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How to turn thermal energy of room into work

  1. May 24, 2015 #1
    So I live in Florida and I don't use my air conditioner, so my apartment gets pretty hot sometimes. It occasionally gets over 90 degrees F.
    I had an idea of how to cool the room, but I'm not sure how feasible it is. My idea is a device that could turn that thermal energy into work, which should cool off the room.
    But I'm not sure where to start on this idea. Anyone have any ideas?
     
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  3. May 24, 2015 #2

    phinds

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    I have one. Post your idea so we know what you are talking about.
     
  4. May 24, 2015 #3
    I have no idea what the device would be or if it's even feasible, so I have no idea of one. That's my question; is it possible to turn the thermal energy of a room into work sufficient enough to cool the room down to a more comfortable temperature?
     
  5. May 24, 2015 #4

    phinds

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    Well, thermodynamic was my worst course in college 50 years ago so I may be missing something but I think to do work with heat energy, you have to have heat differential.

    SO ... you could buy an air conditioner, use it to feed a heat exchanger that sucks heat energy out of the room and then use that energy to power an air conditioner that actually cools the room. Uh ... you see how that sounds ?
     
  6. May 24, 2015 #5

    Baluncore

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    You will not solve this challenge without understanding temperature and entropy. Cooling the air will release energy, but it costs more to get the associated reduction in entropy. If it did not, refrigerators would not need a connection to the power supply. An air conditioner needs about 1kW of power to generate the equivalent of 3kW of cooling. There is no such thing as a free lunch.

    If the air in the room close to the ceiling is hotter than outside air, install a vertical stove pipe chimney to drive air circulation which will draw cooler air in through the door and windows.
     
  7. May 24, 2015 #6
    I'd bet that the 90 degree air is quite hot compared to the soil 15 feet below your home. That seems like a temperature differential to me. Good luck getting much work out of it though, if anything you'd be better off lining your inside walls with a nice heat conductor, say iron, which continues deep into the ground followed by thick insulation on the outside. Call it the heat (cold) shield.

    Or you could just buy and install an air conditioner.
     
  8. May 27, 2015 #7
    So the thermal energy of my room can't be used to power anything, even though there is energy present? Solar panels generate electricity from photons colliding with it. In my room there are air molecules colliding with objects. Why can't those collisions be used to generate electricity?

    Thanks for the replies.
     
  9. May 27, 2015 #8
    Fundamental contradiction to the laws of thermodynamics. Only free energy can be used to do work (hence the term free energy).

    Consider this: what if you could use the random thermal energy at a single temperature to generate electricity? Then do some work with that electricity and everything goes back to heat. Free work for ever by conservation of energy. This is a perpetuum mobile.

    Edit: So basically, you need a thermal gradient to deplete.
     
  10. May 27, 2015 #9
    I thought about that, but since my computer and stove are what heat my house, I figured that's what would be powering the machine. Or if it's a hot day, then the sun is indirectly powering it by heating my room. If the heat is being generated by my appliances or the sun, then it's not free work.

    So what you're saying is that for thermal energy to power something, it has to be set up like a battery's potential difference, but it would be a thermal difference instead. I guess you would be able to generate electricity through the movement of the heat from the hotter position to the colder position.
     
  11. May 27, 2015 #10

    NascentOxygen

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    Fans to move the hot air out of your roof space might be the first step. Fans powered by solar energy could work here (intercept some of the solar energy and make it do work before it heats your apartment).

    That way, you wouldn't have blistering hot air baking the upper side of your ceiling, so that's got to improve conditions inside.
     
  12. May 27, 2015 #11

    phinds

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    Yes, that is exactly what I said in post #4. You have to have a heat differential to get any work out of heat.
     
  13. May 27, 2015 #12

    Baluncore

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  14. May 28, 2015 #13
    Thank you for that link to the Brownian ratchet. That explains why this wouldn't work. Here is the law that it breaks:
    "It is impossible for any device that operates on a cycle to receive heat from a single reservoir and produce a net amount of work."

    And the explanation makes sense.

    "Although at first sight the Brownian ratchet seems to extract useful work from Brownian motion, Feynman demonstrated that if the entire device is at the same temperature, the ratchet will not rotate continuously in one direction but will move randomly back and forth, and therefore will not produce any useful work. The reason is that the pawl, since it is at the same temperature as the paddle, will also undergo Brownian motion, "bouncing" up and down. It therefore will intermittently fail by allowing a ratchet tooth to slip backward under the pawl while it is up."

