methanol fuel cells. Already also in direct methanol fuel cells, however, as already explained above, the separation of a methanol solution from a fluid mixture of gaseous CO ₂ and from liquid methanol solution is also possible. For such a case, the term "hydrophilic" should be understood to mean that the contact angle with a drop of methanol solution is 90 ° or less. Analogously, in embodiments in fuel cells of a different type, in accordance with the liquid flowing through the fluid guide channel, the material will be selected such that a hydrophilicity sets in. Furthermore, the term "hydrophilic" can be understood even further than the feature namely by the fact that the surface of the Flüssigkeitsabführkanalinnenwandung is less hydrophobic than the surface of the Fluidführungskanalinnenwandung.

In a further embodiment, it is provided that the inner cross-sectional area of the Flüssigkeitsabführkanals is smaller, at least in the region of the branch of the fluid guide channel than the inner cross-sectional area of the fluid guide channel in the region of the branch of Flüssigkeitsabführkanals. Such a configuration, if the materials of the fluid guide channel inner wall and the Flüssigkeitsabführkanalinnenwandung equal, or the materials are different, but still the interfacial tension between the present in the fluid guide channel fluid and each of the two inner walls are identical or nearly identical, leads to a formation of capillary forces, which favors the movement of the liquid along the Flüssigkeitsabführkanals, unless further, opposite forces do not lead to a reduction of this effect. Even in cases where the interfacial tension between the liquid and the Flüssigkeitsabführkanalinnenwandung is greater than the interfacial tension between the liquid and the Fluidführungskanalinnenwandung - for example, of the liquid water so the Flüssigkeitsabführkanalinnenwandung is less hydrophilic than the Fluidführungskanalinnenwandung - can thus by sufficiently large differences in the Cross-sectional areas of this effect are overcompensated and the addition of the forces as a whole still lead to a preferred flow of the liquid in the Flüssigkeitsabführkanal. For the design of the geometrical variables as well as the selection of the materials of the channels, in one embodiment it is decisive, for which magnitudes of the adjustable or changing remaining pressures in the system whose influence has already been mentioned above, a change in the flow behavior of the liquid at the branch is sought between the fluid guide channel and Flüssigkeitsabführkanal. In a particularly preferred embodiment, the inner cross-sectional area of the Flüssigkeitsabführkanals about 0.1 mm ^{2 is} large, while the inner cross-sectional area of the fluid guide channel with about 0.2 mm ^{2 is} about four times.

In a further advantageous embodiment, the fuel cell is characterized in that it can be used regardless of location. This means that the functional activity of the fuel cell is maintained regardless of the spatial position in which it is located. A position-independent functionality can be due, for example, to the fact that due to acting capillary forces gravitational forces for influencing the flow direction can be neglected. Furthermore, the position independence can be ensured by the fact that no use of sensors in the fuel cell is necessary, which in turn imply a position dependency. Examples include sensors whose measuring principles are based on hydrostatic pressure, on ultrasound or on a weight measurement.

In a further advantageous embodiment, at least the water separator of the fuel cell is exchangeable. The water separator may be removed from the overall system of the fuel cell, preferably as a complete unit, and thereafter reused after, for example, maintenance. Alternatively, an exchange of the water separator with a completely new replacement system can be provided, for example, in cases where due to contamination of one or more channels Channels the proper functioning of the water separator is no longer guaranteed. In a particularly advantageous embodiment, the replaceable water separator is designed such that it can be introduced via a slide-in system in a recess provided for this purpose.

According to a further aspect of the invention, a method is proposed, by means of which a liquid is recovered from a multiphase mixture which, starting from a cathode outlet of a portable fuel cell, is guided through a fluid guide channel. In this case, the fluid guide channel has at least one fluid guide channel inlet and one fluid guide channel outlet. In addition, at least one Flüssigkeitsabführkanal is disposed on the fluid guide channel. The inflow of a portion of the guided through the fluid guide channel as part of the multiphase mixture liquid in the Flüssigkeitsabführkanal is effected by at least one capillary force occurring. The size of this portion of liquid can be adjusted in the process. For this purpose, an effective pressure is preferably changed, which is in particular from the outside, especially on the multiphase mixture acting pressure. This also relates, for example, to the sum of all pressure components, but preferably at least to the pressures acting on the fluid guide channel inlet, on the fluid guide channel outlet and / or on the liquid outlet channel outlet. Upon action of one or more of these pressures, the forces acting together with all other forces acting, as well as, for example, by the above-mentioned capillary forces, so that by changing the pressures of the proportion of guided in the fluid guide channel as part of the multiphase mixture, the liquid is passed into the Flüssigkeitsabführkanal set, preferably regulated, is.

A method is further provided, which comprises that an increasing amount of liquid in the mixer leads to an increase of the pressure prevailing in the mixer, which in turn leads to an increase of the pressure also prevailing at the liquid discharge channel outlet and vice versa. This pressure manifests itself in one or more forces, which oppose a capillary force pointing away from the fluid guide channel, so that, with an increased pressure, the proportion of the fluid present in the fluid guide channel which is pressed into the fluid discharge channel decreases. Furthermore, it is possible that the same principle applies, if not only in the mixer, but instead or additionally in any further, arranged between the mixer and the Flüssigkeitsabführkanalauslass components, a corresponding pressure increase takes place.

