Thermodynamics

Summary

Thermodynamics is a branch of physics that deals with heat, work, and temperature, and their relation to energy, entropy, and the physical properties of matter and radiation. The behavior of these quantities is governed by the four laws of thermodynamics which convey a quantitative description using measurable macroscopic physical quantities, but may be explained in terms of microscopic constituents by statistical mechanics. Thermodynamics applies to a wide variety of topics in science and engineering, especially physical chemistry, biochemistry, chemical engineering and mechanical engineering, but also in other complex fields such as meteorology. Historically, thermodynamics developed out of a desire to increase the efficiency of early steam engines, particularly through the work of French physicist Sadi Carnot (1824) who believed that engine efficiency was the key that could help France win the Napoleonic Wars. Scots-Irish physicist Lord Kelvin was the first to formulate a concise d

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https://en.wikipedia.org/wiki/Thermodynamics

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Among the plethora of alternatives available, concentrated solar power (CSP) appears as one of the most favourable options. The stability and dispatchability of production achievable by the integration of storage and fuel-solar hybridisation are amidst the major advantages of this technology. Nevertheless, conventional CSP plants are based on stream-turbine cycles which consume large amounts of water. In addition to the low thermodynamic efficiency of this type of cycle, the installation of such plants in water-scarce areas is complicated by their reliance on water resources. Thus, the study of new concepts that overcome these drawbacks is necessary for the future of this technology. The availability of high temperature solar receivers for solar tower systems opens the way for the use of gas-turbines in hybrid solar- natural gas configurations. In order to increase the efficiency of the cycle while keeping the water consumption as low as possible, a promising alternative to the recovery of the waste heat in steam-turbines is to use a low-temperature intercooled-recuperated gas-turbine cycle. This work focuses on the analysis and optimisation of the performance of an innovative hybrid solar gas-turbine power plant with an air-based bottoming cycle (ABHSGT). The evaluation considers thermodynamic performance, economic viability and environmental impact as interrelated concerns. With this in mind, detailed steady-state and dynamic models of the power plant have been developed and validated by comparison with existing components. A second model without bottoming cycle has been built for comparison. A multi-objective optimisation using an evolutionary algorithm has then been performed, optimising both capital cost and specific CO2 emissions and resulting in a Pareto-optimal set of possible designs. The analysis of the trade-off curves resulting from the optimisation reveals promising outcomes. The global minimum for the levelised cost of electricity, found at relatively high solar shares, proves the economic potential of the technology. The integration of the bottoming cycle decreases significantly the levelised cost of electricity and the CO2 emissions of the system compared to the reference plant, and higher efficiencies are achieved. The optimal design selected for an in-depth thermoeconomic and environmental analysis exhibits a levelised cost of electricity of 109 [USD/MWhe] for a solar share of 20% and an overall exergetic efficiency 38.5%. The specific CO2 emissions are reduced by 33% compared to those of a simple gas-fired power plant. The water consumption is kept at very low levels compared to other CSP plants, making the system suitable for the deployment in water-scarce areas. In addition, the environmental impact induced by the land use requirements is considerably lower than that of other renewable energy technologies. The sensitivity analysis performed to assess the consequence of changes in varying financial conditions on the levelised cost of electricity and the net present value reveals that the system studied represents a profitable investment in the presence of feed-in tariffs. In the light of the performance obtained in the three aspects considered (thermodynamic, economic and environmental), it can be concluded that the ABHSGT represents a promising alternative to other renewable energy technologies, especially in water-scarce areas.

2012

Precipitation, Characterization, Kinetic, and Thermodynamic Modelling of Synthetic C-S-H Systems (C-S-H, CASH, CSH+$)

Maya Nicole Harris

This thesis investigates the effects of aluminates, sulfates, and heterogeneous substrates on the nucleation and growth of synthetic calcium-silicate-hydrates (C-S-H) produced by dropwise precipitation. The use of synthetic C-S-H, separate from other cementitious phases, allows formation mechanisms and kinetics effects to be investigated in a simplified system. The addition of other ions and substrates then allows an approach to the more complicated, multi-phase system of cement and concrete. The first goal of this thesis was to reproducibly synthesize single-phase C-S-H with Ca:Si from 1 to 2. Guidelines were described for collection, drying, and handling of the precipitates to prevent the formation of Ca(OH)2 during synthesis and minimize carbonation. Interlayer spacing in precipitates was studied by transmission XRD to compare the effects of Ca:Si, relative humidity, and time, using the 14Å tobermorite structure as a baseline for synthetic C-S-H. The growth and nucleation kinetics of C-S-H were then investigated with the combined thermodynamic and population balance equation model (PBEM). The surface characteristics of C-S-H, C-A-S-H, and C-S-H +

were investigated by acoustophoresis to compare the effects of Ca:Si, washing, and dispersion media on zeta potential measurements. Results showed the necessity of full characterization of both solutions and precipitates in order to understand and compare acoustophoresis results. In the final section of the thesis C-S-H, C-A-S-H, and C-S-H +

were grown on the surface of heterogeneous substrates, quartz and calcite, to approach C-S-H growth in real systems. Overall, the results from this thesis provided helpful information on synthetic C-S-H, C-A-S-H, and C-S-H + $ at the full range of Ca:Si molar ratios, particularly ratios above 1.5, as observed in Portland cement. As we get closer to understanding and controlling C-S-H in real systems, this work will help provide insight as to how heterogenous substrates and aluminates introduced by SCMs, in addition to sulfates, commonly added to cement via gypsum, affect the early age strength and kinetics of C-S-H formation.

EPFL2022

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