FR2914786A1 - Gas flow rate evaluating method for fuel cell of e.g. motor vehicle, involves calculating total molar flow rate of gas with respect to formula comprising parameters of hydrogen concentrations, current, number of cells and Faraday's constant - Google Patents

Abstract

The method involves measuring a hydrogen concentration on a hydrogen inlet branch (14), and measuring another hydrogen concentration on a hydrogen recirculation loop (12). A total molar flow rate of gas is calculated in the hydrogen recirculation loop with respect to a formula comprising parameters of the hydrogen concentrations, current generated by a fuel cell, number of cells in the fuel cell and Faraday's constant. An independent claim is also included for a device for evaluating a flow rate of gas circulating in a hydrogen recirculation loop of a cell in a fuel cell.

Description

"Method for evaluating the flow rates of gases flowing in a loop of

  hydrogen recirculation of a fuel cell cell and associated device ".

  The invention relates to a method for evaluating the flow rates of gases flowing in a hydrogen recirculation loop of a fuel cell. The invention also relates to a device for implementing such a method. A fuel cell is an electrochemical device that converts chemical energy into electrical energy from a fuel, typically hydrogen, and an oxidant, oxygen, or oxygen-containing gas. that air, the only product of the reaction being water accompanied by a release of heat and a production of electricity. Within the fuel cell, the overall chemical reaction resulting from the reactions occurring at the electrodes is as follows: H 2 + 1 O 2 - * H 2 O A fuel cell can be used to supply electrical energy to any device such as for example, a computer, a mobile phone but it can also be used to provide traction for a motor vehicle and / or power for electrical devices contained in a vehicle. A fuel cell consists of an assembly of elementary cells. An elementary cell 1 is schematically shown in FIG. 1. Each elementary cell 1 comprises a protonic conductive electrolyte 2 which is sandwiched between two porous cathode 3 and anode electrodes 4 and which provides the electronic transfer between these two electrodes 3, 4.

  For this purpose, the electrolyte 2 may be a proton exchange polymer membrane with a thickness of 20 to 200 m, the resulting cell being a proton exchange membrane type cell or PEMFC cell.

  The assembly consisting of the electrolyte 2 and the two electrodes 3,4 forms a membrane electrode assembly (AME) 5 which is itself sandwiched between first 6 and second 7 bipolar plates made of an electrically conductive material.

  The two half-reactions leading to the synthesis reaction of the aforementioned water are: At the anode: H2-2H + + 2e- At the cathode: O 2 + 2H + + 2e - H2O. To ensure the operation of such a fuel cell, the cathode must be supplied with oxygen, generally from the air in automotive applications. For this, there is provided an air intake branch 8 and an air outlet 9.

  And the anode must be fed with hydrogen. For this purpose, there is provided a hydrogen intake branch 10 and a hydrogen outlet 11. With reference to FIG. 2, it is known to provide a recirculation loop 12 which makes it possible to ensure the circulation of gases from the 4a outlet of the anode 4 via a circulator 13 and to mix with pure hydrogen circulating in the hydrogen intake branch 14, this intake branch being supplied with hydrogen by a supply of pure hydrogen 15.

  The circulator 13, which may be a pump or an ejector, is subjected to delicate operating conditions and in particular, any presence of liquid water in this circulator 13 must be avoided. This is why a separator 16 makes it possible to collect and separate the water present in liquid form from the other species present in the fluid coming from the outlet 4a of the anode 4, such as hydrogen, nitrogen and steam. of water, the latter being introduced into the recirculation loop 12 described above. This operation of recirculation of the species resulting from the anodic reactions makes it possible on the one hand to mix all these species, and on the other hand to participate in the humidification of the feed gas of the anode circulating in the intake branch 14, these two functions contribute to the proper functioning of the fuel cell.

  To control the humidification of the feed gas, the operation of the circulator must be controlled. For this, it may be necessary to know either the volume flow rate or the mass flow rate of the gas flowing in the recirculation loop 12. A measurement of these flows is possible thanks to a sensor placed on the recirculation loop 12. The presence of such a sensor causes the following disadvantages. Firstly, with regard to the measurement of the volume flow, the pressure drop induced by this sensor causes an oversizing at the level of the circulator 13 in order to compensate for this pressure drop, and thus an electrical over-consumption and a drop in the efficiency. of the consequent system. There are non-intrusive volume flowmeters, that is to say not creating a pressure drop, but this type of flowmeter is not necessarily adapted to a hydrogen recirculation loop of a fuel cell. and result in additional cost and integration constraint. On the other hand, with respect to the measurement of mass flow rates, the flow of gas leaving the anode 4 may have a variable composition with respect to the proportion of different gaseous species depending on the point of operation of the system.

