Heat transfers in a double-skin roof ventilated by natural convection in summer time

International audienceThe double-skin roofs investigated in this paper are formed by adding a metallic screen on an existing sheet metal roof. The system enhances passive cooling of dwellings and can help diminishing power costs for air conditioning in summer or in tropical and arid countries. In this work, radiation, convection and conduction heat transfers are investigated. Depending on its surface properties, the screen reflects a large amount of oncoming solar radiation. Natural convection in the channel underneath drives off the residual heat. The bi-dimensional numerical simulation of the heat transfers through the double skin reveals the most important parameters for the system's efficiency. They are, by order of importance, the sheet metal surface emissivity, the screen internal and external surface emissivity, the insulation thickness and the inclination angle for a channel width over 6 cm. The influence of those parameters on Rayleigh and Nusselt numbers is also investigated. Temperature and air velocity profiles on several channel cross-sections are plotted and discussed

Phase-change materials to improve solar panel's performance

International audienceHigh operating temperatures induce a loss of efficiency in solar photovoltaic and thermal panels. This paper investigates the use of phase-change materials (PCM) to maintain the temperature of the panels close to ambient. The main focus of the study is the computational fluid dynamics (CFD) modeling of heat and mass transfers in a system composed of an impure phase change material situated in the back of a solar panel (SP). A variation of the enthalpy method allows simulating the thermo-physical change of the material properties. The buoyancy term in Navier-Stokes' momentum conservation equation is modified through an additional term which forces the velocity field to be non-existent when the PCM is solid. For validation purposes, isotherms and velocity fields are calculated and compared to those from an experimental set-up. Results show that adding a PCM on the back of a solar panel can maintain the panel's operating temperature under 40 °C for 80 minutes under a constant solar radiation of 1000 W/m2

Volumetric Monitoring and Modeling of Indoor Air Pollutant Dispersion by the Use of 3D Particle Tracking Velocimetry

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Application of two-color LIF thermometry to nucleate boiling

International audienceThe laser-induced fluorescence (LIF) thermometry is applied to measure the temperature field surrounding a single vapor bubble growing at an artificial nucleation site. In order to correct measurement errors due to the non-uniformity of the incident laser intensity, the two-color LIF thermometry technique is used in this nucleate boiling experiment. This technique is based on the use of two fluorescent dyes: the temperature sensitive dye Rhodamine B and the temperature insensitive dye Sulforhodamine-101. The concentration of the dyes is optimized by analyzing the behavior of fluorescence intensities. The mapping between the two images is determined through a geometrical calibration procedure. This technique presents a success in correcting the non uniformities due to the reflection of the light at the bubble surface and to the temperature gradient. The obtained temperature fields show that the two-color LIF is a promising technique in the investigation of nucleate boiling

Solar PV Passive Temperature Control Using Phase Change Materials

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Large scale particle tracking velocimetry for 3-dimensional indoor airflow study

Alors que la majorité des méthodes actuelles de suivi Lagrangien de particules ne permettent que la mesure d\u27écoulements 2D ou 3D à petite échelle, le présent document décrit un algorithme complet de suivi Lagrangien 3D en grand champ. Des bulles de savon gonflées à l\u27hélium servent de traceurs. L\u27éclairement est fourni par des sources continues de type halogènes. Des cameras synchrones sont préalablement calibrées suivant des algorithmes connus afin d\u27avoir un repère 3D commun. Apres suppression du fond, une procédure spéciale supprime les larges spots créées par les images des bulles se rapprochant des caméras. Les centres de masse des bulles restantes sont ensuite calculés. Le suivi temporel se fait soit par corrélation croisée, soit par régression linéaire tempéré par l\u27emploi d\u27un critère de qualité. La triangulation 3D se fait par une méthode de moindres carrés. L\u27algorithme complet est testé dans plusieurs configurations : dans une cellule expérimentale de taille 3.1mx3.1mx2.5m aux parois gris clair(ii) dans une salle de taille 5.5mx3.7mx2.4m aux parois noires(iii) au dessus d\u27une source de chaleur(iv) à l\u27intérieur d\u27une maquette d\u27avion de taille réelle(v) dans un réservoir 100cmx100cm 100cmx rempli d\u27une solution aqueuse en utilisant une lumière continue et des particules creuses de 10um de large. Dans chaque cas, des lignes directrices sont données pour un positionnement optimal des cameras et des sources de lumière. Les résultats montrent que l\u27algorithme est capable de suivre plus de 1400 traceurs dans des volumes allant jusqu\u27a 3mx3mx1.2m

Thermal behavior of a passive solar wall with silica aerogel and phase change materials

International audienceThe present contribution discusses the thermal behavior of a passive solar wall composed of a glazing facing the outside, a channel filled with highly insulating and translucent silica aerogel, and glass bricks filled with an eutectic phase change material (PCM), facing the inside. First, the experimental building with the tested wall, south-oriented, is described. Second, the experimental results on four different weather conditions are detailed. Thanks to a numerical model, the wall behavior is simulated for the French climatic zones of Nice (Mediterranean climate), La Rochelle (oceanic climate), Embrun (inland mountain climate) and Nancy (cold continental climate). Results show that the wall is translucent and may save 27% to 83 % on the building energy bill for heating, depending on the building location. The wall performs best on cold sunny climates, where the phase change can occur all year. The air temperature inside the building is always 1 to 9°C above the external temperature. Overheating may occur in summer for Mediterranean and oceanic climates when the PCM stays in liquid state. This may be prevented by the use of shading

Phase Change Materials (PCM) for cooling applications in buildings: A review

International audienceCooling demand in the building sector is growing rapidly; thermal energy storage systems using phase change materials (PCM) can be a very useful way to improve the building thermal performance. The right use of PCM in the envelope can minimize peak cooling loads, allow the use of smaller HVAC technical equipment for cooling, and has the capability to keep the indoor temperature within the comfort range due to smaller indoor temperature fluctuations. This article presents an overview of different PCM applications in buildings for reducing cooling loads under different climate conditions, and the factors affecting the successful and the effective use of the PCM. Many drawbacks have been found in PCM applications, mainly the intense impact of summer weather conditions over the PCM performance, which prohibits its complete solidification during night, and thus, limiting its effectiveness during the day. Proposed solutions are reviewed in this article. Finally, a topology diagram is presented to summarize the steps leading to an effective use of PCM in building applications

Gestion thermique des batteries Li-ion par l'utilisation de matériaux à changement de phase

International audienceLes batteries lithium-ion nécessitent une gestion thermique pour que soit assuré leur bon fonctionnement. Des systèmes actifs comme un refroidissement par air en convection forcée ou par un liquide de refroidissement sont employés actuellement. L’utilisation de matériaux à changement de phase (MCP), en tant que système passif, représente une alternative intéressante. Dans ce papier une comparaison est proposée entre un système comportant un MCP et un système à air en convection forcée