Effet de serre | EPFL Graph Search

L'effet de serre est un processus naturel résultant de l'influence de l'atmosphère sur les différents flux thermiques contribuant aux températures au sol d'un objet céleste. La prise en compte de ce processus est nécessaire pour comprendre les températures observées à la surface des planètes ou satellites recouverts d'une atmosphère épaisse comme la Terre, Vénus et Titan, et le processus doit se produire aussi pour certaines exoplanètes. Le terme fait référence à l'effet analogue qu'une serre a sur la température des plantations qu'elle abrite. Dans un système stellaire, l'essentiel de l'énergie thermique reçue par une planète ou un satellite provient du rayonnement stellaire. En l'absence d'atmosphère, cet objet en réfléchit une partie, absorbe le reste et rayonne essentiellement comme un corps noir, ce qui conduit à une certaine température au sol. Une atmosphère réfléchit, absorbe et réémet une partie du rayonnement reçu de l'étoile, et réfléchit, absorbe et réémet également une p

Development of the Jungfraujoch multiwavelength lidar system for continuous observations of the aerosol optical properties in the free troposphere

Gilles Larchevêque

Climate changes and global warming are generally associated with the enhanced greenhouse effect, but aerosols can induce a cooling effect and thus regionally mask this warming effect. Unfortunately, the strong variability both in space and in time of the aerosols and thus the difficulty to characterize their global basic properties induce large uncertainties in the predictions of the numerical models. Those uncertainties are as high as the absolute level of the enhanced greenhouse forcing. To solve this problem it is necessary to improve the set of well-calibrated instruments (both in situ and remote sensing) with the ability to measure the changes in stratospheric and tropospheric aerosols amounts and their radiative properties, changes in atmospheric water vapor and temperature distributions, and changes in clouds cover and cloud radiative properties. The quantity used to assess the importance of one compound (greenhouse gases, aerosols) to the variation of the radiative budget of the Earth is the radiative forcing. One of those forcings is the direct aerosol radiative forcing and it depends on the optical depths and the upscatter fraction of the aerosols. Those two parameters depend on the chemical composition and size distribution of the aerosols. Thus the key parameters of this radiative forcing are the chemical composition through its refractive index and the size distribution of the aerosols. This thesis deals with the design and the implementation of one multi-wavelength lidar system at the Jungfraujoch Alpine Research Station (Alt. 3580m asl). This lidar system is a combination of one standard backscatter lidar and one Raman lidar. Its design have been supported by a ray tracing analysis of the receiver part. The laser transmitter is based on a tripled Nd:YAG laser and the backscattered light is collected by one Newtonian telescope for the tropospheric measurements and by one Cassegrain telescope for the future stratospheric measurements. The received wavelengths for each telescope include three elastically scattered wavelengths (355, 532 and 1064nm), two spontaneous Raman signals from nitrogen (387 and 607nm) and one spontaneous Raman signal from the water vapor (408nm). The optical signals received by each of the telescopes are separated spectrally by two filter polychromators. They are build up around a set of beamsplitters and custom design thin band pass filters with high out-of-band rejection. On the visible channel, the adds of a Wollaston prism separates the parallel polarized backscattered signal (532(p)nm) of the perpendicular polarized one (532(c)nm). Photomultiplier tubes perform the detection of the signals for the UV and visible wavelengths and by Si-avalanche photodiodes for the near-infrared signal. The acquisition of the signals is performed by seven transient recorders in analog and in photon counting modes. Within the frame of the EARLINET (European Aerosol Research Lidar Network), hardware and software intercomparisons have been done. The software intercomparison has been divided into the validation of the elastic algorithm and the Raman algorithm. Those intercomparisons of the inversions of the lidar signals have been performed using synthetic data for a number of situations of different complexity. The hardware intercomparison have been achieved with the mobile micro-lidar of the Observatoire Cantonal de Neuchâtel. The present lidar system provides independent aerosol extinction and backscatter profiles, depolarization ratio and water vapor mixing ratio up to the tropopause. Their uncertainties could be smaller than 20% and thus make possible the retrieval of the microphysical aerosol parameters like the volume concentration distribution and the mean and integral parameters of the particle size distribution, (effective radius, total surface-area concentration, total volume concentration and number concentration of particles). This retrieval is performed by one algorithm of the Institute of Mathematic of the University of Postdam based on the hybrid regularization method. The first results of the retrieval of the volume concentration distribution with three backscatter (355, 532 and 1064nm) and one extinction (355nm) profiles has demonstrated promising results. Future upgrades of the system will add ozone concentration and temperature profile up to the stratopause.

