Planification de l'électrification rurale décentralisée en Afrique subsaharienne à l'aide de sources renouvelables d'énergie : le cas de l'énergie photovoltaïque en République de Djibouti

Over the past 40 years, growth of renewable energies benefited of the new world energy frame, which resulted of the questioning about what development of human societies had to be. Furthermore, although human development comes with electricity, the rural condition of many populations of Sub-Saharan Africa incites us to look for suitable power supply alternatives. Eventually, in this specific context, renewable energies can represent a reliable solution to the off-grid electrification of rural people. However, this solution has to be economical and technical, and not only political. The Republic of Djibouti is a little developing country located in the Horn of Africa which perfectly symbolizes the social and energy challenges of rural populations in Sub-Saharan Africa. Instability and limitation of the existing electrical grid, fuel cost and lack of fossil resources point to the geographically diffused solar resource as probably the best way to improve human development and reduce poverty of Djiboutian rural peoples. Therefore, we have considered the study of photovoltaic (PV) systems within the rural off-grid electrification frame. Essentially, within this work, Djibouti was the study case of an overall scientific methodology, whose primary objective is to be reusable by countries showing similar energy and social characteristics. Firstly, in order to evaluate relevance of these systems, it was necessary to estimate the level and repartition of the solar resource across the country. So we developed a solar atlas, i.e. cartography of the hourly solar irradiation reaching the ground, based on satellite-derived irradiance estimates retrieved between 2008 and 2011. For assessing the atlas quality, we compared irradiation estimates against ground measures retrieved on 4 different sites by 2 temporary weather stations deployed between 2010 and 2013. This comparison globally showed good results with, for the daily case, a maximum relative root mean squared error of 8.05 % and a minimum correlation coefficient of 0.8892 through all samples. Finally, yearly map extracted from the atlas showed that, with a daily mean irradiation of 5.87 kWh/m^2.day, the solar potential of Djibouti is one of the most significant in the world. Satellite models are useful for determining solar irradiance at ground level but they don't take into account local topography effects. In order to incorporate these shading effects to the satellite-derived irradiance maps, improving therefore irradiance accuracy and spatial resolution, we used a Digital Elevation Model (DEM). Firstly, this disaggregation process was based on the development of a new fast horizon algorithm which was assessed by means of topographic measures in Corsica Island. This model thus shows higher performances than already existing ones, with not only a better precision but also a substantially reduced CPU time consumption, which makes this one a well suited model for calculation of large mapping areas. Finally, by correcting irradiance with horizon shading and elevation gradient for each pixel of the DEM, we improved geographic information of the solar irradiation atlas. Although solar resource is the first indicator of the photovoltaic potential, other elements, as environmental parameters or endogenous characteristics of photovoltaic modules, also have to be taken into consideration to precisely estimate the energy produced by a PV system. Hence, by means of different models, we evaluated the influence of irradiance and temperature onto the conversion efficiency of a PV generator to finally retrieve the atlas of the PV potential across the country. Furthermore, we developed an empirical method to retrieve ambient temperature from land surface temperature (LST), by training an artificial neural network with satellite estimates and ground measurements. In conclusion, by combining this cartography to a multi-criteria approach comparing relevance between PV systems and classical power supply systems within the rural electrification scheme, we developed the first photovoltaic decision making tool of the country intended for all officials acting in the energy field.La remise en question du mode de développement des sociétés humaines a, sur ces 40 dernières années, profondément transformé le contexte énergétique mondial, se traduisant ainsi par l'instauration d'un nouveau cadre politique, dont les engagements auront notamment favorisé l'essor des énergies renouvelables. Par ailleurs, si l'électricité apparaît comme un vecteur fondamental du développement humain, le contexte des populations majoritairement rurales d'Afrique subsaharienne incite également à la recherche d'alternatives énergétiques adaptées. En substance, si les ressources renouvelables peuvent répondre de manière pertinente au défi de l'électrification décentralisée des zones rurales de la région, elles doivent toutefois représenter une solution qui, avant d'être politique, soit crédible sur les plans technique et économique. La République de Djibouti, petit pays situé dans la corne de l'Afrique, symbolise parfaitement les défis social, économique et énergétique de l'ensemble de la région, et des populations rurales en particulier. Avec un pays pauvre en ressources traditionnelles, mais présentant a priori un gisement solaire intéressant, nous avons alors privilégié l'étude des systèmes photovoltaïques (PV) dans le cadre de l'électrification décentralisée des populations rurales du pays. Comme d'autres ressources, ces systèmes, bien que reposant sur une technologie relativement ancienne, ont réellement pris leur essor au début des années 2000 avec les mesures incitatives du Protocole de Kyoto. Au cours de ces travaux, la République de Djibouti aura servi de cas d'étude à l'élaboration d'une méthodologie scientifique générale, centrée sur l'énergie photovoltaïque dans un cadre décentralisé, dont les résultats principaux ont vocation à être réutilisés par des pays structurellement voisins, c.-à-d. présentant le même contexte socio-énergétique. Evaluer la pertinence de la technologie photovoltaïque nécessitait, dans un premier temps, d'estimer le niveau et la répartition de la ressource solaire au sein du pays. Pour cela, nous avons construit un atlas de l'irradiation solaire horaire incidente sur le territoire, pour la période 2008-2011, à partir d'un modèle satellitaire de rayonnement. Afin de valider les estimations issues de cet atlas, nous les avons comparées aux mesures in situ relevées par deux stations météorologiques temporaires déployées, entre 2010 et 2013, sur quatre sites présentant des caractéristiques distinctes. Les résultats de cette comparaison ont montré que les estimations présentaient une bonne précision, avec notamment, pour l'irradiation journalière, une erreur relative maximale de 8,05 % et un coefficient de corrélation minimal de 0,8892. Finalement, la carte annuelle extraite de l'atlas a confirmé que le gisement solaire du pays, avec une irradiation moyenne de 5,87 kWh/m^2 par jour, constituait l'un des plus importants au monde. Si les modèles utilisant des données satellitaires permettent de retrouver le flux solaire incident au sol, ils ne tiennent en revanche pas compte des effets du relief local sur ce dernier. Ainsi, afin de tenir compte des effets d'ombrage engendrés par le terrain, nous avons développé une procédure dite de désagrégation, couplant l'utilisation d'un maillage numérique de terrain (MNT) aux cartes de rayonnement issues du modèle satellite. Pour ce faire, nous avons élaboré un modèle théorique afin de retrouver l'horizon autour d'un point donné, et validé celui-ci à l'aide d'une campagne de mesures topographiques en Corse. Ce dernier apparaît ainsi plus performant que les modèles existants, avec, d'une part, une précision plus élevée et, d'autre part, un temps d'exécution fortement réduit qui le rend bien adapté au traitement cartographique (nombre important de points). En définitive, en intégrant au rayonnement le gradient d'altitude et l'ombrage dû à l'horizon pour chaque pixel du MNT, nous avons pu enrichir le niveau de l'information fournie par la cartographie de l'irradiation globale. Bien que le gisement solaire soit l'indicateur principal du potentiel photovoltaïque, il est également nécessaire de considérer des paramètres secondaires, éléments de technologie et caractéristiques environnementales, qui permettent d'évaluer avec précision l'énergie électrique produite par un système PV quelconque. En combinant l'utilisation de différents modèles, nous avons ainsi pu intégrer à l'estimation finale du productible l'influence du rayonnement et de la température sur le rendement de conversion d'un module photovoltaïque. A cet égard, nous avons d'ailleurs développé une méthodologie empirique qui, en s'appuyant sur l'utilisation d'un réseau de neurones artificiels, permet de retrouver la température ambiante à travers un territoire à partir de données satellitaires de la température de surface terrestre. Finalement, nous avons construit une cartographie spatiotemporelle du productible PV qui, couplée à une évaluation multicritère de la pertinence de la technologie PV vis-à-vis des alternatives énergétiques classiques en matière d'électrification rurale, est destinée à faciliter la prise de décision pour les différents acteurs, publics ou privés, du domaine de l'énergie