A further embodiment provides that a feed-forward control of the amount of liquid present in the mixer takes place without direct measurement of the amount of liquid. This feed-forward control involves automatic stabilization of the liquid balance in the mixer. However, such a feed-forward control is possible not only in the mixer, but also in other components of the fuel cell. For example, the feed-forward control on the fixation of the condenser temperature or the temperature of the liquid separator may be limited to a certain value. This value is dependent on the dimensioning of the various components of the fuel cell, but can be estimated, as will be explained with reference to an embodiment.

It is further contemplated to use the fuel cell for powering a mobile phone, a portable computer, a remote control, a small aircraft or a small watercraft. Here, the term mobile phone includes all electronic devices that are portable and with which you can access telecommunications applications. The term computer refers here to all portable computers, such as laptops, notebooks, handhelds, tablet computers and the like. Portable home appliances refers to portable devices that are used in the home. This includes, for example, vacuum cleaners, lawn mowers, but also devices for the production or maintenance of personal hygiene, such as dry shaver, hair dryer and similar devices, so that they are put into operation without dependence on the mains. Furthermore, a use for the operation of small vessels or small aircraft, such as drones may be provided. For the operation of the devices, it is not necessary here that the fuel cell be integrated directly into the corresponding components. Rather, it is also possible that the fuel cell is carried as a separate unit, for example in a backpack, and a power connection is made from this to the portable device by means of a cable connection.

The following figures and embodiments relate essentially to an embodiment as a portable fuel cell in a special embodiment as DMFC, within which a water separator between a cathode output downstream cooler and a mixer is arranged. This embodiment has been modeled as a capillary separation apparatus for the recovery of water from a two-phase flow and simulated in the sense of a computer-aided fluid simulation CFD. By means of the CFD simulation, it is possible to model the separation process to the extent that it is possible to determine whether the planned water separator functions independently of location, how the design and process variables influence the ability of gas-liquid separation. Other parameters can also be checked.

The details and features resulting from the subsequent individual figures are therefore not limited to the particular example. Rather, one or more features having one or more features from various figures such as those resulting from the above description may be linked to new designs. In particular, the following statements do not serve as a limitation of the respective scope, but explain individual features and their possible interaction with each other.

Show it:

1 a first model of a DMFC system;

2a) a schematic representation of a water separator in a front view;

2 B) a schematic representation of a water separator in a plan view;

3a) a schematic representation of an influence of a capillary pressure on a behavior of a water droplet and a gas in a hydrophilic Flüssigkeitsabführkanal;

3b) a further schematic representation of an influence of a capillary pressure on a behavior of a water droplet and a gas in the hydrophobic fluid guide channel;

4 a schematic representation of various possible flow types;

5 an initial water distribution in a liquid discharge channel for different gas-liquid volume _ratios used in the simulation R _gl : a) R _gl = 9.5; b) R _gl = 2.5; c) R _gl = 0.6;

6 an approximation of the product of λ and a derivative of the equilibrium temperature T ^cond with respect to λ at different condenser temperatures;

7 a simulated recycling efficiency of a water separator for different directions of gravity;

8th a simulation of a partial separation of a liquid by diverting it into a Flüssigkeitsabführkanal for the case of no gravity;

9 a recycling efficiency with changing number of Flüssigkeitsabführkanälen and inlet speeds;

10 an influence of a pressure difference at the Flüssigkeitsabführkanalauslass and the volumetric gas-liquid ratio on the recycling efficiency using the example of a single-channel water separator;

11 a result of simulating a DMFC system with a capillary water separator with the capillary pressure in the fluid guide channel of the water separator and the counterpressure p ^mix - p ^env acting on the mixer, as a function of time;

12 a temperature along with a recycling efficiency in the water separator as a function of time;

13 a volumetric ratio R _gl of gas and liquid flow, which flow into the water separator.

1 shows an exemplary embodiment of a schematic structure of a portable fuel cell 101 , which is a cathode cycle 113 includes. From the 1 goes a cell stack of anode 103 and cathode 102 , a first connection between an anode outlet and a degasser 107 and a second connection between a cathode output of the cathode 102 and a capacitor 108 , of the in turn comprises a water separator. Between the anode outlet and the degasser 107 and the cathode output and the capacitor 108 is each a cooler 105 . 106 arranged. From the degasser 107 and the capacitor 108 Starting lead lines to a mixer 110 to which also leads from a methanol reservoir coming from an intermediate fuel pump methanol into the mixer 110 leads.

2 shows a principle model of a water separator 202 , A multiphase mixture as a two-phase flow of a liquid and a gas flows into a main channel in the form of a fluid guide channel 201 that has a fluid guide channel inner wall 205 having hydrophobic walls. Preferably, the fluid guide channel 201 a square cross-sectional area with an edge length of 0.2 mm. About three millimeters from a fluid guide channel inlet 203 removed are two Flüssigkeitsabführkanäle 204 arranged, which from the main channel 201 depart. These channels have a Flüssigkeitsabführkanalinnenwandung 206 with hydrophilic walls and a smaller compared to the cross-sectional area of the fluid guide channel inlet cross-sectional area. The cross-sectional area in this embodiment shown is preferably also square and has, for example, an edge length of 0.1 mm. These fluid drainage channels 204 are for the discharge of liquid water from the fluid guide channel 201 designed and transported this one millimeter away at their end. The main channel 201 extends for another two millimeters after the end of the last Flüssigkeitsabführkanals 204 up to a gas outlet.

