The invention relates to a fuel cell system according to the precharacterizing clause of claim 1.
A fuel cell system, in particular a system with fuel cells assembled in modular manner, comprises at least one channel system for the inlet, circulation and outlet of fluid operating substances. In order to achieve a high level of efficiency when generating power, devices are provided which control or adjust pressure and/or temperature and/or humidity and/or the composition of at least one operating substance. To detect the particular actual values of the variables to be controlled or adjusted, sensors are used which are preferably arranged on a storage tank, on a device for producing the substances to be reacted and/or on a channel of a channel system.
DE 100 47 248 A1 shows a typical structure of a proton exchange membrane fuel cell system (PEM fuel cell system). The PEM fuel cell system consists of a cathode electrode, an anode electrode and a matrix, which together form a membrane electrode unit (MEA). The cathode electrode and the anode electrode consist in each case of an electrically conductive body, which serves as a support for a catalyst material. The matrix is arranged between the cathode electrode and the anode electrode and serves as a support for an electrolyte. A plurality of fuel cells are stacked on one another with interposed separator plates. The inlet, circulation and outlet of oxidants (oxidizing agents), reductants (fuel) and coolants proceed via channel systems, which are produced using the separator plates. For each liquid or gaseous operating medium, supply collector channels, distributor channels and outlet collector channels are provided in the fuel cell stack, these being separated from one another by sealing means. Via at least one inlet collector channel, the cells of a stack are supplied in parallel with an oxidant fluid, a reductant fluid and a coolant. The reaction products, excess reductant and oxidant fluid and heated coolant from the cells are fed out of the stack via at least one outlet collector channel. The distributor channels form a connection between the inlet and outlet collector channels and the individual active channels of a fuel cell. The fuel cells may be connected in series to increase voltage. The stacks are closed by end plates and accommodated in a housing, the positive and negative poles being conveyed outside to a consumer unit.
In a PEM fuel cell system, in which the fuel contains hydrogen, a gas mixture of air and water vapour is supplied on the cathode side. The dew point of the gas mixture should preferably be approximately 5° C. below the gas temperature. To achieve this at any desired operating temperatures, a device may be provided for humidity control which has constant actual value determination.
It has already been proposed to use an optical or a capacitive sensor to measure actual humidity. The reliability and measuring accuracy of these sensors is impaired if a liquid film forms on a sensor surface.
It has additionally been proposed to determine the actual value indirectly by measuring temperature and pressure. Such measuring instruments are complex and have low dynamic range of measurement.
As described in DE 101 49 333 A1, the humidity of a gas may be determined with a resistance measuring structure, which cooperates with a layer of carbon black. The speed of measurement is low and the resistance measuring structure can only disadvantageously be used in a narrow channel of a fuel cell system due to its structural size.
A device is known from DE 199 44 047 A1 for measuring the concentration or density and the velocity of particles in a flowing fluid. To measure these variables, an ultrasonic detector is provided on a channel wall. The sound waves issuing from an ultrasonic transmitter in multifrequency operation are reflected by the particles. The reflected waves are received by an ultrasonic receiver, wherein the frequency shift and ultrasonic absorption brought about by the particle motion in accordance with the Doppler principle are evaluated. The particles detectable with the device range in size from 1 μm to 1000 μm.
In a cooking apparatus according to DE 101 43 841 A1, the density of a cooking atmosphere in a cooking chamber is determined by measuring sound velocity, in order to derive therefrom the content of water vapour and thus the humidity of the cooking atmosphere. In this application, account is taken of the fact that the cooking temperature influences the density of the cooking atmosphere. Sound velocity measurement may be performed in a measuring tube, which is connected atmospherically to the cooking chamber. Sound velocity is measured using an ultrasonic transmitter and an ultrasonic receiver, between which a known measurement path is located, and which may be combined into a single unit.
The object of the invention is to develop a fuel cell system with a device for adjusting the moisture content of the working fluid flowing in one direction in a channel system, which system allows reliable and dynamic measurement of the actual humidity.
The object is achieved with a fuel cell system which has the features according to claim 1. Advantageous developments are revealed by the subclaims.
According to the invention, an ultrasonic detector is used to measure the actual humidity. The invention is particularly advantageously applicable to the measurement of the actual humidity on the cathode side of a fuel cell system.
To measure actual humidity, sound velocity is preferably determined in a working fluid which flows in a channel of a channel system, from which a dew point temperature is derived. The ultrasonic detector comprises a transmitter and a receiver, which are arranged at a known distance from one another. At identical temperature, sound velocity in water vapour is higher than in dry air. This means that as the humidity of the working fluid rises, sound velocity increases along the measurement path. Since the vapour pressure of water increases exponentially as the temperature rises, the accuracy of the measurement principle increases on the one hand as the temperature increases and on the other hand as the respective dew point of the working fluid is approached.
The invention will be explained below with reference to exemplary embodiments; in the drawings:
During operation of the fuel cell system 1, the humidity of the air in the cathode channel system 2 is measured constantly using the transmitter 16 and receiver 17. Ultrasonic waves issue from the transmitter 16, these penetrating the moist air in the cathode channel system 2 and impinging on the receiver 17. Transmitter 16 and receiver 17 are at a known distance from one another. From the propagation time of the sound waves and the distance between transmitter 16 and receiver 17, a sound velocity is obtained which is dependent on the humidity of the air at that time. The actual humidity determined from the signals of the receiver 17 in the control device 22 is compared with a stored reference humidity. A difference between actual and reference humidity is compensated by opening or closing the valve 12 to a greater or lesser extent using the control device 22. In this way, more or less water vapour from the water vapour generator 13 is fed into the air conveyed with the blower 10. When determining the control variables for the valve 12, the temperature in the cathode channel system 2 is taken into account. This temperature is obtained from the measured values of the temperature sensor 15.
In the variant according to
According to
The diagram illustrated in
Number | Date | Country | Kind |
---|---|---|---|
10 2007 003 938.9 | Jan 2007 | DE | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
---|---|---|---|---|
PCT/EP08/00283 | 1/16/2008 | WO | 00 | 7/24/2009 |