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Inferno
06-06-2001, 10:34 PM
I stumbled onto this site by accident about a week ago and now I'm completely addicted. This is my very first question so please bare with me. Here goes;

If we can separate water (h2o) into its base elements, hydrogen and oxygen, why can't we build huge facilities that convert water into fuel, and then burn that fuel to run a conventional power plant?

Am I being too ideallistic? Is it simply because the energy it takes to separate water is more than the energy you would get back out of the fuel? If the answer is yes, then I have to ask: Would the same be true on a factory-size scale? What does it take to separate water into hydrogen and oxygen?

Well, I hope I haven't overshot my question quota:) Thanks for stopping by; hope to hear from ya'll real soon!

Cheers -> Dante

CalMeacham
06-06-2001, 11:15 PM
Am I being too ideallistic? Is it simply because the energy it takes to separate water is more than the energy you would get back out of the fuel? If the answer is yes, then I have to ask: Would the same be true on a factory-size scale? What does it take to separate water into hydrogen and oxygen?



That's it in a nutshell. The laws of Thermodynamics being what they are, you can't go through the cycle of disciation and combustion back to your original state and have a net gain. Economies of scale don't help you at all -- it doesn't work at any scale. Hydrogen generation plants mght be a great way to store and later to re-convert energy, but you still need an ultimate source for the energy. (Even so, I have heard that hydrogen does not invariably "burn" completely cleanly to water alone. You get some hydrogen peroxide (H2O2) and other reactive compunds as well.)


. This is my very first question so please bare with me.

Only if you're female.

sailor
06-06-2001, 11:18 PM
>> please bare with me

You go first. I'm a bit shy

sailor
06-06-2001, 11:24 PM
OK, so...

>> If we can separate water (h2o) into its base elements, hydrogen and oxygen, why can't we build huge facilities that convert water into fuel, and then burn that fuel to run a conventional power plant?

So you get back the energy you put in in the firt place minus some losses. Doesn't work. OTOH, if you had large quantities of H2....

Smeghead
06-06-2001, 11:24 PM
It's been pretty well explained now, but if you're still curious, do a search for threads about why hydrogen doesn't burn. Same concept, and there have been a few threads on the subject.

Dr. Lao
06-06-2001, 11:34 PM
Originally posted by Smeghead
It's been pretty well explained now, but if you're still curious, do a search for threads about why hydrogen doesn't burn.Hydrogen burns just fine, but I think you mean "why water doesn't burn." There have been a number of threads on that.

Astroman
06-07-2001, 02:03 AM
To address the question of why we have not yet built power plants to extract energy from water, here it goes:
We actually have built facilities that do this, but they do not "burn" hydrogen or oxygen. Rather, they fuse heavy isotopes of hydrogen into helium, in much the same way that the sun produces energy.
The one reason we don't have fusion power plants cranking out oodles of power is that as it stands now, we do not have the technology developed to create the enormous pressures and temperatures needed for fusion. Experimental reactors have produced a great deal of power, but still, the amount of power needed to start and sustain the reaction is greater than the amount of energy produced.

On a side note, there is a form of power generation that operates in the opposite direction, Inferno. Fuel cells combine oxygen and hydrogen to produce electricity, with the only by-product being water. The only reason we don't see more fuel cells powering things in the world around us is that they are expensive to manufacture.

I hope I cleared some stuff up. If you want to learn more about fuels cells, go to
http://www.sciam.com/explorations/122396explorations.html
To learn more about fusion power generation, go to http://wwwofe.er.doe.gov/

Mangetout
06-07-2001, 06:06 AM
Now converting water to H and O2 on a massive scale then using it to run Hydrogen-powered cars might be a good idea; it would reduce inner-city pollution and it might be possible to get better energy value out of the fossil fuels when burning them in bulk to generate the energy to split the water in the first place.
I any case it would probably be a better idea that electric cars (where 50% of the car's weight is the batteries).