2 shows two Flüssigkeitsabführkanäle 204 , To model in the influence of the number of Flüssigkeitsabführkanäle 204 To be able to examine, on the one hand a model with one as well as a model with five Flüssigkeitsabführkanälen 204 to be examined. The Flüssigkeitsabführkanäle 204 For reasons of comparability they are in each case 0.2 mm apart. The rest of the structure is also analogous to each other.

Due to their simple structure, each of the models was networked with a hexagonal grid. For macroscopic models with a laminar flow profile, as here in this case, the resolution achieved by this proved to be perfectly adequate. After a numerical network validation, the cell size was set to 12.5 μm.

The dimensioning used in both models was chosen as an example, in order to be able to investigate in particular the effect of a capillary force as well as the effects of different pressure settings. In particular, a dimensioning of the water separator with different, to the previously mentioned widths and cross sections, but also lengths different areas is possible. An exemplary embodiment provides, for example, that a cross section of the fluid guide channel in the region of a branch into a Flüssigkeitsabführkanal by at least a factor of 3, preferably by a factor of 4 to 7 is greater than a cross section of the Flüssigkeitsabführkanals. However, this range of size difference is not mandatory for all embodiments. Rather, the dimensions can even deviate considerably. A clue to the more accurate sizing is given for example by the function. This depends, for example, on whether an arrangement takes place in the cathode or anode circuit. This in turn determines the fluids to be separated. Also, the operating point or a work area in the dimensioning can influence the interpretation.

3a) and 3b) show the basic operation of a water separator 308 with a fluid guide channel 301 with a hydrophobic main channel and a Flüssigkeitsabführkanal 302 compared with a hydrophilic separation channel. A multiphase mixture as a two-phase flow from a liquid 304 and a gas 305 flows from the cathode exit through the fluid guide channel inlet 303 into the main hydrophobic channel 301b and passes a portion of the branch from the fluid guide channel 307 , Part of the water droplets gets into the hydrophilic separation channel 302b while the gas passes through the main channel 301 continue to the Fluidführungskanalauslass 306 is guided in the form of a Hauptkanalauslasses. If the capillary forces are not strong enough, you can also use gas 305 in the hydrophilic separation channel 302b can get or vice versa liquid 305 in the form of water droplets pass through this and the water separator 308 leave with the gas as it is in 3b is shown.

• • Der Kapillareffekt in den hydrophilen Trennkanälen 302b hindert das Gas daran, in diese zu strömen, so lange der Gasdruck nicht größer als der Druck in der Flüssigkeit im hydrophilen Trennkanal 302b abzüglich des in Bezug auf die Wirkungsrichtung negativen Kapillardruckes p_c,hyphil ist.
• • In dem hydrophoben Hauptkanal 301b drückt der positive Kapillardruck p_c,hyphob in der flüssigen Phase diese in den Trennkanal 302 unter der Voraussetzung, dass der Druck in der flüssigen Phase größer ist als der der flüssigen Phase im Trennkanal 302.

The surface properties of the hydrophilic separation channels 301b and the hydrophobic main channel 301b serve different goals:

• • The capillary effect in the hydrophilic separation channels 302b prevents the gas from flowing into it as long as the gas pressure is not greater than the pressure in the liquid in the hydrophilic separation channel 302b minus the negative capillary _pressure p _c with respect to the _{direction of} action _{, hyphil} .
• • In the hydrophobic main channel 301b the positive capillary _pressure p _{c, hyphobic} in the liquid phase pushes it into the separation channel 302 provided that the pressure in the liquid phase is greater than that of the liquid phase in the separation channel 302 ,

In 4 various possible flow types are shown schematically. The flow is first assumed as a laminar flow for the embodiment, but without such a condition for the operation of the embodiment would be. The Volume-of-Fluid, abbreviated, VOF model with two Euler phases and the implicit formulation of the volume forces is used for the description of the two-phase plug flow, as it stands 4 evident. In order to investigate the process of water separation, all simulations are set up as transient simulations. As materials, the fluids "air" were selected as the primary phase and "liquid water" as the secondary phase. For the interaction of the fluids with the walls, the wall adhesion was modeled. The surface tension between water and air is set to the constant value of 0.072 N / m, corresponding to the value at 298 K. The contact angle for water has been set at 126 ° for the hydrophobic fluid channel inner walls and at 20 ° for the hydrophilic Flüssigkeitsabführkanalinnenwandungen.

At the inlet of the fluid guide channel, only the entry of air parallel to the fluid guide channel and at a constant rate across the cross-sectional area is permitted. All outlets take a constant pressure. A practical problem arises when using this condition: if a drop reaches the end of the fluid guide channel, then the simulation at that point can not converge due to the higher capillary pressure. However, this does not affect the results of the simulations, since all drops that do not end up in the liquid discharge channels are already lost, and thus all the necessary information has already been compiled. The pressure at the end of the liquid discharge channels may be set directly or as a difference to the pressure at the fluid guide channel outlet. For the simulation, it is necessary to also define the eventual return flows at the outlets, whereby water was defined as the medium for the liquid discharge channels and air as the medium for the fluid guide channel. To ensure the convergence of the simulations, the lower relaxation factors were set to 0.6 for the pressure and 1.0 for the pulse.

mit L_gas als Länge einer einzelnen Gasblase 503 und L_liquid als diese eines einzelnen Wassertropfens innerhalb der Initialisierung im Hauptkanal.In 5 is the initial water distribution in the water separator 501 for the different gas-liquid volume ratios R _gl used in the simulation, where a) R _gl = 9.5; b) R _gl = 2.5; c) R _gl = 0.6. The gas-liquid volume ratio is defined here as the ratio between the lengths of the gas bubble 503 and drops of water 504

with L _gas as the length of a single gas bubble 503 and L _liquid than that of a single drop of water within the initialization in the main channel.

