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Fuel cells:electricity or hydrogen powered

  1. Jun 18, 2008 #1
    The new Honda fuel cell car is out and I am confused. There are now gas cars, electric cars, hybrids, and now hydrogen-to-electricity(fuel cell) cars. I always thought that fuel cell tech was basically filters that produced combustible H from H2O, and after combustion rejoined the combustion products with the oxygen molecules to form water as the waste product. I thought that the only major difference in the concept of an H engine and a gas engine was the fuel used, along with a smaller and specificly reformed engine for H due to the per volume power difference between gas and H.

    The Honda vehicle makes what is an electric car fueled by hydrogen that is bought at a pump, just like we now do with gas. That seems to be a step back or at most a bridge to actual vehicle self-contained, fuel cell production of H. And, isn't stopping to fill up what we are trying to make obsolete? How does H to electricity as propellant constitute an improved answer over burning vehicle self-produced hydrogen in a form of combustion engine? Is not the waste product the same water and no CO2?

    And, aren't we still trying to bring mainstream what was discovered way back in 1837....making propellant power out of water?

    Informed answers would be appreciated.
     
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  3. Jun 18, 2008 #2

    LURCH

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    I think you may have been confused between fuel-cell technology and hydrogen burning technology. Fuel-cells put hydrogen in proximety with oxygen, with a conducting membrane in between. As the hydrogen passes through the membrane to get to the oxygen, only the hydrogen nuclius can go straight through; the electron gets caught in the conducting membrane and has to take "the long way around" to get through the membrane. Along the way, this electron is used to do work.

    This is very different from hydrogen combustion, which simply put hydrogen in proximity to oxygen, and then throws in a match. The resulting explosion is used the same way exploding gasoline is used in most of today's vehicles.

    The advantage of hydrogen combustion is that it is a simple internal combustion system and work some principles nearly identical to those used in vehicles today. The technology is very familiar.

    The advantage of the fuel-cell is that it is a far more efficient reaction. I don't know the exact ratio, but the same amount of hydrogen and oxygen in a fuel-cell vehicle will propel that vehicle many times farther than it would with an internal combustion engine.

    The idea behind both of these technologies is not to "do away with" pulling up to a pump to refuel your car. The idea is to do it with petroleum fuel, or other forms of fossil fuel.
     
  4. Jun 18, 2008 #3
    I guess what we will have is an electric car

    Thanks for your response. So, what it seems we will have then with fuel cell tech as an end product is a car powered by an electric motor which itself is powered by the product of the fuel cell, which is H. If an electric car is the most efficient use of H, then perhaps the ultimate vehicle power solution can be considered done, or maybe not? Will all power sources found still power the same electric motor for propulsion? Maybe that's the weak link. Comments?

    In your opinion, is an electric car fueled by hydrogen considered to be the best end solution to vehicle propulsion and no emissions, or is there even a more efficient way of using H or even common H2O as a power source as has been rumored for many decades if not centuries? Or, even another form of use of magnetism, which is the primary concept in the operation of electric motors. Seems it will be hard to get away from electric motors.

    Thank you again. Or any informed person who wishes to respond.
     
    Last edited: Jun 18, 2008
  5. Jun 18, 2008 #4

    jambaugh

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    You still seem a bit confused with regard to source of energy. The fuel cell takes the chemical energy from the hydrogen and converts it to electrical energy which turns the electric motor.

    H2 is a power source but H20 is not it is rather the waste product from burning H2 (either in an engine or using a fuel cell). Magnetism in and of itself isn't a source of energy but a mechanism for converting electrical energy to mechanical energy as in the typical motor.

    Let me clarify a fuel cell further. Let's make a simple battery. Stick a strip of zinc in a water based electrolyte and also a pencil lead (carbon electrode) and wire each to an electric motor. The zinc will oxidize (the oxygen in the water of the electrolyte) producing electricity which turns the motor.

    Note that the energy from this battery came originally from the process of refining that zinc (probably reduction with carbon from coal or oil). So even if you're using a battery you're burning fossil fuel in a sense.

    Now consider one of those new Zinc-Air batteries. The oxygen used to oxidize the zinc comes from the air and not the electrolyte. (It has the advantage that you don't need to carry the weight of that oxygen as water and you get more energy per weight.)

    Now consider a hydrogen fuel cell. It is just like the Zinc-Air battery except that it uses Hydrogen and not Zinc. But here's the problem. The be$t way to get hydrogen is from fossil fuels... specifically it is usually produced from natural gas. However if we can build more nuclear power plants (and/or wind and/or solar and/or crack the fusion power problem) and/or if the price of fossil fuel goes up high enough then the price of non-fossil fuel electricity would be low enough where electrolyzing water to get hydrogen would be cheaper than getting it from fossil fuels.

