DE102020123975A1 - Device and measuring method for the spatially resolved in-situ detection of liquid water on a support structure and its use in a fuel cell stack and in plate heat exchanger applications - Google Patents

Abstract

The invention discloses a device for the spatially resolved in-situ detection of fluids, in particular liquid water, on a support structure, comprising at least three transducer elements which are used up and/or arranged on the support structure, a control unit for controlling the transducer elements and a signal processing unit for Evaluation of signals recorded by the converter elements, and a measurement method which uses the arrangement according to the invention. The task of being able to detect fluids in a spatially resolved and continuous manner on a support structure during operation is achieved in that the converter elements form transmission paths among themselves and thus cover a measuring range for detecting fluids on the support structure, with the control unit each having a converter element of the at least controls three converter elements as a transmitter and records transmitted signals through the respective other converter elements, the signal processing unit tomographically evaluating the signals of the converter elements and outputting a spatially resolved distribution of the fluid on the support structure.

Description

The invention relates to a device for the spatially resolved in-situ detection of fluids, in particular liquid water, on a support structure, comprising at least three transducer elements which are used up and/or arranged on the support structure, a control unit for controlling the transducer elements and a signal processing unit for Evaluation of signals recorded by the converter elements.

The invention also relates to a measurement method that uses the arrangement according to the invention.

The invention also relates to the use of the device according to the invention in a fuel cell or in a fuel cell stack and in a heat exchanger, for example for the food industry or in a seawater desalination plant.

Lamb waves belong to the acoustic surface waves that propagate over long distances on surfaces. Lamb waves are vibrations of a plate in which deflections occur both in the direction of propagation (longitudinal) and perpendicular (transverse) to it. Therefore, Lamb waves are mixed pressure and shear waves. Depending on the excitation frequency, Lamb waves appear in at least two basic modes, a symmetric and an antisymmetric mode. In the case of symmetrical Lamb waves, the upper and lower sides of the plate move away from (or towards) the center of the plate at the same time, the vibration modes are denoted by S0, S1, S2, etc.; in antisymmetric Lamb waves, the upper side moves away from the center of the plate and the lower side moves towards the center of the plate at the same time (i.e. both upwards or both downwards), the vibration modes are denoted by A0, A1, A2, etc. Lamb waves are dispersive, i. H. their speed of propagation depends on the excitation frequency and the plate thickness. The group and phase velocities are shown in dispersion diagrams. Lamb waves are often used for non-destructive testing of plates and/or plate-like structures. By using Lamb wave attenuation, velocity and mode conversion, information about the sizes and locations of existing defects can be obtained. This allows large-area plate-like structures such as aircraft fuselages, ship hulls, elbows and pressure vessels of nuclear reactors to be quickly inspected for structural defects such as delamination, delamination and corrosion.

Lamb waves have gained great importance in non-destructive testing and structural health monitoring (SHM) and are applied in many different areas for manufacturing and in-situ inspection. Structural health monitoring is an automatic monitoring process that is carried out continuously or periodically in order to determine and monitor the condition of an object and thus prevent damage, e.g. B. cracks or deformations, and to be able to initiate countermeasures.

Since Lamb waves propagate in a plate with several co-existing modes, each with different dispersion properties and also different sensitivity to different defects, it is of practical interest to exploit a specific mode to generate a corresponding tomographic image to a specific Locate a defect present in the disk to be examined, e.g. B. a thickness thinning due to surface corrosion.

In "Damage Identification in Composites based on Hilbert Energy Spectrum and Lamb wave tomography algorithm", IEEE Sensors Journal, Vol. 19, No. 23, December 1, 2019, an arrangement and a method are described, for example, which allow damage location in composite materials on the basis of the Hilbert spectrum and Lamb wave tomography. For this purpose, a sensor with sensor elements arranged in a circle is used, which are arranged evenly on the composite panel, with each sensor element acting alternately as an actuator in order to generate a Lamb wave. The other sensor elements are responsible for detecting the response signal. By using the Hilbert energy spectrum and applying a probability mapping algorithm, the damage location of the damage in the composite panel is calculated.

The method of Lamb wave propagation and the exploitation of its properties should now be used for a completely new area of application.

The focus is no longer on the detection of defects within a panel to be examined. Rather, the aforementioned methods known from the prior art for a spatially resolved detection of fluids, e.g. As milk or water, especially liquid water, can be used. Water is the only chemical compound on earth that occurs naturally as a liquid, solid and gas. The term water or liquid water is used for the liquid state of aggregation. Im tight state one speaks of ice, in the gaseous state of water vapor.

Water can be desirable, disruptive, or totally undesirable in some applications. For example, depending on the application, it is advantageous to detect where there is water or no water.

Water management in fuel cells, for example, is crucial for their performance, efficiency and service life (see: Li, Hui, et al. "A review of water flooding issues in the proton exchange membrane fuel cell." Journal of Power Sources 178.1 (2008): 103 -117).