EPFL2002

Thermo-hydro-mechanical behavior of energy tunnels

Matthias Timothee Stanislas Wojnarowicz

Subsurface geothermal in combination with Ground Heat Exchanger (GHE) and heat pumps systems had proved over the past decade to be an effective way in space heating and cooling. GHE’s embedded in geostructures such as tunnels has become more and more attractive in the current state where we try to mitigate greenhouse gas effects. The scope of this paper is to provide a fully coupled numerical model to assess the different physical aspects of an energy tunnel such as the kinetic and thermal boundaries layers development, thermally induce stress, and the heat generation. To do so, a finite element model is used to assess the heat exchange between, the GHE, the concrete lining, the air within the tunnel, the surrounding ground, and groundwater flow. The numerical model is then confronted with data from real-scale experimentation that was performed in Torino in 2018. The predictions from the numerical model show a quite good match with the data from the experimental prototype. Later on, similar models are generated to assess the effect of the heat exchange on air velocity and temperature within the tunnel as well as the induced thermal stress. The numerical results highlight the formation of both the kinematic and thermal boundary layer and shows that those boundaries are affected by the GHE’s activation. For the kinematic aspect, the lower temperatures observed in the vicinity of the heat exchangers tend to lower the velocity magnitude especially close to the wall surface. This change in velocity may be linked to a buoyancy effect cause by the lower temperature of the air in this regions. Indeed natural convection occurs along the tunnel, cooler fluid is pushed down while the warmer fluid rise; locally a lower temperature tends to decrease the viscosity of the fluid and increase its volumic mass. It was observed that the activation of GHE’s enhances this phenomenon in their vicinity due to the temperature gradient that they produce. Moreover, GHE’s activation tends to increase the overall turbulence of the flow and thus the eddies’ formation. The locations of those disturbance are difficult to predict due to the fact that small disturbance may generate large vorticity within the flow. However, it is shown that after passing the GHE zone, the velocity profile tends to stabilize to the undisturbed case. The kinematic entry length seems to happen further down the stream in the case where GHE’s are activated. This is due to the temperature disturbance generated by GHE’s activation. This phenomenon is increased with respect to the number of GHE loops activated. Furthermore, some probing was performed in the vicinity of the heat exchanger to assess the thermal induce stress due to the heating operation. The recorded stress shows peaks at early stages due to the higher temperature gradient that is generated between the GHE’s and concrete at beginning. Finally, after some time, the temperature reaches a steady state hence the stabilized thermal response recorded.

2020

Decreasing the Stability of Borohydride with Ionic Liquids for Hydrogen Storage and Carbon Dioxide Conversion

Loris Giovanni Lombardo

The world needs to move from a fossil fuel to a renewable energy based society. With the increasing electricity production from wind and PV, short and long term energy storage are becoming one of challenge of the 21st century. While batteries are the preferred method for short term storage, another technology is required for seasonal storage. Hydrogen is a valuable energy carrier owning its high energy density (122 kJ/g). However, compact and safe storage of H2 is challenging. The current solution is to compress hydrogen up to 700 bar for mobile application (fuel cell cars), and metal hydrides for stationary application. Material-based H2 storage avoids using high-pressure cylinder. Complex hydrides are especially attractive for solid-state H2 storage due to their high gravimetric and volumetric hydrogen density. Sodium borohydride (NaBH4) contains 10.6 mass% of H2, but high temperatures are needed to release H2 (505 Â°C) and the reaction is not reversible under moderate conditions. To apply complex hydrides in real applications, they must be modified to improve the kinetics and thermodynamics of H2 desorption. Several possibilities exist such as confinement into scaffolds, catalyst addition, or combination of several hydrides to modify the dehydrogenation pathway. The stability of complex hydrides is determined by the localization of the charge on the central atom of the complex. Therefore, in this thesis the interaction of highly polar or ionic compounds with complex hydrides were investigated. We combine NaBH4 with a special class of organic salts: ionic liquids (IL). Considering their chemical and thermal stability, IL are attractive additive. Using organic additives instead of expensive metal catalysts is another advantage. Two different techniques were employed to combine the IL and NaBH4. The first one consists of mechanochemically milling them to create an interaction between BH4- and IL. The second one is to directly synthesized IL borohydrides ([IL][BH4]) by metathesis reaction. Different ILs, such as vinylbenzyltrimethylammonium chloride ([VBTMA][Cl]), 1-butyl-3-methylimidazolium chloride ([bmim][Cl]), and 1-ethyl-1-methylpyrrolidinium bromide ([EMPY][Br]) were tested. [bmim][BH4] was able to release H2 below 100 Â°C. The hydrogen content ranges between 2.4 mass% and 2.9 mass%. The second challenge of this century is the CO2 problematic. Atmospheric CO2 concentration linearly increases, leading to greenhouse effect. Carbon capture and sequestration (CCS) and utilization (CCU) are primordial research topic to stabilize the CO2 emission. Hydrides are primary reducing agents, thus reduction of CO2 can be accomplished with them. Direct CO2 reduction at ambient conditions is challenging for classical borohydrides. With our [IL][BH4], gaseous CO2 can be captured and reduced under ambient conditions (1 bar, RT). Three CO2 bounds with boron to give triformatoborohydride ([HB(OCHO)3]â. Dilute CO2 (6 vol%) can be used as well. The reaction was followed in real time with a magnetic suspended balance (MSB). Further, CO2 reduction happened when air is used as a carbon dioxide source. Formic acid can be obtained by adding HCl. Thus, ionic liquid borohydrides have great potential as a CO2 absorber and reducer to alternative fuels. In short, we applied ionic liquid to destabilized borohydrides. The resulting materials exhibited potential for solid-state hydrogen storage, as well as CO2 capture and transformation.

EPFL2021