Non-proportional Repartition Rules Optimize Environmental Flows and Energy Production

In order to meet the growing demand for energy, the development of hydropower leads to an increase of the river exploitation by human activities. Thereby, water management has become a major issue in the energy transition. A better definition of the flow release rules is now required to improve the Minimal Flow Requirement (MFR) concept, which has long been used in spite of its environmental inconsistency. In this work, we present a class of non-proportional redistribution rules that broadens the spectrum of dynamic flow releases based on proportional redistribution for run-of-the-river power plants. We adapt the mathematical form of the Fermi-Dirac statistical distribution to engineer a novel class of redistribution functions. In particular, such functions are used to define the fraction of water allocated to the environment depending on the inflow at the intake. The theoretical background as well as the economic interpretation is presented, and the ability to generate variable flow releases carefully discussed. MFR, proportional and non-proportional distribution policies are then applied to a real case study and their respective economic and environmental efficiencies quantitatively compared. We show that non-proportional distribution policies allow for operating conditions actually close to the Pareto frontier, which improve both efficiencies with respect to those obtained from some traditional MFR and proportional policies.

Complexes luminescents de lanthanides avec des dérivés du cyclène comme briques pour l'ingénierie de sondes analytiques biomédicales

This work deals with the engineering of a new luminescent complex of lanthanide, able to be specifically grafted onto biological materials and to emit a characteristic light useful in quantifying the concentration of the target molecules. We have based our molecular design on TbL1, wich possesses a significant quantum yield in water pointing to an efficient intramolecular energy transfer. We have elaborated the synthesis of a ligand derived from L1 and named L3, by functionalizing one of the chromophoric groups into a moiety able to couple the luminescent sensor to biological materials. The synthesis of the complexes was then realized. The thermodynamic study of the ligands L1 and L3, and their terbium complexes in water was made to establish the distribution diagrams and to compare their behaviour, especially at neutral pH. In addition, EuL1 was also investigated and the complexation constants of TbL1, EuL1 and TbL3 in water showed that these molecules are thermodynamically stable in aqueous media. Characterization of EuL3 in the solid state by high resolution luminescence allowed us to determine the geometry of the coordination sphere of the Eu(III) ion, which possesses a distorded C4 symmetry. The detailed study of the Eu(III) ion transitions evidenced two different sites, possibly arising from an equilibrium induced by the competition between a water molecule and the carboxylate function. This interpretation is in line with the measured lifetimes of the Tb(III) and Eu(III) excited states and the calculation of the hydration number of the complexes in aqueous solution. The determination of the energy of the singlet state and triplet states of the ligand, as well as of the emitting levels of the Ln(III) ions in solution showed that their relative values are favorable for a good intramolecular energy transfer. The quantum yields of TbL3 and EuL3, which amount to 2.6 % and 1.4 %, respectively, are lower than those of the complexes with L1, denoting that the presence of the acid function increases losses in the energy transfer. However, these values remain sizeable enough for potential biomedical applications. The study of the intermolecular energy transfer from EuL1, EuL3 and TbL3 to the acceptor Cy5 has proved promising, and the resulting yields were 90 % for EuL1 and 80 % for the complexes with L3.

EPFL2004

Factors influencing photocatalytic drinking water detoxification and disinfection by suspended and fixed TiO2

