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Question on thermodynamics

Former Community Member
Former Community Member
Hello everyone, I have a question on the energy cost of compressing gas.

I didn't find a community about thermodynamics, so I post the question here..


When gas absorbs heat. its volume increases from v1 to v2. When we compress the volume of the same gas from v2 to v1, does the energy required equal to the heat it absorbs? Does the temperature affect the energy required?


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  • Former Community Member
    0 Former Community Member in reply to Chris Pearson

    Lynton Mogridge:

    It sounds like you are running a sub-critical two phase refrigeration cycle. This is similar to your domestic refrigerator. If you look up refrigeration cycle on Wikepedia it will give you the details and the normal performance measure COP (coefficient of performance), more detail in Cengel and Boles Chapter 10.

    Lynton.




    Thanks, I found that the COP in air conditioner is about 3.x,


    If we use 1km copper line, radius is 900 mm2,

    At 20 deg c

    R = 0.0172*10^-6 * 1000 / (900*10^-6) = 0.0191(ohm)

    At 90 deg c, temperature coefficient of resistivity is 0.00393 at 20 deg c

    R = 0.0191*1.00393^70 = 0.02513(ohm)


    The current is 1000A

    P=1000*1000*0.0191=19100(W)

    P90 = 1000*1000*0.02513 = 25130(W)

    Psav = 25130-19100 = 6030(W)


    Per year, the energy saving is

    W = Psav* 24*365 = 52822.8(kw*h)   -- per kilometer


    To keep the line 20 deg c, the input power of compressor should be

    19100/3.5 = 5457.14(W)


    It seems it is worth implementing the solution.

    Am I right? Could anybody working in electricity domain point out the rough difference between the real value and the ideal value?

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