    But it's being treated as if it's getting free energy, when I don't understand why. Something is heating up the fluid that the paddle is immersed in, so the ratchet is being powered indirectly by whatever is heating up the fluid; be that the sun or otherwise. And that's where my idea comes in; the fluid loses energy by doing work, and that's how it cools off. Even if it's moving the paddle forward and backwards netting a zero displacement, it's still having to do work to move it, right? So it would have to be cooling off.
     
  15. May 28, 2015 #14

    NascentOxygen

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    If turning in one direction performs work outside, then turning in the other allows work to be done on this enclosed gas, so no net movement means no net change in energy.
     
  16. May 28, 2015 #15
    No, nothing is heating up the fluid. If a molecule bounces off of the paddle to move it forward, then that molecule will get a backward impulse. If a molecule bounces the paddle backward, then it will get a forward impulse. If the paddle has no net movement, then the molecules have net impulses that add up to whatever was before -- the same amount of movement on a molecular level, i.e. the same heat content and temperature as before.

    The paddle wiggles the molecules back.
     
  17. May 29, 2015 #16

    jack action

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    Let's see how much energy you can retrieve from your room:

    Let's imagine you have a 1000 ft² apt with 10 ft ceiling giving 10 000 ft³ of air (283 m³). The temperature is at 90°F (305 K) and you cool it down to 39°F (277 K), the temperature of the coldest heat sink I can't think off, i.e. deep underneath the ground or a large body of water.

    The specific heat of air is 1.005 kJ/kg/K and its density is about 1.16 kg/m³. This gives you 1.005 * (305 - 277) * 1.16 * 283 = 9238 kJ of energy stored in your room that you can recover. But to recover that energy, you will need a heat engine. Maximum efficiency possible is the Carnot efficiency which is 1 - 277/305 = 9%. A more realistic maximum efficiency would be the endoreversible efficiency, which is 1 - sqrt(277/305) = 4.7% (see heat engine efficiency for more info). Using this last efficiency, this means that you can expect recovering only 434 kJ of the 9238 kJ calculated earlier, or 120 W.h. This is the equivalent of filling a 12 V, 10 A.h, battery.

    This is if you «empty» your apt once, say, at the end of the day. If you do so, the room should be at 39°F at the end of the process. But because the room cools down, the engine efficiency will drop as well, further reducing the energy you can recover.

    If you do this during the day, while the sun, computer and stove heat your apt and you maintain the temperature at 90°F, how many batteries you can fill with your heat engine will solely depend on how much heat the sun and appliances produce in your apt. You could also keep your apt at a cooler constant 70°F as well, but in this case, you would have an even smaller efficiency for your heat engine.

    Such an engine would probably be very costly, if feasible. Engines with small temperature differential cannot usually handle a lot of power, i.e. how much energy they can transform per unit of time. So it would probably take a very large engine to remove enough energy to cool your apt. Some engines that might be used for this could be a thermoelectric generator or a Stirling engine.

    A better way that is actually used with that concept is geothermal cooling, which is a more efficient air conditioning (so it doesn't give you free work, but it costs less energy).
     
  18. May 30, 2015 #17
    What makes you feel cool is both temperature and air movement. if you create the desired convection cycles to power anything at the same time focus on air movement because that's the impact of a fan. no reduction in temperature directly but the evaporation of your body sweet will do the cooling.
    Another very easy and efficient way to cool you room is to similarly evaporate water.... it's incredibly effective and super cheap. draping your room in water absorbing materials will cool the room... the temperature difference can then be used to drive the Stirling engines mentioned before. Depends on the architecture of your room/house. If you have a water reservoir under the room and a heat out let at the top you can use the temperature difference to drive pressure differentials to drive a piston or rotary functions which can be harnessed to do work... So yes, where ever there is heat or energy you can use this to cool the room and produce useful work. It's all about designing the endless flow of energy according to the know paths it likes to take. It's all simple like flowing water...
     
  19. May 30, 2015 #18

    OmCheeto

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    Interesting.

    I've never considered desiccants as a means of air cooling. Open to the air that is.
    I used to be familiar with how Lithium Bromide coolers worked, long, long ago. I suppose it works on a similar principle.

    Of course, this violates the rule of the OP's question: How does one extract energy from a thermally homogeneous environment? I don't think it can be done.

    But 90% less energy is pretty cool. I have a 700 watt AC unit which keeps my bedroom quite comfortable in the summer. That would be really neat to have it run on only 70 watts. That's less than half of my solar capacity. :smile:
     
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