In order to verify the numerical models (both the 1D FC model and the Petri-net degrada- tion model), a PEMFC experimental rig was built. The experimental rig has the capability to accommodate either a single cell or 7 cell stack as per the schematic in Figure 6.1.
Figure 6.1 from left to right shows the gas panel that serves Air, H2 and N2 inlets to the laboratory, and the H2 and Air outlet valves. Every inlet pipe leading to the test rig has a non-return valve to block any gasses trying to return to the gas panel. To facilitate the choice between H2 or N2 going into the mass flow controller on the anode side, two ball valves were installed. The opening of one, and the closing of the other would allow the choice of one of the feed gasses entering the mass flow controller. After the gasses pass through the mass flow controllers, they pass a gas pressure sensor before entering the humidification bottles. Before entering the cell, pressure, temperature and humidity sensors interact with the gas flow. After leaving the cell, the gasses make their way to their associated outlet valves. Individual cell potential voltmeters were used dependent upon the type of test being undertaken.
H2in Air in P-1 F H2 F Air N2in Fuel Cell H2BPV out Air BPV out Relative Humidity Sensor Pressure Transducer Mass Flow Controller Ball Valve Water Pump Coolant Header Tank Deionised water tank w/ water heater (HW) Moisture collector Non-Return Valve Gas Regulator Valve Thermocouple HW Multimeter BIP thermocouples
Individual cell potential
Humidifier F
HW
For example, one is used to monitor single cell performance, however two are used for two cells which allows to monitor individual cell potential. For single cell operation, the hot water tank pumps deionised water throughout the cell to bring it to a desired operating temperature. A PEMFC stack was purchased from Pragma industries (see Figure 6.2), comprising of 7 cells. The cell active area was 100cm2 providing a nominal power output of 470W.
Figure 6.2: Pragma 7 cell, 470W research development stack
The stack contained pre-made MEAs from the manufacturer of the stack. The membrane was Nafion XL with a thickness of 27.5µm, sandwiched with a catalyst layer of Platinum ink with a loading of 0.2 mg Pt/cm2. Carbon paper type GDL were used to finish the MEA (See Figure 6.3).
The stack is water cooled, or heated, using deionised water sent through a serpentine channel in between each cell in the stack, as in Figure 6.4.
The gas flow field plates were of a square serpentine configuration as shown in Figure 6.5. H2 was sourced from a 350 bar gas bottle, and was 99.99999% pure hydrogen as per the international standard [38]. Air was used as the cathode feed gas, sent from an air compressor with filters for oil and water. Both gases were regulated through Hastings mass flow controllers, to accurately control the flow rates of the gas entering the stack at any time.
The gasses were humidified through Nafion tube, bubbler type humidifiers, and were speci- fied to humidify the feed gases at the range of flow rates to be used. Dry gas is passed through the bottle which is filled with grade 1 deionised water. The gas then picks up moisture and is passed through a heated tube section before entering the cell/stack. Temperature was controlled both at the bottle and the heated length of tube. Modifying these two temperatures means that a level relative humidity can be obtained for experimentation.
An electronic load bank was used to ‘waste’ the energy developed by the stack and log/dis- play the energy use in real time. A TDI Power Systems load bank was used, with a range of 0-150A and 0-800W, as shown in Figure 6.6.
The gas leaving the humidifiers was passed through a custom manufactured humidification sensor T-piece, as seen in Figure 6.7. These sensors measure the humidity levels in the gas stream before entering the stack.
Figure 6.3: MEA assembly
Figure 6.4: Coolant channels in between each cell
Figure 6.5: Gas channels in between each cell
Figure 6.7: Humidity sensor T-piece detail
Figure 6.8: Complete test rig
Two set-ups for heating or cooling the stack were built. If the test rig was using a stack, the heat that the stack would produce during operation would require the use of a cooling system. Therefore the water flowing through the stack is passed through an oil cooler type radiator with a funnel cowling exhausting the warm air up into a fume cupboard (see Figure 6.9) with the help of dual 15cm electric fans. A thermocouple is used on the coolant exit from the stack to monitor stack temperature.
If the rig is used for only a single cell, the cell cannot create enough heat to operate at the required temperature, and therefore cell heating is used. A heating element in the header tank
heats the ‘coolant’ water to the desired temperature for the cell, and is pumped through the system, creating heat.
Figure 6.9: Stack cooling system
The finished single cell test rig is presented in Figure 6.8 and shows all contributing com- ponents in the fume cupboard, which is used for leak extraction.
A fuel cell monitoring software package was created to monitor fuel cell parameters in real time, and log these variables for data analysis at a later date (see Figure 6.10). The software was built in LabView, and uses data logging equipment supplied by National Instruments. The data logged is shown in Table 6.1.
Figure 6.10: Front end of the software developed for the fuel cell test rig
The desired flow rates for the gasses into the cell can be chosen by entering the flow rate into the central control block seen below the fuel cell image in Figure 6.10. This sends a signal of between 0 and 5 Volts to the mass flow controllers to meter gas flow to the rig. The exact value of the flow rate is displayed and logged to file four times per second. The gas pressure is measured before it enters the humidification bubblers as to make sure that the upper limits of the humidifiers are not exceeded during operation. The gas pressure, temperature and humidity is displayed and logged before it enters the cell. The cell temperature is set by the heated water tank and pump system. This is done by entering the value the operator desires into the central control box located under the fuel cell image. This sends a signal to a heating element that uses simple logic to turn off when a target temperature is met. To increase the accuracy of this system, a thermocouple is located at the cell inlet to accurately turn off the heater tank when the temperature set point is achieved.
All gauges and indicators were verified by external calibration of each instrument used.