Fuel Cell Characterization Lab

Transcription

1 SJSU E 10 Introduction to Engineering Fuel Cell Characterization Lab What is a Fuel Cell? Before we answer that question, let s first review the process of electrolysis. In the process of electrolysis, electrical energy is used to separate hydrogen and oxygen from water, H 2 O. The theory of conservation of energy states that the electrical energy used for this process is contained, at least partially, in the new state - hydrogen and oxygen. Intuitively we should be able to recombine these two gases, and while doing so, release electrical energy. This intuition is indeed correct. The fuel cell that we will use in this lab is capable of combining oxygen and hydrogen gases (becoming water) and releasing electrical energy during this recombination process. Electrolysis can be thought of as a charging process and the recombination (by a fuel cell) a discharging process. Similarly, the gases can be thought of as a way of storing energy. One way to transport the energy from a remote wind farm, for example, is to use the electrical power generated from the wind turbines to produce hydrogen and oxygen. The gases can then be stored, transported and distributed to the consumer in the same way that gasoline is processed. In this lab, you will experiment with the electrolysis process to produce hydrogen and oxygen and use a fuel cell to recover electrical energy from these gases. Determining the input energy to an electrolysis process The fuel cell that we will use in this lab is also capable of performing the electrolysis process. The same power meter used in the previous labs will be used to measure the electrical power produced from the electrolysis process. The total energy used in this process is: Energy = Power Time However, there is a problem with applying this formula. You will find that the power reading varies with time. Fortunately, since the rate of variation is not high, the reading essentially stays constant over a period of, say, 10 seconds. For this reason, the total energy will need to be determined by the following formula: Total Energy = P 1 10 sec. + P 2 10 sec. + P 3 10 sec. +.+ P n 10 sec. where P 1 is the reading at t = 10 second, P 2 is the reading at t = 20 second, and P n is the reading at t = (n x10) second. In the following procedure, you will need to record power readings every 10 seconds over a time period of 6-7 minutes. Procedure (filling the fuel cell with water): 1) If not already connected, attach the hoses according to Figure 1, and fill the water reservoir with the provided distilled water. To avoid spills, you may use a measuring cup to fill the water reservoir. Do NOT use tap water or bottled water in this lab! Doing so will damage the fuel cell.

2 2) Locate the short lengths of tubing protruding from the fuel cell that are ged with red s. Starting with one side of the fuel cell, pinch closed and hold the short tube, and remove the. Insert the tip of the syringe tubing into the tubing you are pinching. Once the syringe tubing has been inserted, you can release the short length of tubing you were pinching. Slowly draw back the plunger of the syringe. What you should see is the water from the reservoir being drawn up into the corresponding gas tank. You need to continue sucking with syringe until the ALL the air has been removed from the gas tank, and the tubing and fuel cell are filled with water. You may need to pinch off the short tube, remove the syringe, reset its plunger, and continue removing air until the process is complete. Empty any water that you suck into the syringe into one of the waste buckets in the lab. Once all the air has been removed from the gas tank and the tubing, and you can see that the fuel cell membrane is set, pinch off the short tube, remove the syringe, and reinstall the red. 3) Repeat Step 2 for the other side of the fuel cell. 4) Suck out any additional water from the reservoir. You should leave the reservoir about ¼ full. Fuel cell H 2 tank O 2 tank Charging (Electrolysis) Process hose water reservoir Figure 1 Hose connection for the fuel cell set up We will use the power supply unit (Agilent E-3630A) on the work bench for the the electrolysis process. This power supply unit consists of three variable voltage sources. Figure 2 shows the functional diagram of this unit. As shown, these three voltage sources share a common connection (marked COM ). One of the three voltage sources has an adjustable range of 0v to 6v while the other two have the range 0v to 12v (but with different polarity). Figure 3 shows the front view of the unit. The voltage marked on the output terminals (+6v, +12v, -12v) are the maximum output voltages, NOT the actual output voltage. The actual output voltage and current are displayed on the front panel. Since there is only one set of displays (voltage and current), one needs to select a voltage source to display. This is accomplished by pressing one of the three push-button switches (marked METER ). For this lab, we will only use the 0 to 6v supply, so the button marked +6v should be pushed in. The output connections for this lab should be made from the connectors marked +6v and COM. The one marked +6v has the higher voltage (the + side), and the one marked COM has the lower voltage (the - side). The output voltage is set by turning the knob marked +6v under the label voltage adjust. Page 2 of 8

3 +12v 0~12 0~12 0~6-12v +6v COM Figure 2 Functional diagram of the Agilent E-3630A power supply Display select Power switch Voltage and Current of the selected source Voltage adjustment Connect the black wire here Connect the red wire here Figure 3 Front view of Agilent E-3630A power suply unit We will use 2.5V for the electrolysis process. Since the power supply is a voltage source, the voltage level is maintained at 2.5V regardless of the current drawn by the load circuit (the electrolysis setup, in this case), so long as it is within the maximum output current capacity of the unit. This constant voltage greatly simplifies the data recording in the following steps. As mentioned above, the total power can be determined by the following formula. Total Energy = P 1 10 sec. + P 2 10 sec. + P 3 10 sec. +.+ P n 10 sec = (V 1 I 1 10) + (V 2 I 2 10) + + (V n I n 10) = 2.5V 10sec ( I 1 + I I n ) This formula allows us to determine the total energy used for the electrolysis process by taking the current reading (recorded from the panel of the power supply) every 10 seconds for a period of several minutes. Procedure: 5) Change the voltage/current display to +6v by pushing the +6v button. Again, this +6v push-button switch and the label +6v above the output connector do NOT mean that the output voltage is 6v. They refer to the voltage source (one of the three) that can be set to any voltage between 0v and +6v. Page 3 of 8

