WO2007102031A1

WO2007102031A1 - Fuel cell voltage monitor

Info

Publication number: WO2007102031A1
Application number: PCT/GB2007/050117
Authority: WO
Inventors: Otto Franklin Carlisle; Gerard Peter Theodore Sauer
Original assignee: Afc Energy Plc
Priority date: 2006-03-09
Filing date: 2007-03-09
Publication date: 2007-09-13
Also published as: GB2449040A; GB2449040B; GB0604802D0; GB0815943D0

Abstract

A voltage monitor for monitoring the voltages of fuel cells (10) in a stack, is structured as a hierarchy of monitoring units. It comprises several slave monitoring units (60), each comprising a voltage- measuring circuit (60) and a plurality of voltage- measuring connections (5), each connection (5) being connected to one of the fuel cells, and the circuit (60) being arranged to generate a signal indicative of the voltage. A master monitoring unit (40) is arranged to receive the signals from all the slave monitoring units (50) and to monitor them for deviation from an acceptable range.

Description

Fuel Cell Voltage Monitor

The invention relates to fuel cells, and to a method and apparatus for monitoring cell voltages.

Fuel cells are a type of electrochemical energy conversion device. Another type of electrochemical energy conversion device is the battery. In contrast to a battery, fuel cells do not store all of the chemicals that are to be converted; rather, at least one of the chemicals is supplied externally, usually from a fuel tank. Consequently, fuel cells do not run out of (internal) chemicals and so do not become "dead", as batteries do.

Fuel cells are known in a number of different varieties. Fuel cells based on hydrogen and oxygen are particularly common today; fuel cell types include alkaline fuel cells (AFCs), phosphoric-acid fuel cells (PAFCs), solid oxide fuel cells (SOFCs) and molten carbonate fuel cells (MCFCs). Fuel cells based on hydrogen and oxygen are generally believed to be the most promising candidates for use in powering vehicles, for example cars and buses. The present inventors have demonstrated that AFCs may offer a viable design for use in such vehicles.

Fuel cells are designed to produce voltages within a pre-selected range. When the cell is in use it is very important to keep the cell's output voltage within that range. If the voltage falls, for example, it can be an indication that the cell has become contaminated or that its integrity is breaking down (allowing premature mixing of fuels) . In prior-art voltage monitoring systems, cells are either monitored individually by dedicated monitoring units or else the voltage of a group or stack of cells is monitored, with a many-to-one relationship between the cells and the stack monitor. However, when a large number of cells are used, monitoring individual cells requires a correspondingly large number of monitoring units, which is expensive; similarly, a single monitor suitable for monitoring a large number of cells will be an expensive monitor.

It is desirable to be able to provide an apparatus and method for monitoring the individual voltages of a plurality of fuel cells whilst mitigating, or eliminating, at least some of the above-mentioned disadvantages .

A first aspect of the invention provides a voltage monitor for monitoring the voltages of fuel cells in a stack, the monitor comprising a hierarchy of monitoring units comprising: a plurality of slave monitoring units, each slave unit comprising a voltage-measuring circuit and a plurality of voltage-measuring connections, each connection being arranged for electrical connection to one of the fuel cells, and the circuit being arranged to generate a signal indicative of the voltage of said fuel cell; and a master monitoring unit arranged to receive the signals from the plurality of slave monitoring units and to monitor said signals for deviation of the voltage of any of the fuel cells from a selected range of voltages .

The monitor may comprise a time-division multiplexer arranged to multiplex signals from the cells and to output the multiplexed signals to the voltage measuring circuit. The monitor may comprise a storage circuit for recording the signals generated by the voltage- measurement circuits. The storage circuit and the voltage-measurement circuit may be comprised within one integrated-circuit chip; for example, they may be provided together in the form of a programmable integrated circuit (PIC) .

The monitor may comprise a slave controller for reading the recorded signals and outputting them to the master monitoring unit. The slave controller may be comprised within the slave monitoring unit and output the signals through the interface. Alternatively, the slave controller may be separate from the slave monitoring unit. In that case, the slave controller may be connected to the slave monitoring unit by an interface bus, for example a controller area network bus.

The signals generated by the voltage-measuring circuits may be digital signals. Alternatively, the signals generated by the voltage-measuring circuits may be analogue signals. The analogue signals may be digitised within the slave monitoring units after the signals are generated.

The slave monitoring units may be galvanically isolated from each other. The slave monitoring units may be galvanically isolated from the master monitoring unit.

