IMTC 2005 – Instrumentation and Measurement Technology Conference Ottawa, Canada, 17-19 May 2005
A Microcontroller-Based System for the Monitoring of a Fuel Cell Stack Giovanni Bucci, Member, IEEE, Edoardo Fiorucci, Member, IEEE, Fabrizio Ciancetta and Francesco Vegliò (*) Dipartimento di Ingegneria Elettrica (DIEL) tel. + 9.0862.4 46 4, fax + 9.0862.4 440 , {bucci, fiorucci,ciancetta}@ing.univaq.it (*) Dipartimento di Chimica, Ingegneria Chimica e Materiali tel. + 9.0862.4 422 , fax + 9.0862.4 420 ,
[email protected] Università di L'Aquila, Poggio di Roio, 67040 L'Aquila ITALY
Abstract – This paper describes the development of a monitoring and managing system for a PEM fuel-cell (FC) stack. This is a stand-alone unit that embodies two microcontrollers, hardware and sensing circuits, a LCD and a numeric pad. The proposed system acquires the most important gas parameters: mass flow rate, pressure and temperature. The output current and voltage are also acquired. The system design complies with the requirements of a portable FC based application. Additionally, results obtained during the experimental test are presented. Keywords – Fuel cells, monitoring, microcontroller
I.
INTRODUCTION
A fuel cell is an electrochemical device that converts a fuel’s chemical energy directly to electrical energy, with no internal moving parts, with a power ranging from some watt up to 250 kW, depending on the application. The FCs can replace the internal combustion engines in transportation systems, reducing pollution and increasing efficiency. In stationary applications, FCs can supply single and two-family homes with electricity and heat and generate back-up power for hospitals or telecommunication systems. Recently they have been applied in micro-systems, such as to supply power to mobile telephones, CD players and laptop computers. These devices are an attractive alternative to uninterruptible power supplies (UPS), power-line filters, or energy storage systems used to condition the grid electricity. In recent years different types of technologies have been developed [1,2]. One of the most diffused, the PEM fuel cell, has a high proton conductivity membrane as electrolyte that allows protons to be transmitted from one face to the other when supplied by hydrogen and oxygen. Each single cell produces about 0.6 V and can be combined in a fuel cell stack to obtain the required electrical voltage and power. The operating temperature is in the range of 70°C to 100 °C. The safety is an important aspect to be considered in the design of a FC application. Hydrogen is highly flammable and explosive and in high concentrations may cause asphyxiation and death, so there are hazards associated with its use, even if it has been utilized for decades in a wide variety of industrial applications. The autoignition temperature in air is relatively high (560 °C), the volume concentrations for flammability in air is 4.1 % to 74 %. Its risk level as a fuel at atmospheric pressure is similar to that of
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fuels such as natural gas and propane; however, it is a potentially dangerous substance because its low volumetric energy density requires high pressure and liquid storage. In designing a FC-based system some safety measures must be adopted to minimize the safety hazards during the working: pressure of both hydrogen and oxygen must be monitored and gas leakage detected. Moreover, for the management of the power generation other FC parameters should be measured and controlled. To increase the system performance and reduce the cost in low- and medium-power applications, a suitable monitoring and managing system must be incorporated into the FC system. To this aim in this paper we propose a system that monitors and manages the main FC electrical and chemical quantities, implemented using compact transducers and a microcontroller-based data acquisition and processing unit. In the paper the main features of the implemented system are described and results from experimental tests are presented. II. THE PEM FUEL CELLS UNDER TEST PEM fuel cell requires hydrogen and oxygen as inputs, though the oxidant may also be ambient air; input gases must be humidified. It operates at a temperature lower than other fuel cells (limitations imposed by the thermal properties of the membrane), so it requires cooling and management of the exhaust water to function properly. The PEM can be contaminated by CO, reducing the performance and damaging catalytic materials within the cell. A number of fuel cells can be linked in serial/parallel mode to implement a stack with the required power. The main features of the stack under consideration (Fig.1) are: i) 10 cells for stack, ii) electrode area of 64 cm2, iii) nominal power of 150 W, iv) nominal voltage of 6 V, v) nominal current of 25 A, vi) reactants H2/air, reformate/air, H2/O2; vii) max operating temperature of 70 °C; viii) operating air pressure of (0 to 4.4) kPa; ix) operating H2 pressure of (0 to 4.4) kPa; x) self-humidified stack. III. THE PROPOSED MONITORING SYSTEM The monitoring and control of a FC stack requires a suitable data acquisition system, to precisely handle its
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operations [ ]. The most important parameters to control are related to the gases: mass flow rate, pressure, temperature and stoichiometry. The stack temperature is another important parameter, because the FC performance is directly related to the temperature at which the chemical reaction takes place. The monitoring and control of the electrical load parameters (current and voltage) are also required.
processors with 8 input channels at 10 bits, on-chip Flash program memory (16K instructions) and RAM (1.5Kbytes).
