Exergy analysis — Definitions

The first and second principles of thermodynamics are essential "laws" for solving energy-related problems. The first principle stipulates the equality of the various forms of energy (thermal, mechanical, electrical, etc.) and leads to the examination of the energy flows to which the various systems are subjected, and then to the writing of the balance sheet that must translate the conservation of energy. However, while there is quantitative equality between the various forms of energy, the quality of the various forms of energy varies from one form to another, and even within a given form, and also varies according to the situation under consideration. For example, a megajoule of thermal energy at 1,000 ˚C does not represent the same energy "potential" as a megajoule of the same thermal energy at 20 ˚C. Similarly, the potential use of a megajoule of mechanical energy quickly appears, to the user who is the engineer, to be different from the potential use of a megajoule of thermal energy. While mechanical energy can be spontaneously transformed into thermal energy (by "degradation", for example), the opposite, non-spontaneous transformation requires a very precise procedure.

All these elements, linked to the quality of energy and the processes of energy transfer and transformation, make up the second principle of thermodynamics, also considered as a principle of evolution.

The physical quantity linked to this evolution is entropy, which is created as soon as operations take place outside of strict equilibrium, i.e. for all industrial operations which must necessarily have a certain kinetics if they are to take place in a finite time. Thus, the greater the imbalance in a process (heat transfer in an exchanger, for example), the greater the power involved, all other parameters being equal. While this may appear to be a very positive aspect, there is, as you might expect, a downside: high transfer kinetics are paid for by high energy "degradation" (spontaneous and irreversible transformation of energy deemed "noble" into heat) and high entropy creation.

So, for a very long time, entropy creation has been used by scientists to measure the degradation of energy caused by the irreversibility of energy transfers and transformations. However, for the engineer, accustomed to reasoning in terms of energy, i.e. joules, megajoules or kilowatt-hours, or even in terms of power, i.e. watts, kilowatts or megawatts, this measurement is impractical. Indeed, entropy, or its evolution over time, is measured in units of energy, or power, per kelvin (J · K ^-1 ; W · K ^-1 ). This fact is at least one of the reasons why the notion of exergy is so useful in dealing with these...