La circulation atmosphérique en Afrique de l'Ouest est caractérisée par des vents de sud-ouest (mousson) pendant la saison humide et par des vents de nord-est (harmattan) pendant la saison sèche. Cette alternance des saisons est due aux variations de pression liée à l'état des surfaces (rugosité, albédo, végétation) en réaction au forçage solaire. Ces mêmes états de surface génèrent une variabilité de flux turbulents de surface et des circulations secondaires qui rendent complexes les analyses des mesures effectuées sur place en vue de documenter les interactions surface-atmosphère. La modélisation fine échelle (LES) couramment utilisée dans l'étude de la couche limite atmosphérique est requise pour pouvoir palier à ces difficultés en raison de sa capacité à prendre en compte les flux turbulents en 3D et sur topographie complexe. Notre site d'étude est le bassin versant d'ARA située au Nord du Bénin dans un contexte Soudanien avec des propriétés de surface variables. Une analyse climatique est effectuée sur la base des observations de radiosondage, de radar UHF et de stations au sol afin d'extraire des données composites représentatives des saisons sèche et humide. Ces données composites ont servi par la suite à forcer le modèle Méso-NH dans sa version LES. Pour pouvoir caractériser les échelles de longueur des flux turbulents de surface relatives aux saisons sèche et humide, les données standard de forçage de surface de Méso-NH que sont le relief GTOPO30 (1km de résolution) et la végétation ECOCLIMAP (1km de résolution) ont été respectivement remplacer par le SRTM (90m de résolution) et les données de SPOT/HRV (20m de résolution) reéchantillonné à 90m de résolution. A l'aide d'outils statistiques comme la variographie 2D et le suivi Lagrangien, il ressort que la variabilité spatiale de la chaleur sensible H est gouvernée par le couple vent-relief tandis que celle de la chaleur latente E est difficile à mettre en lien sur végétation hétérogène (SPOT/HRV) en saison sèche. En saison humide, la variabilité spatiale du champ H dépend du vent tandis que celle du champ E dépend de la végétation. Cette étude révèle dans tous les cas que les échelles caractéristiques de ces deux champs diffèrent dans les mêmes conditions de forçage de surface et atmosphérique. / West Africa atmosphere circulation is characterized by south-westerly wind (monsoon regime) during the wet season and north-easterly wind (harmattan regime) during the dry season. This alternation of wind regime is due to surface pressure variability linked to surface heterogeneities. Surface heterogeneities generate surface flux variability, secondary circulation and make complex analysis when trying to document surface-atmosphere feedbacks. LES modelling usually used for boundary-layer studies due to its potential to take into account 3D turbulence over complex topography, is used here to overcome these difficulties. Our site of interest is located in north of Benin characterized by Soudanian climate and heterogeneous surface properties. Climate analysis are first performed with radiosoundings, UHF radar, and EC station data in order to extract composite profile representing dry and wet season.. These composite profiles are then used to force atmosphere part of the Méso-NH LES model. To characterize turbulent fluxes length scales relative to dry and wet season, standard surface forcing data with Méso-NH like GTOPO30 orography (1km ) and ECOCLIMAP vegetation (1km) are respectively replaced by SRTM (90m) and SPOT/HRV vegetation data (20m) resampled to 90m. Along with statistical tools like 2D variography and Lagrangian, we notice that during dry season on heterogeneous vegetation, sensible heat flux H is more driven by wind and orography while we not able to discuss the latent heat flux E case. During wet season with the same surface forcing, it appears that H is driven by wind while E is more dependent to vegetation variability. Our study concludes in all case that H and E are not characterized by the same length scale.

