Bilan carbone | EPFL Graph Search

Le bilan carbone lié à la fabrication d'un produit ou à l'activité d'une entité humaine (individu, groupe, collectivité) est un outil de comptabilisation de ses émissions de gaz à effet de serre. Il tient compte de l'énergie primaire et de l'énergie finale de ces produits et services. Le bilan carbone vise à renseigner les systèmes d'échange de type bourse du carbone, à poser les bases de solutions efficaces de réduction de la consommation énergétique, et peut optimiser la fiscalité écologique (taxe carbone…) et certains mécanismes de compensation carbone. Le bilan carbone permet également détudier la vulnérabilité d'une activité économique ou d'une collectivité, particulièrement sa dépendance aux énergies fossiles, dont le prix devrait augmenter dans les prochaines années selon les prospectivistes. L'outil est géré et diffusé par l'Association pour la transition Bas Carbone (ABC) et est notamment utilisé en France pour le calcul du Bilan des émissions de gaz à effet de serre (BEG

A mixture model to take into account diluted gas in liquid flow: applications to aluminium electrolysis

Emile Tryphon Pierre Soutter

Aluminium is a metal sought in the industry because of its various physical properties. It is produced by an electrolysis reduction process in large cells. In these cells, a large electric current goes through the electrolytic bath and the liquid aluminium. This electric current generates electromagnetic forces that set the bath and the aluminium into motion. Moreover, large quantities of carbon dioxide gas are produced through chemical reactions in the electrolytic bath: the presence of these gases alleviates the density of the liquid bath and changes the dynamics of the flow. Accurate knowledge of this fluid flow is essential to improve the efficiency of the whole process.The purpose of this thesis is to study and approximate the interaction of carbon dioxide with the fluid flow in the aluminium electrolysis process.In the first chapter of this work, a mixture-averaged model is developed for mixtures of gas and liquid. The model is based on the conservation of mass and momentum equations of the two phases, liquid and gas. By combining these equations, a system is established that takes into account the velocity of the liquid-gas mixture, the pressure, the gas velocity and the local gas concentration as unknowns.In the second chapter, a simplified problem is studied theoretically. It is shown that under the assumption that the gas concentration is small, the problem is well-posed. Moreover, we prove a priori error estimates of a finite element approximation of this problem.In the third chapter, we compare this liquid-gas model with a water column reactor experiment. Finally, the last chapter shows that the fluid flow is changed in aluminium electrolysis cells when we take into account the density of the bath reduced by carbon dioxide. These changes are quantified as being of the order of 30% and explain partially the differences between previous models and observations from Rio Tinto Aluminium engineers.

EPFL2022

The effects of land use and climate change on the carbon cycle of Europe over the past 500 years

Jed Oliver Kaplan, Kristen Marie Krumhardt

The long residence time of carbon in forests and soils means that both the current state and future behavior of the terrestrial biosphere are influenced by past variability in climate and anthropogenic land use. Over the last half-millennium, European terrestrial ecosystems were affected by the cool temperatures of the Little Ice Age, rising CO2 concentrations, and human induced deforestation and land abandonment. To quantify the importance of these processes, we performed a series of simulations with the LPJ dynamic vegetation model driven by reconstructed climate, land use, and CO2 concentrations. Although land use change was the major control on the carbon inventory of Europe over the last 500years, the current state of the terrestrial biosphere is largely controlled by land use change during the past century. Between 1500 and 2000, climate variability led to temporary sequestration events of up to 3Pg, whereas increasing atmospheric CO2 concentrations during the 20th century led to an increase in carbon storage of up to 15Pg. Anthropogenic land use caused between 25Pg of carbon emissions and 5Pg of uptake over the same time period, depending on the historical and spatial pattern of past land use and the timing of the reversal from deforestation to afforestation during the last two centuries. None of the currently existing anthropogenic land use change datasets adequately capture the timing of the forest transition in most European countries as recorded in historical observations. Despite considerable uncertainty, our scenarios indicate that with limited management, extant European forests have the potential to absorb between 5 and 12Pg of carbon at the present day.

2012

Novel Interfacial Characterization and Surface Engineering in Semiconductor Electrodes for Optimized Solar Fuel Production

Yongpeng Liu

Harvesting sunlight, the ultimate renewable power source, in a cost-effective way has been long recognized as a necessary route to meet the global energy challenges. Solar energy can be transformed into electricity by means of photovoltaic devices to supply the electricity grid or alternatively it can be stored in batteries. However, given the limitation of current battery technology, methods to store sunlight for long duration and at large scale are still under development. Instead, solar energy can be directly converted into chemical fuels, for instance hydrogen from water splitting or carbon-based products from carbon dioxide reduction, using a semiconductor-liquid junction (SCLJ) in a photoelectrochemical (PEC) cell. PEC water splitting for sustainable hydrogen production has garnered growing attention ever since the first demonstration of photocatalytic water splitting on semiconductor surface. However, light harvesting through a single-absorber system is limited by the large semiconductor band gap energy required to afford overall water splitting. In contrast, a dual absorber PEC tandem cell consists of photoanode and photocathode with different band gaps that can complementarily utilize a wide range of solar spectrum. Despite considerable efforts to boost the solar-to-hydrogen (STH) conversion efficiency for unassisted solar water splitting, the performance and stability of PEC tandem cell devices are still insufficient for commercialization. To further improve the performance of a PEC tandem cell, it is of great importance to have rational design of photoelectrodes and comprehensive understanding of the SCLJ where the photo-synthetic reaction is happening. The work presented in this thesis aims at developing and utilizing novel operando spectroelectrochemistry to characterize the semiconductor/electrolyte interface. These interfacial characterization techniques provide fundamental understanding of the SCLJ and identify the performance bottleneck of the photoelectrodes, which establish the roadmap for optimizing the solar fuel production process. Taking hematite as a model photoanode material, we first explore the potential of intensity-modulated photovoltage spectroscopy (IMVS) for characterization surface recombination process in Chapter 2. Then in Chapter 3 we study the effect of electrolyte pH on the performance of metal oxide photoanode by a set of operando spectroelectrochemical tools. In Chapter 4 we evaluate the charge carrier behavior of the emerging spinel copper ferrite photoanode whose features limiting the solar water oxidation response are revealed for the first time. In addition, to deal with the late onset potential that restrain spinel zinc ferrite photoanodes for tandem cell application, we demonstrate that a sub-nanometer aluminum oxide layer operates as an efficient surface passivation agent improving the onset potential by 100 mV in Chapter 5. Likewise, guided by spectroelectrochemical measurements, we establish a new benchmark on lanthanum iron oxide photoanode by rational surface and bulk engineering in Chapter 6. Finally, combining computational methods and operando spectroelectrochemistry, we identify for the first time the catalytic active sites and surface trap states on chalcopyrite photocathodes in Chapter 7.