Fig. 1 The PEM FC stack
Fig. The implemented µC-based system
The Fig.2 shows the implemented monitoring and managing system. We adopted as digital mass flow meterscontrollers the Bronkhorst DMFC Mod. F-201C-RAB-22V. These devices measure and control the gas flow, ranging from ml/min up to 1250 l/min; the accuracy is ± 0.5 % of reading plus ± 0.1 % of FS and the settling time is 1 s. Two electronic back-pressure controllers (DBPCs) regulate the pressure of filtered air and H2. A distilled water pump is used for the stack cooling; it requires about a 4 l/min water flow rate, for a supplied current of 25 A at 45 °C. All the described flow meters are connected to a MCU (Micro-Controller Unit) system using a RS-485 link. Moreover, the MCU system (Fig. ) keeps data from analog signals coming from the temperature sensors and LEMs, and generates power signals for the actuators (cooling water pump and cooling fan), using condition circuits. The MCU system embodies two PIC18F452 microcontrollers (MCU_A and MCU_B), 10 MIPS
The system also includes a CY7C462A FIFO memory, to buffer the 8Ksamples acquired by the system, a 24FC256 EEPROM, two RS-2 2 and RS-485 ports, digital and analog buffers. The MCU_A, the master, has been designed to manage LCD, numeric pad and RS-485 data communication. Moreover it acquires sample data from the Analog to Digital converter, manages the two I/O digital lines, accesses to EEPROM memory but can’t access directly to FIFO memory. The MCU_B, the slave, controls the condition and power circuits and acquires input signals. It accesses to EEPROM and to FIFO memories, manages the RS-2 2 interface and communicates with the other MCU. The system uses the I2C bus to transfer data between the two MCUs and to the EEPROM. This bus transfers data to a particular device, identified using a specific ID number (address) on the beginning of the data packet. The MCU system can works in stand alone mode, without connection to the PC, in order to comply with the requirements of a portable FC based system. To interface with the user, a two-line, 16 character LCD and a numerical pad are used. Through the user interface, the operator can act a precise selection of the main working parameters. On the front panel a 50 V differential input and the related attenuator knob are also included. The MCU measures the temperature of both the stack and reaction products with Jtype thermocouples. The current and voltage transducers are compact size devices for PCB mounting, based on the Hall effect: the LEM LAH 50 P (with a Fig.2 The implemented monitoring and managing system for the PEM stack nominal current of 50 A) and LEM LV200-
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AW (with a nominal voltage of 20 V). A gas sensor Zelweger mod. EEX monitors the hydrogen leakage. The control and measurement software, developed using the assembler language, performs the following tasks: i) communication with the gas controllers, for both measurement and control of flow rate and pressure of the reactants; ii) acquisition of voltage, current and temperature, iii) acquired data processing and visualization.
120
Power [W]
100 80 60 40 20 0 0
IV. EXPERIMENTAL RESULTS
300
600
900
1200
1500
time [s]
As part of the development phase, the implemented system was tested in the laboratory with the described PEM fuel cells. Different load conditions have been produced, in order to investigate about the aptitude of the proposed system to manage the FCs during both static and dynamic working conditions. Linear and step current increase and decrease tests have been carried out at different temperatures, in order to investigate about the better FC working condition for a specific electrical load. Time delay in the MCU’s command actuation has been also measured.
Fig.5 The FC power during static test
• the adopted PEM fuel cells have two different voltamperometric characteristics, depending on the increasing or decreasing of current (Fig.6); • the H2 consumption waveform has a shape similar to the current one, with a delay due to the chemical kinetics; • the cells are very sensitive to the heat, and the measurement of temperatures of both the stack and the reaction products can be used for the evaluation of the efficiency; • the measurement of the delay between the current variation and the corresponding voltage response can be adopted for the implementation of dynamic control systems for automotive applications.
A. Static measurements The static characterization is used to evaluate the PEM cell performance: the relationship between voltage and current shows the PEM efficiency at different load conditions. During this test the flow rate has been kept fixed, while the supplied power ranged over the entire scale during a specific time interval. The tests have been conducted to simulate steady state conditions, with a slow variation of supplied current. When the test starts, the output current increase from 0 A to 25 A (run up), then decrease to 0 A (run down). During this test the FC voltage decreases proportionally to the current and reaches the minimum corresponding to the maximum current (Fig.4). The FC voltage power increases proportionally to the current and the maximum is obtained at the maximum current (Fig.5).
Voltage [V]
10 8 6 4
Fig.6 The FC hysteresis effect
2 0 0
300
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900
1200
1500
time [s]
Fig.4 The FC voltage during static test
The analysis of the first results, obtained with the proposed system, suggests pointing out the following features:
The hysteresis is a secondary effect of the FC temperature variation. Specifically, the temperature increases and decreases proportionally to the supplied power, but the decrease occurs with a lower speed, because of the thermal inertia. During the run up and run down the FC shows a different behaviour. To reduce this phenomenon, we refrigerated the FC in order to fix the temperature, making stable the FC working conditions. The Fig.7 shows the two phases of the test. In this case the hysteresis effect is quite negligible, because of the temperature stabilization.