Dans les années récentes, des politiques visant à promouvoir l’efficacité énergétique ont été instaurées en réponse aux obligations réglementaires européennes et internationales. Le stockage d’énergie thermique s’est révélé être une technologie qui permet une amélioration de l’efficacité énergétique, en particulier celle des installations techniques pour le conditionnement d’air, le chauffage et l’eau chaude sanitaire pour le bâtiment. Parmi les différents types existants, le stockage thermique par chaleur sensible est le plus ancien et le plus répandu sur le marché. Or, l’intégration du ballon de stockage dans les installations énergétiques s’avère délicate tant dans la phase de conception que de l’exploitation de ces installations. Par ailleurs, il convient d’évaluer – pour les systèmes et équipements techniques du bâtiment – leurs consommations énergétiques annuelles (ou saisonnières). Pour répondre à l’ensemble de ces exigences, le recours à la modélisation et simulation dynamique des composants et systèmes énergétiques devient indispensable. Le travail de la présente thèse présente une approche de modélisation et de simulation dynamique d’un ballon de stockage d’eau par chaleur sensible qui répond aux contraintes particulières suivantes : assurer une modélisation fine à partir de la résolution des équations de Navier-Stokes d’un composant – le ballon de stockage – dans lesquels les mécanismes de transfert et d’écoulement sont complexes et réaliser une modélisation dynamique d’un système thermique associant des divers composants techniques d’un circuit et ceci avec des temps de calcul raisonnables, compatibles avec les pratiques courantes des bureaux d’étude spécialisés en conception d’installations . Le travail réalisé associe donc une analyse fine du comportement dynamique du ballon grâce au développement d’un modèle CFD, la détermination d’un modèle réduit à partir de ce modèle – qui permet la construction d’un champ dynamique de température – et enfin une modélisation sous Modelica adaptée à la simulation d’un système énergétique complexe. Dans les différentes phases de cette étude, les résultats issus de la simulation sont alors confrontés aux résultats déduits de divers travaux expérimentaux. La validation de la démarche suite à cette confrontation calculs/expériences permet d’envisager l’application des outils présentés à des projets techniques notamment au projet « PV cooling » de climatisation des bâtiments avec une ressource solaire photovoltaïque, projet réalisé en parallèle de ce projet de thèse et porté par les acteurs industriels qui soutiennent cette recherche. / In recent years, policies to promote energy efficiency have been introduced in response to European and International regulatory obligations. Thermal Energy Storage has proven to be a technology that improves energy efficiency, particularly for the air conditioning, heating and domestic hot water utilities in buildings. Among the existing types, sensible heat storage is the oldest and most widespread on the market. The integration of the storage tank into energy installations may be tricky in both the design and operation phases of these installations. Moreover, the annual (or seasonal) energy consumption of the building's technical systems and equipment should be evaluated. To meet all these requirements, dynamic modeling and simulation of energy components and systems becomes essential. The work of this thesis presents a dynamic modeling and simulation approach of a sensible heat water storage tank which respond to the following particular constraints: To ensure a fine modeling based on the resolution of the Navier-Stokes equations of a component – the storage tank – in which the flow and transfer mechanisms are complex, and to carry out a dynamic modeling and simulation, with reasonable computational time, of a thermal energy system associating various technical components of a circuit and compatible with the usual practices of the specialized system design offices. Thus, the carried out work combines a detailed analysis of the dynamic behavior of the storage tank through the development of a CFD model, the development of a reduced model from the previous CFD model that allows the construction of temperature dynamic fields and finally a Modelica modeling adapted to the simulation of a complex energy system. In the different phases of this study, the results from the simulation are compared to the results deduced from various experimental works. The validation of the approach following this comparison between calculations and experimental results makes it possible to consider the application, of the presented tools, in technical projects and in particular the project “PV cooling” for buildings air conditioning with a photovoltaic solar resource, a project that is carried out in parallel with this thesis by the industrial players supporting this research.

