Hi Jesus,

Basically the Exergy destruction and exergy loss are two different tersm, however, they are being used for same meaning sometimes. Any ways, exergy destruction rate whithin a process (lets say evaporation, condensation, ... ) is related to Entropy generation rate via Gouy-Stodla theorem as        Ex_dot_dest = T_0 * S_dot_gen                    where T_0 refers to your systems dead state temperature. These two terms are not the same, but related and represent eachother.

There are two ways to calculate your exergy destruction way through control volume concept for a stream:

1) Exergy balance equation: Ex_in + Ex_thermal_in = Ex_out + Ex_dest + Ex_thermal_out + Ex_dest

2: Entropy balance equation: s_in + Q_in /T_0 + S_dot_gen = s_out + Q_out/T_0

and then: Ex_dest = T_0 * S_dot_gen from above eqaution.

Formulas seem to be easy, they are, but there are many important point you need to consider to develpp your balance equations and also selecting you source and sind temperatures as T_0 for entropy balance equation.

Anyways, if you wanna learn more about exergy and entropy stuff, let me know, I have good resources to share with you.

Best Regards

Reza Soltani

Dear Jesus,

I'd like to add an important fact with a view to your question raised and a question I am about to bring up. Energy can be transformed from one form to another, but cannot be created or destroyed. So exergy as a transformed or processed form of energy can only be "re"transformed into thermal energy for example. I would say the ratio of proportionality is depending on the closed system itself and used materials and substances that have different energetic densities and pressure has an influence too.

Hi Jesus,

Basically the Exergy destruction and exergy loss are two different tersm, however, they are being used for same meaning sometimes. Any ways, exergy destruction rate whithin a process (lets say evaporation, condensation, ... ) is related to Entropy generation rate via Gouy-Stodla theorem as        Ex_dot_dest = T_0 * S_dot_gen                    where T_0 refers to your systems dead state temperature. These two terms are not the same, but related and represent eachother.

There are two ways to calculate your exergy destruction way through control volume concept for a stream:

1) Exergy balance equation: Ex_in + Ex_thermal_in = Ex_out + Ex_dest + Ex_thermal_out + Ex_dest

2: Entropy balance equation: s_in + Q_in /T_0 + S_dot_gen = s_out + Q_out/T_0

and then: Ex_dest = T_0 * S_dot_gen from above eqaution.

Formulas seem to be easy, they are, but there are many important point you need to consider to develpp your balance equations and also selecting you source and sind temperatures as T_0 for entropy balance equation.

Anyways, if you wanna learn more about exergy and entropy stuff, let me know, I have good resources to share with you.

Best Regards

Reza Soltani

Dear Axel,

I would like to point out that, Energy and exergy cannot be view from the same point. You are right about energy but, exergy CAN be transformed and also destroyed. Its why second law is different from the first law,if not, they could be the same, so, exergy is always destroyed cause there is no reversible process in planet.

Lets say, a compressor, shaft rotates and compresses air, the temperature increases cause of frictions, the total energy is same, just transfered from mechanical energy to thermal energy, but, exergy is destroyed, because that produced heat does not work for us to increase the pressure and only is an obstacle, so its useless for us, it decreases the available compression potential, so, it is exergy destruction. 

All the best

Reza

Entropy production can be obtained using common thermodymanis equation, and it can be used in both case exergy loss and exergy destruction, but they have different meaning:

Exergy destruction is an internal phenomenon, refers to the exergy destroyed due to irreversibilities within a component system (like irreversibilities inside a combustion, exergy destruction during the combustion process as function of engine operating and design parameters)

Exergy losse is seen as external irreversibilities (Transfer of exergy from the overall system to its surroundings. This exergy transfer is not further used in this or another system)

Entropy: A concept that is not a physical quantity

https://www.researchgate.net/publication/230554936_Entropy_A_concept_that_is_not_a_physical_quantity

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