The fuel cell is of particular importance in the transition to renewable energies, as it is suitable for both stationary and mobile applications, can be used decentrally, is environmentally friendly and has a high electrical efficiency and is therefore suitable for storing and distributing energy from renewable sources sources of particular interest.

For example, if liquid water collects in the channels of the fuel cell, the reaction educts are distributed unevenly and the performance drops. In order to control humidity, liquid water must be detected/measured in the fuel cell during operation (in situ).

A fuel cell, in particular a PEM fuel cell 1 (proton exchange membrane), consists of a membrane 2 on which electrodes 3 with a catalyst are applied on both sides (see Fig 1 ). Finely distributed noble metals on carbon serve as catalysts, mostly Pt on the cathode side and Pt/Ru on the anode side. The membrane 2 with the electrodes 3 on both sides is referred to as an electrode-membrane unit (EME) or membrane electrode assembly (MEA) 4 . A gas diffusion layer (GDL) 5 is pressed on both sides of the MEA 4 . The porous and therefore gas-permeable GDL 5 often consists of a graphite fiber fabric (0.1 to 0.3 mm) and fulfills several tasks. First of all, the GDL has to distribute the two reaction gases as optimally as possible to the entire active electrode surface. Furthermore, the GDL 5 must dissipate the electric charge (electrons) generated in the electrodes 3 . And finally, the GDL 5 regulates the water (vapor) transport away from the membrane 2. More or less complicated channel structures in the so-called bipolar plates 6 realize the planar gas supply to the GDL 5. The designation bipolar plate 6 makes it clear that on the opposite surfaces a reverse voltage applied. This becomes clear in the arrangement of several individual cells. The channel structures 7 worked out of the bipolar plates 6 are referred to as “flow field”.

Depending on the load connected, the voltage generated by a cell is between half a volt and about one volt. This voltage can be increased to a usable value by electrically connecting several individual cells in series to form a so-called stack. Fuel cell stacks consist of five (laboratory scale) to several hundred individual cells. In a bipolar stack structure 8, individual cells 1 are in electrical contact with each other through a shared bipolar plate 6 (see Fig 2 ). The bipolar plate 6 carries the fuel gas on one side and air or oxygen on the other side to the respective electrodes 3. The name bipolar plate comes from the voltage applied to both sides with different signs: negative pole on the anode side (hydrogen) and positive pole on the cathode side (oxygen). The sum of the individual cell voltages is called the stack voltage. This is dissipated by current collectors at the beginning and end of the cell stack.

The properties of a fuel cell stack depend on a large number of parameters. On the one hand by the properties of the individual components from which it is composed. On the other hand, the operating parameters with which the stack is run have a decisive influence. These include the following parameters: The flow field in the bipolar plates has a significant impact on the performance of the fuel cell. The reaction gases should be available in sufficient quantities at every point and evenly on the membrane. Therefore, water management also plays a crucial role in current membranes. The proton conductivity of the membrane and thus the performance of the stack increases in proportion to the water content of the membrane. Therefore, the membrane must be kept moist, it must not dry out. The water content of the membrane can be increased, for example, by humidifying the reaction gases. On the other hand, the cell must not become too humid so that the diffusion of the reactants to the reaction site is not restricted due to water standing in the porous structures. In order to increase the cell/system efficiency and increase the service life, it is of crucial importance to optimally adjust the water balance in the cells of a fuel cell stack. An obvious and standard solution is the periodic/continuous blowing out of water droplets by applying forced convection, i.e. increasing the air volume flow. However, this solution entails an increase in energy consumption due to that required for convection Liche fan in purchase and consequently reduces the system efficiency.

A detailed measurement of the water distribution of a cell within a cell stack has not yet been possible in order to understand the water distribution as a function of material properties, geometry and operating parameters. If a measuring system were available, which in-situ, i. H. If the water distribution could be recorded locally in a cell or a cell stack in real operation, measures for optimal water management could be developed and there is a perspective possibility of reacting actively and specifically to the water distribution in the channels of the fuel cell or the fuel cell stack.

Existing approaches for direct detection of liquid water can so far only be applied in the laboratory (ex situ_measurement) since they either require a modification of the cell structure to allow optical accessibility for optical measurements or are only used in stacks with few cells, especially when the methods are based on ionizing radiation. These methods therefore do not allow in-situ measurement for a fuel cell stack or in other areas of application in which fluids are to be detected in places that are difficult to access.

Another area of application in which the localization of water plays a role is in seawater desalination plants, particularly in the area of heat exchangers, where the water vapor enriched with salt condenses or where the water supplied evaporates. The area of the heat exchanger available for condensation or evaporation should be as large as possible. Defects where water vapor no longer condenses or evaporates should therefore be detectable at an early stage by spatially resolved information about water-free areas on the heat exchanger being recorded.

