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Theoretical Limits of Alcohol Stoves


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  • #1230405
    Eric Fredricksen
    BPL Member

    @efredricksen

    Locale: Silicon Valley

    I've been playing with different alcohol stoves. Building them, researching them, etc. My main goal is minimal fuel to boil one pint of water (using some lightweight cooking set). Boil time isn't much of a concern for me.

    The real competitor to alcohol stoves in terms of fuel weight are Esbit stoves; they'll boil a pint using a 14 gram fuel tablet, and can probably be marginally lighter than an alcohol cookset. (The negatives are filthier burning and the cost of the fuel.) So it's my goal to boil a pint with 14g of alcohol (by which I mean ethyl alcohol, aka ethanol). Ethyl alcohol has a density of 0.789 g/cm3, so that's about 0.63 fluid ounces by volume. Quite doable, I'm sure, but I haven't done it yet.

    That got me thinking about the theoretical limit for a 100% efficient cookset. (And let me say right off the bat that I am not a chemist of any sort.) Alcohol burns per the reaction
    C2H5OH(g) + 3 O2(g) → 2 CO2(g) + 3 H2O(l) releasing −1409 kJ/mol. Once calorie is 4.184 J, and the molar mass of ethanol is 46.06844 g/mol, so that's 7310 cal/g of ethanol burned.

    Meanwhile, a pint of water is 472g. A calorie is (by definition) the energy need to raise one gram of H20 one degree Celsius. Let's assume we're at NIST standard temperature, or 20°C (68°F). At sea level, water boils at 100°C; higher up, boiling point goes down, but so does the starting temperature around dinner or breakfast time, broadly speaking, so we shouldn't be too terribly far off from what might happen up in the mountains.

    In any case, to raise 472g of water 80°C will take 37760 cal. A 100% efficient alcohol stove then would need 37760 cal / 7310 cal/g = 5.17g of alcohol fuel. The stove I'm looking for (which will burn 14g), then, would be 37% efficient. Stoves that consume 1 fluid oz of alcohol, or 23.3g (a fairly common case), are 22% efficient.

    There you have it – if you can boil a pint of water with 1/4 fl oz of ethanol, you've got a hell of a stove on your hands that's getting close to the theoretical maximum efficiency.

    (One minor wrinkle in all this is that the water produced by the combustion is liquid. I suspect that that water is inevitably vaporized during combustion, so that might need to be subtracted out of the energy produced by the combustion. If so, the heat of vaporization of water is 0.65 kJ/mol. Each mole of alcohol burned produces three moles of liquid water, so the net energy produced my burning ethanol would be 1409 kJ/mol – 3 * 0.65 kJ/mol, or 1407 kJ/mol. It's pretty much a non-issue.)

    #1445050
    Bob Ellenberg
    Member

    @bobthebuilder

    I read your post but skipped most of the #'s. I have nearly finished my newly refined set up (I have used alcohol for years) but it will be October before I get to run some test and then I will publish details and my findings here. But here is the crux of it: Conserving Heat = Conserving Fuel = Less Weight. Minimizing heat loss from the time you light your stove until you consume your cooked food or drink is the way to reduce heat loss and save fuel thereby saving weight." I think my newly refined setup is going to maximize every stage.

    I am using the Packafeather Featherfire (TRUE simmering and ability to snuff it out and recapture any unused alcohol) with a cone heat reflector that comes to the top of the pot snugly and thereby maximizing heat gain in the pot and minimizing heat loss up the sides and then doing thermos cooking with super efficent light weight insulated cups (MYOG–no commercial ones available). My initial impressions are that my already low fuel usage is going way down and I have saved some equipment weight as well over my previous set up.

