Machine thermique | EPFL Graph Search

Une machine thermique est un mécanisme qui fait subir à un fluide des transformations cycliques au cours desquelles le fluide échange avec l'extérieur de l'énergie sous forme de travail et de chaleur. La théorie des machines thermiques s'attache à la description et à l'étude physique de certains systèmes thermodynamiques qui permettent de transformer l'énergie thermique en énergie mécanique, et vice versa. Fondée au milieu du siècle, elle s'appuie sur la thermodynamique, et en particulier sur ses deux premiers principes. De nombreuses machines thermiques sont d'un usage courant : le moteur thermique ou (sous la forme d'un moteur à combustion externe tel que la machine à vapeur et la turbine à vapeur, ou sous la forme d'un moteur à combustion interne tel que le moteur à essence, le moteur Diesel, le moteur à réaction et la turbine à gaz), le réfrigérateur et la pompe à chaleur. Naissance C'est le désir de mettre en équation les machines thermiques (avec Carnot notamment

THERMOECONOMIC ANALYSIS AND OPTIMISATION OF AIR-BASED BOTTOMING CYCLES FOR WATER-FREE HYBRID SOLAR GAS-TURBINE POWER PLANTS

Raphaël Michel Adrien Marie Sandoz

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

Electrochemical systems for hydrogen fuel cell and battery electric vehicle infrastructure

Yorick Ligen

Thermodynamics and heat engines are the core disciplines which enabled the development of the thermo-industrial society during the 20th century. Liquid hydrocarbon fuels are one of the easiest and most convenient solutions offered by the thermophysical constraints of our world. However, an alternative to these dense energy carriers is required to enable a transition to a low carbon transport sector. Climate change mitigation, local air pollution reduction and energy independency are some of the key advantages that hydrogen fuel cell and battery electric vehicles can bring in this context. Beyond a shift from fossil fuels, the entire value chain, from primary energy to powertrains must be reconsidered, redesigned and redeployed to include renewable energies, robust powergrids, charging stations and electric powertrains. The prominent role of the infrastructure in terms of energy efficiency is demonstrated in chapter 2, introducing a grid to mobility segmentation for life cycle studies. In addition, the shortcomings of local and off-grid solutions are highlighted in the same chapter. Nevertheless, the grid integration also requires innovative solutions to comply with the physical constraints of current networks. In particular, the role and the sizing of stationary buffer batteries is detailled in chapter 3. The stochastic nature of charging events is used to develop a battery sizing algorithm including grid tie constraints. This research was intrinsically motivated by the perspective of infrastructure operators. A full scale demonstrator is at the core of the scientific questioning of this thesis. The design, the construction and the operation of a grid to mobility demonstrator is reported in chapter 4. Including a 200 kW / 400 kWh Vanadium redox flow battery, a 50 kW alkaline electrolyser, and a hydrogen refueling station, this demonstrator enabled a better understanding and characterization of process engineering, of control and programming and of electrochemical phenomenons. In particular, the data analysis on the electrolysis side and purification solutions are reported in chapter 5. Finally, the intrinsic characteristics of air driven gas boosters, a robust small scale compression solution, are analyzed in chapter 6 and the challenges for a full scale hydrogen mobility are discussed with an economic and logistics perspective.

EPFL2020

Multicriteria Optimisation of Small Hybrid Solar Power System.

Daniel Favrat, El Hadj Malick Kane

A new concept of small hybrid solar power system (HSPS) has been successfully demonstrated in the context of a project called SPS (Solar Power System). This plant integrates two rows of solar collectors, two superposed Organic Rankine Cycles (ORC) each equipped with a scroll hermetic expander-generator and a heat engine. In operation with solar energy only, the heat is supplied by a thermal fluid (presently pressurized water) heated in the vacuum insulated focal tubes of sun following, flat concentrators made of series of thin plate mirrors (CEP). In hybrid mode additional heat is supplied by heat recovery from the exhaust gases of the engine in series with the solar network and by a separate network recovering heat from the cooling of the engine block at an intermediate temperature level. This paper presents the results of a multi-criteria optimization of a 22 kWe HSPS, including aspects such as energy performance, economic and financial analysis, and environemental aspects. The so called mini-maxi methodological approach with genetic algorithm is used considering three principal criteria such as the energy efficiency of the superposed ORCs, the minimal cost of the installation and the minimum emission of CO2 . Taking into account of the solar radiation time dependence, the elecricity supply variation and the change of configuration (night and day operation), the performance analysis is based essentially on the yearly energy simulation in which the off-design physical models of components are considered. A comparison of HSPS with pure fossil fuelled Plants (DEPU-Diesel Engine Power Unit) is reported for the same electrical power load curve, with an economic sensitivity analysis. Results show that the solar electricity costs are still high and depend considerably on the size of the Solar Field (the HSPS Levelized Electricity Cost with 5 to 16% of annual solar share is about 17% to 49% higher than a similar size Diesel Engine Power Unit). However, a reduction of CO2, emission up to 26% could be obtained when replacing the Diesel Engine Unit by a similar HSPS. Those hybrid solar thermal power systems may already be competitive if a tax of about 42 Swiss cts/kgCO2 would be considered.

2002