Another application where the localization of fluids plays an important role is in (plate) heat exchangers in the food industry. In the milk processing industry in particular, milk is heated in heat exchangers to eliminate microbial risks and extend product shelf life. During heating, proteins denature and agglomerate or salts precipitate on the heat-transferring wall. These deposits are called fouling and reduce heat transfer, which means that the specified product heating can only be guaranteed if the temperature of the heating medium is gradually increased. However, this can lead to a reduction in product quality and sensory properties due to burning. It would therefore be desirable to detect the locations in a spatially resolved manner where no fluid, e.g. B. milk, due to fouling can be heated more to take appropriate countermeasures.

It is therefore desirable and therefore the object of the present invention to specify an arrangement or device and a method that make it possible to detect fluids, such as water, in particular liquid water, in a spatially resolved and continuous manner on a carrier structure during operation. This arrangement should be particularly suitable for detecting liquid water in a fuel cell or in a fuel cell stack or for detecting defects in a heat exchanger structure, such as those used in a seawater desalination plant or in the food industry.

On the assembly side, the object is achieved by a device for the spatially resolved detection of fluids, in particular liquid water, on a support structure according to independent claim 1 . The device comprises at least three converter elements which are attached and/or arranged on the support structure, a control unit for controlling the converter elements and a signal processing unit for evaluating signals picked up by the converter elements, with the converter elements forming transmission paths among themselves and thus spanning and covering a measuring range for detecting fluids, in particular liquid water, on the support structure, with the control unit respectively controlling one converter element of the at least three converter elements as an actuator and recording transmitted signals through the other converter elements, the signal processing unit tomographically evaluates the signals of the transducer elements and outputs a spatially resolved distribution of the fluid, in particular the liquid water, on the support structure.

More than three transducer elements are preferably formed on the carrier structure, the carrier structure being understood to mean plate-like structures whose thickness is small compared to the areal extent of the structure itself. Depending on the use, the carrier structure can also be a heat exchanger or another flat structure. The transducer elements are applied to or arranged on the support structure. The more transducer elements are formed on the carrier structure, the better the measurement range that is spanned by the transmission paths of the transducer elements is covered. The path between a first transducer element and another transducer element is referred to as the transmission path, on which the Lamb wave emitted by the first transducer element propagates to the other transducer element. With n transducer elements, there are accordingly (n-1)*n transmission paths, e.g. B. with n=4 transducer elements, there are 3*4=12 transmission paths that span the measurement area or the measurement area on the carrier structure (see, for example 3 ).

The evaluation of the measurement signals recorded by the signal processing unit allows a spatially resolved determination of fluids, in particular liquid water, on the support structure or a statement can also be made as to where there is no fluid. Tomographic methods are used for the evaluation. One possibility is the classic procedure for tomographic reconstruction. The modes to be examined are separated from one another by frequency filtering or differentiation based on transit time, and signal characteristics such as amplitude, phase/transit time, and dispersion are extracted. If necessary, a differential measurement is carried out for calibration (with/without fluid). For the tomographic reconstruction, depending on the selected arrangement, a grid is set up in which each pixel has a specific value for, for example, attenuation or the speed of sound. A system of equations can then be set up according to the transmission paths with the measured propagation times or attenuations. This is solved to determine the spatially resolved sound velocity or attenuation. This value can be converted into a fluid allocation via a calibration. The calibration can be made for each fluid occupancy or interpolated from interpolation points (flooded and completely empty).

In a variant of the device according to the invention for the spatially resolved detection of fluids or liquid water on a support structure, the converter elements are attached to a periphery of the support structure and arranged such that the entire surface of the support structure is covered by the measuring area of the converter elements.

The periphery of the support structure is to be understood in particular as an edge region of the support structure which still belongs directly to the support structure but is not directly required for the functionality of the structure in a specific application or would have an unfavorable effect. The transducer elements are applied and arranged in such a way that the largest possible area is covered and detectable by the transmission paths spanned between the transducer elements.

In another variant of the device according to the invention, the transducer elements are made from piezoelectric macro-fiber composites (MFCs) for generating Lamb waves.

A piezoelectric MFC transducer element consists of rectangular piezo ceramic rods sandwiched between layers of adhesive, electrodes and polyimide film. The electrodes are attached to the foil in an interdigitated pattern, whereby the MFC structure of the applied voltage is transmitted directly to and from the ribbon rods. The MFC can also be applied as a thin, surface-conforming plate to various types of structures, e.g. B. glued, or embedded in the support structure. When a voltage is applied, it generates oscillations called Lamb waves. A particular advantage of the MFCs is their particularly thin design, which allows integration, e.g. B. in a fuel cell stack, as well as the particularly high bandwidth, which allows optimal adjustment of the operating point.

