Loss due to inefficiency

  1. I am just double checking an idea that I've always figured was true, but never really asked someone who knows about. In basically any machine that is not 100% efficient, the only "place" the lost energy (due to the inefficiency) can go is into its surroundings as heat energy, or radiation, correct? If this is so, how easily/effectively can radiation be contained and transferred back into heat energy? How is this done? (I assume just very reflective surfaces surrounding the thing that is emitting radiation)

    Thanks.
     
  2. jcsd
  3. Andrew Mason

    Andrew Mason 6,864
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    If you are talking about heat engines, principles of thermodynamics prevent you from using a portion of the 'waste' heat to do work. So a portion is inherently inaccessible. If you contain or capture it, you just make the process more inefficient. It has to flow to a cooler reservoir in order to get work out of the heat. That is not to say that the heat cannot be used for purposes of heating. This is frequently done for example, in your car, and in industrial co-generation schemes.

    AM
     
  4. Indeed, and that's what I'm referring to. So my initial assumption is true, though?
     
  5. Andrew Mason

    Andrew Mason 6,864
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    Yes. The unused energy (ie. energy that is not used to do work) is dispersed into the environment in various forms (sound, heat, radiation). If your question is how to convert heat radiation (infra-red) into heat, you just absorb it with something. Infra red does not have much penetrating power.

    The energy loss due to radiation is:

    [tex]E = \sigma T^4[/tex]

    where [itex]\sigma = 5.67051 \times 10^{-8} W/m^2K^4[/itex]

    So if the temperature (eg steam) is in the order of 400K, you are looking at a maximum of 1.5 kilowatts/m^2 of radiation loss, or about 2 horsepower.

    AM
     
  6. Clausius2

    Clausius2 1,479
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    A compressor can be assumed as an adiabatic machine. There are no losses by radiation nor heat conduction to the environment. But the compressor communicates an irreversibility to the flow. That's another loose we haven't mentioned: the proper increasing of entropy of the flow through a turbomachine or heat engine.
     
  7. Q_Goest

    Q_Goest 2,976
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    Just a thought here that goes along with your other post. If that "machine" is a compressor (or pump or expander), the energy could also go into your working fluid. I suppose one could also say that eventually it will go into the environment, but for the purposes of engineering, the energy going into the working fluid due to a machine not being 100% efficient is an important consideration.
     
  8. So basically there's only sound, heat, and infrared to take into consideration?
     
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