After initialization, the geometry was filled with sections of water, which are the water droplets 504 represent that are to be recovered. The filling patterns for the investigated three different gas-liquid ratios are in 5 shown. The total simulation time was set to 15 ms seconds. The courant number has been set to the value 0.1.

The efficiency of the water separator is defined by the effectiveness or the efficiency of the water recovery, determined as the mass ratio of the recovered water to the amount of water at initialization.

The water separator, as described in the previous paragraphs, is aimed at recovering water droplets from a multiphase stream consisting mainly of gas. For this reason, in this embodiment, the water separator is connected only to the air outlet where this problem occurs. This water separator is also referred to as Kondensatseparator.

Another separator, also referred to as degasser, on the other hand has the task of separating gas from a liquid stream having a volumetric gas to liquid ratio close to 1: 1. In the embodiment, it is assumed that the phases are ideally completely separated from each other.

The efficiency of the separator to recover liquid from the fluid mixture depends, according to the results according to one embodiment, mainly on the pressure differences at the outlets. To find out how the water separator affects the dynamics of the water balance, it is advantageous to find a relationship between the amount of water in the system and the pressure difference. The differential pressure between the outlets of the water separator can be equated according to an embodiment with the atmospheric pressure difference in the mixer. Provided that the pressure at the gas outlet corresponds to the atmospheric pressure and the pressure loss between the water separator and the mixer can be neglected. If the existing volume in the system is considered to be ideally stiff for the liquid, the pressure goes to infinity with additional liquid addition. In order to set up a physically meaningful model, however, it is advantageous to take into account the imperfect behavior of real materials. Although all liquids are modeled as being incompressible and the mixer as being ideally stiff, the mixer is connected to other system components via hoses, which are usually made of a flexible plastic, having a much lower modulus of elasticity than the materials of the mixer or other system components. Similarly, the same applies to compression modules of liquids. For this reason, it is assumed in a development of the invention that the increase in volume which is available to the liquid can only be attributed to the extent of the lines. Starting from a uniform elastic hose with a circular cross-section, the current volume can be expressed by the nominal volume V and the tension ε:

The balance of the pressure difference between the inside and outside of the hose and the tension can be written as: (p ^mix -p ^env ) * d _in = (d _out -d _out ) * E * ε

wobei angenommen wird, dass im Falle von negativen Differenzdrücken der Schlauch zusammenfallen wird, und sich somit der Differenzdruck zu Null ergibt. Es werden die folgenden Werte beispielsweise für den Schlauch angenommen: d_out = 4,8 mm, d_in = 3,2 mm und E = 1,9 MPa, was den Werten entspricht, wie sie aus dem Datenblatt von Tygon 3350-Silikonschläuchen entnommen werden kann. In den Simulationen wird der Mischer jeweils als vollständig mit Lösung gefüllt gestartet, während die Schläuche ein nominelles Volumen von 5 cm³ aufweisen.By combining the above two equations by the elimination of ε, the differential pressure in the mixer can be expressed as follows:

assuming that, in the case of negative differential pressures, the hose will collapse and thus the differential pressure will be zero. For example, the following values are assumed for the hose: d _out = 4.8 mm, d _in = 3.2 mm and E = 1.9 MPa, which are the values as taken from the data sheet of Tygon 3350 silicone hoses can be. In the simulations, the mixer is started in each case as completely filled with solution, while the hoses have a nominal volume of 5 cm ³ .

wobei der hydraulische Durchmesser d_h das Verhältnis von Querschnittsfläche zum vierfachen benetzten Umfang darstellt; im Falle der quadratischen Querschnittflächen, wie sie in den CFD-Simulationen genutzt wurden, entspricht der hydraulische Durchmesser der Seitenlänge.In order to be able to separate the liquid phase from the two-phase stream, it is provided in a preferred example that the water separator in the fluid guide channel has a capillary pressure which is greater than the counter-pressure in the liquid discharge channels. In general, the capillary pressure is given by:

wherein the hydraulic diameter d _{h represents} the ratio of cross-sectional area to four times wetted perimeter; in the case of the square cross-sectional areas, as used in the CFD simulations, the hydraulic diameter corresponds to the side length.

The CFD simulations assume a constant surface tension due to the very short simulation time, but the system simulation time extends over a much wider range and therefore accounts for the slow temperature change and associated surface tension dependence. The surface tension of water is related to the temperature by:

Assuming that the pressure at the gas outlet is equal to that of the environment and that at the fluid outlet is equal to that at the mixer, the condition is that no gas enters the fluid discharge channel as follows: p _{c, hyphile} = p ^mix - p ^env , where p _{c, hyphil is} the capillary pressure in the liquid _{discharge channel} . It is assumed that this condition is always true as soon as p _{c, hyphil} <0 and p ^mix ≥ p ^env .

Likewise, the condition for the water separation is that the capillary pressure in the fluid guide channel is greater than the pressure in the mixer: p _{c, hyphob} > p ^mix - p ^env .

With the accumulation of water in the system, p ^mix may increase to such an extent that this condition is no longer met and further water recovery is blocked. The water separator is therefore according to a development modeled so that it separates only a liquid flow from the fluid guide channel; that is, any liquid that arrives is diverted into the liquid _{discharge channels} as long as p _{c, hyphob} > p ^mix - p ^{env is} met. Otherwise, no liquid will pass into the liquid discharge channel.

