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Homework Help: Thermoelectric Compost Generator to Charge Batteries

  1. Jun 11, 2015 #1
    Hello, new to the forums. I am in school, as an Ecology major, but recently endured a long-awaited realization: I LOVE to build things! In my free time I find myself (attempting) figuring out how to build TEGs for our school's compost pile and creating all sorts of fancy aquaponics systems. I am taking physics and math courses for the remainder of my stay as an undergrad, and even staying longer due to this, in the hopes to get accepted into an ERE program after my bachelor's program is finished. We'll see. ;)

    This question isn't necessarily a homework question, but I figured belongs here as to not produce unneeded noise in other sections. I am too excited for this to wait for school to start up again to attend a physics club meeting so I figured I'd ask here: Note: While experimenting with these devices, I accidently spilled water on my laptop and.. it's dead. Fantastic. My responses may be a bit delayed as I need to ride my bike to the uni library.

    1. The problem statement, all variables and given/known data

    Two TEC modules(DC) - 12V ~15Vmax
    Pb-Acid battery(DC)- 6V 4.5A.

    The TEG module output is relative to the heat differentiation, as you probably know.

    My problem is that with my limited knowledge, I cannot figure out how to set up this system, or how to change it, in order to charge the battery with these 12V TEC devices. Upon testing the output of both of the TEGs(together), I found I can build up to around 1.5V output. (~32degreesF / ~130degreesF | Large compost piles can become 160-180 degreesF) - Sorry about this being in Fahrenheit.

    In short, can I charge a 6V battery with a 1.5V source? I know that a simple circuit will not, because the voltage potential is higher in the battery. In fact, if I tried, all I would end up doing would be creating the Peltier effect instead of the Seebeck effect. But If I connect a 6V battery to 1.5V source, the TEG's would use the power (and heat up my compost, which is a plus), and the battery would lose it's potential, correct?


    Once the battery drops to 1V, let's say, wouldn't the potential difference switch and the TEGs would charge it back to 1.5V or so, creating an equilibrium? Leading to another question: If I connected a battery to a battery, while the first battery charges the second battery to 1.5V, it loses voltage, so the TEGs connected to the first battery can continue to charge it, while it loses power to the second(or third, fourth, etc battery, etc.), eventually having a decent amount of charge in the combined batteries to use to light the bulbs? The same as using low voltage batteries, maybe?

    My second thought was interesting. What if I were to place a resistor(or light bulbs) between the battery and TEGs(TEGs--->battery --> Resistors -->TEGs) to create voltage drops so much so that the output of the TEGs may override the input? This is where my inexperience will surely shine.

    Since TEGs creating varying amounts of V, would the addition of a capacitor be of any help? Maybe switching to AC, going through a capacitor(from my understanding, AC will by itself discharge?), switching back to DC... but that sounds like a lot of lost energy, something this system can't really withstand. Could it fill the "peaks and vallies" creating by the TEG output? Can a capacitor store more energy than the output is releasing?

    I'm in over my head for sure, but am enjoying this so much so that I can't wait to continue studying. I apologize for my lack of terminology and such...

    Thanks so much for reading! I hope I can, with help, figure this out. It must be possible!!!
    I'll check back soon,
    Connor John Schmitz.
  2. jcsd
  3. Jun 11, 2015 #2


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    Connect two TEG's in series, and you get 2*1.5V = 3V.
    It's not complicated to make an electronic circuit that steps up these 3V to say 9V, but:
    When you load the TEG's, the warm side will get colder and/or the cold side will get warmer. Thus after a short time the TEG's will yield 0V.
    It's stated that if you can make a "machine" just running by thermal energy (heat), then you have made a perpetual machine of 2. degree. Put it into a vulcano and you have electricity the rest of your life.

    You can make a "machine" producing electricity if you heat it (on one side) and cool it (on the other side) at the same time. You can heat your TEG's in the pile, but you cannot cool them (unless you with patience sit and cool them with ice cubes).

