Harrington Heights Laboratories (HHL) has been hired to provide independent testing of a new fuel cell energy storage and generation system. The manufacturer claims the system has an overall efficiency of 52%, which would be a revolutionary jump beyond current technology. You have been asked to find the true efficiency of the system. I. Introduction
PEM fuel cells use a proton exchange membrane (PEM) to convert the chemical energy contained in H2 and O2 gases into water and electricity (or vice versa). H2 on the anode side of the membrane is split catalytically:
H2 2H+ + 2e-
The protons (H+) pass through the membrane to the cathode side, where it reacts with oxygen:
½ O2 + 2H+ + 2e- H2O
The electrons produced on the anode side pass through an external circuit (where the current is used to power some device) that is connected on the other end to the anode side. The overall reaction is thus:
2H2 + O2 2H2O
When operated in this manner (generating electricity and H2O from H2 and O2) the device functions as a fuel cell. If, instead, a current is applied, the device will run in reverse (generating H2 and O2 from H2O and electricity) and functions as an electrolyzer.
A solar cell and electrolyzer together allow solar energy to be converted into electricity (which is not easily stored) and then into chemical energy in the form of H2 and O2 (which can be stored for later use). Then when the energy is needed a fuel cell can convert the chemical energy in H2 and O2 back into electricity. The overall conversion process, however, is not perfectly efficient as energy is lost at each conversion step.
II. Procedure
1. Study the experimental apparatus and electrical diagram. Make sure you understand how the system works.
2. Fill each of the two lower tubes of the electrolyzer with distilled water up to the "20" line. Add additional water to each tube until the level is a little below the 3-way valves. Do NOT overfill. Three-way valves provide a way to bleed air out of the tubes while filling. Both valve handles should be "up" when done filling.
2. Connect the red and black wires to the + and – connections on the voltage regulator and the electrolyzer. The "tracking" switch should be turned off. Flip the "Solar-Regulator" switch down towards "Regulator". Turn off the "Fan" switch. Turn the voltage "coarse" knob counter-clockwise to stop. Turn on the voltage regulator and adjust the voltage dial until the "Voltage Regulator Mode" voltage reaches 2.5 V. It can be fine-tuned using the "current" knob.
3. Open the outlet valves on the fuel cell. This will flush the H2 and O2 tubes and the fuel cell with pure gases, replacing any air that may be present. Continue until the "Charging Voltage" reaches 0.7 volts. Turn on the fan and keep running for another 5-10 minutes.
4. Turn off the fan. Close the outlet valves. As the electrolyzer splits the water, gases will build up in the lower cylinders, forcing water upward into the upper cylinders. This is normal. Wait until the water level in the H2 tube reaches the "20" line. Now you are ready to begin recording a charging/discharging cycle.
5. Start your stopwatch to time the charging event.
6. As you are charging, record the water level in each tube, "Voltage Regulator Mode" voltage, and "Electrolyzer Amperage".
7. Once the lower cylinders have been filled with 10-20 ml of gas (5-6 minutes using the voltage regulator) turn off the voltage regulator or halogen lamp and flip the "Solar-Regulator" switch to the middle position to stop charging the system.
8. Flip on the "Fan" switch, set your stopwatch and record the "Charging Voltage", "Fan Amperage", and water levels. Take data untilthe water levels return to the "20" line.
9. Turn off the fan and repeat steps 5-8 to get multiple trials.
III. Calculations
Power input or output (P) can be calculated from the voltage (V) and current (I):
P = V*I
Energy input or output can be calculated from:
E = ∫Pdt
Efficiency of the electrolyzer/fuel cell cycle will be determined as the fraction of Energy input that is available as Energy output
Efficiency = Eout/Ein
IV. Report
1. Power generated by the Voltage Regulator
2. Plot a graph showing input and output power vs. time for each trial
3. Calculate and report the average overall efficiency of the electrolyzer/fuel cell system
Electrical Diagram of System (may or may not be entirely accurate)
O2
H2
Fuel Cell
Electrolyzer
Electrolyzer Amperage
Regulator Mode
Solar
Mode
Solar/
Regulator
switch
A
V
Voltage
Regulator
V
Solar
Cell
Fan Amperage
Charging Voltage
A
V
Fan
switch
Fan
10