In a further variant of the device according to the invention, the converter elements are formed from interdigital converters made of piezoelectric material for generating Lamb waves. The interdigital transducers (interdigital transducers-IDT) have a comb-like electrode structure that has very advantageous properties in terms of mode selectivity. The IDTs are made of piezoelectric material such as polyvinylidene difluoride (PVDF), which is very flexible and inexpensive.

In another further variant of the device according to the invention, the converter elements are formed from piezoelectric discs for generating Lamb waves. Small piezo discs are glued to the support structure. This variant is very cheap and easy to implement.

In a variant of the device according to the invention, the converter elements are arranged on one side of the support structure and/or on both sides of the support structure. In the simplest case, the transducer elements are attached to one side of the plate-like support structure. For mode separation and differentiation of the effects below and above the support structure, it is also possible to additionally attach converter elements below the support structure, that is to say on the opposite side of the support structure.

In a further variant of the device according to the invention, the converter elements are designed and operable as transmitters and receivers, so-called transceivers, or separately as transmitters or receivers.

• a) Ansteuern eines ersten Wandler-Elementes mit einem elektrischen Signal mittels der Ansteuereinheit, wobei das Wandler-Element entsprechend seiner Richtcharakteristik eine Lamb-Welle auf der Trägerstruktur aussendet,
• b) Empfangen der Lamb-Welle durch mindestens die weiteren Wandler-Elemente, welche die Lamb-Welle wieder in ein elektrisches Signal umwandeln und von der Signalverarbeitungseinheit erfasst werden,
  • - Wiederholen der Schritte a) und b) bis alle Wandler-Elemente auf der Trägerstruktur eine Lamb-Welle ausgesendet haben,
  • - abschließend Verarbeiten und Auswerten der transmittierten elektrischen Signale der Wandler-Elemente durch die Signalverarbeitungseinheit, wobei die Signalverarbeitungseinheit eine tomographische Rekonstruktion der Verteilung des Fluides, insbesondere des Flüssigwassers, auf der Trägerstruktur vornimmt.

The object is also achieved by a measurement method which the invention Device uses, wherein the measuring method has the following steps:

• a) Controlling a first converter element with an electrical signal by means of the control unit, wherein the converter element emits a Lamb wave on the support structure according to its directional characteristic,
• b) receiving the Lamb wave by at least the other converter elements, which convert the Lamb wave back into an electrical signal and are detected by the signal processing unit,
  • - Repeat steps a) and b) until all transducer elements on the support structure have emitted a Lamb wave,
  • - finally processing and evaluating the transmitted electrical signals of the transducer elements by the signal processing unit, wherein the signal processing unit performs a tomographic reconstruction of the distribution of the fluid, in particular the liquid water, on the support structure.

The transducer elements are preferably arranged on the periphery of the support structure. This has the advantage that the Lamb wave, which propagates in or on the support structure due to the electrical control of a transducer element by the control unit, covers/covers the largest possible area of the support structure. The Lamb wave has different modes as already described. For the detection of fluids, especially liquid water, the S0 and A0 modes are used first, as these can be easily distinguished in a specific frequency range. Higher modes can also be used if the different contributions in the frequency range can be separated from one another. The Lamb wave that is excited and emitted by the converter element is received and detected by all other converter elements by generating an electrical signal in the respective converter element, which signal is detected by the signal processing unit. A measurement cycle, which consists of steps a) and b), is repeated until all transducer elements on the support structure have emitted a Lamb wave, which is detected by the other transducer elements. Finally, the transmitted electrical signals from the transducer elements are processed and evaluated by the signal processing unit. If a fluid, in particular liquid water, is present on the support structure, this can be identified by a change in the signal curve compared to a support structure without fluid or completely with fluid, or it can be identified where there is no fluid if there is fluid everywhere else. These signal changes are used for the tomographic reconstruction of the distribution of the fluid, in particular the liquid water, on the support structure. For this purpose, the measuring range is locally discredited and each pixel is assigned a value for attenuation or sound velocity. The transmission paths can be viewed as line integrals through the measurement area. Consequently, a system of equations can be set up which describes the attenuation or sound velocity along the transmission path. By solving the system of equations, one obtains the spatially distributed damping or speed of sound, which can be transferred to the fluid/water layer using a calibration function or a model.

In a first variant of the measuring method according to the invention, the converter elements are controlled one after the other in a defined order by the control unit. This variant is less complex, less error-prone and reproducible.

In another variant of the measurement method according to the invention, the converter elements are controlled in a randomized manner by the control unit. This variant is less sensitive to fluctuations in attenuation or sound velocity due to other influences such as temperature, pressure, etc.

In a further variant of the measurement method according to the invention, the tomographic reconstruction of the distribution of the fluid, in particular the liquid water on the support structure, is carried out using previously recorded calibration data, or using a model-based numerical simulation, or using an artificial neural network.

