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Harnessing the power of heat? [discussion]

  1. Aug 2, 2010 #1
    I've only taken an introduction to physics classes so excuse any ignorance on my part.

    Recently, I've been wondering why all we have figured out is an indirect way to harness the energy of heat.

    My reference is to nuclear power mostly; We use the enormous amounts of heat to create steam from water and then use the steam to turn a turbine which powers a generator. I am sure this indirect way loses most of the energy and all that waste heat (the steam) is just going up in the air.

    I have searched for answers on google for a few hours and I have came across another indirect way to harness the power of heat using a thermocouple (junction between two different metals that produces a voltage related to a temperature difference). It's my understanding that this process normally produces minuscule results in general.


    When I think of a fire, it is so hot; it seems to be so much energy packed into it. To think all we can get from this overload of energy is a few watts is disappointing.

    I got to thinking; Heat is basically the "vibration" of molecules in an object. When I think of vibration, I think of movement, and with movement comes kinetic energy (ke = 1/2mv2). So, based on this reasoning lets take a 1KG metal weight and heat it up so its bright red. Lets assume the total mass of an object refers to ALL the molecules in the object. The velocity would be very very small, because we don't see the object move. KE = (1/2)(1 Kg)(1-1000m/s)2 .... This basically comes out to basically nothing. One would think something that is THAT hot would have some form of energy, yet its kinetic energy is almost nothing. So I am thinking you can't really apply kinetic energy to this and maybe thermal energy isnt based on kinetic energy, or atleast not that equation. However, reading this page: http://www.school-for-champions.com/science/thermal_energy.htm confuses me. It states: "Thermal energy is the total internal kinetic energy of an object due to the random motion of its atoms and molecules. It is sometimes confused with internal energy or thermodynamic energy. They consist of the sum of the internal kinetic energy (thermal energy) and the potential energy of an object.".

    What is this energy called from the movement of all the molecules when an object heats up?

    How do we measure this energy?

    How can we harness it more directly?

    All replies and discussions are welcomed, as the point of this post is for me to learn and get a better grasp of physics.
  2. jcsd
  3. Aug 2, 2010 #2
    Because it is impossible. Its not that "we haven't figured it out", actually it was figured out in ~1826. These are equivalent statements:

    It is impossible to convert heat completely into work in a cyclic process.

    Heat generally cannot flow spontaneously from a material at lower temperature to a material at higher temperature.

    In a system, a process that occurs will tend to increase the total entropy of the universe.

    In the neighbourhood of any equilibrium state of a thermodynamic system, there are equilibrium states that are adiabatically inaccessible.

    It is impossible for a heat engine to produce net work in a complete cycle if it exchanges heat only with bodies at a single fixed temperature.

    Or equivalently: No engine operating between two reservoirs can be more efficient than a Carnot engine operating between the same two reservoirs, which is always less than unity.

  4. Aug 2, 2010 #3


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    Radio active plutonium buttons that generate heat and thermalcouples used to convert that heat into electricity are used on some satellites, mostly ones that leave earth orbit where solar panels are not a reasonable option. Essentially this is a battery with a very long life time.
  5. Aug 2, 2010 #4
    But thermocouples are very inefficient and also limited by the 2nd law. We have a professor working on those and another professor working on solar panels in our MSE department.
  6. Aug 2, 2010 #5
    Your replies are a bit confusing to me. I understand I probably asked alot of questions in the discussion, but in general my main questions were in bold at the bottom (thats why they were in bold).

    What's impossible? I think you are saying that its impossible to convert all of the heat energy. All being the keyword, as expressed by "It is impossible to convert heat completely into work in a cyclic process." which is expanded on Wikipedia further stating "Thus, a heat engine with 100% efficiency is thermodynamically impossible."... Which makes sense and I understand. I am just asking why there hasn't been a more direct way to harness SOME of the energy from heat besides thermocouples or using steam and in a better way.

    Yes. I know. I am not sure what your trying to state here, I guess it might of been a rebuttal to Curl. I stated in the original post that I knew of thermocouples.