    (A lot of "if"s here but also a lot of "or" so its worthwhile exploring each and all.)

    But hydrogen may not be the best way to store electricity. There are other types of fuel cells possible [See final note below] and there is also the regular ole rechargeable battery.

    I personally think that new battery tech will likely be the solution to automotive energy storage.

    What hydrogen fuel does for us right now is help local emissions problems like in the LA basin where the exhaust gets trapped. But we are a long way yet from independence from fossil fuels even if everyone drove electric or hydrogen fuel cell cars tomorrow. At best these along with hybrids provide means of more efficiently using the original fuel.

    One final note: I recently ran across mention of the Liquid Tin Anode Solid Oxide Fuel Cell (LTA-SOFC) which can burn almost "anything".
    http://www.celltechpower.com/technology.htm
    It isn't likely to be used in cars anytime soon but potentially it could replace the steam turbine power plants and indeed allow people to electrify their house off of piped gas or fuel oil or even lawn trimmings. Its high temp and size probably won't let it be practical for personal automobiles but I could see it used on trains or maybe eventually in larger trucks.

    The key idea is that tin oxide is reduced by the fuel and the tin is then oxidized in the solid electrolyte fuel cell. Its pretty neat IMNSHO!
     
  6. Jun 18, 2008 #5
    Very generous explanation. Thank you. Candu reactors? Low grade uranium. Nothing even close to weapons grade. Used all over the world. Good track record. What then?
     
  7. Jun 18, 2008 #6

    russ_watters

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    Just to highlight the main points there:

    -Fuel cells that run on hydrogen extracted from water are just batteries.
    -Fuel cells that run on hydrogen extracted from methane (or methane itself) are fossil fuel burners.
    -Heat engines (engines that use the heat of combustion to drive the car) can only be around 40% efficient.
    -Electric motors run around 95% efficient.
    -Batteries are upwards of 90% efficient.
    -Fuel cells are 50-80% efficient.

    So the bottom line is that at the car, by far the best way to power the wheels is with an electric motor. The best way to power the electric motor is also still an open question (batteries or fuel cells?). And we must remember that if the goal here is to get off fossil fuel, we need an overhaul of our electric power generation systems. They need to be entirely non-fossil fuel. Right now, the US gets roughly 70% of its electricity from fossil fuels.
     
  8. Jun 18, 2008 #7
    Russ,
    As I see it, nuclear is the way to go. but the Canadian nuclear , CANDU , is much better than the ones we're building. My problem with nuclear is that I don't trust people to run them safely. People have that attitude about being sloppy. I don't know what it is. It's human nature to screw up. They'll do almost anything for money. our credo has to change, I guess. I know my credo sucks.
    I like the idea of using intermediate steps to get away from fossil fuel. Off the shelf technology. We should get our ideas from the countries that are really suffering with the energy crisis. Sorry my dog is barking. out.
     
  9. Jun 18, 2008 #8
    Oh, Russ,
    Excellent post. Focused me right up. Thanks.
     
  10. Jun 18, 2008 #9

    mgb_phys

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    In fact one great use for Candus is to burn weapons grade U and even Pu that is left over form weapons programs. It's the safest way to dispose of it.
     
  11. Jun 18, 2008 #10
    Is there a reason that we're not building them here? Also if Iran was constrained to use CANDUs, they wouldn't have to get any real % of 235. What's the deal.
     
  12. Jun 18, 2008 #11

    mgb_phys

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    Candu do produce waste which contains Pu, you can process this to extract the Pu.
     
  13. Jun 19, 2008 #12

    russ_watters

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    Politics.
     
  14. Jun 19, 2008 #13

    vanesch

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    CANDU reactors have nothing special about this. A CANDU reactor is a moderated (thermal, not fast) reactor using heavy water, meaning one can use natural uranium if one wants to (was the original motivation), but one actually uses slightly enriched uranium to use the fuel more efficiently. A LWR (light water reactor) needs slightly enriched uranium, because it can't work on natural uranium. Both can work on plutonium, but both use it badly.

    Turning weapon-grade uranium into something safer is extremely easy: remix it with depleted uranium (it was separated from) and make, to your liking, again natural uranium or slightly enriched uranium - smartest move is the last of course, you can use it in any type of thermal reactor.

    Best way of using weapon grade plutonium... mix it again with depleted uranium, to make MOX fuel - however, this stuff can be separated again using chemistry. Eventually, mix it with reactor-grade plutonium, to decrease its weapon-usability (although all plutonium apart from Pu-238 is weapon-usable, the easiest is Pu-239 in high purity, which is called weapon-grade).

    Most efficient way to use plutonium and depleted uranium: use it in a fast sodium reactor. You win a hundred-fold in fuel efficiency. However, these research programs have had a lot of stupid political opposition in the last decades, which means we're not ready yet to build a large park of fast reactors.