A second aspect of the invention provides a voltage monitor for monitoring the voltages of fuel cells in a stack of fuel cells, the monitor comprising a hierarchy of monitoring units comprising:

(a) a plurality of slave monitoring units, each slave unit comprising a time-division multiplexer for reading sequentially the voltages of a plurality of fuel cells within the stack and for generating signals indicative of said voltages and a store arranged to receive and store the signals as they are generated by the time-division multiplexer;

(b) a store reader arranged (i) to read from the store the stored signals, when the store contains a complete set of stored signals for all of the plurality of fuel cells read by the multiplexer and (ii) to transmit, on request, the signals read from the store; and

(c) a master monitoring unit arranged (i) to request sequentially the store readers to transmit the signals read from the store, (ii) to receive the signals from the store and (iii) to monitor said signals for deviation of the voltage of any of the fuel cells from a selected range of voltages.

The store reader may be integrated with the slave monitoring unit. Alternatively, the store reader may be separate from the slave monitoring unit.

A third aspect of the invention provides a method of monitoring the voltages of fuel cells in a stack, the method comprising: using a hierarchy of monitoring units, the hierarchy comprising a plurality of slave monitoring units and a master monitoring unit; connecting a first of the slave monitoring units to a first plurality of the fuel cells; measuring, using the first slave monitoring unit, the voltage of each of the first plurality of fuel cells and generating signals indicative of said voltages; connecting a second of the slave monitoring unit to a second plurality of the fuel cells; measuring, using the second slave monitoring unit, the voltage of each of the second plurality of fuel cells and generating signals indicative of said voltages; transmitting the signals generated by the first and second slave monitoring units to a master monitoring unit; monitoring the signals transmitted to the master monitoring unit for deviation of the voltage of any of the fuel cells from a selected range of voltages.

The signals generated in respect of each fuel cell may be combined by time-division multiplexing within the relevant slave monitoring unit; and the multiplexed signals may be stored in a storage circuit within said slave monitoring unit. It may be that the stored signals are read from the storage circuit only when a complete set of signals for the relevant plurality of fuel cells has been stored. The signals may be digitised within the slave monitoring units after the voltages are measured.

Each slave monitoring unit may monitor 45 cells or fewer; or may monitor fifteen cells or fewer. The monitoring unit may be 24 V dc supply. The monitoring unit may output a digital representation of the voltages of the monitored fuel cells. The output may be continuous .

The monitoring unit may comprise a transient suppressor to reduce the possibility of transient overvoltages in the monitored cells.

The unit may be suitable for use with cells having a voltage operating range of -0.5 V to 1 V. The unit may be arranged to issue an alarm when the measured voltage of a cell crosses a threshold, which may be for example set at a low voltage and or a negative voltage.

The monitoring unit may be cooled with ambient air. The unit may operate at an operating temperature range of -40° and to +5O⁰C. It will be appreciated that aspects of the present invention described in relation to the method of the present invention are equally applicable to the apparatus of the present invention and vice versa.

Certain illustrative embodiments of the invention will now be described in detail, by way of example only, and with reference to the accompanying drawings in which:

Fig. 1 is a block diagram showing an example of a voltage monitor according to the invention;

Fig. 2 is a detail of a part of Fig. 1 ;

Fig. 3 is a block diagram showing a second example of a voltage monitor according to the invention; and

Fig. 4 shows a flow diagram of a fuel cell stack assembly.

The example voltage monitor 20 of Fig. 1 is used to measure the voltages of individual fuel cells within three fuel-cell stacks 10a, b,c. Each stack 10a, b,c comprises fifteen individual fuel cells. As discussed above, it is very important that fuel cells remain in their designed operating range. There are many reasons why they might not and therefore it is important that the voltages are monitored to ensure the safety of the cells and surrounding bodies. Constant monitoring of the individual cells allows the monitor to identify a faulty cell and report the problem or shut down the system.

Monitor 20 comprises slave monitors 20a, b,c. Each slave monitor 20a, b,c comprises multiplexer 50a, b,c and a store for storing stack voltage data, the stores being in the form of programmable integrated circuits (PICs) 60a, b, c .

Each cell in stack 10a is connected, via electrical connectors 5a,b,c , to multiplexer 50a. Multiplexer 50a is connected to slave PIC 60a. In the same way, stacks 10b, c are connected to multiplexers 50b, c, which are in turn connected to storage circuits in the form of slave PICs 60b, c.

Slave PICs 60a, b,c are each connected to a slave controller in the form of master PIC 70, which acts as a store reader, reading data stored in PICS 60a, b,c. Master PIC 70 is connected via Controller Area Network (CAN) interface bus 30 to master controller 40. Master PIC 70 gives an output (without any request from the master controller 40) in the format of the CAN bus 30. Those data are representative of the monitored analogue voltages of the individual fuel cells 10a, b,c. Master controller 40 stores those data on stacks 10a, b,c into an array for display and control of the fuel cells.

