Exergy

what we actually live of


På svenska

The Second law of Thermodynamics tells us that the quality of energy is degraded every time energy is used in any process.

This 'energy quality' has been named Exergy. Thus, another name for the Second Law could be the 'Exergy law'

The amount of energy in the universe remains constant (First Law), but exergy is constantly used up (Second Law). In the end (very long time from now), exergy is used up in the universe, and no processes can run.

From the Unified Principle, we can also derive that the most efficient pathway for exergy consumption available, automatically will be chosen

The energy of the universe is constant, but exergy is canstantly consumed. This can be compared with a tooth-paste tube: When you squeeze the tube (= conduct any process) the paste (= exergy) comes out. You can never put the paste back in the tube again (try!), and in the end you have only the tube itself (= low-exergy energy) left.
When you squeeze the tube, the depressions (= entropy) will increase. (The entropy of a system increases when exergy is lost) But you can never take the depressions in the tube and 'un-brush' your teeth. (I.e. entropy is not negative exergy.)

When you buy energy from the electricity network, you actually buy exergy. You can find the energy as room temperature heat after some time, but you can not take that room temeperature energy back to the electricity company and ask for money back. They won't accept it.

For the more scientific-minded, the following definition of exergy (E) might be used:

Where U, V, S and ni stands for the extensive parameters of the system, (that is internal energy, volume, entropy
and number of moles)
while po, To and µo stands for the intensive parameters of the environment (pressure, temperature and chemical potential)
and eq stands for "in thermodynamic equilibrium with the environment".
This equation shows that the exergy includes extensive as well as intensive parameters, i. e. amounts of energy and potentials.
It is important to note and understand this fact.

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Updated:
2013-01-06