River, lakes and   streams account for more carbon dioxide emissions than all other freshwater   reservoirs together. However, there is still lack of knowledge of the   physical processes that control the efficiency of the air-water exchange of   CO2 in these aquatic systems. In the more turbulent water sections   of a river, the gas transfer is thought to be governed by the river’s   morphology such as bottom topography, slope and stream flow. Whiles for wider   sections of the river, the gas transfer could potentially be influenced by   atmospheric forcing (e.g. Wind speed). The main purpose of this project is to   study the fluxes of carbon dioxide and how (wind speed and stream discharge)   influence the CO2 fluxes in the river. In this study, direct and   continuous measurements of CO2 emission was conducted for the   first time in a controlled boreal river in Kattstrupeforsen (Sweden) from   18th April to 10th May 2018. A unique measurement setup which combines eddy   covariance techniques, general meteorology and in situ water variables (for   high accuracy emission measurements) was used. The results show that   in the late winter, an   upward directed CO2 fluxes measured in the river was approximately   2.2 μmol m−2 s−1. This value agrees with many other small and   large rivers where CO2 fluxes has been studied. The river can be   said to serve as source of CO2 to the atmosphere in the day due to   the dominant upward fluxes recorded during the daytime. The results also show   that carbon dioxide fluxes increase with increasing wind speed notably at   wind speed above 2 m s-1. There   was no relation between CO2 fluxes and stream discharge. This   indicates that wind speed could be one principal factor for air- river gas   exchange. The findings in this work on river gas exchange will provide   a basis for a regional estimate and be applicable for many river systems on a   global scale. / <p>2018-07-09</p>

L'impact des modifications climatiques et de l'augmentation de la démographie sur le cycle de l'eau et de l'énergie dans la région ouest africaine passe par la quantification des échanges entre les différents couverts de la surface continentale et l'atmosphère. Toutefois, la compréhension du rôle des interactions entre la surface et l'atmosphère dans la dynamique de la mousson ouest africaine est limitée par le manque d'observations dans cette région. Cette thèse porte sur l'étude des flux turbulents, en particulier l'évapotranspiration réelle, en climat soudanien. Les deux couverts étudiés sont une forêt claire (site de Bellefoungou) et une mosaïque de culture/jachère (site de Nalohou), situés dans la région du Nord – Bénin. On dispose de quatre années de mesures (2007 à 2010). Les sites d'étude font partie de l'observatoire hydro – météorologique AMMA – CATCH.Les données de flux turbulents de l'atmosphère ont été mesurées avec la technique d'eddy covariance. La partition énergétique des flux a été examinée à travers la fraction évaporative (EF) et le rapport de Bowen (β) aux échelles diurne, saisonnière et inter-annuelle. Des caractéristiques de surface (conductance de surface et aérodynamique) et le coefficient de découplage ont été calculés pour interpréter la dynamique de l'évapotranspiration réelle.L'analyse des résultats est basée sur un découpage du cycle saisonnier suivant quatre phases du cycle de la mousson : la saison sèche, la saison humide, les phases d'humidification et d'assèchement de l'atmosphère. Aux échelles diurne et saisonnière, on montre que le taux d'évapotranspiration réelle de la forêt est toujours supérieur à celui de la mosaïque de culture/jachère quelle que soit la saison. L'évapotranspiration réelle demeure non nulle en saison sèche sur le site de Nalohou malgré les conditions de surface peu favorables à ce processus. En saison humide, après le saut de mousson, la partition énergétique des flux atteint un régime stationnaire avec une moyenne égale à 0,75 à Bellefoungou et 0,70 à Nalohou pour les 4 années étudiées. Le rapport de Bowen pris dans le même ordre est environ de 0,4 et 0,6 traduisant ainsi, en dépit des conditions humides, la part non négligeable du flux de chaleur sensible sur les deux couverts végétaux. La