The monitor 20 uses an input supply of 24V (+-20%) and the monitor 20 is designed to measure cell voltages within the range of -0.5 V to 1 V.

To illustrate operation of the monitor 20, Fig. 2 shows approximately one third of the whole device 20. The process begins with multiplexer 50a sequentially reading at connectors 80 the analogue voltages of the individual cells 10a using a time-division multiplexing method. Those voltages are output (at output 90) as analogue signals. The voltage from each cell is output when its address is selected through address lines 100 by slave PIC 60a. In slave PIC 60a, the analogue voltages are measured, converted to a digital format and stored. Slave PIC 60a cycles through each cell in the stack 10a. The reading, conversion and storage happens repeatedly until the master PIC 70a selects the slave PIC 60a by setting the chip select line 120 high. The master PIC 70a then clocks in the data representing the cell voltages using a generated clock source 110 for synchronisation. Once the master PIC 70a holds the data for all 15 channels it sends that data out to the master controller 40 via the CAN bus 30. In the complete circuit of Fig. 1, the master controller 40 then selects the next slave PIC 60b, and the process repeats with cells 10b. Each of the three sections 50a, 60a, 50b, 60b and 50c, 60c are galvanically isolated from each other and also from master PIC 70. Galvanic isolation helps to reduce the risk of short circuits across cells being formed between those three sections or the sections and the master PIC 70. Galvanic isolation also helps to reduce the number of components required and hence production costs.

Fig. 3 shows another example embodiment. In this example, slave monitors 320a, b,c are separate units, which makes galvanic isolation easier to achieve. Each unit comprises multiplexer 350a, b,c and slave PIC 360a, b, c. As with multiplexers 50a, b,c in the first embodiment, multiplexers 350a, b,c are connected to fuel cell stacks 10a, b,c. In Fig.3, however, the separate slave monitors 320a, b,c are connected to the slave controller 370 by controller area network CAN bus 330. Slave controller 370 is connected to master controller 40 by CAN bus 30, as before. In this embodiment, however, slave controller 370 comprises a further multiplexer 350' and slave controller in the form of PIC 360'. In use, digital signals representing fuel-cell voltages are stored in PICs 360a, b,c as before. When a complete set of data for stacks 10a, b,c has been stored, PICs 360a, b, c, without polling, pass the data over CAN 330 to slave controller 370, where it is received by multiplexer 350'. Multiplexer 350' multiplexes the data received from all the slave monitors 320a, b,c. The signals from each slave monitor are output from multiplexer 350' when the slave monitor's address is selected through address lines by slave PIC 360'. When all the voltage data have been received, slave PIC 360' outputs, without any polling or other request from controller 40, the data over CAN interface bus 30 to master controller 40. Thus multiplexer 350' and slave PIC 360' process signals from slave monitors 320a, b,c in a similar way to the way in which slave monitors 320a, b,c process signals from their respective fuel cell stack 10a, b, c .

For convenience of illustration, the above embodiments show only three fuel cell stacks 10a, b,c. In practice, the invention may be used to monitor the voltages of a larger or smaller number of stacks. The embodiment of Fig. 3 is particularly well adapted to monitor the voltages of a large number of stacks, as use of the CAN bus 330 allows connection of a large number of stacks to slave controller 370 without the need for direct wiring. The number of stacks monitored by the monitor is limited only by the capacity of multiplexer 350' (and further multiplexers in further slave PICs may be connected to master controller 40 via bus 30).

The above embodiments utilise PICs to act as the slave monitoring unit and the slave controller. In other embodiments, other suitable kinds of microprocessor may be used. The use of the CAN bus 30 or the CAN bus 330 has the benefit of providing a flexible architecture for the voltage monitor, as it enables the monitor to be readily expanded or modified, for example to change the number of cell stacks 10, or to change the cell configurations within a stack.