Out 6 shows an approximation of the product of λ and the derivative of the equilibrium temperature T ^cond relative to λ at different condenser temperatures. For the example system off 1 it has already been shown that the water balance is subject to metastable dynamics. This means that feedback control was indispensable in previous systems. Therefore, a way to measure the water level had to be implemented. Such measurements are difficult to implement in portable systems and require a specific orientation of the tank in space. The development of portable systems, which should work independent of orientation, is prevented by such requirements.

However, the introduction of the water separator in the illustrated embodiment alters the dynamics of the water balance in the system. The ability of the water separator to recover water and then supply it to the mixer depends, according to a suggested example, on the pressure in the mixer, according to the following dependency p _{c, hyphob} > p ^mix - p ^env , where the condition for the water separation is: The capillary pressure in the fluid guide channel must be greater than the pressure in the mixer.

welche bereits weiter oben erklärt wurde.The pressure in the mixer is in turn dependent on the water content in the system with the relation

which has already been explained above.

This results in the effect that the water recovery rate now depends directly on the water level in the system: more water in the system leads to higher pressures in the mixer; which in turn blocks the recovery of more water from the water separator. The dynamics of the water balance is thus stabilized by a change in the system properties.

zu setzen.Once the dynamics are stable, it is possible to control them through feed-forward processes. As a result, can be dispensed with a measurement of the water level. A preferred feed-forward control algorithm is that, the capacitor temperature to a value given from the standpoint of autonomy

to put.

In the above equation are included: the humidity of the environment (r ^env ), which has only a small influence; λ, which has a strong influence but is often set constant; and the mixing ratio in the condenser (r ^cond ), which in turn is a function of the condenser temperature, which in turn is the only adjustable amount to control the amount of water. λ is the excess air ratio, which is the ratio of oxygen in the inflow airflow to what is actually required theoretically in the fuel cell. It should be noted here that this definition includes both reactions: the main reaction to generate electricity and the side reaction, also known as crossover. The λ control is a feed-forward technique that explains the stability and short time constants for the dynamics in the oxygen concentrations in the cell.

The autonomy condition is, strictly speaking, only valid if the temperature of the two separators, ie condenser and degasser, in 1 are the same. For different temperatures, however, the error for the calculated water separator temperature is in most cases below 1K. For the simulations, the DMFC stack consists of 3 cells under a constant load of 2 A. The methanol solution has a nominal concentration of 1 M. The nominal temperature The fuel cell is 333 K. The excess air ratio is λ = 3. The ambient conditions are 60% relative humidity and at a temperature of 298 K. It is not a cell-stack bound size for the determination of the condenser temperature crucial to keep the amount of water constant. The system is stabilized only so long as additionally recovered water leads to an increase in pressure in the mixer. The dynamics lose their stability if the mixer or hoses are not filled to their nominal value.

It is therefore preferred, for safety reasons, to achieve a lower temperature than is apparent from the teachings of autonomy to ensure that the mixer and hoses are sufficiently filled with liquid, thus stabilizing the system. It has previously been shown that the water balance in a DMFC system is in equilibrium when the autonomous education is met. This equation links the excess air rate, the ambient humidity and the temperature in the condenser. For a given humidity and excess air rate, it is possible to invert the autonomy relationship and find the temperature in the condenser which keeps the water balance in balance. The autonomy relationship is derived with some assumptions, which in practice makes one with an uncertainty in the capacitor temperature of 1 K. This is a first safety cut that must be subtracted from the condenser temperature.

Another conservative simplification is the assumption of no humidity in the environment. Humidity usually has only a very small influence on the autonomous relationship, and assuming no humidity in the environment will in any case lead to a lower equilibrium condenser temperature. Furthermore, this allows the abandonment of a built-in control system humidity sensor.

wobei r^cond das Mischungsverhältnis von Wasser im Kondensator als

und y env / O₂,dry als Molenbruch von Sauerstoff in trockener Luft definiert ist.Autonomy education can therefore be simplified and differentiated in the following way:

where r ^{cond is} the mixing ratio of water in the condenser as

and y env / O₂, dry is defined as the mole fraction of oxygen in dry air.

For all significant values for λ, for y env / O₂, dry << 3 · λ applies, the following simplification is allowed:

erhalten, welcher in 6 dargestellt ist, und welcher genutzt werden kann, um den Sicherheitsbereich abzuschätzen, um welchen die Kondensatortemperatur gesenkt werden sollte, wenn eine Unsicherheit von δλ in der λ-Kontrolle existiert, welche ebenfalls mit einer Feed-Forward-Kontrollstrategie implementiert ist und zu einem Fehler im stationären Zustand führen kann. This expression, assuming that the gas phase in the condenser is saturated, the law of Raoult is valid, and the Antione equation is used to calculate the saturation vapor pressure, can be explicitly written in the sense that the mixing ratio r ^cond of the temperature depends. As a result, an expression of the form becomes:

received, which in 6 and which can be used to estimate the margin of safety by which the capacitor temperature should be lowered, if there is an uncertainty of δλ in the λ control, which is also implemented with a feed forward control strategy, and an error in can lead to a steady state.

For a given relative uncertainty in the excess air ratio δλ / λ is the required safety range for the capacitor temperature according to one embodiment ΔT _λ = 18 K · δλ / λ, where the 18 K is approximately the minimum value 6 matches. For example, for an uncertainty of λ of 5%, a haircut of 0.9K results.