A near-real-time tropical deforestation monitoring algorithm based on the CuSum change detection method

Tropical forests are currently under pressure from increasing threats. These threats are mostly related to human activities. Earth observations (EO) are increasingly used for monitoring forest cover, especially synthetic aperture radar (SAR), that is less affected than optical sensors by atmospheric conditions. Since the launch of the Sentinel-1 satellites, numerous methods for forest disturbance monitoring have been developed, including near real-time (NRT) operational algorithms as systems providing early warnings on deforestation. These systems include Radar for Detecting Deforestation (RADD), Global Land Analysis and Discovery (GLAD), Real Time Deforestation Detection System (DETER), and Jica-Jaxa Forest Early Warning System (JJ-FAST). These algorithms provide online disturbance maps and are applied at continental/global scales with a Minimum Mapping Unit (MMU) ranging from 0.1 ha to 6.25 ha. For local operators, these algorithms are hard to customize to meet users’ specific needs. Recently, the Cumulative sum change detection (CuSum) method has been developed for the monitoring of forest disturbances from long time series of Sentinel-1 images. Here, we present the development of a NRT version of CuSum with a MMU of 0.03 ha. The values of the different parameters of this NRT CuSum algorithm were determined to optimize the detection of changes using the F1-score. In the best configuration, 68% precision, 72% recall, 93% accuracy and 0.71 F1-score were obtained

Azimuthal anisotropy of charged jet production in root s(NN)=2.76 TeV Pb-Pb collisions

We present measurements of the azimuthal dependence of charged jet production in central and semi-central root s(NN) = 2.76 TeV Pb-Pb collisions with respect to the second harmonic event plane, quantified as nu(ch)(2) (jet). Jet finding is performed employing the anti-k(T) algorithm with a resolution parameter R = 0.2 using charged tracks from the ALICE tracking system. The contribution of the azimuthal anisotropy of the underlying event is taken into account event-by-event. The remaining (statistical) region-to-region fluctuations are removed on an ensemble basis by unfolding the jet spectra for different event plane orientations independently. Significant non-zero nu(ch)(2) (jet) is observed in semi-central collisions (30-50% centrality) for 20 <p(T)(ch) (jet) <90 GeV/c. The azimuthal dependence of the charged jet production is similar to the dependence observed for jets comprising both charged and neutral fragments, and compatible with measurements of the nu(2) of single charged particles at high p(T). Good agreement between the data and predictions from JEWEL, an event generator simulating parton shower evolution in the presence of a dense QCD medium, is found in semi-central collisions. (C) 2015 CERN for the benefit of the ALICE Collaboration. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).Peer reviewe