différence de rugosité entre les deux couverts végétaux entraîne une conductance aérodynamique nettement supérieure à Bellefoungou par rapport à Nalohou. On montre également que la végétation du site de Nalohou est plus efficace en transpiration pendant la saison humide que celle du site de Bellefoungou. A l'échelle inter-annuelle, on n'a pas pu mettre en évidence une relation entre flux de chaleur latente et pluviométrie pour les quatre années étudiées qui sont toutes des années excédentaires. Cependant nous avons observé que le rayonnement net explique la majeure partie de la variabilité inter-annuelle des flux turbulents.Enfin, nous avons également montré avec le coefficient de découplage que la surface soudanienne et l'atmosphère restent couplées toute l'année. Ce fonctionnement de l'interface surface – atmosphère reflète le rôle majeur que jouent les conditions de surface dans la variabilité saisonnière de l'évapotranspiration réelle. Les résultats issus de cette étude donnent une première estimation des flux de chaleur latente et de chaleur sensible sur une forêt claire et une mosaïque de culture/jachère en climat soudanien. Ils sont d'une importance capitale pour la paramétrisation et la validation des modèles de surface ainsi que pour la quantification robuste de la ressource en eau disponible en surface pour l'agriculture, principale activité génératrice de revenus des populations locales. / Assessing the impact of climate and anthropic changes on the water and energy cycles, mainly rely on the quantification of the transfer between the various land covers and the atmosphere. Nevertheless the land – atmosphere interactions in the West African monsoon dynamic is not yet well understood because of the lack of observations in this region. This thesis focuses on the analysis of the sensible and latent heat fluxes under Sudanian climate. The two studied land covers are a clear forest (Bellefoungou) and a cultivated area (Nalohou), located in northern Benin, during four years (2007-2010). The study sites are a part of the hydro – meteorological AMMA – CATCH observatory.Turbulent fluxes were measured with the eddy covariance technique.The flux partitioning was investigated through the evaporative fraction (EF) and the Bowen ratio (β) at diurnal, seasonal and inter-annual scales. Surface characteristics (surface and aerodynamical conductance) and the decoupling factor were calculated to interpret the dynamic of the actual evapotranspiration.The analysis was performed according to four different stages of the monsoon cycle: dry and wet seasons drying and moistening intermediate stages. At diurnal and seasonal scales, actual evapotranspiration was always higher on the forest than on the cultivated area. It remained non zero during the dry season at Nalohou despite surface conditions which were not favorable to this process. During the wet season, after the monsoon onset, EF remained steady with a mean seasonal value of 0.75 at Bellefoungou and 0.70 at Nalohou for the four studied years. The Bowen ratio was 0.4 and 0.6 respectively, thus the sensible heat flux was significant on the two contrasted vegetation covers during the wet season. The contrasted roughness length of the two vegetation covers led to a highest aerodynamic conductance at the clear forest site. The mixed of crop/fallow was shown to be more efficient than the clear forest regarding wet season transpiration. At the inter-annual scale, no relationship can be evidenced between evapotranspiration and annual rainfall for the studied period (2007-2010), which was rather rainy. Nevertheless, the net radiation explains the main part of turbulent fluxes inter-annual variation.Finally, complete surface atmosphere decoupling was never observed. This property of the surface – atmosphere interface underlines the key role of the surface conditions in the actual evapotranspiration. Our results provide a first estimate of the latent and sensible heat fluxes over a clear forest and a mixed crop/fallow under sudanian climate. They are relevant to land surface models parametrisation or evaluation and to a robust quantification of the water resources for agriculture, the main economic activity in this region.

