I was thinking about Hydrogen balloons and that large ones which are used for weather balloons which sometimes go up to 100,000 ft (approx 30km). Then I was wondering, how much potential energy has the balloon gained with the balloon and the weight it carries to get up to 100,000 ft. It seems the object would have a lot of potential energy at that height. If the object was rolled down a ramp from that height, it would generate a lot of energy going down a 30km height ramp. Then how much energy was used to get it to lift, to produce the hydrogen in the first place?
So my question is, could there be any situation where the potential energy the balloon and its cargo gained exceed the energy it took to make the hydrogen in the first place? Then, if so, how could a cycle be set up where the lifting energy of the hydrogen is used to liberate more hydrogen and produce energy.
Here is another idea, what if the balloon started at the bottom of the ocean, a Electrolysis device is separating hydrogen and oxygen from the water down there. A balloon collects the hydrogen and oxygen and pulls upwards. The balloon is attached to a string which it pulls up and turns a pully (wheel) at the bottom as it goes up. Could the rotation of the wheel gain more energy than the cost to extract the hydrogen. I guess the weight of the string would be a factor to consider as well.
My thought is that all of this is very unlikely, as it seems like a perpetual motion device, as the hydrogen and oxygen could be re-combined and it would fall back downwards as water and the cycle would be repeated. The question would be, where does the energy come from? it has to come from somewhere, so this seems very unlikely. I cannot think of where the energy comes from.
But can anyone work out the calculations even for a very basic calculation?
Best Answer
I haven't done the calculations, but I doubt that this scheme would generate net energy. As was pointed out, electrolysis uses a lot of energy. However, after the H2 and O2 rises up the water column, you could get some of the energy back with a fuel cell that would convert the hydrogen and oxygen back to water and supply additional electric power, but inefficiencies at each stage would have to be made up from the energy gained by the rising gases in the column of water.
To show that there has to be a net overall loss of energy, consider the following modification to your problem. Imagine a long vertical pipe filled with water. Now instead of electrolysis, let's say you instead use an air pump to inflate a balloon near the bottom of the water filled pipe. This takes work which goes into lifting the column of water up such that the surface of the water rises enough to allow for the volume of the ballon. Once the ballon has risen through the water column the water level will go back to it's former position. That fall in water level is the source of the energy that the rising ballon could generate. So there is no free lunch or perpetual motion machine here - inefficiencies at each stage will insure that there is a net loss of energy.
By the way, I do not know this for a fact, but by this thought experiment, I would predict that electrolysis of water under high pressure would take more energy than under lower pressure. I say this because the electrolysis is effectively inflating a balloon against the pressure of the water - which will thus take more energy at high pressure.