Production of He-4 and (4) in Pb-Pb collisions at root(NN)-N-S=2.76 TeV at the LHC

Results on the production of He-4 and (4) nuclei in Pb-Pb collisions at root(NN)-N-S = 2.76 TeV in the rapidity range vertical bar y vertical bar <1, using the ALICE detector, are presented in this paper. The rapidity densities corresponding to 0-10% central events are found to be dN/dy4(He) = (0.8 +/- 0.4 (stat) +/- 0.3 (syst)) x 10(-6) and dN/dy4 = (1.1 +/- 0.4 (stat) +/- 0.2 (syst)) x 10(-6), respectively. This is in agreement with the statistical thermal model expectation assuming the same chemical freeze-out temperature (T-chem = 156 MeV) as for light hadrons. The measured ratio of (4)/He-4 is 1.4 +/- 0.8 (stat) +/- 0.5 (syst). (C) 2018 Published by Elsevier B.V.Peer reviewe

Forward-central two-particle correlations in p-Pb collisions at root s(NN)=5.02 TeV

Two-particle angular correlations between trigger particles in the forward pseudorapidity range (2.5 2GeV/c. (C) 2015 CERN for the benefit of the ALICE Collaboration. Published by Elsevier B. V.Peer reviewe

Event-shape engineering for inclusive spectra and elliptic flow in Pb-Pb collisions at root(NN)-N-S=2.76 TeV

Peer reviewe

Extracorporeal Membrane Oxygenation for Severe Acute Respiratory Distress Syndrome associated with COVID-19: An Emulated Target Trial Analysis.

RATIONALE: Whether COVID patients may benefit from extracorporeal membrane oxygenation (ECMO) compared with conventional invasive mechanical ventilation (IMV) remains unknown. OBJECTIVES: To estimate the effect of ECMO on 90-Day mortality vs IMV only Methods: Among 4,244 critically ill adult patients with COVID-19 included in a multicenter cohort study, we emulated a target trial comparing the treatment strategies of initiating ECMO vs. no ECMO within 7 days of IMV in patients with severe acute respiratory distress syndrome (PaO2/FiO2 <80 or PaCO2 ≥60 mmHg). We controlled for confounding using a multivariable Cox model based on predefined variables. MAIN RESULTS: 1,235 patients met the full eligibility criteria for the emulated trial, among whom 164 patients initiated ECMO. The ECMO strategy had a higher survival probability at Day-7 from the onset of eligibility criteria (87% vs 83%, risk difference: 4%, 95% CI 0;9%) which decreased during follow-up (survival at Day-90: 63% vs 65%, risk difference: -2%, 95% CI -10;5%). However, ECMO was associated with higher survival when performed in high-volume ECMO centers or in regions where a specific ECMO network organization was set up to handle high demand, and when initiated within the first 4 days of MV and in profoundly hypoxemic patients. CONCLUSIONS: In an emulated trial based on a nationwide COVID-19 cohort, we found differential survival over time of an ECMO compared with a no-ECMO strategy. However, ECMO was consistently associated with better outcomes when performed in high-volume centers and in regions with ECMO capacities specifically organized to handle high demand. This article is open access and distributed under the terms of the Creative Commons Attribution Non-Commercial No Derivatives License 4.0 (http://creativecommons.org/licenses/by-nc-nd/4.0/)

Elliptic flow of muons from heavy-flavour hadron decays at forward rapidity in Pb-Pb collisions at root s(NN)=2.76TeV

The elliptic flow, v(2), of muons from heavy-flavour hadron decays at forward rapidity (2.5 <y <4) is measured in Pb-Pb collisions at root s(NN)= 2.76TeVwith the ALICE detector at the LHC. The scalar product, two- and four-particle Q cumulants and Lee-Yang zeros methods are used. The dependence of the v(2) of muons from heavy-flavour hadron decays on the collision centrality, in the range 0-40%, and on transverse momentum, p(T), is studied in the interval 3 <p(T)<10 GeV/c. A positive v(2) is observed with the scalar product and two-particle Q cumulants in semi-central collisions (10-20% and 20-40% centrality classes) for the p(T) interval from 3 to about 5GeV/c with a significance larger than 3 sigma, based on the combination of statistical and systematic uncertainties. The v(2) magnitude tends to decrease towards more central collisions and with increasing pT. It becomes compatible with zero in the interval 6 <p(T)<10 GeV/c. The results are compared to models describing the interaction of heavy quarks and open heavy-flavour hadrons with the high-density medium formed in high-energy heavy-ion collisions. (C) 2015 CERN for the benefit of the ALICE Collaboration. Published by Elsevier B.V.Peer reviewe