    #1445054
    Lynn Tramper
    Member

    @retropump

    Locale: The Antipodes of La Coruna

    I think any 'snuffable' stove, especially one like the packafeather with nozzle to draw back unused fuel will be optimal. I've used this system with a Caldera Cone and got far better than 14g to boil a pint (can't remember off the top of my head as I was using metric units). I have found that the BEST fuel efficiency comes from slightly over-fuelling the stove….I don't understand the physics of this, but by putting in around an ounce of fuel, boiling, tsnuffing, then recovering unused fuel is the way to go IMHO (the Caldera just increases the heat transfer). This system also works with the AGG 12-10 stove, using the same snuffer cap and fuel recovery method.

    #1445063
    Tony Beasley
    BPL Member

    @tbeasley

    Locale: Pigeon House Mt from the Castle

    Hi Eric,

    I have done a bit on alcohol stove design and and efficiency, I heated 0.5l, water 80ºC using 9.3g ethanol.

    check this old thread out

    http://www.backpackinglight.com/cgi-bin/backpackinglight/forums/thread_display.html?forum_thread_id=12503&skip_to_post=91913#91913

    Tony

    #1445071
    René Enguehard
    BPL Member

    @ahugenerd

    Locale: Newfoundland

    You assume that your water is pure and deionized, which it isn't. It would therefore take more fuel to boil the same amount of "normal" water, making your theoretical lower limit higher. Complete combustion would be required, which can only realistically be accomplished by heating the fuel first, so you could expect a "heat up" period, bringing up your theoretical limit even higher.

    #1445080
    Lynn Tramper
    Member

    @retropump

    Locale: The Antipodes of La Coruna

    "You assume that your water is pure and deionized"..

    and at sea level, etc….S.T.P

    "which can only realistically be accomplished by heating the fuel first"

    Which is why, per ml boiled it is more efficient to heat larger amounts of water. Also, the size, shape (relative to the flame pattern of the stove) and material that the water is held in will have a big impact on efficiency. Something along the lines of a JetBoil may be the most fuel efficient (ie a heat exchanger and insulated pot), but not very light to carry. Then there's the question of wind, which is where a system like the Caldera Cone has advantages.

    Basically, the difference between what's theoretically possible and what is practical in the real world is huge!

    #1445088
    Eric Fredricksen
    BPL Member

    @efredricksen

    Locale: Silicon Valley

    Rene: Yeah, I do assume idealized water, which I should have mentioned. But I have no idea how to quantity the effect (or how to mine Wikipedia). I am clearly not a chemist, but I guesstimate there'd be a fairly insignificant difference.

    As for complete combustion, I'm even less of a chemist in this paragraph, but since alcohol burns so apparently cleanly, even from a tealight stove, it seems like the combustion might be pretty complete.

    Tony: Thanks; interesting info. Especially the part about the efficiency curve of the alcohol stove being pretty flat vs. boil time (with insulation). I didn't really expect anyone to have run these numbers before me but in retrospect that I should have.

    Allison: Hmm. Strange – the overfilling phenomenon doesn't make any sense to me. Perhaps has to do with the complete combustion question. Very interesting.

    Bob, etc: Yeah, I expect a Caldera Cone type thing to be a big efficiency win.

    #1445120
    Roger Caffin
    BPL Member

    @rcaffin

    Locale: Wollemi & Kosciusko NPs, Europe

    You haven't mentioned what sort of pot you want to use. If it's a narrow beer can, expect very low efficiency. If it is a wide heat-exchanger pot, expect far higher efficiency. But they are heavier.

    Cheers

    #1445190
    Tony Beasley
    BPL Member

    @tbeasley

    Locale: Pigeon House Mt from the Castle

    I also have seen evidence in my testing that surface finish/emissivity can be a factor in heat transfer into the pot and out of the pot.

    This paper by Berick 'heat loss in a cook pot at constant temoerature' is worth reading

    http://www.bioenergylists.org/en/taxonomy/term/840

    this is my post on stove/flame size vs pot size

    http://www.backpackinglight.com/cgi-bin/backpackinglight/forums/thread_display
    .html?forum_thread_id=11467&skip_to_post=85849#85849

    Tony

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