Principes de la thermodynamique

vignette|Entropie d'un corps à 0 K (à gauche) Corps avec une température supérieur à 0 K (à droite) Les principes de la thermodynamique sont les principales lois (principes en fait, car non démontrés) qui régissent la thermodynamique :

premier principe de la thermodynamique : principe de conservation de l'énergie ; introduction de la fonction énergie interne, U ;
deuxième principe de la thermodynamique : principe d'évolution ; création d'entropie, S ;
troisième principe de la thermodynamique ou principe de Nernst : l'entropie d'un cristal parfait est nulle à T = 0 K.

On évoque parfois deux autres principes :

principe zéro de la thermodynamique : la température est une grandeur repérable ;
quatrième principe de la thermodynamique, comme nom donné à la théorie d'Onsager : il existe une relation linéaire entre les courants volumiques et les forces thermodynamiques.

de:Thermodynamik#Kurze Zusammenfassung der Hauptsätze

Conceptual Design of a Combined Cycle Power Plant Using Existing Gas Turbines

The main goal of this master is to define and analyse the best layout options for a combined cycle power plant. The gas turbines used for this combined cycle are known. Methods for power augmentation of the gas turbine are considered. Different layouts and thermodynamics parameters of the steam cycle must be optimised.

2007

Energy integration study of a multi-effect evaporator

François Maréchal, Vanessa Palese, Zoé Périn-Levasseur

In the pulp and paper industry, multi-effect evaporators are used to evaporate water from black liquor solutions to allow its recycle as chemicals and fuel for the process. The thermodynamic principle of the multi-effect evaporator consists in a serie of reboilers operating at different pressures; the water evaporated at one stage is condensed and used as the heat source for another stage. Due to its strong integration with the process, it is worth to analyse the integration of the multi-effect evaporator with the rest of the process. To do so a thermo-economic analysis model of the evaporation system has been developped. The example is based on the evaporation system of a calcium bisulfite pulp manufacturing mill located in Switzerland. This system involves 3 multi-effect evaporators fed at different concentrations of black liquor. A systematic analysis of the system Grand composite curve has been developed to identify pertinent process modifications. From this analysis, several modifications like decreasing the deltaTmin of a stream, increasing or decreasing pressures of evaporation effects have been evaluated. For each of these measures, we have analysed the thermo-economic aspects, adapting the pinch analysis rules to account for the thermo-econmic benefit of integration. By these measures, the minimum energy requirement of the multi-effect evaporation system can be reduced by up to 20%. Resulting from the integration of the utility system, the related utility cost can be diminished of up to 23% from the base case model. The integration of heat pumping system and the utilities has then been analysed in order to reduce the exergy degradation in the energy conversion system. A thermoeconomic analysis including operating and investment cost estimation, evaluation of the Net Present Value and Payback Time of different process configurations corresponding to different energy savings scenarios has been performed. The integration of a heat pump system shows a reduction of the Net Present Value and an acceptable Payback Time compared to the base case model. A sensitivity analysis on electricity and natural gas prices has then been performed in order to better understand the economic condition of the integration.

2008

A theoretical study of disorder and decomposition in multinary nitrides hard coatings materials

Björn Alling

The development of multinary nitrides materials has revolutionised the hard coatings industry over the last 20 years. Especially important materials systems in this matter have been TiAlN and CrAlN which shows higher hardness, better oxidation resistance and can perform at higher temperatures as compared to TiN. When synthesised through physical vapour deposition techniques these system form cubic rock salt structured Ti1-xAlxN and Cr1-xAlxN solid solutions as a metastable phase over a large part of the concentration range. One of the main objectives during the optimisation of the coatings has been to increase the amount of Al in the coating while still keeping the rock salt structure, avoiding phase separation and the formation of hexagonal wurtzite AlN. However, in Al-rich TiAlN coatings it was found that isostructural decomposition within the cubic phase was in fact beneficial for the coatings performance at working temperatures just below 1000 °C. The reason was that the formation of strained coherent c-AlN domains within the grains initiated a age-hardening of the coating. In the CrAlN coatings it is possible to solve a larger amount of Al in the cubic phase. Possibly connected to this fact is that no isostructural decomposition and in principle no age hardening has been observed in rock salt structured Cr1-xAlxN. Although large series of experimental investigations have been performed on these systems, no systematic theoretical study has yet been undertaken. This theoretical work is an attempt by means of first-principles calculations together with thermodynamics considerations within the framework of alloy theory, to close or decrease the knowledge gap between experimental observations of cutting performance of various coatings and the fundamental quantum mechanical and thermodynamics processes that governs it. We first consider the structural properties of the treated mixed nitride systems. The concept of atomic misfit or volume difference, which is well known in the community is studied and the physics that leads to a positive deviation from Vegard's rule is revealed. Then the TiAlN and CrAlN systems are studied in detail. A clear connection between the development of the electronic structure with composition and the mixing enthalpy of the alloys, and thus the tendency for decomposition, is found for TiAlN. The importance of magnetic effects on the thermodynamics of mixing in CrAlN is established leading to a qualitative lower tendency for decomposition. Since the nitrogen composition can deviate substantially from perfect stoichiometry in these systems, a study of the influence of nitrogen vacancies on the decomposition pattern of TiAlN in the cubic phase is performed. The results imply that a presence of nitrogen sub-stoichiometry in Al-rich TiAlN will enhance the tendency for isostructural decomposition. The achieved results including those for the systems ScAlN and HfAlN are compared and discussed especially by considering volume misfit and electronic bandstructure effects as driving forces for coherent and incoherent decomposition.

EPFL2009