Referring now to figure 4, a fuel cell stack assembly is shown. A fuel cell stack 400 is supplied with hydrogen gas from cylinder 402, which is regulated by 2-stage regulator 404 and controlled by control valve 406. Each cell in the stack 400 consists of electrodes (an anode and a cathode) on opposite sides of an electrolyte chamber, the electrodes separating the electrolyte chamber from respective gas chambers; in the preferred embodiment each electrode consists of a permeable hydrophilic polymer substrate, with a conductive coating and a catalyst on the side facing the gas chamber. Hydrogen is supplied to the chambers next to the anodes, and air is supplied to the chambers next to the cathodes. The hydrogen supplied to the hydrogen chambers of the cells in the stack 400 is maintained at a low positive pressure of approximately +2.0 kPa (above atmospheric pressure) by means of the regulator 404 and the control valve 406. The hydrogen does not normally flow out of the hydrogen chambers (as it undergoes reaction there) . There may be a buildup of contaminants within the hydrogen chambers, in which case a purge valve 408 is opened to allow a brief flow of hydrogen through the chambers, so that the hydrogen and contaminants are vented through a purge exhaust 410. The air chambers of the cell stack 400 are supplied with air by air blower 412 at a pressure of +1.8 kPa. Air blown by blower 412 is cleaned by passing through scrubber 414 and filter 416 before it reaches cell stack 400. Air and entrained evaporated water are exhausted through air exhaust 420. A solution 425 of potassium hydroxide (KOH) in water, which is the cell electrolyte, is circulated by a pump 430 between the cell stack 400 and a tank 432 via heat exchanger 434, which removes excess heat. In the preferred mode of operation the concentration of the electrolyte is constantly approximately 6 M. A depression pump 440 maintains a negative pressure in the electrolyte circuit and is exhausted through depression exhaust 442. The pump 440 removes water vapour that has evaporated in the tank 432, and any gases. Hence in operation of the assembly, the depression pump 440 maintains the electrolyte at a pressure below atmospheric pressure, such that the pressure in the electrolyte chambers of the cell stack 400 is at -10 kPa (below atmospheric pressure) taking into account the effects of the pump 430, and the heat exchanger 434. The cell stack 400 generates electricity, supplied to an external circuit (not shown) through terminals 450.

The pressure differential of about 12.0 kPa between the gas chambers and the electrolyte chamber is selected so the interface between the potassium hydroxide electrolyte and the hydrogen and air gases occurs at the catalyst layer, the interface being regulated by the pressure differential. At each anode (the negative electrode) a chemical reaction occurs between the hydrogen gas and the hydroxide ions of the electrolyte, the products of which are water and electrons. At each cathode a chemical reaction occurs between oxygen gas in the air chamber, water and electrons, the product of which is hydroxide ions. The electrons travel from the anode to the cathode via the terminals 450 and an electric circuit (not shown) , so there is an electric current. The negative electrolyte pressure in the electrolyte chamber and the hydrophilic substrate create conditions in which water produced at the anode is drawn into the electrolyte chamber, and the excess water produced in the reactions evaporates at the cathode and is removed from the fuel cell stack 400 in the air exhaust 420.

It will be appreciated that the manner in which the pressure differential is achieved may differ from that shown in figure 4. For example in a modification the recirculation pump 430 is, instead, installed at the outlet from the cell stack 400, and a restriction valve (not shown) is arranged at the electrolyte inlet to the cell stack 400; in this arrangement the recirculation pump 430 and the restriction valve produce the negative pressure within the electrolyte in the cell stack 400, while the electrolyte in the tank 432 may be at atmospheric pressure.

The current flowing in the circuit and the voltage between the terminals 450 are monitored by sensors 452 and 454 connected to a microprocessor 455 which provides control signals to the depression pump 440 (these electrical connections being represented by broken lines). Hence the pressure of the electrolyte 425 is adjusted to ensure the optimum position of the interface between the electrolyte 425 and the gases, ensuring that the interface is within the catalyst layer of each electrode .

The cell stack 400 is also provided with a cell voltage monitor as described in relation to figures 1 to 3 (not shown in figure 4), whereby the voltage of each individual cell within the (or each) stack 400 is monitored; the monitoring system is also provided with data from the current sensor 452. If some cells within a stack 400 have voltages which are less than the acceptable range (taking into account the current flowing, and the cell configuration) , then a sequence of recovery operations is carried out. Firstly the hydrogen chambers are purged by opening the purge valve 408 to eliminate any gaseous contaminants; then the depression pump 440 is adjusted so as to adjust or oscillate the pressure differential; and thirdly the air blower 412 is adjusted to increase the airflow and so to increase the evaporation of water. If these steps, individually or collectively, do not return the cells to acceptable voltages, then the next step is to disconnect the external circuit from the terminals 450, so that no electrical current flows. After allowing the cells to stand in this open circuit state for a short period (eg 10 min) then the hydrogen purge, pressure differential adjustment, and airflow adjustment are repeated.

Whilst the present invention has been described and illustrated with reference to particular embodiments, it will be appreciated by those of ordinary skill in the art that the invention lends itself to many different variations not specifically illustrated herein. For that reason, reference should be made to the claims for determining the true scope of the present invention.