• – Lösen der Autonomiebeziehung unter der Annahme keiner Luftfeuchte für die Kondensatortemperatur T^cond;
• – Abziehen von 1 K als Sicherheitsabschlag für die Modellierungsfehler,
• – Abziehen von ΔT_λ wie es aus Gleichung ΔT_λ = 18 K· δλ / λ für den Implementationsfehler von der λ Kontrolle hervorgeht.

To summarize, to get the condenser temperature set point for the feed-forward solution volume control, the following procedure is suggested:

• - solving the autonomic relationship assuming no air humidity for the condenser temperature T ^cond ;
• Deducting 1 K as a safety margin for the modeling errors,
• - subtracting ΔT _λ as shown in Equation ΔT _λ = 18 K · δλ / λ for the implementation error from the λ control.

For this embodiment, the total safety discount for modeling and implementation with 2 K is assumed. The proposed feed-forward algorithm according to this example preferably requires only the knowledge about the λ-value, but no measurement.

In 7 the recycling efficiency of the water separator for different directions of gravity is listed. In order to investigate the influence of gravity on the planned design, simulations with defined values for the gas-liquid ratio and the inlet velocity under different directions of gravity were investigated. The parameters were chosen so that several drops were introduced in the fluid guide channel and the resulting water recovery efficiency is far from the 0% and 100% range limits. The results should not reflect those that lead to a design with the maximum separation efficiency, but rather that they emphasize the effect of gravity.

Given is a water separator having a single liquid discharge channel, a volumetric gas-to-liquid ratio of _Rgl = 2.5 and an inlet velocity of 0.354 m / s. The assumption of orientation independence for the separation process is validated by setting up the same simulation with different directions of gravity. The results are in 7 to see. Although the effective direction of gravity has a quantifiable, but only very small influence on the separation efficiency. The standard deviation for the recycling efficiency for the different orientations is less than ± 1%, which in practice is very difficult to measure and easy from non-idealities and other practical constraints, such as leaks in the system, drops which do not reach the water separator to the extent anticipated , Temperature gradients that can cause evaporation, etc., are superimposed. The influence of gravity can therefore be neglected with the proposed system.

8th shows the simulation of the partial separation of the liquid by diverting into the Flüssigkeitsabführkanal in the case of no gravity. The original water droplet distribution is identical to that of 5b , After 2 ms, the first water stopper reaches the separation channel. After 8 ms, two out of four drops of water have passed the separation channel and are considered as a loss from the system to the environment.

9 shows the recycling efficiency with changing number of separation channels and inlet rates. For an inlet velocity of, for example, 0.354 m / s, a monotone, almost linear dependence of the recycling efficiency on the number of channels can be observed. Complete separation of gas and liquid is achieved at lower inlet velocities or with an increasing number of fluid discharge channels. The inlet velocity values are chosen to represent an increase in the number of parallel fluid flow channels by 50% and 100%. As a result, there is a reduction in the intake speed. In a water separator, this would be expressed in a stack of separation units from liquid feed channels. For example, two water separators reduce the speed by 50%. It can be seen that a complete recovery of the liquid water can be achieved either by increasing the number of Flüssigkeitsabführkanälen or by the number of multiple fluid guide channels. The latter has been realized by means of reducing the inlet velocity in the simulations

10 shows the influence of the pressure difference at the outlet of Flüssigkeitsabführkanäle and the volumetric gas-liquid ratio on the recycling efficiency using the example of a water separator with only one Flüssigkeitsabführkanal. The simulations were carried out with a water separator with only one liquid discharge channel and an inlet velocity of 0.354 m / s. For a R _gl of 9.5, complete separation is achieved up to a differential pressure of 0.55 kPa. For lower gas-liquid ratios, the recycling efficiency shows a linear dependence on the differential pressure. In general, the recycling efficiency increases with increasing gas-liquid ratio and falling differential pressure. High pressures lead to negative recycling efficiencies, that is, already separated liquid from the Flüssigkeitsabführkanälen is pushed back into the fluid guide channel. In most cases, a high volumetric gas-to-liquid ratio results in complete separation, which can be attributed to the longer time intervals between the drops. At lower ratios, the recycling efficiency gradually decreases. Once the counterpressure in the mixer reaches the hydrophobic capillary pressure, the recycling efficiency versus the volumetric gas-liquid ratio shows a reverse behavior: the largest volumetric gas-liquid ratio even evokes negative recycling efficiencies since the capillary pressure is insufficient to supply the water to keep in the Flüssigkeitsabführkanälen. Instead, water is pushed by the Flüssigkeitsabführkanälen in the fluid guide channel.

For smaller volumetric gas-liquid ratios, the droplets in the fluid guide channel that have already passed the liquid discharge channel or channels produce positive positive pressure between the separation point and the environment: this difference adds to the hydrophobic capillary pressure and results in a positive recycling efficiency.

In the following, a zero-dimensional approximation of the 3D separator model is integrated into a model of a DMFC system. The aim of a system simulation is to understand how the special properties of the water separator, taking advantage of capillary forces, affect the dynamics and control of the system, which is particularly explicitly related to the already mentioned water balance.

