Keywords: bâtiment, paroi ventilée, convection Building, ventilated wall, convection Nomenclature q densité de flux, W.m -2 h coefficient d

est destinée au dépôt

Mise en place d'un dispositif expérimental pour la caractérisation énergétique d'une paroi bioclimatique ventilée

Dwinanto Sukamto, Francois Gloriant, Monica Siroux

To cite this version:

Mise en place d'un dispositif expérimental pour la caractérisation énergétique d'une paroi bioclimatique ventilée

Installation of an experimental device for the energetic characterization a ventilated bioclimatic wall Dwinanto SUKAMTO 1* , François GLORIANT 1 , Monica SIROUX 1

Introduction

Le changement climatique est l'enjeu majeur que devra affronter le monde dans l'avenir. Dans ce contexte, la France place le secteur du bâtiment au coeur de sa stratégie pour relever ce défi [START_REF]Bâtiments à énergie positive et réduction carbone[END_REF]. L'économie d'énergie est une priorité dans les pays développés. Le secteur du bâtiment est responsable de 40% de la consommation d'énergie en Europe [START_REF] Commission | Evaluation of the Energy Performance of Building Directive[END_REF]. L'enveloppe du bâtiment est l'élément de construction qui a le plus grand impact sur la consommation énergétique globale du bâtiment [START_REF] Manioǧlu | Economic evaluation of the building envelope and operation period of heating system in terms of thermal comfort[END_REF] [START_REF] Pacheco | Energy efficient design of building: A review[END_REF]. Il faut tenir compte du fait que la façade représente l'interface entre les facteurs environnementaux extérieurs et les exigences intérieures des occupants.

Ces dernières années, l'architecture a montré un intérêt particulier pour les parois ventilées. On ne peut nier qu'il existe un nombre considérable d'études et de publications sur les "enveloppes ventilées" [5][6] [START_REF] Lai | Solar façades: A review[END_REF], en particulier sur les façades double peau [START_REF] Barbosa | Perspectives of double skin façades for naturally ventilated buildings: A review[END_REF][9] [START_REF] Shameri | Perspectives of double skin façade systems in buildings and energy saving[END_REF], le photovoltaïque intégré aux bâtiments [START_REF] Quesada | A comprehensive review of solar facades. Opaque solar facades[END_REF][12] [START_REF] Zogou | Experimental validation of an improved concept of building integrated photovoltaic panels[END_REF], les cheminées solaires, les murs solaires et les murs Trombe [START_REF] Harris | Solar chimney and building ventilation[END_REF][15] [START_REF] Saadatian | Trombe walls: A review of opportunities and challenges in research and development[END_REF], ou encore les capteurs solaires de façade [START_REF] Gunnewiek | Flow distribution in unglazed transpired plate solar air heaters of large area[END_REF] [START_REF] Leon | Mathematical modeling and thermal performance analysis of unglazed transpired solar collectors[END_REF].

La façade ventilée est choisie par les architectes et les entrepreneurs comme solution d'enveloppe dans une large variété de types de bâtiments, de climats et de configurations de conception. Le système offre une grande diversité de revêtements extérieurs et la possibilité de sélectionner une grande variété de matériaux, de couleurs et de tailles de panneaux. Outre l'aspect esthétique, le but premier de ce type de façade est de protéger les matériaux isolants en dissipant l'humidité.

L'objectif d'une conception bioclimatique d'un bâtiment est d'obtenir un confort intérieur de la manière la plus « naturelle » possible. Pour un site donné, il ne s'agit plus uniquement de se protéger des aléas du climat mais également de valoriser ses bienfaits. Dans cette démarche, on cherche donc à adapter au mieux le bâtiment avec son environnement. La paroi ventilée répond à cette démarche. Dans cette étude, nous cherchons à valoriser la lame d'air de la paroi pour assurer le préchauffage et le renouvellement d'air en période de chauffe. La paroi bioclimatique ventilée fait alors office d'organe d'entrée pour le renouvellement d'air du bâtiment qui en est muni. L'écoulement a lieu sous l'effet du tirage thermique ou à l'aide d'une VMC si le bâtiment concerné en est muni. Ce type de paroi est étudié depuis plusieurs décennies mais son utilisation pour assurer le préchauffage de l'air neuf reste encore rare. De plus, ces performances sont encore sujet à débat. Un dispositif expérimental a été mis en place pour la caractérisation énergétique de ce type de paroi innovante. L'originalité de notre dispositif repose en partie sur l'utilisation des fluxmètres qui permet une analyse plus précise des phénomènes de convection. Notre objectif à terme est d'étudier l'influence de l'épaisseur de la cavité ainsi que du débit d'air sur les performances de la paroi. Pour cela, nous devons auparavant nous assurer que le dispositif est pertinent et opérationnel. L'objet de cet article est donc de présenter notre dispositif expérimental et de montrer à travers plusieurs essais préliminaires sa viabilité.