Claims

Cl aims

1. A voltage monitor for monitoring the voltages of fuel cells in a stack, the monitor comprising a hierarchy of monitoring units comprising:

a plurality of slave monitoring units, each slave unit comprising:

a voltage-measuring circuit and a plurality of voltage-measuring connections, each connection being arranged for electrical connection to one of the fuel cells, and the circuit being arranged to generate a signal indicative of the voltage of said fuel cell; and

a master monitoring unit arranged to receive the signals from the plurality of slave monitoring units and to monitor said signals for deviation of the voltage of any of the fuel cells from a selected range of voltages.

2. A monitor as claimed in claim 1, comprising a time- division multiplexer arranged to multiplex signals from the cells and to output the multiplexed signals to the voltage-measuring circuit.

3. A monitor as claimed in claim 1 or claim 2, comprising a storage circuit for recording the signals generated by the voltage-measurement circuits.

4. A monitor as claimed in claim 3, in which the storage circuit and the voltage-measurement circuit are comprised within one integrated-circuit chip.

5. A monitor as claimed in claim 3 or claim 4, comprising a slave controller for reading the recorded signals and outputting them to the master monitoring unit .

6. A monitor as claimed in claim 5, in which the slave controller is comprised within the slave monitoring unit.

7. A monitor as claimed in claim 5, in which the slave controller is separate from the slave monitoring unit.

8. A monitor as claimed in claim 7, in which the slave controller is connected to the slave monitoring unit by an interface bus .

9. A monitor as claimed in any preceding claim, in which the signals generated by the voltage-measuring circuits are digital signals.

10. A monitor as claimed in any of claims 1 to 8, in which the signals generated by the voltage-measuring circuits are analogue signals.

11. A monitor as claimed in claim 10, in which the analogue signals are digitised within the slave monitoring units after the signals are generated.

12. A monitor as claimed in any preceding claim in which the slave monitoring units are galvanically isolated from each other.

13. A monitor as claimed in claim 12 in which the slave monitoring units are galvanically isolated from the master monitoring unit.

14. A voltage monitor for monitoring the voltages of fuel cells in a stack of fuel cells, the monitor comprising a hierarchy of monitoring units comprising: a plurality of slave monitoring units, each slave unit comprising a time-division multiplexer for reading sequentially the voltages of a plurality of fuel cells within the stack and for generating signals indicative of said voltages and a store arranged to receive and store the signals as they are generated by the time-division multiplexer;

a store reader arranged (i) to read from the store the stored signals, when the store contains a complete set of stored signals for all of the plurality of fuel cells read by the multiplexer and (ii) to transmit, on request, the signals read from the store; and

a master monitoring unit arranged (i) to request sequentially the store readers to transmit the signals read from the store, (ii) to receive the signals from the store and (iii) to monitor said signals for deviation of the voltage of any of the fuel cells from a selected range of voltages.

15. A monitor as claimed in claim 14, in which the store reader is integrated with the slave monitoring unit.

16. A monitor as claimed in claim 14, in which the store reader is separate from the slave monitoring unit.

17. A method of monitoring the voltages of fuel cells in a stack, the method comprising:

using a hierarchy of monitoring units, the hierarchy comprising a plurality of slave monitoring units and a master monitoring unit;

connecting a first of the slave monitoring units to a first plurality of the fuel cells; measuring, using the first slave monitoring unit, the voltage of each of the first plurality of fuel cells and generating signals indicative of said voltages;

connecting a second of the slave monitoring units to a second plurality of the fuel cells;

measuring, using the second slave monitoring unit, the voltage of each of the second plurality of fuel cells and generating signals indicative of said voltages;

transmitting the signals generated by the first and second slave monitoring units to a master monitoring unit ;

monitoring the signals transmitted to the master monitoring unit for deviation of the voltage of any of the fuel cells from a selected range of voltages.

18. A method as claimed in claim 17, in which the signals generated in respect of each fuel cell are combined by time-division multiplexing within the relevant slave monitoring unit.

19. A method as claimed in claim 18, in which the multiplexed signals are stored in a storage circuit within said slave monitoring unit.

20. A method as claimed in claim 19, in which the stored signals are read from the storage circuit only when a complete set of signals for the relevant plurality of fuel cells has been stored.

21. A method as claimed in any of claims 17 to 20, in which the signals are digitised within the slave monitoring units after the voltages are measured.

22. A voltage monitor for monitoring the voltages of fuel cells in a stack, the monitor comprising a hierarchy of monitoring units substantially as hereinbefore described with reference to, and as shown in, the accompanying drawings .