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Thread: How many watts/sq inch?

  1. #11
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    Sep 2008
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    159
    BeaverRat,

    You don't want to mix up power density, current density and power dissipation. They are all different things and are not interchangeable. Power density is a volumetric variable that is measured in watts per cubic inch, centimeter, liter or whatever volume metric you want to use and has nothing to do with surface area. There is also weight based power density, but that is something else. Surface area is only used to calculate current density and power dissipation.

    Unless you want to try and predict how hot your reactor will get, all you need to be concerned with is current density using only amps per square inch and the active surface area of one face of one plate. 0.25 AMPS (not watts) per square inch is the recommended operating parameter to limit plate degradation and 0.5 amps per square inch is the recommended absolute limit.

    For our purposes, heat is not a real factor in plate degradation and it can get very complicated trying to figure in all the other necessary variables required to determine how hot your reactor will get such as ambient temperatures, air flow, thermal condutivity of all the materials and the percentages each material will contribute to the whole heat dissipation picture. You don't even want to try and go down that road.

    Looking at your opening post, your predicted 50 amps will be divided among 4 stacks with 12.5 amps going to each stack and you will have a surface area of 30.25 square inches...so 12.5 amps divided by 30.25 square inches gives you a current density of 0.41 amps per square inch. That's below the absolute limit, but well above the recommended safe limit. With your configuration you should shoot for a total amp draw of 30 amps or so to play it safe.
    "Sell your cleverness and purchase bewilderment"

  2. #12
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    Jan 2010
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    141
    So power density doesn't matter... only current density shows the deterioration status of the plates?

    If I configured the cell as

    -NNNN+NNNN-NNNN+NNNN-NNNN+NNNN-

    That is 31 plates with each one a reactive area of 5.75x5.75 inches (33 inches squared) (will 1/8th inch gaskets work?)

    50 amps split between 6 stacks is 8.33 amps per stack. 8.33/33= amp density of .25

    This setup can safely run 50 amps at 12 volts, correct?

  3. #13
    Join Date
    Sep 2008
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    159
    That's right. More than likely, however, you will probably have about 13.2 volts most of the time which will give you 2.6 volts per cell and with that I expect it to pull more than 50 amps. You will need a PWM, but with that voltage per gap, I'm guessing 1/8" gaskets should work just fine. Depending on your vehicle's electrical system, you could see voltages and amp draws even higher than that.
    "Sell your cleverness and purchase bewilderment"

  4. #14
    Join Date
    Jun 2009
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    Fort Worth, TX
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    Well, power density does kind of matter, but not the density you're thinking. The voltage per gap isn't what is important, but the voltage potential from plate surface to electrolyte. The REAL power density is this voltage potential, times the current.

    However, since you're using stainless, the voltage potential is going to be a constant, so the equation simplifies down to the only changing variable - current density.

    In other words, what IM2L844 and Larry have said... LOL

  5. #15
    Join Date
    Jan 2010
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    It actually will be exactly 12 volts (not 13.8) because it is running off of a converted ATX PSU. That is why I am using 4 Bi-polars instead of 5.

    Oh, and I did it again... I meant 1/8th gasket width to allow for a reactive area of 5.75x5.75 versus 5.5x5.5. Not Gasket thickness.

  6. #16
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    Jan 2010
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    Ok, I am even more confused now. Larry mentioned power density as to not go over 1 watt per inch squared of total plate area including both sides of the plates. IM2L said that you only need to worry about the current, not the watts. IM2L said to not go over 1 amp per 4 inches squared on one face. WHich is correct, 1 amp per 4 square inches or 1 watt per 4 square inches? They are completely different.

  7. #17
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    Sep 2008
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    Anchorage Ak
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    Quote Originally Posted by BeaverRat View Post
    Ok, I am even more confused now. Larry mentioned power density as to not go over 1 watt per inch squared of total plate area including both sides of the plates. IM2L said that you only need to worry about the current, not the watts. IM2L said to not go over 1 amp per 4 inches squared on one face. WHich is correct, 1 amp per 4 square inches or 1 watt per 4 square inches? They are completely different.
    From all my testing 1 watt is the rule of thumb. 1 amp is very different depending on voltage.

    Larry
    2008 Nissan Frontier 4X4 Nismo. 12 MPG baseline with my normal commute and heavy stop and go daily driving. Generator installed and working on 3/29/2009

    Up to 14.5 MPG with no enhancers. Still testing the effects of lots of HHO and no electronic enhancers.

  8. #18
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    Jan 2010
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    How exactly do you calculate your power density?

  9. #19
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    Jan 2010
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    Shane, did you mean power? because amps X volts is watts. What method is used to calculate how many plates you need? Power density, or amp density?

    IM2L says to use current density (amps/inch squared) to determine how many amps can travel through a certain cell without damaging i. The amps in one stack, divided by the surface area of one plate face. For instance, If you have a cell with plates that are 1x1" (excluding gaskets) configured as -NN+NN-NN+NN- running off at 12 amps, then the current density would be 12 amps divided over 4 stacks, or 3 amps per stack divided by the surface area of one plate face (1 square inch). So, it would be 3 amps per square inch which is terrible, he recommended .25 amps per square inch.

    HHOPWR says to use power density (watts/inch squared) to dictate how much amp draw can go through a cell without deteriorating it. You achieve this by taking the total surface area or (total plates - 2) X (Surface area of one face) X (2) + (2 X one plate face surface area) and then doing Watts/Total surface area. He says to use 1 watt per 4 inches squared.

    Which one is correct?

  10. #20
    Join Date
    Sep 2008
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    159
    I don't know where this watts per square inch thing, in the context of the electrolysis of water, originated, but there are tons of reference materials from all the generally accepted authorities (i.e. Bob Boyce, Tero Ranta, et al) going back many years clearly stating that current density (Amps/sq. inch) is the correct metric to use. A quick search of Bob's Hydroxy group forums turned up more than 200 references over 4 years that agree with me, but I would suggest that you research it for yourself rather than taking any one person's word for it.

    Later, I'll see if I can find a couple of links to get you started.
    "Sell your cleverness and purchase bewilderment"

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