The use of calibration data for the tomographic reconstruction of the distribution of the fluid has the advantage that this variant represents a clear physical equivalent of the actual state of the fluid distribution, is easy to handle and is therefore comprehensible, practicable and real-time capable. With model-based numerical simulation or full wave inversion, a very precise reconstruction and high spatial resolution can potentially be achieved, since more information from the signal is used than with the previously recorded calibration data, but this variant is very time-consuming, not real-time capable and requires a very precise modeling ahead. When using a neural network, a very fast and potentially accurate reconstruction is possible with very little time expenditure at the time of the measurement, which is also real-time capable, but the creation of training data sets is initially very complex.

In a particularly advantageous manner, the device according to the invention is used for the spatially resolved detection of liquid water on a support structure in a fuel cell or in a fuel cell stack, the support structure being designed as a bipolar plate in the fuel cell stack and the liquid water being distributed on the bipolar plate by means of the converter elements , which are arranged on the bipolar plate, can be determined in a spatially resolved manner by means of tomographic reconstruction.

The solution according to the invention makes it possible to detect liquid water in a fuel cell or in a fuel cell stack in situ. It is particularly advantageous that this measurement can take place during operation of the fuel cell and that no modifications have to be made to the design of the fuel cell in order to be able to visually visualize the water distribution. Until now, the flow field, which is the focus of the investigations, had to have an optical access, e.g. through acrylic glass, in order to examine the liquid water distribution. This is no longer necessary with the invention. The use of MFCs or IDTs or piezo disks is inexpensive and does not have a negative effect on the performance and efficiency of the fuel cell or fuel cell stack. The investigations can be significantly simplified by the solution according to the invention, so that subsequent model creation, for example for simulations of the flow field, can also be significantly simplified. With the arrangement and the method according to the invention, numerical models for the formation and removal of liquid water in the operation of a fuel cell stack can be easily created and validated, the optimization of the operating parameters (material flows) of a fuel cell stack and the optimization of the flow field structure with regard to the formation of liquid water ( or the two-phase flow of water and steam) becomes possible.

In a further advantageous manner, the device according to the invention is used for the spatially resolved detection of liquid water on a support structure in a seawater desalination plant, the support structure being designed as a plate in a heat exchanger of the seawater desalination plant and the liquid water being distributed on the plate by means of the converter - Elements that are applied and arranged on the plate can be determined in a spatially resolved manner by means of tomographic reconstruction, whereby defects on the plate of the heat exchanger where no more water condenses or evaporates can be detected.

The solution according to the invention makes it possible to monitor the condition of hard-to-reach surfaces by monitoring a heat exchanger in a seawater desalination plant to determine whether water can still condense or evaporate over the entire surface or whether the efficiency of the plant is impaired due to deposits.

In another advantageous way, the device according to the invention is used for the spatially resolved detection of fluids on a plate heat exchanger in the food industry, in particular for the detection of milk or points on the plate heat exchanger where milk can no longer be heated.

The solution according to the invention makes it possible to monitor the condition of surfaces that are difficult to access in such systems in the food industry by monitoring a heat exchanger to determine whether milk can still be heated all over the surface, or the heat transfer to the milk or other liquids to be heated due to fouling is reduced.

The solution according to the invention can be used in any heat exchanger structure in which fluids or locations where no fluid is present are to be detected.

The device according to the invention is also referred to as a sensor for detecting liquid water in a fuel cell stack or for detecting faults in plate heat exchangers where there is no longer any fluid present.

The invention will be explained in more detail below using some exemplary embodiments.

• 1 Aufbau einer PEM-Brennstoffzelle nach dem Stand der Technik;
• 2 Aufbau eines Brennstoffzellenstapels nach dem Stand der Technik;
• 3 Transmissionspfade für verschiedene Anordnungen der Wandler-Elemente auf der Trägerstruktur;
• 4 Trägerstruktur mit peripher angeordneten Wandler-Elementen zur Detektion von Flüssigwasser gemäß der Erfindung;
• 5 Modifizierter Brennstoffzellenstapel zur Detektion von Flüssigwasser;
• 6 Modifizierter Wärmeübertrager in einer Meerwasser-Entsalzungsanlage zur Detektion von Fehlstellen, an denen kein Wasser mehr verdampft bzw. modifizierter Plattenwärmeübertrager in der Lebensmittelindustrie zur Detektion von Fouling-Stellen auf selbigem.

