Would this be perpetual motion:
1) You make hydrogen from water using energy.
2) you let the H₂ rise through a pipe capturing all its energy.
3) you burn the H₂ in a fuel cell at its highest height.
4) Then you let the water from the fuel cell run down a pipe capturing all of its energy.
5) Now all the energy you captured from the H₂ rising, the fuel cell burning,and the water falling is used to make more H₂.

Asked by: Gary Alan

Answer

First, one point: As asked, the scheme COULD work. Why? Simple, because when you produce hydrogen from water, you also produce oxygen. If you just send the hydrogen up through the water, and burn it up above, combining it with new oxygen, and let it drop back, you will be using the gravitational energy from oxygen. Thus you have a perpetual motion machine not too different from the Energizer bunny, which works with an external energy source. To form a complete cycle, one needs to send to oxygen together with the hydrogen, up through the water. This does not alter the question much.

Now, why won't the scheme work? First, let me point out that the energy obtained from the burning is exactly equal to the energy spent for splitting water into hydrogen and oxygen. That settles the score between the burning and splitting.

Second, one important fallacy is that hydrogen (and oxygen), being gases, will readily rise through water. This is wrong, because they don't come 'expanded'. They need to do some work in order to expand under water after their production. If we imagine this happening in a bowl of water, this expanding will have to raise the level of the water - (or you can say expanding under pressure) thus needs to spend energy for it. Then, it can be set for its journey up the liquid. When it pops out of the water, the water level returns to what it was, and the gas is just re-paid what work it exerted to expand under water, given the rising was 'slow'. Then, it is burned, and turned into water. You can really only let the water drip into the bowl, and you will gain no energy from letting it go down the depth of the water, since to force it back down you need to displace that much water -- just the energy you would gain from dropping the water that height. So, after one cycle, you are back pretty much where you started. Now, we can assert that, given it is a break-even situation (as must be) even the tiniest losses will cause the machine to slow down and stop.
Answered by: Yasar Safkan, Ph.D. M.I.T., Software Engineer, Istanbul, Turkey

Unfortunately, there would be losses of energy at every stage of the process. As the hydrogen rose, it would interact with air molecules, warming the air slightly. When you burned it, some of the heat would escape from the system. As the water fell there would be frictional losses which would also warm the pipe. You could never capture all the heat produced by these losses, no matter how tightly you insulated the system - some of it would escape to the outside. In addition, there would be frictional losses in converting the motion energy into electrical energy, and losses in the electrical circuits from the resistance of the wires and components. As a result of all these losses, the system would quickly run down - that is, there would be less available energy, and less hydrogen produced with each cycle.
Answered by: Ed Gibbs