The drawback is the volatile explosive nature of pure hydrogen; you'd have to make a virtually indestructible fuel tank to keep it in, otherwise relatively minor traffic accidents would be catastrophic.

If the power to split the water came from solar panels, now there's an idea...

Una Persson
06-07-2001, 07:18 AM
Originally posted by Mangetout
and it might be possible to get better energy value out of the fossil fuels when burning them in bulk to generate the energy to split the water in the first place.

Due to the cascading inefficiencies involved, probably not.

The drawback is the volatile explosive nature of pure hydrogen; you'd have to make a virtually indestructible fuel tank to keep it in, otherwise relatively minor traffic accidents would be catastrophic.

Experimental vehicles using hydrogen have very well-designed tanks, and this is only a minor concern.

Inferno - the simple reason comes down to this - money and efficiency. We can do it, it just isn't cost-effective or the most efficient use of the energy right now. Yes, hydrogen is a very useful and clean-burning fuel, and someday either the benefits will outweigh the cost, or some new process will be developed to lower the cost, but for the near future hydrogen is essentially a niche fuel only. It's great for generating Popular Science articles like "Wouldn't it be great if..." and "Homer Crevice from Asshole, Alabama recently converted his 1952 Packard Convertible to hydrogen - why don't all of you too!"

Phobos
06-07-2001, 09:52 AM
Originally posted by Mangetout
If the power to split the water came from solar panels, now there's an idea...

gasp! you dared mention the 's' word in the presence of sailor?!? ;)

Anthracite - Judging from your "sigh", I hope you're not getting too bored with these kinds of threads. The SDMB folks definitely appreciate your expertise in power generation techs.

erislover
06-07-2001, 10:15 AM
The drawback is the volatile explosive nature of pure hydrogen; you'd have to make a virtually indestructible fuel tank to keep it in, otherwise relatively minor traffic accidents would be catastrophic.
This isn't a monster concern (what, you mean we're carrying around 20gallons of explosive, flammable liquid?). But if you're extra worried about it Chrysler and some others are working on using methanol as fuel. There is then an intermediate phase where the hydrogen is gleaned from the methanol before final stages of water production.

Whack-a-Mole
06-07-2001, 10:15 AM
Astroman wrote:
The one reason we don't have fusion power plants cranking out oodles of power is that as it stands now, we do not have the technology developed to create the enormous pressures and temperatures needed for fusion.

We sure do have fusion reactors and we do have the technology to start the reaction. Search for links on Tokomak Rings or Reactors and you should find a lot of material. (And of course there is always a Hydrogen Bomb that gets fusion going but that doesn't really count for this discussion.)

As Astroman noted in the sentence following what I quoted above the problem is that we have to put more energy in to get the reaction started than we get out thus making it worthless for energy production (as things stand now).

The problem is the hydrogen fuses at such incredibly high temperatures (100 million degrees celsius...hotter than the core fo the sun) and pressures the plasma can't actually touch anything. No man made material can withstand those temperatures and not melt (or burn or generally suffer annihilation). As a result the whole thing is kept in a magnetic containment bottle (which is what the Tokomak Ring achieves) that keeps the hydrogen plasma from actually touching anything. The power required to start and maintain all of this stuff (the intense magnetic fields and so on) are more than can be gotten back out. (Also, does anyone know if you can even add fuel (deuterium) once the reaction is running?)

I've read some stuff on a Spheromak (http://www.llnl.gov/str/Hill.html) reactor that would be cheaper to build and operate than a Tokomak Ring would. The linked article talks about its benefits but also mentions that the Tokomak concept is still the leading contender for workable fusion energy till Spheromak is more proven.

Fusion energy is a Holy Grail of energy production given that there are vast amounts of hydrogen about (and even though deuterium, a hydrogen isotope, is only a small percentage of the total amount of hydrogen there is still a helluva a lot of it as well). Also, there are not really any nasty by-products that can't be easily dealt with (comparatively speaking...I think the radioactive material produced by fusion plants have a half-life of only a few decades...not the tens of thousands of years of current nuclear waste).