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Then, we tested the system with decreasing current steps and the Figs.11 show the voltage and power measured with a FC temperature of 55°C.
10
runup rundown
8 6 4 2 0 0
5
10
15
20
25
Cur r ent [ A ]
Fig.7 The hysteresis effect at 47°C
B. Dynamic measurements The static conditions represent only one aspect of the problem related to the monitoring and management of a PEM FC system, because the electrical load can have high dynamic variations. In order to investigate about the MCU performance, we carried out sequential increasing current steps: (0 to 5) A, (5 to 10) A, (10 to 15) A, (15 to 20) A and (20 to 25) A, at 40 °C, 55 °C and 65 °C, as shown in Figs.810.
Fig. 10 The FC power at 65 °C
Fig. 8 The FC power at 40 °C
Fig. 11 The FC voltage and power at 55°C
Fig. 9 The FC power at 55 °C
The fuel cell system has a better behaviour for greater value of initial current. In particular during the transition from 5 A to 0 A, the voltage increases of 1.5 V, because of the fuel in excess in the pipe. During the tests starting at greater values of initial current (25 A, 20 A and 15 A), the FC generates lower values of overvoltage. The decrease shape is more linear then the increase one, so the cell work well for this kind of variation. The obtained results are tabulated in Tab.1, for the FC working without the temperature control and with a temperature fixed at 40 °C, 55 °C and 65 °C.
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Tab.1 Results obtained during the dynamic tests
Current
Test without temperature control
Test at 40°C
Test at 55°C
Test at 65°C
ampere
V volt
P watt
∆ watt
V volt
P watt
∆ watt
V volt
P watt
∆ watt
V volt
P watt
∆ watt
0 -5
7
33
33
7
33
33
7.2
35
35
7.4
36
36
5-10
5.6
60
27
5.5
55
22
5.6
57
22
5.8
58
22
10-15
5
80
20
4.8
72
17
5
74
17
5.2
75
17
15-20
4.6
90
8
4
80
8
4.2
82
8
4.4
84
9
20-25
3.3
90
0
2.8
78
-2
3
90
8
3.2
93
9
The time required to close a valve is of around 2.0 s, and a similar time is required to close valves, as shown in Fig.1 .
B. Valve response We measured the delay time spent to transfer a command from the MCU to a DMFC device and to execute it. The result is that this delay depends on the topology and length of RS-485 bus, and number of linked devices. We measured the delay time during both open and close operations, in order to understand the system response at every variation of reagents. The time required only for measuring a mass flow is of 71 ms. The system has required a different time to open or close the valve or, better, to increase to the maximum the flow during the opening and to decrease to zero the flow during the closing. Specifically, the difference between the two times depends on the activation phenomena of the chemical reagents in the fuel cell when the FC starts to operate. The time required by the device to open a valve is of around . s, to have maximum capacity; the overall time required to open valves is of around 5 s, as shown in Fig.12.
Fig. 1 The transients during the closing of
valves
V. CONCLUSIONS The PEM fuel cells can be successfully adopted for distributed generation of electric power, UPS and electric transportation applications. A high number of parameters have to be monitored for the management of a fuel cell based system. In the paper we propose the use of a compact lowcost microprocessor based system for the monitoring and management of PEM fuel cells and describe the main blocks. In the paper some results obtained during both the static and dynamic working of the FCs are reported and discussed. REFERENCES [1] Fig. 12 The transients during the opening of valves
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Ellis, M.W; Von Spakovsky, M.R.; Nelson, D.J.; “Fuel cell systems: efficient, flexible energy conversion for the 21st century”, Proceedings of the IEEE, Volume: 89 Issue: 12, Dec. 2001.
[2] [ ]
Laughton, M.A.;” Fuel cells”, Power Engineering Journal, Volume: 16 Issue: 1, Feb. 2002. Yaw Chung Cheng; Win Hsiao Yuen; Lin, A.S.; Jiann Tsarng Tsay “Automation and control for the PAFC fuel cell components and the power plant”, Industrial Automation and Control: Emerging Technologies, 1995. International IEEE/IAS Conference on, 22-27 May 1995.
[4]
[5]
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Correa, J.M.; Farret, F.A.; Canha, L.N.; “An Analysis of the Dynamic Performance of Proton Exchange Membrane Fuel Cells Using an Electrochemical Model”, Proceedings of IEEE IECON’01. Pukrushpan, J.T.; Stefanopoulou, A.G.; Huei Peng; “Modeling and control for PEM fuel cell stack system”, Proceedings of the American Control Conference 2002, Volume: 4, 8-10 May 2002.
