Available Energy (Exergy) Calculation

I am stuck in between a problem. Part of where I am doubtful is as follows.

400KJ of energy is transferred from a source at 1000K to a 2 kg gas contained in an enclosed vessel. The gas temperature is 350K and atmospheric temperature is 300K. What is the available energy of gas?

The solution is-
1. Calculate change in entropy of gas (ds).

2. From that calculate unavailable energy= 300*ds.

3. available energy= 400-300*ds.

But what I think is- available energy= Energy of gas at 350K+400-300*ds.

where 300=atmospheric temperature.

#-Link-Snipped-# sir can you help?

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  • Ramani Aswath
    Ramani Aswath

    @ramani-VR4O43 • Nov 15, 2012

    Since the gas is already at 350 degK, it already has some available energy = 2 x (350-300) x sp.Ht.
    You have added another 400kJ.
    Add this to the above, which should give you the total available energy.

    Have I missed something here?

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  • ISHAN TOPRE
    ISHAN TOPRE

    @ishan-nohePN • Nov 15, 2012

    bioramaniSince the gas is already at 350 degK, it already has some available energy = 2 x (350-300) x sp.Ht.
    You have added another 400kJ.
    Add this to the above, which should give you the total available energy.

    Have I missed something here?

    Yeah, this is what I think too, but the text says do not consider energy of system i.e., (350-300)*sp heat. Calculate from 400KJ.

    Will check once again and clear the confusion.

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  • Ramani Aswath
    Ramani Aswath

    @ramani-VR4O43 • Nov 16, 2012

    IssueYeah, this is what I think too, but the text says do not consider energy of system i.e., (350-300)*sp heat. Calculate from 400KJ. Will check once again and clear the confusion.

    Usually available energy refers to the energy available above the ambient temperature, since this can be harvested. If this is correct then mCpdelT cannot be ignored.

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