Figure 8.
Mass
Transport
State.
Figure 9. Beginning and Ending Electrical Data for Five Minutes.
Efficiency Experiments
Now, let’s determine our fuel cell’s
efficiencies (Energy and Faraday) by first
generating 6 ml of hydrogen this time.
We want to set up the Test Bed with a
five ohm load (two 10 ohm resistors in
parallel or equivalent). Then, we need to
time the rate of hydrogen consumption
with a wristwatch or your computer’s
clock. Start by generating the 6 ml of
hydrogen; when you’re ready, connect
the fuel cell to the Test Bed and begin
timing the process. We want to allow
the hydrogen to be consumed over
exactly five minutes or 270 seconds.
When the time is up, disconnect
the fuel cell from the Test Bed and
visually measure the remaining hydrogen
left in the storage cylinder. During the
timing, record both the starting and
ending voltages and currents. Figure 9
shows the start and stop values that I
measured; your data may differ. Now,
let’s plug in some numbers to see
how efficient our fuel cell performed.
voltage (E) and
current (I) must be
determined. The
averages are computed between the
readings at time zero and at 270
seconds. Table 1 illustrates these
averages based on the data taken.
Your values may be different.
Entering these values into the
equations:
between the theoretically calculated
volume of hydrogen consumed by
the load (at a certain current) and the
experimentally calculated volume of
hydrogen consumed. Faraday
Efficiency is computed as:
η F = H2 Volume (theoretical) / H2
Volume (experienced) where …
Electrical energy in Joules H2 =
E (average) x I (average) x time
(seconds)
Electrical energy in Joules H2 =
0.539 x 0.119 x 270 = 17. 3 J
H2 Volume (theoretical) = [Electrical
charge in Coulombs (C)] x [H2
Volume per mol]
Therefore, the volume of H2
used can be found as:
Electrical Charge in Coulombs (C) =
I (ave) x time (sec) = 0.119 x 270
= 32.13C
H2 Vol (theoretical) = ( 32. 13 C /
193,000 C/mol) x 24,000 ml/mol
= 3.99 ml
Volume of H2 consumed in mL /
24,000 mL/mol = mol of H2
5 mL / 24,000 mL/mol = 0.00021
mol H2
Therefore, the Faraday Efficiency
we measured is:
The energy content consumed
H2 is given by:
η F = H2 Volume (theoretical) / H2
Volume (experienced)
η F = 3.99 / 5.0 = 0.798 x 100%
= 79. 8
Energy Efficiency
Calculations
The energy efficiency of a fuel cell
is the ratio of the electricity produced
by the consumed hydrogen compared
with the calculated theoretical energy
contained in the consumed hydrogen
(expressed as a percentage). In order to
compute the required values, the average
Energy content of consumed H2
(theoretical) = mol H2 x
286,000J/mol
Energy content of consumed H2
(theoretical) = 0.00021 x 286,000
= 59. 6 J
Energy efficiency can be
computed as:
A value like 79.8% efficiency
looks pretty good, but it can be
better or worse depending on the
electrical current used in the
experiment. If you care to, modify
the experiment with a different load
resistor and repeat it.
Time
1 second
270 seconds
E (volts) I (amps) H2 (ml)
0.580 0.124 6(starting)
0.497 0.114 1 (remaining)
0.539
(avg)
0.119
(avg)
5
(consumed)
Table 1. Averaging Readings.
η E = Electrical energy in Joules H2 /
Energy content of consumed H2
(theoretical)
η E = 17. 3 / 59. 6 = 0.2903 x 100%
= 29.03
Fuel Cell Stacks
Faraday Efficiency
Calculations
50
June 2010