Description du dispositif expérimental mis en place

Présentation de la paroi ventilée et de la boîte chaude gardée

La paroi ventilée est une paroi avec un canal non hermétiquement isolé participant à l'apport d'air neuf au sein du bâtiment; un échange convectif a lieu dans la cavité selon l'axe de la paroi, qui vient perturber le flux thermique radial entre les ambiances intérieure et extérieure (Figure 1). L'air en écoulement à l'intérieur de la cavité est alors préchauffé et introduit à l'intérieur du bâtiment. Ce dispositif permet alors la diminution des besoins en chauffage du bâtiment. L'échantillon étudié est composé de deux parois de couleur grise de 1m de hauteur et de largeur. La première est fixe tandis que la seconde est mobile permettant ainsi de faire varier l'épaisseur de la cavité ventilée. Ces deux parois sont constituées d'un alliage d'aluminium et de polyéthylène de 3 mm d'épaisseur et présentent une résistance thermique est de 0.008 m².K.W -1 . Leurs surfaces sont lisses (faible rugosité) avec une émissivité de 0.9.

La lame d'air ventilée est introduite au sein d'une boîte chaude gardée (Figure 1). Une boîte chaude gardée est un dispositif composé de deux enceintes climatiques capables de reproduire des conditions de température et d'humidité relative choisies. La plage en température et en humidité va de -30°C à +60°C et de 10% à 98%. Chaque enceinte possède son propre groupe frigorifique garantissant la production de froid avec une précision sur la température de ± 2 °C. Chacune possède également 3 résistances électriques de 2.5 kW, garantissant la production de chaleur avec une précision sur la température de ± 3 °C [START_REF] Entreprise | Manual Usuario[END_REF]. La boîte chaude gardée est connectée à un ordinateur permettant de contrôler les paramètres de température et d'humidité de chacune des enceintes.

Instrumentation et système d'acquisition

Pour caractériser le comportement thermique de l'échantillon étudié, la paroi ventilée est équipée de thermocouples de type K et de fluxmètres de la marque Captec Entreprise®, permettant la mesure conjointe des températures de surface, de l'air à l'intérieur comme à l'extérieur de la cavité, ainsi que l'estimation des flux de chaleur de part et d'autre de la cavité ventilée (Figure 2). Dans chacune des cellules climatiques, 3 thermocouples permettent de déterminer les températures d'ambiance. Au niveau de chacune des parois de la cavité ventilée, 12 thermocouples et 4 fluxmètres ont été installés sur 4 niveaux de hauteurs différentes et sont également répartis sur la largeur. Pour la mesure de la température d'air, 3 thermocouples ont été disposés à l'entrée et 3 autres à la sortie de la cavité. A l'intérieur et au milieu de la cavité, se trouvent également 4 thermocouples. L'écoulement d'air au sein de la cavité est généré à l'aide d'un ventilateur contrôlé par un variateur de tension et la mesure du débit est estimée à l'aide d'un anémomètre à fil chaud CTV 210 du fabricant KIMO. L'ensemble des capteurs sont connectés à un multimètre Keithley 2700® permettant une mesure en tension de l'ordre du dixième de microVolt. Le multimètre est lui-même relié à un ordinateur par le canal de communication série RS-232. L'acquisition et le traitement des données sont alors effectués via le logiciel LabVIEW.

Une caméra infrarouge est également utilisée afin d'avoir une estimation de la température de la paroi froide du dispositif.

Températures de consigne de la boîte chaude gardée

Dans un premier temps, une vérification de la température de consigne dans chacune des cellules de la boîte chaude gardée a été réalisée. On cherche à obtenir une différence de température d'au moins 20°C entre les 2 cellules de façon à obtenir des flux de chaleur suffisamment élevés pour être correctement mesurés par les fluxmètres [START_REF] Gloriant | Study Of The Performances Of A Supply-Air Window For Air Renewal Pre-Heating[END_REF].

Figure 3 : Mesures de la température ambiante dans la cellule froide (à gauche) et dans le cellule chaude (à droite)

Avec les consignes considérées (20°C dans la cellule fixe et 0°C dans la cellule mobile), on constate pour chacune des cellules, une différence non négligeable sur la mesure de la température ambiante entre le capteur intégré de la boîte chaude gardée et les mesures des thermocouples (Figure 3). On ne peut donc pas se fier à la sonde de température intégrée à la boîte chaude gardée car celle-ci surestime la température ambiante de 2,6°C pour la cellule mobile et de 4,3°C pour la cellule fixe. Cette imprécision peut en partie s'expliquer par le fait que les cellules ne sont pas étanches puisque l'on a créé artificiellement un écoulement d'air entre elles. Or pour une utilisation standard de la boîte chaude gardée, il ne devrait pas y avoir d'échange d'air entre les cellules ; le système de régulation du dispositif s'en trouve alors perturbé.