  And the mass flow sensors are generally calibrated for a given gas composition.

  These sensors therefore become inoperative or too inaccurate when the composition of the gas mixture is

  different from the one with which they were calibrated.

  To overcome these drawbacks, the method of the invention makes it possible to evaluate flow rates of gas flowing in a hydrogen recirculation loop which ensures the circulation of the gases coming from the anode of a cell of

  fuel cell to the hydrogen inlet branch of this anode, and in which circulates residual hydrogen, nitrogen and water vapor, this method being essentially characterized in that it comprises at least the steps of: measuring the concentration of hydrogen X 1 on the intake branch, measuring the concentration of hydrogen X 2 on the recirculation loop, and

  calculation of the total molar flow rate of gas in the recirculation loop with respect to the following formula: where X 1 is the hydrogen concentration on the intake branch, where X 2 is the hydrogen concentration on the recirculation loop, and

  CONSO where Q is the molar flow rate of hydrogen consumed by the H2

  And N is the fuel cell generated current, N is the number of fuel cells, and F is the Faraday constant.

  The method of the invention may further comprise at least the step of:

  calculation of the molar flow rate of hydrogen circulating in the recirculation loop (12) with respect to the following formula: QH2 = X2, total where Q is the total molar flow rate of gas flowing in total

  the recirculation loop, and where X 2 is the hydrogen concentration on the recirculation loop. The method of the invention may also comprise at least the additional steps of: measuring the saturation temperature Tat or the relative humidity of the gas flowing in the recirculation loop;

measuring the pressure P, and

  calculation of the volume flow rate of water vapor circulating in the recirculation loop by the following relationships. Q VAP = X VAP Qtotai where Q is the total molar flow rate of gas flowing in the total recirculation loop, and where XVAP responds to the following formula. where Tsat is the saturation temperature (in Kelvin) in the recirculation loop 12. With Pvap, which is the vapor pressure and which is given by the law of dv. Antoine: PVAP = exp (23, 1961, 3816, 44) And advantageously, the method of the invention also comprises at least the step of:

  calculation of the molar flow rate of nitrogen in the recirculation loop with respect to the following formula: QN2 = QTOTAL QVAP ù QH2 where Q TOTAL the total molar flow rate of gas in the TOTAL recirculation loop, where Q is the molar flow rate of water vapor in the VAP recirculation loop, and where Q is the molar flow rate of hydrogen in the loop of H2 recirculation. Finally, the method of the invention may further comprise at least the step of: calculating the mass flow rate of hydrogen with respect to the following formula: mass QH2 = QH2xM (H2) Where M (H2) is the molar mass hydrogen and / or - calculation of the mass flow rate of water vapor with respect to the following formula: ## EQU1 ## where M (H 2 O) is the molar mass of the water and / or - calculation of the mass flow rate of nitrogen with respect to the following formula: mass than N2xM (N2) QN2

  Where M (N2) is the molar mass of nitrogen The invention also relates to a device for implementing the previously described method, said device comprising at least one hydrogen recirculation loop which ensures the circulation of gases. from the anode of a cell stack to

  Fuel to the hydrogen inlet branch of this anode in which recirculation loop circulates residual hydrogen, nitrogen and water vapor. This device is essentially characterized in that it comprises a measuring device of

  The hydrogen concentration at the hydrogen inlet branch and an apparatus for measuring the hydrogen concentration at the recirculation loop.

  To be able to evaluate the volumetric flow of steam

  As the water circulates in the recirculation loop, the device of the invention also comprises an apparatus for measuring the saturation or relative humidity temperature of the gas flowing in the recirculation loop and a device for measuring the pressure of the recirculation loop. circulating gas

  30 in this recirculation loop.

  The invention will be better understood, and other objects, features, details and advantages thereof will appear more clearly in the following explanatory description made with reference to the drawings.