    In short: It will not work without cooling.
  4. Jun 11, 2015 #3
    Ah. However- using a liquid to cool the other side- especially during the winter would work- no? I was thinking clear ammonia(and is not dangerous if spilled) since it has such a low freezing point for these northeastern winters.. we have so much compost heat is generated throughout the year.. Which actually would set the parameters even more apart than my experiments. Say, 0F to an internal 120F. Quite the temperature differential! Hah I can hope it gets that hot though. Ammonia is also one of the only liquids I could find that has a higher specific heat capacity than water if I remember correctly from chemistry. Wouldn't freeze but would cool one side while the compost internal energy being produced by the chemical reaction heats the other? Or even experiment with heat sinks and CPU fans to help get heat away from the cool side.

    Thankfully, compost generates a constant amount of heat, always providing the system with much more energy compared to the outside air. A well suited thermo-conductor on both sides( I am not sure if that is an actual term ) would relay these temperatures to a well placed TEG, no? One into compost and one sticking up into air/ammonia bath? the compost pile will always be warmer than the outside environment/ammonia. what if the surface area of the ammonia was large and better at losing it's heat to the colder environment carried through the TEG.. Hmmm I enjoy so much- this thinking. Visualizing, guessing and experimenting within your own brain world..

    As for the machine. In this scenario I don't personally perceive that the modules are acting as a machine but instead are utilizing the chemical machine that already exists- the compost pile that creates the heat. It's (attempting) to utilize natural machinery and use its byproduct for the creation of heat. I suppose by definition... But it's a weak man who doesn't question authority;^).
    What I am trying to create is a mere addition, too modest to deme a machine underneath what evolution has built over life's Uh, lifetime. Heh.

    Nature has done the tough work for us really imagine trying to invent the process by which organic matter decomposes? hehe if he's out there- Id like to meet this master of engineers. (Oh yeah! I have already, duh! I am myself coded by such a similar substance and such a process heheh) ANYWAY..

    These TEGs I bought are weak and cheap and were for my amusement and thought id humor myself by attempting to fix them to the compost pile. I can do this- one thing I've learned from learning about science is that when you ask yourself, "this will not work, will it?" the only correct answer to that question is, "not until you figure it out." But I think maybe some stronger modules/more are necessary for this task. In the meantim:

    Thanks for your input! Unfortunately it's a measly 1,5V for both. Hehe. But like I said they were cheap. I guess im still confused about how to charge a battery with a lower voltage source, though. I was wondering if you could explain that further? What about from 1,5V to 3V? I could light an LED with that kind of intensity! Heheh good fun, all this learning.
    Connor john Schmitz
  5. Jun 11, 2015 #4


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    Yes, if you cool one side sufficiently it will work ( maybe with higher voltage than 1.5V ).

    The electronic voltage step up circuit can be made as simple as:
    You must make a small circuit giving pulses to the transistor, T. Above the transistor you see a transformer with primary windings, L1, and secondary windings, L2.
    When T is switching current on, current will be drawn through L1, which magnetizes the transformer. When T switches the current off, voltage will be induced in L2.
    If number of turns in L2 > L1, then voltage on the secondary side will be stepped up, compared to the voltage on the primary side. The circuit will fit into a matchbox.
    Last edited: Jun 11, 2015
  6. Jun 12, 2015 #5


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    Just to add that the circuit is called a DC to DC step-up converter. Google it. You can buy modules.
  7. Jun 12, 2015 #6
    This is perfect! Thanks very much for your help. I look forward to one day being able to help answer questions on this forum.

    I will look for higher output TEGs. I don't need much- If I can get 3-6V output, and then step it up, that would be enough for my needs I think! Maybe more, depending on the cost and quality of the TEGs.

    I appreciate your time.

    Thanks for giving a name to the image, CWatt- but trying to build it sounds fun, too ;^)

    Connor John Schmitz.
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