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Hydrogen energy convervion

  1. Jun 23, 2014 #1

    Does anyone know what is the energy loss of splitting water to hydrogen and then convert it back to energy?

    hpw much energy do you need for making 1 KWh?

    thank you,

  2. jcsd
  3. Jun 23, 2014 #2


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    More than 1KWh. How much more depends on the efficiency of the process that you use, so there's really no good answer to your question unless you provide some more specifics.
  4. Jun 23, 2014 #3
    Ideally, the energy you spend to split water is regained when you burn the hydrogen.
    So the balance is zero. You "make" exactly zero energy. So you need 1 KWh to make 1KWh.

    If you consider real conditions (as opposite ideal ones) then you spend more energy than you get back.
  5. Jun 23, 2014 #4


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    Since energy is always conserved you always get out what you put in. The problem is the energy you get out may not be in a useful form.

    If you use electricity to split water...


  6. Jun 23, 2014 #5
    For thermodynamic processes, you do get out what you get in, but often times, what you get out is in the form of heat, that cannot be 100% efficiently converted back to energy. It depends on the process, and the temperature.
  7. Jun 24, 2014 #6

    I want to know, what is the best method (efficiant vs cost) known in real world to spit the hydrogen from water and then convert/use the hydrogen back to energy. in other words how much energy I use (Kwh) in order to make 1KWh.

    I would like to know if hydrogen is a good method to store energy or there is a better method to do that.

    Thank you,

  8. Jun 24, 2014 #7


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    what do you specifically want to do with the hydrogen ?
    Then decide if its better just to use the electrical power to directly power the
    other system and skip the very inefficient hydrogen making step.

  9. Jun 24, 2014 #8


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    Just 1kW? To some extent the efficiency depends on the scale (eg big electrolysis plants are likely to be more efficient). Lets say 80% at best using a large plant. See above.

    Do you also need the hydrogen to be compressed for storage? That takes more energy. The process is likely to be more efficient if you don't compress the hydrogen but then you need a huge tank which is a problem of it's got to be mobile. If I understand this paper (which I've only scanned) you loose about 25% of the energy in the hydrogen reaction to liquefy it. So that's 75%.


    Then you need to "burn" the hydrogen to make electricity. Again the efficiency will depend on the scale. Some waste heat will be produced and you may only be able to reclaim that energy if you do it on a large scale...


    So a quick stab at the numbers suggests the best you might achieve is...

    80% * 75% * 80% = 48%

    So for 1kW out you need 2kW input.

    If you can't reclaim heat lost at various stages because it's small scale you might be lucky to achieve nearer..

    50% * 75% * 50% = 19%

    So for 1kW out you need 5.3kW input

    My numbers could be way out because I've only done what you could have done and googled the efficiencies of the various steps. I may also have missed out some losses - such as the energy costs of wear and tear on the fuel cell? or ???
  10. Jun 24, 2014 #9
    Hello. I hope you don't mind if I reverse the order in quoting you but from my POV this is the proper sequence.

    No. Hydrogen is not a good method for storing energy, at least on planet Earth, since it is a gas at Earth-normal temperatures and lower density translates into less energy storage capability per volume. This is mild over-simplification but if you consider the thermal storage of Air vs/ Water it makes easy sense.

    Hydrogen is only good for storing energy in Stars like our own Sol, and partly for similar reasons, ie: high density due to high gravity. Obviously the thermonuclear angle doesn't hurt either :tongue2:

    This does bring up what is possibly the best real world alternative to storing and using energy which is Solar Power. Whether stored as thermal or electrical, it is very efficient and getting better all the time. The insolation on a square meter of surface area per day is roughly 1KW/M^2 which equals the energy stored in a 55 Gallon Drum of Oil in a fairly short period of time, and the Sun does all the "drilling".

    I apologize in advance if I'm taking a tack in which you have little or no interest, but it seems when considering energy alternatives for the real world we really have to look at infrastructure and how this energy can be distributed. It is impractical (and dangerous) to store hydrogen and there is almost zero infrastructure in place, anywhere. OTOH most countries have considerable infrastructure for storing and distributing electrical power, and the technology for storing it locally (batteries) is improving all the time. This is important because while the energy input required for a given output is extremely low, the efficiency is as yet fairly low, on the order of 20%, but it is continuous and low maintenance, and it is improving.

    If this intrigues you at all and you'd like an introduction that while rooted in actual Science is aimed at laymen and available in many languages, go Here > http://www.solar-facts-and-advice.com/solar-energy.html
  11. Jun 24, 2014 #10


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    I think you have to define what "good" means. There is a table here showing the "energy density" for various methods of storing energy.


    Note that there are different figures for the "energy per kg" and "energy per litre". For some applications the weight of the energy store is more important that it's volume.

    Uranium has an energy density of 80,620,000 MJ/kg compared to just 142 MJ/kg for Hydrogen. Does that make Uranium 500,000 times better than Hydrogen?
  12. Jun 24, 2014 #11
    As far as I know, there is essentially no storage of electrical power anywhere, other than a few pumped storage facilities. Did you have something specific in mind here?
  13. Jun 24, 2014 #12
    You're right and I stand corrected regarding electrical storage, especially AC. The distribution system however, is second to none.
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