Par ailleurs, on constate que le brassage de l'air est suffisant pour obtenir une répartition homogène de la température dans le temps et dans l'espace. En moyenne, la différence de mesure maximale constatée entre les thermocouples est de 0,04°C dans la cellule mobile et de 0,7°C dans la cellule fixe.

Températures de surface

Dans un second temps, on cherche à vérifier la mesure des températures de surface effectuée par les thermocouples. Pour cela, on la compare à celle obtenue à l'aide d'une caméra infrarouge. La zone couverte par la caméra infrarouge présente une surface de 10 cm 2 . Sur la partie haute et sur la partie basse de cette surface, se trouvent respectivement les thermocouples T8 et T5 (Figure 4). On peut observer que les mesures de température de chacun des capteurs sont stables dans le temps et sensiblement proches. Les moyennes sur la période considérée pour les thermocouples T5, T8 et la caméra infrarouge sont respectivement de 12,14°C, 12,11°C et 12,18°C. C'est un résultat attendu que d'obtenir une température légèrement supérieure au niveau du thermocouple T5 par rapport au thermocouple T8 [START_REF] Gloriant | Study Of The Performances Of A Supply-Air Window For Air Renewal Pre-Heating[END_REF]. En effet, l'air en circulant dans la paroi se réchauffe peu à peu en récupérant une partie du flux thermique venant de la cellule chaude. Ainsi, la paroi adjacente à la cellule froide qui est en contact avec cet air de plus en plus chaud, se réchauffe également le long de la cavité. En faisant une moyenne de température avec les thermocouples T5 et T8, on obtient une moyenne de la température sur la surface couverte par la caméra infrarouge. On constate alors que l'écart de mesure entre ces 2 types de sondes est de 0.05°C. Les mesures des températures de surface par les thermocouples sont donc en adéquation avec les mesures réalisées par caméra infrarouge et confirme la fiabilité de l'instrumentation des parois.

Calcul du coefficient d'échange de chaleur h à la surface des parois

Pour finir, on cherche à quantifier le coefficient d'échange de chaleur entre les parois de notre échantillon et les ambiances de chacune des cellules. Pour ce travail, nous avons considéré 2 épaisseurs de lame d'air distinctes (0.005 m et 0.01 m) et 3 débits d'air (10 m 3 .h -1 , 20 m 3 .h -1 et 30 m 3 .h -1 ). L'utilisation conjointe des thermocouples et des fluxmètres permet une estimation expérimentale de ces coefficients d'échange sans avoir recours à des corrélations existantes. C'est un des intérêts de notre dispositif. Pour estimer ces paramètres, on se base sur la loi de Newton : h=q/(Ts-Ta)

(

Selon cette relation, l'estimation du coefficient d'échange pariétal h, nécessite la connaissance de la densité du flux de chaleur q traversant la paroi, de la température ambiante de l'air dans la cellule considérée Ta ainsi que de la température à la surface de la paroi Ts.

Chaque paroi de l'échantillon étant munie, selon différentes hauteurs, de 4 fluxmètres et de 4 thermocouples, il est possible d'obtenir 4 estimations locales du coefficients h par cellule. Pour la température ambiante dans chacune des cellules, on considère une unique température d'air correspondant à la moyenne des mesures des 3 thermocouples installés dans chaque cellule.