In 11 is the result of a simulation of a DMFC system with a water trap utilizing capillary forces. The capillary pressure in the fluid guide channel of the water separator and the counter pressure p ^mix - p ^env acting from the mixer are shown as a function of time. The temperature in the condensate separator changes from the initial 298K to 314K during the simulation, causing the change in capillary pressure in the fluid guide channel, as in FIG 11 you can see. From the beginning of in 11 The simulation shown returns recovered water to the system, which is stored in the expanded hoses connected to the water separator. As the mixer is considered to be filled at the beginning of the simulation, the differential pressure in the mixer rises very rapidly as a consequence. After 20 s, the differential pressure reaches the value of the capillary pressure in the water separator. As a result, the driving force for the separation disappears and excess water is released to the environment. During operation of the system, the capillary pressure does not change significantly, even if it reduces slightly with increasing temperature. Furthermore, it can be seen that the differential pressure in the mixer increases with the proposed feed-forward control to the value of the capillary pressure. The dynamics of the water balance is therefore stable. The shape of the transition of the differential pressure in 11 is influenced by the reaction rate in the fuel cell, tube dimensions, especially their geometry and elasticity, and the controller settings. An increase in the reaction rate leads to increased water production, which accelerates the transition.

führt ein vergrößertes Schlauchvolumen zu einem langsameren Übergang, da der selbe Wert für kondensiertes Wasservolumen durch ein größeres nominelles Volumen V ~^tub geteilt wird und einen kleineren Gegendruck hervorruft. Ein äquivalenter Effekt kann für ein kleineres Verhältnis von innerem und äußerem Durchmesser oder einer kleineren Elastizität erreicht werden.In compliance with the equation

Increasing the tubing volume results in a slower transition as the same condensed volume of water is shared by a larger nominal volume V ~ ^tub and ^produces a smaller backpressure. An equivalent effect can be achieved for a smaller ratio of inner and outer diameter or smaller elasticity.

The feed forward control setpoint was derived with several safety margins: one for ambient humidity, one for modeling errors, and one for errors in the implementation of λ control. This area allows the accumulation of water in the system: a larger safety margin results in a reduced set point for the separator temperature, resulting in increased water condensation and a faster increase in back pressure in the mixer. Conversely, a smaller security area leads to a slower transition. If the area becomes infinitesimal and the setpoint exceeds the water equilibrium temperature, the system will begin to lose water and eventually dry out. The main disadvantage of large security areas is the increased cooling capacity compared to the theoretically necessary minimum; Depending on the current implementation, such increased cooling demand would lead to higher costs and, for very large security areas, even unrealistic. For example, it is not possible to lower the separator temperature below ambient when a passive radiator uses the ambient air for cooling. However, it is also possible to provide a cooling below the ambient temperature. For this purpose, an active cooling can be used, for example by using a Peltier cooler.

In the 11 values shown are consistent with CFD simulation results showing a transition from complete to no separation in the range of 0.5 kPa to 1 kPa counterpressure at the mixer. This backpressure is not particularly high, and perhaps less than the pressure fluctuations, such as those produced by a circulation pump, or the hydrostatic pressure generated by the height difference between the mixer and the water separator. If mixers and water separators are also arranged in unfavorable positioning relative to one another, it is still possible to modify the system by means of smaller liquid discharge channels or porous membranes in such a way that the capillary pressure can assume significantly greater values.

In 12 the temperature along with the recycling efficiency in the water separator is shown as a function of time. As has already been stated, the ability of the system to keep the water balance constant is, in one embodiment, largely independent of operating conditions and of the design of the system and the fuel cell. The results presented can thus be regarded as universal. Also, the recycling efficiency increases with increasing condenser temperature, since more water is in the gas phase, and thus, according to definition, no longer enters into the calculation of the recycling efficiency.

In 13 the volumetric ratio R _gl of gas and liquid flow is shown, which flows into the water separator. The simulated values for the volumetric gas-liquid ratio in 13 are more than two orders of magnitude larger than those used in the CFD simulations. Smaller values, on the other hand, would have rendered the design obsolete, in that they would have led to a lower recycling efficiency, as was the case 10 is apparent. Higher values, however, are unproblematic to use. These state that the distribution of droplets coming from the fuel cell is much farther apart than in 5 accepted. Would values for R _gl from the system simulation has been applied to the CFD simulations, this would have led to a much longer fluid guide channel, which in turn would have had considerably longer computing times and no added value to the result.

The modeling of a water separator by means of CFD makes it possible to investigate the influence of various operating parameters and design on the selective separation of the liquid phase from a gas-liquid stream. The spatial orientation of the water separator has with respect to gravity only a very small and negligible influence on the separation performance. By simulating different configurations it could be shown that a complete separation can be achieved by an adequate design of the water separator. To achieve this, according to one embodiment, the proportion of gas should be at least 10 times greater than the volume of liquid. The pressure in the Flüssigkeitsabführkanälen here should not exceed the limit pressure, which is close to the hydrophobic capillary pressure in the fluid guide channel.

A zero-dimensional model of the water separator was then implemented in a DMFC system simulation to determine the effects on the dynamics of the system. The minimum volumetric gas-liquid ratio condition implies a large safety margin for the water separator of the DMFC system, so that complete separation can be assumed as long as the back pressure at the mixer does not exceed the hydrophobic capillary pressure in the water separator. This is not the case for the degasser, which has near-1 gas-liquid ratios, and therefore requires a different design.

One conclusion is that by incorporating a water separator utilizing capillary forces into a DMFC system, the dynamics of the water balance could be stabilized, which was previously metastable and required feedback control in combination with measurements to be made. The stabilization allows the introduction and the test of a feed-forward control. Added to this is a sensorless control algorithm, which is much faster and cheaper to implement than this would be possible by means of a feedback solution, for example.