Tato diplomová práce se zabývá tématem výměny tepla v zásobníku tepla typu ”packed bed”. Cílem je popsat přenos tepla v zásobníku tepla obsahující kamínky malých průměrů, skrz který proudí horký vzduch. Toto je modelováno v prostředí MATLAB. Na začátku je krátký úvod do problematiky zahrnující ukládání tepla a jeho možné využití. Dále je uveden krátký přehled o základech přenosu tepla, typech přenosu tepla a termofyzikální vlastnosti systému vzduch-kámen. Ve třetí kapitole je představen zásobník tepla typu ”packed bed” a rozličné modely a dané podmínky jsou vysvětleny. Další kapitola se zabývá s numerickými metodami, převážně s metodou konečných diferencí použitou v této práci. Pátá kapitola se zaměřuje na obecnou optimalizaci daného problému přenosu tepla. Populačně založený metaheuristický optimalizační algoritmus zvaný Genetický algoritmus je popsán. Sestavení modelu je ukázáno v šesté kapitole, stejně jako prezentace výsledků získaných z programu MATLAB. V poslední kapitole je pak diskutován závěr a doporučení.

This master‘s thesis contains a comprehensive laboratory task about saving a heat energy with a use of changing a state of matter. The thesis is written for the object Chosen passages of renewable resources and saving a heat energy. Part one presents a theoretical part of the topic, terminology and relations needed to make this laboratory work. It also contains a created procedure of partial tasks necessary to take laboratory measurements. The second part of the thesis is about measuring and evaluation of acquired values according to procedure described in the theoretical part of the thesis. Different variations considered during the creation of measuring procedure are also named here. The final part of the thesis describes a laboratory equipment necessary to complete the task and an example of filled protocol is attached.

<p>Evaporative cooling is a highly energy efficient alternative to conventional vapor compression cooling system. The sensible cooling effect of evaporative cooling systems is well documented in the literature. Direct evaporative cooling however increases the relative humidity of the air as it cools it. This has made it unsuitable for data centers and other applications where humidity control is important. Desiccant-based dehumidifiers (liquid, solid or composites) absorb moisture from the cooled air to control humidity and is regenerated using waste heat from the data center. This work is an experimental and theoretical investigation of the use of desiccant assisted evaporative cooling for data center cooling according to ASHRAE thermal guidelines, TC 9.9. The thickness (depth) of the cooling pad was varied to study its effect on sensible heat loss and latent heat gain. The velocity of air through the pad was measured to determine its effect on sensible cooling. The flow rate of water over the pad was also varied to find the optimal flow for rate for dry bulb depression. The configuration was such that the rotary desiccant wheel (impregnated with silica gel) comes after the direct evaporative cooler. The rotary desiccant wheel was split in a 1:1 ratio for cooling and reactivation at lower temperatures. The dehumidification effectiveness of a fixed bed desiccant dehumidifier was compared with that of a rotary desiccant wheel and a thermoelectric dehumidifier. A novel condensate recovery system using the Peltier effect was proposed to recover moisture from the return air stream, (by cooling the return air stream below its dew point temperature) thereby optimizing the water consumption of evaporative cooling technology and providing suitable air quality for data center cooling. The moisture recovery unit was found to reduce the mass of water lost through evaporation by an average of fifty percent irrespective of the pad depth.</p> <p> </p>

The energy exchange in the soil-snow-vegetation-atmospheresystem was studied to improve the quantitative knowledge of thegoverning processes. The lack of such knowledge contributes tothe uncertainty in the applicability of many existing modelsindependent of the temporal or spatial scale. The theoreticalbackground and available methods for measurements and numericalsimulations were reviewed. Numerical simulation models andavailable data sets representing open land and boreal forestwere evaluated in both diurnal and seasonal time-scales.Surface heat fluxes, snow depth, soil temperatures andmeteorological conditions were measured at an agriculturalfield in central Sweden over two winters, 1997-1999. Twoone-dimensional simulation models of different complexity wereused to simulate the heat and water transfer in thesoil-snow-atmosphere system and compared with the measurements.Comparison of simulated and observed heat fluxes showed thatparameter values governing the upper boundary condition weremore important than the formulation of the internal mass andheat balance of the snow cover. The models were