4 6) Turn the knob labeled +6v under VOLTAGE ADJUST, so that the voltage display shows 2.50v. At this setting, the voltage between the output terminal 6v and COM is 2.5v. Please note the voltage setting can t be set to more than 3v. If this happens, the overload light will be turned on (amber light), so immediately reduce the voltage setting. 7) Connect the COM terminal of the power supply to the negative terminal of the fuel cell (see Figure 4) but DO NOT connect the positive terminal to the fuel cell yet. Fuel cell H 2 tank O 2 tank hose water reservoir Figure 4 Wiring diagram for the electrolysis process 8) Have your stopwatch, paper, and pen ready before you connect the + side (the +6v terminal) to the positive side of the fuel cell. Assign one person to write down the data and one person to keep the time. You may use Table 1 (on page 8) for your data recording. As soon as you make the connection to the positive terminal, start to record the current reading every 10 seconds for several minutes until the gas tanks are full. The voltage may dip momentarily after you make the connection. This is expected. Ignore this voltage drop. Also, in the next step, take care that the water reservoir does not overflow. Use the syringe to remove some water if it gets close to the top. Have some paper towels ready to absorb the additional water. To prevent it from overflowing, you can also use the syringe to draw some water out from the reservoir. 9) Disconnect both contacts (positive and negative) from the power supply immediately as soon as you see the gas tanks are filled with hydrogen and oxygen gases. In an ideal situation, the gas tanks will be completely filled with gases in less than 3 minutes. However, if the efficiency of the fuel cell is poor, it will take a longer time to fill up the gas tanks. In your report to be turned in next week, use Excel to plot a current (I) versus time (t) curve and calculate the total input energy by using the formula given on page 3. The generated gases will be used in the following steps. To minimize gas leakage, proceed to the next step immediately, and be careful not to jostle the tanks. Please do not use the lab time to prepare your report. Turn in a copy of your recorded data at the end of the period. Page 4 of 8

5 Discharging (recombining gases) Process For this part of the lab, we will use the gases generated by electrolysis to power a fuel cell -- a device that combines hydrogen and oxygen, and, in the process of doing that, generates (or, more precisely, recovers) electrical power and pure water. 10) Prop up the front wheels of the car so that they are completely off the tabletop. 11) In the next step, you will connect the fuel cell to the motor through the power meter as shown in Figure 5. Before you make the connection, have a pen and paper ready. Again, assign someone to write down the measurements and someone to keep time. As soon as you complete the connection, the motor will start to turn (and start to use energy). The power provided by the fuel cell is measured by the power meter. As soon as the motor starts to turn, record the power reading every 10 seconds until the motor stops running. This will take several minutes (15-18 minutes). You may use Table 2 on page 9 for your data recording. 12) Now complete the connection shown in Figure 5, and start recording the power every 10 seconds. motor Power meter Fuel cell H 2 tank O 2 tank Prop up the front wheels Figure 5. Power generated by the fuel cell is used to provide energy for the drive motor. In your report (to be turned in next week), use Excel to plot a power versus time (t) curve and calculate the total input energy by using the formula given below. Do not prepare the report now. Output Energy = P 1 10 sec. + P 2 10 sec. + P 3 10 sec. +.+ P n 10 sec Your report should include the efficiency of this energy storage system: Efficiency = Output Energy / Input Energy where input energy is the energy determined in the electrolysis procedure. Page 5 of 8

6 Fuel Cell Car Race The last step of this lab is --- a car race!! Race #1: 13) Charge up the gas tanks again (steps 1~9). You may not have to draw much air/water. Just make sure the tanks and tubing are completely full of water, the fuel cell is wet, and there reservoir is ¼ full. 14) Find an open area in the lab. Line up all the cars. Connect the fuel cell output directly to the motor as shown in Figure 7. The first car to cross the finish line wins!! Race #2: Figure 7 Fuel cell output is directly connected to the drive motor. 15) Charge the gas tanks again. 16) Turn the front wheel at an angle so the car runs in a circle. 17) For this competition, the car that runs longest time wins. Properly carrying out the following step will not earn you any grade points for the lab, but not doing it properly will cost you points. 18) Clean up your work area. Pour the water from the reservoir into the buckets (NOT back into the water bottles!). Use the syringe to suck out any water in the tubing and fuel cell (both sides). Use the paper towels to wipe off the water on the table. Return the fuel cell and the car to your instructor. Page 6 of 8

7 Table 1 Record Current Readings Every 10 Seconds (Charging Gas Tanks) Time (s) Current (ma) Time (s) Current (ma) Time (s) Current (ma) Page 7 of 8

8 Table 2 Record Current, Voltage and Power Readings Every 10 Seconds (Dischaging) Time (s) Current (ma) Voltage (mv) Power (mw) Time (s) Current (ma) Voltage (mv) Power (mw) Page 8 of 8