In 14 are incidentally some uses for a portable fuel cell 1406 shown. For example, a use of the fuel cell on or in a mobile phone 1401 , a portable computer 1402 , a remote control 1403 , a small aircraft 1404 or a small watercraft 1405 be provided for the respective power supply.

Claims (10)

Portable fuel cell system, comprising at least two in a circuit of a fuel cell ( 101 . 1406 ) arranged liquid separator ( 107 . 108 ), wherein in a cathode cycle ( 113 ) between a cathode ( 102 ) and a mixer ( 110 ) the liquid separator ( 108 ) is arranged, and in an anode circuit as a degasser of the liquid separator ( 107 ) between an anode ( 103 ) and the mixer ( 110 ), wherein the liquid separator ( 108 ) - at least one fluid guide channel ( 201 . 301 ) with a fluid guide channel inner wall ( 205 . 309 ), a fluid guide channel inlet ( 203 . 303 ) and a fluid guide channel outlet ( 306 ), for guiding a multiphase mixture of at least two immiscible fluids, and - at least one of the fluid guide channel ( 201 . 301 ) outgoing liquid discharge channel ( 204 . 302 . 502 ) having a Flüssigkeitsabführkanalinnenwandung and a Flüssigkeitsabführkanalauslass ( 310 ) for discharging liquid of the multiphase mixture, and wherein an inflow of in the fluid guide channel ( 201 . 301 ) guided liquid into the Flüssigkeitsabführkanal ( 204 . 302 . 502 ) is effected by means of capillary forces and in this case the in the Flüssigkeitsabführkanal ( 204 . 302 . 502 ) discharged portion of the through the fluid guide channel ( 201 . 301 ) by changing at least one pressure, at the fluid guide channel inlet, at the fluid guide channel outlet ( 306 ) or at the Flüssigkeitsabführkanalauslass ( 310 ), and wherein a sensorless feed-forward control for the liquid separation is implemented such that the temperature of the liquid separator is fixable. Portable fuel cell system according to claim 1, characterized in that one or more liquid deposition influencing, variable pressure adjuster are arranged in a circuit of the fuel cell. Portable fuel cell system according to claim 1 or 2, characterized in that a design of one of the liquid separator ( 202 . 308 . 501 ) is such that at least one acting capillary force in the at least one Flüssigkeitsabführkanal ( 204 . 302 . 502 ) allows a neglect of a gravitational force. Portable fuel cell system according to one of the preceding claims, characterized in that the fluid guide channel inner wall ( 205 . 309 ) is hydrophobic. Portable fuel cell system according to one of the preceding claims, characterized in that the Flüssigkeitsabführkanalinnenwandung is hydrophilic. Portable fuel cell system according to one of the preceding claims, characterized in that the internal cross-sectional area of the liquid discharge channel ( 204 . 302 . 502 ) at least in the area ( 307 ) of the branch from the fluid guide channel ( 201 . 301 ) is smaller than the internal cross-sectional area of the fluid guide channel ( 201 . 301 ) in the region of the diversion of the Flüssigkeitsabführkanals ( 204 . 302 . 502 ). Portable fuel cell system according to one of the preceding claims, characterized in that the liquid separator, in particular the water separator ( 202 . 308 . 501 ), is interchangeable. A method for recovering a liquid from an outlet, preferably a cathode outlet or an anode outlet of a portable fuel cell ( 101 . 1406 ) to a fluid guide channel ( 201 . 301 ) with a fluid guide channel inlet ( 203 . 303 ) and a fluid guide channel outlet ( 306 ) as a multiphase mixture comprising the liquid ( 304 ) and at least a first gas ( 305 ), wherein for recirculating at least a portion of the liquid of the multiphase mixture into a circuit of a fuel cell system comprising the portable fuel cell, via a mixer ( 110 ) of the fuel cell, an inflow of a portion of the liquid ( 304 ) into one at the fluid guide channel ( 201 . 301 ) arranged Flüssigkeitsabführkanal ( 204 . 302 . 502 ) with a Flüssigkeitsabführkanalauslass by in the Flüssigkeitsabführkanal ( 204 . 302 . 502 ) occurring capillary forces is effected, and this in the Flüssigkeitsabführkanal ( 204 . 302 . 502 ) inflowing portion of the liquid ( 304 ) by changing at least one pressure, in particular at the fluid guide channel inlet ( 203 . 303 ), at the fluid guide channel outlet ( 306 ) or at the Flüssigkeitsablasskanalauslass, and wherein a feed-forward control one in the mixer ( 110 ) existing amount of liquid without direct measurement of the amount of liquid. Use of a portable fuel cell system comprising at least one fuel cell ( 101 . 1406 ) according to one of claims 1 to 7 and / or with a method according to claim 8 for the power supply of a portable device, in particular a mobile telephone ( 1401 ), a portable computer ( 1402 ), a remote control ( 1403 ), a small vehicle, in particular a small aircraft ( 1404 ) or a small vessel ( 1405 ). Simulation model for the design of a portable fuel cell system with the features of one of claims 1 to 7 and / or with the features of claim 8, wherein the simulation model in addition to a capillary force takes into account an influence of a pressure for feed-forward control of a water level, - the pressure in a mixer is calculated as a function of the water content in the fuel cell system as follows:

and - the capacitor temperature is set with a feed-forward control algorithm to a value that results from the following autonomy consideration:

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