    Concerning proliferation, in fact a LWR is better than a CANDU, because a CANDU is build in such a way that one can continuously refuel it, which makes it suitable as a plutonium production reactor if you want to use it that way. With a LWR reactor, that's way harder, as you have to open up the pressure vessel each time. So you will get out reactor-grade plutonium instead of weapon-grade plutonium. Warning: also reactor grade plutonium can be used for a weapon, but it is more difficult. This is if the reactor is in a sensitive place, where it could be misused. A reactor in a safe place is not an issue (you don't sneak in some uranium for plutonium production without anyone knowing at the plant).
     
  15. Jun 19, 2008 #14

    vanesch

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    The Iran issue is different. But we're getting way off topic here.
    These are the elements: As Iran is a signatory of the Non-Proliferation-Treaty, it obtained the unalienable right to use all kinds of peaceful nuclear technology, including enrichment and fuel reprocessing. The counter side is that Iran has to declare all nuclear activities to the IAEA and not have a military nuclear program. Iran has been found faulty with its declarations to the IAEA in the past, and there is strong suspicion that they want a military nuclear program.

    The safest way to handle Iran would be (but there's no legal basis for that) to require them to use LWR, and sell them the low-enriched uranium fuel, and take back the spend fuel. As such, they would have nuclear power, and not have any suspicious technology (the bookkeeping can be done on the fuel). Of course, that would make Iran dependent on other nations to sell them their fuel, but it would be the only way to be sure that they don't use the civil nuclear power for military purposes. That wouldn't stop them of course from having a separate military program, hidden from the rest of the world.

    If Iran had CANDU's, it could use it to produce plutonium, and as it could work on natural uranium, the bookkeeping would be more difficult (who says that there hasn't been used more natural uranium than they say ?). LWR are safer in that respect: they need LEU (lightly enriched uranium), they can't easily be refueled and one can do more easily the bookkeeping of the fuel elements that way.
     
  16. Jun 19, 2008 #15
    VANESCH,
    Thank you for the response. I understand what you're saying. From a purely scientific point of view, is a CANDU a better machine than LWR?
     
  17. Jun 19, 2008 #16
    we do need to switch to nuclear energy as our source power plants... atleast until we can harness the energy of fusion technology. people are far too affraid of nuclear power because of incidents in the past... Everything is dangerous unless it is respected... we must regulate everything. Natural gas used to be very dangerous as well because people did not respect it enough or know enough about it... now that we know much more about nuclear technology we can regulate how it is handled and make it much safer.
     
  18. Jun 19, 2008 #17

    mgb_phys

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    It's less thermally efficient, it runs at a lower temperature, and recovers less of the energy from the fuel - although the fuel is reused in a fuel cycle and it can efficently burn used LWR fuel. The core is larger so it's expensive to build and the heavy water isn't cheap, but you don't need an enrichment plant. It's also proven a very safe and conservative design.

    Probably the biggest advantage is that it's relatively simple and easy to build, especially because you don't need a huge pressure vessel. It looks like the capacity of the one plant in the world to produce reactor pressure vessel forgings is going to be a limit for other designs.
     
  19. Jun 19, 2008 #18
    It looks like the capacity of the one plant in the world to produce reactor pressure vessel forgings is going to be a limit for other designs.[/QUOTE]

    What plant? What other designs? Other designs instead of what? Explain, please.
     
  20. Jun 19, 2008 #19

    vanesch

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    I wouldn't say so, although a CANDU reactor has certain advantages but also some problems.
    I would add a few comments to mgb_phys' post.
    Concerning thermal efficiency, I'm surprised: I thought that the CANDU outlet temperature was about 310 degrees, as is that of a PWR.

    The main original advantage of a CANDU was the fact that it could run on natural uranium, but the problem is that you can *hardly* have any burnup with natural uranium: from the moment that you deplete it a bit, you loose criticality. Hence now CANDU's run on LEU, just like LWR. Of course, thanks to the heavy water, they can run down the fuel more than can a LWR, so I guess that with LEU they have a slightly higher fuel efficiency than a LWR.

    The main disadvantage is the heavy water. It is not only costly, it is also tritium-producing. So the primary water is much more active than the primary water of a LWR. Now, if there's a market for the tritium, that can be an advantage of course.

    Concerning safety, it is sure a safe design - although I don't know how it compares to say, an EPR. The CANDU reactor is by now a pretty old design after all.
     
  21. Jun 19, 2008 #20
    You know, the way I think sometimes we don't need to be testing new reactor designs in the field, if you know what I mean. We need safe energy now. Other countries have been using them for years without problems. I'd say jump into the pool and ramp up the knowledge curve. Opinions?
     
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