Show the accompanying drawings

• 1 Structure of a PEM fuel cell according to the prior art;
• 2 Structure of a fuel cell stack according to the prior art;
• 3 transmission paths for different arrangements of the transducer elements on the support structure;
• 4 Support structure with peripherally arranged transducer elements for the detection of liquid water according to the invention;
• 5 Modified fuel cell stack for detection of liquid water;
• 6 Modified heat exchanger in a seawater desalination plant to detect defects where water no longer evaporates or modified plate heat exchanger in the food industry to detect fouling spots on the same.

the 4 shows the arrangement according to the invention for the spatially resolved detection of a fluid, in particular liquid water, on a carrier structure 6, 13, 16. The carrier structure 13, on which the spatially resolved distribution of liquid water 12 in particular is to be detected, has at least three transducer elements 11 which are applied and arranged on the support structure 13, preferably on its periphery 10. The converter elements 11 span transmission paths 14, as is shown in 3 is shown schematically. The more transducer elements 11 that are used, the easier it is to localize the fluid 12 or liquid water on the structure to be examined. The transmitter converter is controlled by suitable control electronics (not shown) with an electrical signal, the parameters of this signal depend on the Lamb wave modes used. For example, for a 0.5mm thick aluminum flow field, the S0 and A0 modes are excited at 2 MHz, which propagate at sound speeds of 4550 m/s and 2700 m/s, respectively. Due to the different speeds of sound, there are different propagation times, which makes it possible to distinguish between the modes. The excitation should be as narrow-band as possible in order to selectively excite modes and avoid dispersion effects. The excitation voltage is ±100 V.

The Lamb waves (modes S0 and/or A0) propagate over the support structure 13, in the case of use in a fuel cell stack 8 over the bipolar plate 6 ( 5 ), from according to the directional characteristics of the IDT or MFC or piezo disc and are converted again into an electrical signal by the second IDT or MFC or piezo disc on the opposite side, amplified, digitized and stored. An AD converter with f _s =50 MHz is used in the signal processing unit (not shown).

• 1) Für die Kalibrierung wird das Verfahren nach „Messung“ mindestens für den Zustand ohne Flüssigwasser auf der Trägerstruktur bzw. Bipolarplatte und gegebenenfalls für den Zustand der komplett gefluteten Trägerstruktur bzw. Brennstoffzelle durchgeführt und die aufgenommenen Signale gespeichert. Während der späteren Messung werden die Signale mit diesen beiden Kalibriersignalen verglichen. Beispielsweise kann dabei die Amplitude der S0 Mode ausgewertet werden. Um auf die Wasserauflage zu schließen wird eine lineare Regression mittels der beiden im Kalibrierexperiment ermittelten Amplituden der S0 Mode durchgeführt.
• 2) Es kann auch eine modellbasierte Kalibrierung genutzt werden. Die Schallausbreitung entlang des Transmissionspfades auf der Trägerstruktur bzw. Bipolarplatte wird mittels numerischer Simulation modellhaft abgebildet. Während der Messung erfolgt eine Optimierung der Modellparameter, beispielsweise mittels der Full wave inversion Methode. Aus den Modellparametern kann die Wasserauflage bestimmt werden.
• 3) Eine dritte Möglichkeit ist die Verwendung eines künstlichen neuronalen Netzes (KNN). In Experimenten werden mehrere Szenarien mit bekannter Wasserauflage eingestellt und die entsprechenden Signale gespeichert. Durch ein neuronales Netz kann der Zusammenhang von Wasserauflage und dem komplexen Signalverlauf mittels Lernstichprobe abgebildet werden. Zum Zeitpunkt der Messung wird dem KNN das gemessene Signal vorgegeben und es wird die Wasserauflage ausgegeben.

Calibration is necessary in order to infer from the received signals a fluid deposit, in particular liquid water deposit, on the bipolar plate 6 . There are different approaches for this:

• 1) For the calibration, the procedure according to "measurement" is carried out at least for the state without liquid water on the support structure or bipolar plate and, if necessary, for the state of the completely flooded support structure or fuel cell and the recorded signals are saved. During the subsequent measurement, the signals are compared with these two calibration signals. For example, the amplitude of the S0 mode can be evaluated. In order to deduce the water layer, a linear regression is carried out using the two amplitudes of the S0 mode determined in the calibration experiment.
• 2) A model-based calibration can also be used. The propagation of sound along the transmission path on the support structure or bipolar plate is modeled using numerical simulation. During the measurement, the model parameters are optimized, for example using the full wave inversion method. The water layer can be determined from the model parameters.
• 3) A third option is to use an artificial neural network (ANN). In experiments, several scenarios with a known water level are set and the corresponding signals are saved. A neural network can be used to map the connection between the water surface and the complex signal curve using a training sample. At the time of the measurement, the measured signal is given to the ANN and the water level is output.

Liquid water along the transmission path 14 between two transducer elements 11 affects various physical properties of the Lamb wave propagation. Possible properties are the attenuation, the speed of sound, the dispersion or the scattering/reflection of the Lamb waves. These properties can be determined over time using the signal parameters, such as maximum amplitude of the modes, energy of the modes, time of arrival of the modes or the spectral composition of the modes, resulting in an integral measurement of the liquid water layer along a transmission path 14 .

The liquid water layer is then calculated according to the respective calibration strategy.