Damien Gumy

The work presented in this thesis was carried out in the framework of two European projects: SOLWATER, and AQUACAT. The global objective of those projects, including the present work, was to set an autonomous system for drinking water production in developing countries using solar energy. More specifically, the technology of drinking water decontamination by photocatalysis using inexpensive TiO2 catalyst and free of charge sun energy was investigated. In details, the research project dealt with the correlation of the TiO2 physicochemical characteristics with its photocatalytic activity on both organic and bacterial removal, the comparison of the photoactivity of suspended and fixed TiO2, and the development of a large scale prototype for fixed catalyst system. Finally, this work also included the creation of a mathematical model to predict the photo and photocatalytic bacterial inactivation. In photocatalytic experiments, phenolic compounds (phenol, 4-nitrophenol, 4-hydroxybenzoic acid and gallic acid) and benzoic acid were used as model organics, and E. coli as model bacterium. The titania physicochemical characteristics, crystalline structure, specific surface area (BET), particle size, aggregate size, and isoelectric point (IEP) of 13 different suspended TiO2 samples were determined and correlated with photocatalytic activity for chemicals and bacteria abatement. Organics degradation kinetics were found to be significantly affected by the TiO2 surface charge, i.e. the IEP. Indeed, the organic compounds degradation was greatly enhanced by negatively charged TiO2 samples. BET affected differently the primary compounds degradation and the total organic carbon mineralization. The primary degradation kinetics of strongly adsorbed pollutants was enhanced by large BET. Whereas, there was an optimum BET value for a total organic carbon removal, for all organic compounds studied. In addition, the mixed crystalline structure anatase-rutile, in the form of the commercially available Degussa P25, was more active for the purpose of organic degradation than pure anatase TiO2. Bacteria abatement was not influenced by BET specific surface area, particle and aggregate size of suspended TiO2. However, surface charge was crucial for the TiO2 and bacteria interaction. Indeed, negatively charged TiO2 were not efficient in inactivating E. coli, probably due to electrostatic repulsions with the negatively charged E. coli outer membrane. Depending on the nature of the contaminants, different catalyst characteristics have to be considered with regards to TiO2 efficiency on water detoxification and disinfection. Thus, it is not possible to draw a general scheme to determine which photocatalyst is the most suitable for the decontamination of particular water. Comparison of suspended and fixed catalyst revealed that the system with suspended TiO2 is more efficient for bacteria inactivation, yet the post-treatment catalyst filtration presents a serious disadvantage compared to fixed TiO2. Large scale experiments in compound parabolic collector (CPC) prototype reactor with fixed TiO2 revealed that a significant photocatalytic effect appeared only at a low bacterial initial concentration (≤ 103 CFU/ml). High organic compounds removal efficiency with fixed TiO2 was observed at laboratory and large scales, with kinetics similar to the ones obtained with suspended TiO2. A mathematical model was developed to predict bacterial photo-inactivation as a function of the light intensity and of the bacteria physiological state. The modelled curves fitted well with the experimental results for both exponential and stationary phases. Moreover, bacteria were more sensitive to light irradiation in the exponential than in the stationary state. Solar photocatalysis with fixed TiO2 was shown to be an efficient technology for organic pollutant removal and could be used as complement to other treatment process. Indeed, due to the slow bacteria inactivation kinetics observed, the technology needs further development to be used as a unique drinking-water treatment.

EPFL2007

Amorphous Molybdenum Sulfide Films as Hydrogen Evolution Catalysts in Water

Daniel Andreas Merki

Molecular hydrogen is a promising candidate to replace fossil fuels as the energy carrier. Hydrogen does not exist in its molecular form on earth and must therefore be generated, starting from hydrogen-rich compounds. Water would be a renewable resource for hydrogen. It can be split into oxygen and hydrogen. Water splitting, however, needs electrical energy. If this energy is produced from renewable resources, such as solar or wind power, water splitting would be a sustainable and green process. One key step in water splitting is the reduction of protons to form hydrogen. To achieve a high efficiency in this reaction, catalysts are required. This dissertation is devoted to the development of hydrogen evolution catalysts that are made of earth-abundant and inexpensive materials. First, transition metal complexes of tetrathiotungstate and tetrathiomolybdate were studied as potential homogeneous catalysts for hydrogen evolution in organic or aqueous solutions (chapter 1). The redox activity of these complexes was investigated by cyclic voltammetry in the absence and presence of an acid in organic solutions. The first reduction peak of (Pr4N)2[Ni(MoS4)2] became more intense and was shifted to less negative potentials upon addition of an acid, e.g. p-cyanoanilinium tetrafluoroborate. This was considered as a sign for catalytic proton reduction. Electrolysis of a solution containing (Pr4N)2[Ni(MoS4)2] and an acid produced hydrogen. However, the complex turned out to be a precursor of the real catalytically active species, which was deposited on the working electrode’s surface during the electrochemical experiment. Consecutive cyclic voltammetric scans were found to be a good method to deposit the catalytically active film in a controllable manner from an aqueous solution of (Pr4N)2[Ni(MoS4)2]. Furthermore, a catalytically active film could be deposited from a solution of MoS42-, i.e. in the absence of Ni2+. Chapter 2 describes the investigation and characterization of films made using MoS42- as the precursor. The films consist of amorphous molybdenum sulfide. They are efficient hydrogen evolution catalysts in water. The films were characterized by XPS and electron microscopy. Whereas the pre-catalysts could be MoS3 or MoS2, the active form of the catalysts was identified as amorphous MoSx, with x close to 2. Significant geometric current densities were achieved at low overpotentials (e.g., ca. 15 mA/cm2 at η = 200 mV) using these catalysts. The catalysis was compatible with a wide range of pH values. The current efficiency for hydrogen production was quantitative. A Tafel slope of 39 mV/dec was observed, suggesting a rate-determining Heyrovsky step. The turnover frequency per active site was estimated.

EPFL2012