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This discussion relates to Technology Review's article More-Efficient Thermoelectrics.

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  ms
  

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  07/25/2008 03:58 PM

  efficiency

  The greatest fraction of heat energy you can turn into electricity is given by the temperature difference divided by the higher [absolute] temperature. (Example: moving heat from 25C to 0C, you'd be able to convert about 25/298 (about 8%) to electricity.) When we speak of efficiency, are we talking about the fraction of this maximum fraction, or something else?

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    rhapsodyinglue
    

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    07/26/2008 09:01 PM

    Re: efficiency

    Every time I read about waste heat harvesting, I hope the article is honest/rigorous enough to define the use of the "efficiency" metric they invariably cite.  None ever seem to.  I suspect you are correct that they are stating an efficiency relative to the thermodynamic limit.
    
    Far more impressive to talk about double digit efficiencies than to admit what small portion of the actual energy contained in the heat is getting converting into something useful.
    
    Perhaps the author could clarify?

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      Siphon
      

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      07/29/2008 06:34 AM

      Re: efficiency

      The author makes it clear that it's about heat-electric efficiency. What's important to realize is what the other guy said, that thermo-electrics efficiency varies over the temperature difference. What's needed is a heat to electrical converter that is inexpensive and operates efficiently at relatively low temperature differences, say 200 or less degrees Kelvin delta T. Thermo-electrics are not the best contender here, and won't likely be for many years to come.

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        HaPPI
        

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        07/29/2008 12:37 PM

        Re: efficiency

        Any idea how it's efficiency (present and speculative) compares to the Stirling engine?  I know they operate at lower temperature differentials, but with moving parts, albeit far fewer than ICE generators.

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          w.d.reynolds@att.net
          

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          07/30/2008 06:09 PM

          Re: efficiency

          The efficiency of the solar-Stirling engine is 31% while the current efficiencty of the best TE device is 3-5%. Even this paper would double that to about 6-10% that is far less than the Stirling engine.

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      Kevin Bullis
      

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      07/31/2008 02:29 PM

      Re: efficiency

      As Siphon says, it's heat-electric.

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  sfitzge308
  

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  07/29/2008 04:37 PM

  Brake retrofit?

  Could this material be inexpensively added to existing brake discs/drums or built into replacement brakes to extract some electricity from the plethora of waste heat? That sounds like an opportunity, a "partial hybrid retrofit" for existing vehicles..although no electric motor drive would be present, the alternator could perhaps be disconnected or removed, lightening engine load and improving efficiency, if the brakes can charge the battery... :  )   S

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  flyingmonster
  

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  07/29/2008 08:59 PM

  small aircraft

  So what is the weight of these on an exhaust system where they could replace an alternator? Small aircraft recip. engine plugs are energized by magnetoes but today's instruments do require a reasonable amount of power. Aircraft engines are generally run at 60% power or more so there is lots of excess heat and at any reasonable altitude Delta T with ambient air temp. is even greater. Another bonus would be improved reliability, something obviously more important for pilots than auto drivers. These, coupled with a Jumo junkers type diesel could be an aviators dream for reliability and efficiency. Call me anytime for a test aircraft.

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    sfitzge308
    

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    09/15/2009 12:19 AM

    Re: small aircraft

    I agree, like brake shoes, any internal combustion engine's exhaust system is another source of waste heat that's just "lying around"... even a small amount of electricity would be great, when obtained for free...

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  mobile karen
  

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  08/31/2009 05:01 PM

  Emerging technology

  If these thermoelectrics could be used to fuse with current energy sources, could they lower necessarily fuel needs by around 35%?

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