The spatially resolved determination of the distribution of the liquid water 12 on the support structure 13 is made possible by the fact that numerous transmission paths 14 are spanned at different angles for the measurements of the water layer. i.e. one converter element 11 acts as an actuator and the others act as receivers, in the next step another converter element 11 acts as an actuator and the others in turn act as receivers until all converter elements 11 were once actuators. However, it is not absolutely necessary for all converter elements to be actuators; with n transducer elements, for example, 1 to n transducer elements can be transmitters of Lamb waves, which are driven sequentially. The parameters determined on different paths 14 can then be used for a tomographic reconstruction of the liquid water distribution in order to determine the spatially resolved distribution of the water 12.

Of course, it is also possible to carry out the measurement only along a transmission path, ie on a line between two transducer elements. On the one hand, this measurement can be used to determine whether and on the other hand how much water is on the carrier structure along the transmission path.

It is particularly advantageous to use the invention in fuel cell stacks 8, since knowledge can thus be obtained as to the prerequisites/circumstances/boundary conditions under which water is formed when and where in the fuel cells. With the findings, design adjustments can be made or parameter sets for the operation of fuel cell stacks or other operating parameters can be derived where the formation of water in an arrangement can have an adverse effect. As a consequence, measures can be derived from this to prevent water formation in some places or, if necessary, to promote it.

In a further exemplary embodiment, the device according to the invention can be used as an icing sensor in a fuel cell stack 8 . When using fuel cells (BSZ) in regions with temperatures below freezing, freezing liquid water in the flow field 7 of the fuel cell 1 can cause damage. Therefore, when switching off the BSZ, it must be ensured that no liquid water remains in the channels of the BSZ. This can easily be done using the device according to the invention.

The device according to the invention can also be used to determine whether there is ice in the flow fields 7 of the fuel cell stack 8, since ice deposits also change the transmission properties of the BSZ. This allows appropriate countermeasures to be taken to ensure optimal operation of the BSZ.

Another application example of the device according to the invention is the use in a seawater desalination plant ( 6 ). Seawater desalination plants contain (plate) heat exchangers 16, each of which has one side on which the water evaporates and one side on which the water condenses. On the condensation side, the detection of passivated points is not critical, as mostly only water condenses there. On the other hand, on the evaporator side of the heat exchanger structure 16, the sea water (depending on how well it is pre-processed) is supplied with salt and algae. There, contamination and thus passivation play an important role. The plate heat exchanger 16 is therefore also referred to or operated as a falling film evaporator on the evaporator side. In contrast to use in a fuel cell stack, the device according to the invention can be used to detect precisely those locations/points where water can no longer evaporate and the efficiency of the system is therefore reduced.

• Im Fall 1) wird ein Medium 1 auf der Platte verdampft und liegt vorher als Rieselfilm auf der Platte vor, wie es in Meerwasserentsalzungsanlagen der Fall ist, wobei ein Phasenwechsel von gasförmig zur flüssig stattfindet.

Another application example of the device according to the invention is the use in a plate heat exchanger 16 for the food industry. The fluids 12 to be detected can therefore not only be liquid water, but also milk or the like. In this exemplary embodiment, similar to use in a seawater desalination plant, the local distribution of the fluid 12 is used indirectly to detect spatially resolved defects on the heat exchanger structure 13, 16 , where due to contamination just no more fluid 12 is available and the structure 13, 16 is passivated. 6 shows a plate heat exchanger 16, which uses the device according to the invention. The white arrow 17 indicates the direction of flow of medium 1 and the two black arrows 18 indicate the direction of flow of medium 2, which is a heating or cooling medium depending on the application. A distinction is made between the following cases:

• In case 1), a medium 1 is vaporized on the plate and is previously present on the plate as a falling film, as is the case in seawater desalination plants, with a phase change from gaseous to liquid taking place.

In case 2) medium 1 is condensed on the plate, whereby a phase change also takes place.

In case 3), medium 1 is cooled or heated, with no phase change taking place.

In all cases, fouling can occur. This leads to reduced heat transfer. With the present invention, this can be detected easily and reliably and appropriate countermeasures can be taken.

Reference List

1

fuel cell

2

membrane

3

Electrode (cathode or anode) with catalyst

4

MEA - membrane electrode assembly

5

gas diffusion layer

6

bipolar plate

7

Channel structures, flow field

8th

fuel cell stack

9

connected load

10

Periphery of the support structure

11

transducer element

12

Fluid, especially liquid water or milk

13

support structure

14

transmission path

15

transceivers

16

plate heat exchanger

17

Flow direction medium 1

18

Flow direction medium 2

Claims (15)