So keep your fingers crossed for this but I wouldn't hold your breath either.

Una Persson
06-07-2001, 12:09 PM
Originally posted by Phobos
Anthracite - Judging from your "sigh", I hope you're not getting too bored with these kinds of threads. The SDMB folks definitely appreciate your expertise in power generation techs.

No, it was not due to that. Just tired that early in the morning.

Akatsukami
06-07-2001, 12:42 PM
Whack-a-Mole writes: Fusion energy is a Holy Grail of energy production given that there are vast amounts of hydrogen about (and even though deuterium, a hydrogen isotope, is only a small percentage of the total amount of hydrogen there is still a helluva a lot of it as well). Also, there are not really any nasty by-products that can't be easily dealt with (comparatively speaking...I think the radioactive material produced by fusion plants have a half-life of only a few decades...not the tens of thousands of years of current nuclear waste).

Do not forget that, even if we assume that Bush opens the floodgates of research funding and that future Presidents keep them open, there isn't a snowball's chance in Hell that we'll see a commercially-viable D-D reactor in this century. D-T fusion is the only likely prospect in the near term (i.e., the next several decades). No problem with breeding sufficient tritium from 6Li, but I'll bet that many who sing the praises of fusion now will strike up a different tune when it is impressed upon the public that tritium is a radioactive gas.

"Current nuclear waste" doesn't contain isotopes that have half-lives of tens of thousands of years, either (not in any significant amounts; I don't have access to my usual sources here and now, so I won't swear that some isotope produced by fission doesn't have a half-life in that range). Plutonium isotopes do, but 239Pu isn't a waste product of fission any more than gasoline is a waste product of refining crude oil. As reprocessing in Europe shows, there's no particular problem in separating it from genuine reactor waste.

Whack-a-Mole
06-07-2001, 03:23 PM
Akatsukami wrote:
"Current nuclear waste" doesn't contain isotopes that have half-lives of tens of thousands of years, either (not in any significant amounts; I don't have access to my usual sources here and now, so I won't swear that some isotope produced by fission doesn't have a half-life in that range).

According to the EPA ( http://www.epa.gov/radiation/yucca/faqs.htm#howlongradioactive )

Spent nuclear fuel and high-level radioactive waste contain short- and long-lived radionuclides. Most radionuclides in this waste will decay to insignificant levels within several hundred years. Some radionuclides will take many thousands of years to decay to non-threatening levels.

Not very detailed, I know, but it establishes that the long lived stuff is in there.

It seems the trick here is that most people (me included) focused on the radiation hazard of nuclear waste. As the following quote from Canada's nuclear program indicates it isn't the radiation, it's the highly poisonous nature of the waste that os problematic for long-term storage. Leak that stuff into a water supply and you've got trouble.

Source: http://www.ncf.carleton.ca./~cz725/

About three meters of water are sufficient to absorb the radiation emitted initially by the spent fuel, while in the dry-storage phase about a meter of concrete suffices. Unshielded, the radiation dose measured at a distance of 30 cm from a CANDU fuel bundle would be [1] about 50 - 60 Sv/h (5000 - 6000 rem/h) after a year following discharge, which is extremely lethal. This drops to about 1 Sv/h after 50 years, 0.3 Sv/h after 100 years, and less than 0.001 Sv/h (100 mrem/h) after 500 years. At this time the major hazard from the spent fuel is no longer one of external exposure; for example, by these estimates, spending an hour about a foot away from a 500-year-old CANDU fuel bundle would result in an exposure that is about 1/4 of the average background exposure, and thousands of times less than what is known to lead to physical harm. However, a significant hazard is associated with internal exposures (for example, from long-lived plutonium isotopes), and therefore an effective long-term protective strategy addresses the need to isolate the spent fuel and prevent its uptake into the biosphere.

In short, nuclear waste is nasty stuff for a long, long time. The waste from a fusion plant isn't nearly so bad. That said I agree with Akatsukami that viable fusion power generation is a long way off yet. I just hope it's quicker than a 100 years or more.