Xogen-phoenix or failure ?

[Seeker]

quote: What you wrote about using a pulsed current, makes me remember some comments I've come across somewhere, to the effect that the value of a pulsed current would be in preventing heat from building up in the electrolysis solution.

I don't usually reply to my own posts,but I just realized that where I got this info about a pulsed current being used for the purpose of reducing heat in the electrolysis solution...was from yop (just a few messages up). Thanks yop

Seeker

[hodgepoj]

quote:Originally posted by Seeker


Hi Dr J,

Thanks for shedding some more light on this electrolysis question. What you wrote about using a pulsed current, makes me remember some comments I've come across somewhere, to the effect that the value of a pulsed current would be in preventing heat from building up in the electrolysis solution. The build up of heat, does seem to be a problem with common electrolysis approaches.

The rate of heat buildup in the solution is the difference between the rate of heat creation and the rate of heat dissipation by convection. The rate of heat creation is the power drawn from the current source minus the rate of molecular dissociation (the power expended in breaking up water molecules into gas molecules). The power drawn from the current source is the steady DC voltage between the electrodes times the steady DC current.

If the current is not steady DC (if it is pulsed), the power drawn from the source would be the product of the voltage and the "effective" current (the square root of the average of the square of the current pulses).

The only way, then, that a pulsed current would reduce the rate of heat buildup is if it expended less power than a steady DC current would. This not only decreases the rate of heat creation but provides more time for the heat to be convected away.

quote:
As far as the hydrogen bubbles sticking to the electrodes, maybe what I had gotten this confused with, was the fact that hydrogen bubbles are near the electrodes.

The following quote seems to suggest that hydrogen bubbles in an electrolysis solution present a resistance to the flow of electrical current:

"The evolution of gas bubbles at the electrodes is often exploited to drive the electrolyte through the electrolyser. But a large volume fraction of bubbles increases the electrical resistance of the electrolyte, which increases the loss of energy. The consumption of energy is perhaps the most critical problem in the electrochemical process industry."

So possibly there would be a rationale for an electrolysis approach which could remove the bubbles from the solution as quickly as possible (maybe especially the ones which are near the electrodes)

An accumulation of gas bubbles around the electrodes does, indeed, increase the resistance of the electrolyte. But rather than pulsing the current to attempt to decrease the bubble density, it would be more effective (generate gas at a greater rate) to draw off the gas as fast as it is generated. For example, make the electrodes hollow pourus cylinders and use a pump to reduce the pressure inside the cylinders.

I'm not trying to shoot down your good ideas, Seeker. I think you have a remarkably good understanding of the process of acquiring hydrogen and oxygen by electrolysis. I'm simply trying to show you that there are parameters and considerations other than those you see.

Dr. Paul O. Johnson
Senior Exhibit Developer
The Science Place
Dallas, Texas 75210

[yop]

Bubbles in solution like this are interesting. Have you ever noticed that the bubbles in a glass of champagne are all roughly the same size? That's because of the interplay of forces affecting bubble behavior. On the one hand, you have the uplift force due to the density difference between bubble and champagne. On the other hand, you also have the hydrophobic effect which tries to pin the bubbles to the glass. Once a bubble reaches a certain size, the uplift force overcomes the hydrophobic effect, and the bubble will escape the glass surface and rise upwards. So all the bubbles in a glass of champagne are just slightly larger than the size where these two forces balance out.

Some other points of interest:
Vortex shedding: We're all familiar with this phenomenon, but we may not be aware of it. Vortex shedding is what makes a flag flap in the wind. Higher wind speeds cause faster flapping. If this flapping rate resonates with a bridge, you get Tacoma Narrows. Modern bridges avoid this by being designed to be very stiff, so that it would take an extremely strong wind, with a velocity of hundreds of miles an hour, to cause resonance.

Hydrogen generation: Hydrogen is exactly like electricity in that it is only as environmentally friendly as its source. If you generate your hydrogen by: Solar=>Electricity=>Hydrogen (from electrolysis of water), then the process is completely clean. Currently, hydrogen is not a completely clean fuel. The most common process for hydrogen generation today is the following: fossil fuel + water => carbon monoxide + hydrogen. The carbon monoxide can be used as a feedstock to produce chemicals, or it can be used as a source of energy and burned to carbon dioxide. The fossil fuel can be anything, usually coal or natural gas because they are abundant and cheap. So in order for hydrogen to be a truly clean technology, independent of fossil fuels, we still need to develop alternative energy sources.

[hodgepoj]

quote:Originally posted by yop

Bubbles in solution like this are interesting.

Vortex shedding: We're all familiar with this phenomenon, but we may not be aware of it. Vortex shedding is what makes a flag flap in the wind.

Hydrogen generation: Hydrogen is exactly like electricity in that it is only as environmentally friendly as its source. If you generate your hydrogen by: Solar=>Electricity=>Hydrogen (from electrolysis of water), then the process is completely clean. Currently, hydrogen is not a completely clean fuel. The most common process for hydrogen generation today is the following: fossil fuel + water => carbon monoxide + hydrogen. The carbon monoxide can be used as a feedstock to produce chemicals, or it can be used as a source of energy and burned to carbon dioxide. The fossil fuel can be anything, usually coal or natural gas because they are abundant and cheap. So in order for hydrogen to be a truly clean technology, independent of fossil fuels, we still need to develop alternative energy sources.

A very interesting message, yop. Another source of info on hydrogen as a fuel is at http://www.eren.doe.gov/consumerinfo...iefs/a109.html

Dr. Paul O. Johnson
Senior Exhibit Developer
The Science Place
Dallas, Texas 75210

[don c.]

An examination of some of the issues pertinent to the Hydrogen Economy can be found at:

The Future of the Hydrogen Economy: Bright or Bleak?

Also check out http://www.tinaja.com/glib/resbn88.pdf