Device for the spatially resolved detection of fluids (12), in particular liquid water, on a support structure (13), comprising at least three converter elements (11) which are attached to the support structure (13) and/or arranged thereon, a control unit for controlling the converter elements (11) and a signal processing unit for evaluating signals recorded by the converter elements (11), characterized in that the converter elements (11) form transmission paths (14) among themselves and thus a measuring area for detecting fluids (12), in particular liquid water on the support structure (13) and cover it, with the control unit controlling one converter element (11) of the at least three converter elements (11) as a transmitter and recording transmitted signals through the respective other converter elements (11). , wherein the signal processing unit tomographically evaluates the signals of the transducer elements (13) and a spatially resolved Ver Division of the fluid (12), in particular the liquid water on the support structure (13). Device for the spatially resolved detection of fluids (12) on a support structure (13). claim 1 , characterized in that the converter elements (11) are arranged integrated on a periphery (10) of the support structure (13) such that the entire surface of the support structure (13) is covered by the measuring range of the converter elements (11). Device for the spatially resolved detection of fluids (12) on a support structure (13). claim 1 , characterized in that the transducer elements (11) made of macro-fiber composites, MFCs, are designed to generate Lamb waves. Device for the spatially resolved detection of fluids (12) on a support structure (13). claim 1 , characterized in that the transducer elements (11) are formed from interdigital transducers made of piezoelectric material for generating Lamb waves. Device for the spatially resolved detection of fluids (12) on a support structure (13). claim 1 , characterized in that the transducer elements (11) are formed from piezoelectric discs for generating Lamb waves. Device for the spatially resolved detection of fluids (12) on a support structure (13). claim 1 , characterized in that the converter elements (11) are arranged on one side of the support structure (13) and/or on both sides of the support structure (13). Device for the spatially resolved detection of fluids (12) on a support structure (13). claim 1 , characterized in that the converter elements (11) can be operated as transmitters and receivers or separately as transmitters or receivers. Measuring method for the spatially resolved detection of fluids (12), in particular liquid water on a support structure (13), which the device according to one of Claims 1 until 7 uses, the measuring method having the following steps: a) activating a first converter element (11) with an electrical signal by means of the activation unit, the converter element (13) emitting a Lamb wave on the carrier structure (13) according to its directional characteristics , b) receiving the Lamb wave by at least one further converter element (11), which converts the Lamb wave back into an electrical signal and is detected by the signal processing unit, - repeating steps a) and b) until all converter elements on the support structure (13) have emitted a Lamb wave, - finally processing and evaluating the transmitted electrical signals of the converter elements (11) by the signal processing unit, the signal processing unit performing a tomographic reconstruction of the distribution of the fluid (12), in particular liquid water on the support structure (13). Measuring method for the spatially resolved detection of fluids (12) on a support structure (13). claim 8 , characterized in that the converter elements (11) are controlled one after the other in a defined order by the control unit. Measuring method for the spatially resolved detection of fluids (12) on a support structure (13). claim 8 , characterized in that the converter elements (11) are controlled in a randomized manner by the control unit. Measuring method for the spatially resolved detection of fluids (12) on a support structure (13). claim 8 , characterized in that the tomographic reconstruction of the distribution of the fluid (12), in particular liquid water, on the support structure (13) takes place using previously recorded calibration data, or using a model-based numerical simulation, or using an artificial neural network. Use of the device for the spatially resolved detection of fluids (12), in particular liquid water, on a support structure (13) in a fuel cell stack (8), the support structure (13) being designed as a bipolar plate (6) in the fuel cell stack (8) and the distribution of the liquid water (12) on the bipolar plate (6) by means of the converter elements (11), which are arranged on the bipolar plate (6), can be determined with spatial resolution by means of tomographic reconstruction. Use of the device for spatially resolved detection of fluids on a support structure (13) in a seawater desalination plant, the support structure (13) being designed as a plate in a heat exchanger (16) of the seawater desalination plant and the distribution of the fluid (12). of the plate (16) by means of the converter elements (11), which are arranged in the plate (16), can be determined in a spatially resolved manner by means of tomographic reconstruction, with defects on the plate (16) of the heat exchanger (16) where no fluid (12 ) more condensed or evaporated, can be detected. Use of the device for the spatially resolved detection of fluids on a support structure (13) in the food industry, the support structure (13) being designed as a plate in a heat exchanger (16) of the food industry plant and the distribution of the fluid (12), in particular milk of the plate (16) by means of the transducer elements (11) arranged on the plate (16) can be determined in a spatially resolved manner by means of tomographic reconstruction, with defects on the plate of the heat exchanger (16) at which no fluid (12) is heated any more is, can be detected. Use of the device for the spatially resolved detection of fluids on a support structure (13), the support structure (13) being designed as a plate in a heat exchanger (16) and the distribution of the fluid (12) on the plate (16) by means of the converter Elements (11) arranged on the plate (16) can be determined in a spatially resolved manner by means of tomographic reconstruction, it being possible to detect defects on the plate of the heat exchanger (16) at which fluid (12) is no longer heated or evaporated.

Images