    I guess what I am saying is couldn't there be a way to actually use the kinetic energy of the molecules themselves? For instance, those flashlight that you shake and the magnet or what not goes through the copper wires (I am not 100% how it works) but this is done via shaking the flashlight (vibrating it). Couldn't something like this be done in a nano scale where we could use the vibrations of the molecules to do work and thus create energy.
  7. Aug 2, 2010 #6


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    How is a thermocouple indirect? You convert heat directly to electricity - what's indirect about that?
    You misunderstand. Heat is kinetic energy. The problem is that it is all random. All of the molecules in an object are essentially vibrating in different directions so the motion of the object as a whole itaverages out to zero net motion. But the energy is still there.
  8. Aug 2, 2010 #7


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    Staff: Mentor

    The OP didn't say convert completely, s/he said convert directly. To me that just means without intermediate steps, not with 100% efficiency. Though s/he is confused enough about how energy works it is tough to know for sure what the goal actually is here.
  9. Aug 2, 2010 #8
    Ah yeah, I forgot about that. I am thinking of speed and not really velocity when I said it's not really moving. The net distance is just zero, or very small. Understandable.

    Also thermocouples are a lot more direct than using steam. I just figured it would still be indirect because you are using metals as well. When I commented on my vibrating idea, I understand there has to be some intermediate you can't just turn heat into electricity magically. So Yeah, thermocouples are pretty direct. Sorry.

    Indeed. I am talking about directly not completely. I do admit that I am confused on how energy works but I am just taking an ordinary look at things and thinking that with electricity comes heat, so why cant with heat come electricity. I guess when I think of energy I think of mostly electricity, when its not just that.

    The main goal is to answer the questions I had in bold.
  10. Aug 2, 2010 #9
    You don't have to apologize for being ignorant - at least you are thinking about things. To get started in finding answers, get a good book on "Thermodynamics." Start with an easy one and then find a harder one.
  11. Aug 2, 2010 #10
    More research has led me to this discussion: http://hypography.com/forums/physics-and-mathematics/5002-electricity-from-heat.html

    Which is basically the same discussion.
    One mention powerchips: take a look at the FAQ:
    http://www.powerchips.gi/faq.shtml and http://www.powerchips.gi/technology/pcalc.shtml

    "For comparison purposes, a typical thermoelectric (Peltier device) generator has a Carnot efficiency of about 5-8%. A Rankine cycle generator, such as a gas turbine, has a typical Carnot efficiency of about 30%, while a diesel or gasoline generator is only about 10-15% efficient in Carnot terms. Power Chips are projected to achieve 70-80% of Carnot efficiency."

    .. They even state to have real devices built. What is your impression on the site and theory behind it? (I know it probably isn't true, because it would have gained more attention; just want to discuss if it would be possible).
    Last edited by a moderator: Apr 25, 2017
  12. Aug 2, 2010 #11


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    Just pointing out an example of radiation and thermcouples being used as a battery on satellites, without any implication about the efficiency of such devices.
  13. Aug 2, 2010 #12
    I also believe in possibility of extracting kinetic energy from moving molecules - according
    to something resembling PM2 (Perpetuum Mobile 2nd kind). I don't agree with arguments
    from those denying this possibility, like maintaining "Heat cannot by itself flow from lower to higher temperature" . At least at molecule level, a slower molecule may very well by collision increase the speed of faster molecule. Although I understand they mean statistically. But how can scientists state impossibility regarding such a complex issue?

    Also a steam engine may have appeared as an impossibility in the beginning: How can the piston return against the steam pressure and make a new stroke? Using a valve that trick was possible - but certainly not obvious before that machine was invented.

    An interesting machine, very much resembling a PM2, is the "Drinking bird" acting on
    evaporation. There we have a machine where everything is free and at equal temperature:
    The "bird", the water and surrounding air. But it works. A common objection is the temperature difference occuring at surface - and therefore no PM2. But that temperature difference is created by the system and nothing added. But the water, although "free",
    makes this not a true PM2. :eek:
  14. Aug 3, 2010 #13
    I think what you are looking for is "en.wikipedia.org/wiki/Pyroelectricity"[/URL]. These both cut out the middle man.

    The Power Chips site seems a little dodgy, but that that could just be me being paranoid and skeptical.
    Last edited by a moderator: Apr 25, 2017
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