Originally Posted by Geof  No, no matter what you or Bob think you cannot get around the laws of thermodynamics. I have come across the misconception you have here many times. In a chemical reaction you are dealing with two separate properties; the rate of the reaction and the energetics of the reaction. They are connected but only in the sense that a reaction which releases a lot of energy will often proceed very fast; this is why some reactions are explosive or burn very fast. But the amount of energy involved in a reaction is not dependent on how fast it proceeds.
When electrolysing water to generate hydrogen and oxygen the rate of this energetically unfavorable reaction can be influenced by the electrode design, temperature, solute composition. So you can produce hydrogen faster or slower but the amount of energy going into breaking the hydrogen-oxygen bond is the same. All the energy that is released when the hydrogen reacts with oxygen has to be put into the system in the first place just to get the hydrogen.
And it is actually worse than that because during the electrolysis some of the electrical energy supplied to the electrolysis cell is wasted as heat so for every 100 units of electrical energy in you get fewer than 100 units of chemical potential energy out in the hydrogen. Then when you utilize the hydrogen in a fuel cell you lose more of the energy as heat. There is no way around it. The full cycle from electrical energy used for the electrolysis back to electrical energy has an overall efficiency of 50% or less for the polymer fuel cells proposed for the 'hydrogen economy'. So half the energy is lost even before the compression, storage and transport of the hydrogen is taken into account. |
Hi Geof,
You paint a bleak picture indeed. Usually I'm just trying to get around the speeding laws, these thermodynamic laws I'm a little hazy on.
So you can produce hydrogen faster or slower but the amount of energy going into breaking the hydrogen-oxygen bond is the same. All the energy that is released when the hydrogen reacts with oxygen has to be put into the system in the first place just to get the hydrogen.
OK, but use what Mariss said, about a 50% efficiency. Wouldn't this work? It is after all a method of storing energy. I realize you're going to lose some in the translation, but it is a method of storing energy for use vehicles.
Donna