useful toevaluate the lack of energy balance closure in the observedsurface heat fluxes, which underlined the importance ofimproved accuracy in eddy correlation measurements of latentflow during winter conditions. The representation of boreal forest in the land surfacescheme used within a weather forecast model was tested with athree-year data set from the NOPEX forest site in centralSweden. The formulation with separate energy balances forvegetation and the soil/snow beneath tree cover improvedsimulation of the seasonal and diurnal variations of latent andsensible heat flux compared with an older model version.Further improvements of simulated surface heat fluxes could beexpected if the variation of vegetation properties within andbetween years and a new formulation of the boundary conditionsfor heat flux into the soil is included. Keywords: Surface energy balance, Snow, Boreal forest,SVAT models, Eddy-correlation Measurements, Latent heat flux,Sensible heat flux, Net radiation, Soil temperature,Aerodynamic roughness, Surface resistance / QC 20100614

<p>The energy exchange in the soil-snow-vegetation-atmospheresystem was studied to improve the quantitative knowledge of thegoverning processes. The lack of such knowledge contributes tothe uncertainty in the applicability of many existing modelsindependent of the temporal or spatial scale. The theoreticalbackground and available methods for measurements and numericalsimulations were reviewed. Numerical simulation models andavailable data sets representing open land and boreal forestwere evaluated in both diurnal and seasonal time-scales.Surface heat fluxes, snow depth, soil temperatures andmeteorological conditions were measured at an agriculturalfield in central Sweden over two winters, 1997-1999. Twoone-dimensional simulation models of different complexity wereused to simulate the heat and water transfer in thesoil-snow-atmosphere system and compared with the measurements.Comparison of simulated and observed heat fluxes showed thatparameter values governing the upper boundary condition weremore important than the formulation of the internal mass andheat balance of the snow cover. The models were useful toevaluate the lack of energy balance closure in the observedsurface heat fluxes, which underlined the importance ofimproved accuracy in eddy correlation measurements of latentflow during winter conditions.</p><p>The representation of boreal forest in the land surfacescheme used within a weather forecast model was tested with athree-year data set from the NOPEX forest site in centralSweden. The formulation with separate energy balances forvegetation and the soil/snow beneath tree cover improvedsimulation of the seasonal and diurnal variations of latent andsensible heat flux compared with an older model version.Further improvements of simulated surface heat fluxes could beexpected if the variation of vegetation properties within andbetween years and a new formulation of the boundary conditionsfor heat flux into the soil is included.</p><p><strong>Keywords</strong>: Surface energy balance, Snow, Boreal forest,SVAT models, Eddy-correlation Measurements, Latent heat flux,Sensible heat flux, Net radiation, Soil temperature,Aerodynamic roughness, Surface resistance</p> / QC 20100614

To understand and to predict the weather and climate, numerical models are important tools and it is crucial that the controlling processes are described correctly. Since 70% of the global surface is covered with water the description how the ocean and atmosphere communicates has a considerable impact. The ocean–atmosphere exchange occurs through transport of momentum (friction) and heat, governed by turbulent eddies. The sea surface is also an important source of turbulence in both directions. The scales of the turbulent eddies cannot be resolved in ocean and climate models. Therefore, the turbulent exchanges have to be related to mean variables, such as wind speed and temperature differences. By using measurements, new methods to describe the air–sea exchange during two specific processes were developed. These processes are the so-called UVCN-regime (Unstable Very Close to Neutral stratification) and swell, i.e. waves which are not produced by the local wind. These processes were included in an ocean model and in a regional atmospheric climate model and the impact was investigated. The UVCN-regime enhances the heat transport significantly during the autumn and winter months in the ocean model. This results in a shallower well-mixed surface layer in the ocean. Wind-following swell reduces the surface friction, which is very important for the atmosphere. Some secondary effects in the climate model are reduced low-level cloud cover and reduced precipitation by more than 10% over sea areas. Locally and for short periods the impact is large. It is important to include the UVCN-regime and the swell impact in models, to make simulations more reliable.