  FIGS. 1 is a diagrammatic representation of a fuel cell cell of the prior art, FIG. 2 is a representation of the invention, and FIG. schematic of a device for recirculating hydrogen of the prior art, - Figure 3 is a schematic representation of the device of the invention according to a first variant; and Figure 4 is a schematic representation of the device of the invention according to a second variant. The elements common to the device of the prior art of FIG. 2 and to the device of the invention of FIGS. 3 and 4 will bear the same references. As shown in FIG. 3, the device of the invention provides, as in the device of the prior art, a hydrogen recirculation loop 12 making it possible to ensure the circulation of the gas coming from the outlet 4a of the anode 4 to the intake branch 14 fed by a hydrogen supply system 15. On this recirculation loop 12 are mounted a circulator 13 and a separator 16. According to the invention, a first hydrogen sensor 25 is disposed on the intake branch 14 near the inlet 4b of the anode 4 and a second hydrogen sensor 21 is disposed on the recirculation loop 12 between the separator 16 and the circulator 13. These two sensors 20, 21 are able to measure the hydrogen concentration in the gas flow under consideration. From these two measurements, it is then possible to calculate the value of the total molar flow rate of the gas flowing in the recirculation branch 12 in the following manner. 35 We have. intake admission QH2 Xi Q TOTAL admission where QH2 is the molar flow rate of hydrogen in intake branch 14, X1 is the measured hydrogen concentration in intake branch 14, and admission is the total molar flow rate of gas in the intake branch 14. We also: recirculation recirculation QH2 X2 TOTAL recirculation where QH2 is the molar flow rate of hydrogen in the recirculation loop 12, 15 X2 is the measured concentration of hydrogen in the recirculation loop 12, and Recirculation is the total molar flow of gas in the 20 By neglecting the material transfers through the membrane not shown in Figure 3, the volume flow of hydrogen in the inlet branch 14 admission L. H 2 and the volume flow of hydrogen in the loop of Q TOTAL Q TOTAL recirculation loop 12 that one seeks to calculate. Recirculation L. H 2 are linked by the following recirculation formula 12: intake recirculation consumed _ NI QH2 H2 QH2 2F Q consumed H2 stack, is the molar flow rate of hydrogen consumed by where N is the number of cells in the fuel cell, where I is the current generated by the battery, and where F is the Faraday constant. From the foregoing, the TOTAL recirculation flow rate Q circulating in the recirculation loop 12 can be calculated by the following total recirculation gas ratio 1 where X 1 total X 1 X 2 CONSO CONS O NI with H 2 H 2 F The device shown in FIG. 4 This device also has a pressure sensor 22 and a saturation or relative humidity temperature sensor 23 capable of respectively measuring the pressure in the recirculation loop 12 and the temperature of the recirculation loop 12. saturation or relative humidity in this loop 12. From these measurements, it is possible to calculate the fraction of vapor Xvap in the gas mixture circulating in the recirculation loop 12 by the following formula: pvap X VAP = P With Pvap, which is the vapor pressure and which is given by Antoine's law: PVQP = exp (23,1961 to 3816,44 It is then possible to determine the molar flow rate

  recirculation of water vapor Q circulating in the VAP loop

  recirculation 12 with regard to the following relation: T at ù 46,13 ~ Where Tsat is the saturation temperature (in Kelvin) in the recirculation loop 12. 11 recirculation recirculation Q VAP = X VAP Qtotai Total recirculation is the total molar flow circulating gas or in the recirculation branch 12 previously calculated. Circulating circulation H2 and the molar flow rate of hydrogen in the recirculation loop 12 can also be calculated by the following formula: 2 recirculation Q total recirculation QH 2 Where X2 is the measured concentration of hydrogen in the recirculation loop 12. recirculation Finally, the molar flow rate of nitrogen Q in the N2 recirculation loop 12 is then also possible to be determined from the following relation: recirculation recirculation recirculation recirculation QN2 total QVAP QH 2 From these three flow rates, density and flow can be deduced total mass of the gas flowing in the recirculation loop 12 by the following formulas. mass = Q H2 xM (H QH 2 2 where M (H2) is the molar mass of massaceous hydrogen 30 Q = v, xM (H2O) wine, where M (H20) is the molar mass of the bulk water QN2 N2xM (N2) Where M (N2) is the molar mass of nitrogen And finally: mass (j / mass mass mass ùTOTAL ù L ~ N2 ~ VAP ~ H2 P 'Mmixture P mixture R • T with Mmixture = XH2 • M (H2) + XH2O • M (H2O) + XN2 • M (N2)

  Where M is the molar mass, P is the pressure and T is the temperature of the gas flowing in the recirculation loop 12.

