07/12/2010

From energy to exergy

jcv080100025.jpgThermodynamics is the science dealing with the study of matter, as well as that of all phenomena involving work, heat and energy in general. In particular, thermodynamics includes the study of matter taking into account the influence of temperature upon its characteristics.

It also examines the conversion of one form of energy into another (mechanical, thermal, chemical or electrical energy). Moreover, it deals with the design and the operation mode of thermal machines.


In addition to the general law of mass conservation, thermodynamics is based on four fundamental laws:

- the Zeroth Law, dealing with the notion of thermal equilibrium;
- the First Law or Law of Equivalence, which concerns the conservative character of energy;
- the Second Law or Carnot Law, which deals with the notion of irreversibility and the concept of entropy;
- the Third Law or Nernst Law, concerning the properties of matter near the temperature of absolute zero.

Thermodynamics may be approached either phenomenologically or statistically:

Phenomenological thermodynamics is based on macroscopic considerations. It seeks to establish macroscopic laws based on a reduced number of empirical laws and postulates that are derived from experimental observation. It also determines the specific properties of matter by means of measurements.

Statistical thermodynamics is based on molecular considerations and probability calculations. It seeks to establish the fundamental laws and to study in agreater depth the structure of matter in order to explain its properties in terms of more general laws of nature.

This book aims at teaching thermodynamics to mechanical engineering students and to allow engineers in industry to supplement or update their knowledge of the subject. For this reason, it deals essentially with the study of problems involving mechanical and thermal energy, and also predominantly involves phenomenological thermodynamics.

When Wisdom comes, her first lesson is, “There is no such thing as knowledge; there are only glimpses of the Infinite Deity.” Practical knowledge is a different thing; this is real and serviceable, but it is never complete. Therefore to systematise and codify is necessary but fatal. Sri Aurobindo

This book is the result of more than half a century of teaching and research in thermodynamics at the Swiss Federal Institute of Technology (Ecole Polytechnique Fédérale of Lausanne), known as the EPFL, in Switzerland. Its objective is to facilitate the teaching of thermodynamics to engineering students and to allow engineers from industry to update their knowledge in this domain. It deals primarily with the study of problems of energy conversion of chemical, mechanical and thermal energies in an approach known as phenomenological thermodynamics.

As it is well known, thermodynamicists are law-addicted people. But what helps them is that there are only four main Laws. The First Law of thermodynamics is a reassuring one for the engineer. It expresses the equivalence between work and heat and satisfies our wish for order and unity. But it is by far insufficient to characterize the notions of energy quality and of energy levels. It leads to definitions of losses and efficiencies that are not adequate for the evaluation of the thermodynamic quality of a process. The Second Law of thermodynamics brings the essential enlightenment to grasp the notions of irreversibility and of energy degradation.

The exergy theory simultaneously implements the First and Second Laws. It is now recognized that it is an extremely fruitful theory. Exergy accounting is the only way to accurately calculate the thermodynamic losses of a given process and to unambiguously define a thermodynamic efficiency expressing its level of perfection. It also allows for the evaluation of the thermodynamic quality of an energy system when considering energy policies and economics, independent of the size, complexity and the nature of the phenomena being looked at. That is why we devote particular care to exergy theory and to its generalisation.

At a time weighted with increasing concerns about the present and future energetic, environmental and geopolitical challenges, it is particularly vital to prioritize our technological choices towards a more rational use of our non-renewable as well as our renewable resources. This implies improvements of both our methodological and technological tools. From the methodological viewpoint a more rational and sustainable use of the available resources is only possible if engineers, architects, industrialists and decision makers can rely on coherent indicators among which the exergy efficiency is bound to play a major role. It is rather disappointing that in this beginning of the 21st century a major part of the practitioners are still using only performance indicators based exclusively on the First Law of thermodynamics. For example, simple boilers for house heating are labelled with efficiencies very close to 100% (apparent perfection!), while it is technologically possible, with each same unit of fuel, to provide about twice as much heat. Conversely, the exergy efficiency allows a coherent ranking of the technical options, with values always below 100%, independent of the domain and the energy service supplied. From a technological standpoint, the notion of exergy also allows a better characterization of the sources of internal losses, and therefore leads to better target designs and retrofitted projects.

The education of engineers in thermodynamics should be based on the three following conditions: the basic concepts as clear as possible, an accurate terminology and an efficient nomenclature. Encouraged by a strong wish for a logical, systematic and aesthetic approach, we have occasionally found it appropriate to propose formulations that deviate from tradition. Some new concepts or grouping of variables have been introduced. A special effort has been made throughout the years to establish a nomenclature as coherent as possible, since we agree with Leibnitz when he wrote: “In symbols one observes an advantage in discovery which is greatest when they express the exact nature of a thing briefly and, as it were, picture it; then indeed the labor of thought is wonderfully diminished.”

Our constant preoccupation has been to present – in a clear, precise and simple way – a complex science, related to a vast domain, which includes subtle concepts, and is often considered arduous. Finally, our ambition has also been to include some aesthetical considerations in this ocean of laws and formulas. Indeed we think that there can also be some fundamental elements of aesthetics in a scientific development. In science there are some daring schematic representations as well as particularly enlightening demonstrations which display conciseness and clarity.

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Les auteurs :

Lucien Borel a obtenu son diplôme d’ingénieur mécanicien de l’Ecole polytechnique fédérale de Lausanne (EPFL) en 1950. Puis il a exercé une activité dans l’industrie suisse jusqu’à sa nomination en tant que professeur de thermodynamique et de machines thermiques à l’EPFL en 1954. Il y a dirigé le Laboratoire de Thermodynamique et d’énergétique et effectué de nombreuses études dans le domaine de la thermodynamique, des turbomachines et de l’énergie. Il a été professeur honoraire de 1988 à son décès à Lausanne, le 26 septembre 2007.

Daniel Favrat est ingénieur mécanicien diplômé (1972) et Dr es sciences (1976) de l’EPFL. Après 12 années passées dans des centres de recherche industriels au Canada et en Suisse, il est nommé professeur et directeur du Laboratoire d’énergétique industrielle de l’EPFL en 1988. Il y a aussi dirigé l’Institut des sciences de l’énergie de 2001 à 2007 et y dirige actuellement l’Institut de génie mécanique. Il est membre de l’Académie suisse des sciences techniques et vice-président du comité énergie de la Fédération mondiale des organisations d’ingénieurs.

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