Energy lost to hydrogen fuel cell So I have a question about an experiment we did in class.
We used a fan to blow air at a little wind turbine to generate electricity, we connected the turbine to a reversible PEM fuel cell to electrolyse water and generate some hydrogen and oxygen. We timed this process and recorded the average power output from the turbine to calculate the total energy produced by the turbine: 59.4 J.
For the second part of the experiment we passed the hydrogen through another PEM fuel cell which would power a little electric motor until all the hydrogen was consumed, and measured the power and time it ran for with the same device. Again, calculated the energy consumed and it gave only 2.4 J.
Which would make it seem like a lot of energy was lost, mainly I guess during the electrolysis, the fuel cell and I guess the motor itself. But can so much energy be lost during this process? If so, how?
 A: During electrolysis, there is over-voltage on the electrodes, meaning that electrolysis doesn't start until a certain voltage is applied (which is NOT zero volts).  Since circuit power is given by P=IV, you will note that there is a substantial loss during electrolysis because that over-voltage does NOT produce any products.  This means that the efficiency of the overall process is low from the start.  That efficiency can be improved somewhat by using exotic metal electrodes (e.g., platinum), but it can't be driven to 100%.
Note that the efficiency of your school experiment can be improved somewhat if you use the energy from the wind turbine to directly charge a battery, which will then be used to drive the electric car.  This simply means that the talk around hydrogen as a "renewable" energy source has been "hyped" to a certain degree.  Given the relatively low efficiency of hydrogen generation via electrolysis, and the difficulty of storing enough hydrogen in a tank to drive a car over a reasonable range, there are probably better technologies to use for transportation than hydrogen fuel cells.
A: To measure efficiency properly, you should have endclosed the electrlysis cell and fuel cell in a calorimeter to measure losses due to heat.  There are heat losses in any kind of energy coversion.  If you add up the joules of energy or calories lost to heat, you should arrive at the end result as predicted by the laws of thermodynamics.
Electrolysis of water, even with the most suitable catalyst and electrolyte is a highly inefficient process to produce hydrogen.  Most commerical hydrogen is produced from natural gas, ie, methane.  The electrical energy obtain from the PEM fuel cell is an efficeint energy conversion process, with 50% efficiency possible with the right catalysts, electrodes, and memebrane.  Your main loss was in the electrolysis and motor.