  It may, according to another variant, be considered to double these measurements by the presence of volume and mass flow rate sensors thereby improving the reliability of the system. The device and method of the invention allow by simple and inexpensive means to evaluate the volume flow in the recirculation loop 12 of a fuel cell system. This advantageously makes it possible not to use an expensive, bulky volumetric volume flow sensor, resulting in significant losses of loads. This method thus makes it possible to eliminate the yield losses resulting from the increase in the electrical consumption of the circulator and also allows gains in mass and volume that are particularly advantageous for an application in the automotive field. By this method it is also possible to determine the mass flow rates in the recirculation loop, including during the operating phases of the system during which the composition of the gas mixture exiting the stack is variable in time.

  Finally, the method and the device of the invention make it possible to control the circulator in the best possible manner in order to respect the humidification stress of the gases at the inlet of the anode.

Claims (7)

  A method for evaluating the flow rates of gas flowing in a hydrogen recirculation loop (12) which circulates the gases from the anode (4) of a fuel cell cell (1) to the hydrogen intake branch (14) of this anode (4), and in which residual hydrogen, nitrogen and water vapor circulates, said method comprising at least the steps of: - measuring the concentration of hydrogen X1 on the intake branch (14), - measurement of the hydrogen concentration X2 on the recirculation loop (12), and - calculation of the total molar flow rate of gas in the recirculation loop (12). ) with respect to the following formula: = 1 where X1 is the hydrogen concentration on the intake branch (14), where X2 is the hydrogen concentration on the recirculation loop (12), and CONSO where Q has the following formula: H2 CONSO NI QH2 2F 30 where I is the current generated by the pil n is the number of fuel cells, and F is the Faraday constant.   2. Method according to claim 1, characterized in that it further comprises at least the step of: calculating the molar flow rate of hydrogen circulating in the recirculation loop (12) with regard to the following formula: QH2 = X2 Q, where Q is the total molar flow rate of gas flowing in the total recirculation loop, and Xz is the hydrogen concentration on the recirculation loop (12)   3. Method according to claim 1, characterized in that it further comprises at least the steps of: measuring the saturation temperature Tat or relative humidity of the gas flowing in the recirculation loop, measurement of the pressure P and calculating the molar flow rate of water vapor circulating in the recirculation loop (12) by the following relationships. Q VAP = X VAP Qtotai gives the following formula: vAp pvap XVAP = p With Pvap which is the vapor pressure and which is given by Antoine's law: PVAP = exp ~ 23,1961u 3816,44 T at ù 46,13 ~ Where Tsat is the saturation temperature (in Kelvin) in the recirculation loop 12. 15 20 25   4. Method according to claims 2 and 3, characterized in that it further comprises at least the step of: - calculation of the molar flow rate of nitrogen in the recirculation loop (12) with respect to the following formula: QN2 = QTOTAL QVAP ù QH2 where Q TOTAL the total molar flow rate of gas in the recirculation loop TOTAL 10 (12), where Q is the molar flow rate of water vapor in the recirculation loop VAP (12), and where Q is the molar flow rate of hydrogen in the H2 loop recirculation (12).   5. Method according to one of claims 2, 3 and 4, characterized in that it further comprises at least the step of: calculating the mass flow rate of hydrogen with respect to the following formula: mass QH2 - QH2 xM (H2) Where M (H2) is the molar mass of hydrogen and / or - calculation of the mass flow rate of water vapor with respect to the following formula: mass QVAP VAP xM (H20) Where M (H20) is the molar mass of the water and / or - calculation of the mass flow rate of nitrogen with respect to the following formula. Where M (N 2) is the molar mass of nitrogen   6. Apparatus for carrying out the process according to any one of claims 1 to 5, said device comprising the hydrogen recirculation (12) of the gases from the anode (4) fuel (1) to the Hydrogen (14) of this anode minus a loop which circulates an inlet branch cell (4) in which recirculation loop (12) circulates residual hydrogen, nitrogen and water vapor, characterized in that it comprises an apparatus for measuring the hydrogen concentration (20) at the hydrogen inlet branch (14) and a device for measuring the concentration of hydrogen (21) at the recirculation loop (12).   7. Device according to claim 6, for carrying out the method according to any one of claims 3 to 5, characterized in that it comprises an apparatus for measuring the saturation temperature or the relative humidity of the gas ( 22) circulating in the recirculation loop (12) and a gas pressure measuring apparatus (23) circulating in this recirculation loop (12).