Applied thermodynamics - First principle. Energy. Enthalpy

Thermodynamics is one of the fundamental sciences. It can be applied to a wide range of situations, whether in mechanics, physics or chemistry. It enables us to analyze the transformations of systems as diverse as engines, bodies undergoing chemical reactions, and even living beings, in relation to their exchanges of matter and energy with the environment. More specifically, it can be said that this science is the basis of energetics, since it is used to describe and analyze the functioning of energy systems, and to seek out optimized ways of operating them.

The aim of the paragraphs devoted to thermodynamics in this article is not to give an exhaustive presentation, but to describe the formalism that can be used to analyze problems directly related to energy systems such as thermal machines and heat exchangers, or even their coupling. This objective excludes, among other things, the chemical aspects of thermodynamics, which are nonetheless useful in the study of combustion, for example. Readers interested in these problems will find the corresponding presentation in the "Chemical thermodynamics" articles of the Physics of Energy treatise.

The first principle of thermodynamics is both a principle of equivalence and a principle of conservation. On the one hand, it states that all energies are quantitatively equivalent, in particular thermal and mechanical energy. On the other, it states that, overall, energy is conserved: it can neither be created nor destroyed, which implies that any exchange of energy between a system and its environment must result in an equal variation in the energy contained in the system. On the other hand, this principle authorizes and enables us to study the transformation of one type of energy into another, using special systems such as electrical machines (motors or alternators – electrical/mechanical energy), thermal machines (motors, turbines, refrigeration machines, heat pumps – thermal/mechanical or electrical energy), etc. In contrast to the first principle, the second principle of thermodynamics highlights the difference in quality between the various forms of energy.

After recalling a number of definitions essential to a proper understanding of the presentation, this article shows how the principle of energy conservation derives from the (older) principle of equivalence and leads to a very broad concept. The third part focuses on thermomechanical transformations of closed systems (with no exchange of matter with the environment) and, in particular, links classical mechanical principles (kinetic energy principle) to the principle of energy conservation. The notion of enthalpy is introduced, a key factor in technical thermodynamics. Finally, the last section deals with open systems and the energy and enthalpy...