La figure 5 présente les estimations du coefficient h à la surface de la paroi dans chacune des cellules et pour des épaisseurs de cavité et des débits d'air différents. Le coefficient h varie entre 15 W.m -2 .K -1 et 24 W.m -2 .K -1 sur l'ensemble des essais avec une moyenne de 20,4 ± 2,7 W.m -2 .K -1 pour la cellule mobile et 18,4 ± 1,5 W.m -2 .K -1 pour la cellule fixe. Cet ordre de grandeur est cohérent par rapport aux corrélations que l'on peut trouver dans la littérature [START_REF] Padet | Convection thermique et massique Principes généraux[END_REF]. On peut remarquer que ni l'épaisseur de cavité, ni le débit d'air dans la paroi, n'ont véritablement d'influence sur les estimations du coefficient h. Cela signifie que ces 2 paramètres n'ont pas d'influence sur les ambiances des 2 cellules climatiques. Par contre, le coefficient h dépend de la position à laquelle il a été calculé et on peut constater que, pour une hauteur donnée, les estimations du coefficient h sont relativement homogènes. On peut penser que, dans chacune des cellules, les mouvements d'air provoqués par le système de régulation, ne sont pas uniformes près des parois de l'échantillon, provoquant des variations locales du coefficient h. Les résultats concernant les incertitudes sur les estimations du coefficient h (Tableau 1) sont globalement satisfaisants. Une incertitude de 5% a été considérée pour les fluxmètres et une incertitude de 0.02°C pour les thermocouples. On peut constater que les incertitudes sont plus faibles dans la cellule chaude dans la mesure où les flux et les différences de températures mis en jeu sont plus importants. C'est dans la cellule froide, au niveau de la partie basse de la paroi que les incertitudes sont les plus importantes ; à cet endroit, les températures entre la paroi et l'ambiance de la cellule froide sont très proches et souvent de l'ordre du dixième de degré.

Conclusion

Cette étude a pour objectif de montrer la pertinence de notre dispositif expérimental pour l'étude des parois bioclimatiques ventilées. A travers plusieurs essais préliminaires, nous avons cherchons à caractériser notre dispositif. Nous avons tout d'abord remarqué que le système de régulation de la boîte chaude gardée n'est pas très précis et qu'il est indispensable d'instrumenter chacune des cellules pour en déterminer les conditions d'ambiance. Concernant la mesure des températures de surface, les résultats sont semblables, que l'on utilise des thermocouples ou une caméra infrarouge. Pour finir, nous avons cherché à estimer la valeur du coefficient d'échange h au niveau de chaque paroi de notre échantillon. Ce dernier travail a été complété par une analyse des incertitudes de mesure qui s'est révèle satisfaisante.

Par la suite, nous allons entamer une série d'expérimentations de façon à identifier la configuration optimale de la paroi ventilée en termes d'épaisseur de cavité et de débit.

Figure 1 :

 1 Figure 1: Présentation de la paroi ventilée (à gauche) intégrée à la porte-échantillon de la boîte chaude gardée (à droite)

  sens de circulation de l'air entre les deux cellules flux thermique en partie récupérée par l'air

Figure 2 :

 2 Figure 2: Instrumentations du dispositif

Figure 4 :

 4 Figure 4 : Mesure des températures de surface

Figure 5 :

 5 Figure 5 : Estimation du coefficient h à la surface de la paroi dans la cellule froide (à gauche) et dans la cellule chaude (à droite)

  Tableau 1 : Calcul des incertitudes sur la mesure du coefficient d'échange h à la surface de la paroi dans la cellule froide et dans la cellule chaude (en W.m -2 .K -1 )

				cellule froide					cellule chaude
	hauteur sur la paroi (m)	cavité de 5 mm pour un débit (en m 3 .h -1 ) de	cavité de 10 mm pour un débit (en m 3 .h -1 ) de	cavité de 5 mm pour un débit (en m 3 .h -1 ) de	cavité de 10 mm pour un débit (en m 3 .h -1 ) de
		10	20	30	10	20	30	10	20	30	10	20	30
	0.8	1.5	1.5	1.7	1.5	1.4	1.7	1.5	1.5	1.4	1.8	1.8	1.6
	0.56	2.7	2.3	2.7	2.2	2.0	2.4	1.2	1.2	1.1	1.3	1.3	1.2
	0.345	4.6	3.0	3.0	3.1	2.7	3.2	1.5	1.6	1.5	1.6	1.6	1.4
	0.123	5.1	2.8	2.7	3.1	2.9	3.4	1.4	1.3	1.5	1.4	1.5	1.2

Dwinanto Sukamto, Francois Gloriant, Monica Siroux. Mise en place d'un dispositif expérimental pour la caractérisation énergétique d'une paroi bioclimatique ventilée. 28e Congrès français de thermique SFT, Belfort, France, 9-12 juin 2020, Jun 2020, Belfort, France. 10.25855/SFT2020-037. hal-

Remerciements

Les auteurs souhaitent exprimer leur gratitude à la fondation ARCONIC et l'INSA de Strasbourg.

L'archive ouverte pluridisciplinaire HAL,


Paolo Cabras

Pierre Auloge

Erik Dumont

Benoit Wach

Élodie Breton

Afshin Gangi

Jonathan Vappou

A new versatile MR-guided high-intensity Focused Ultrasound (MRgHIFU) device: system performance assessment and clinical feasibility

des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

A NEW VERSATILE MR-GUIDED HIGH-INTENSITY FOCUSED ULTRASOUND (MRgHIFU) DEVICE: SYSTEM PERFORMANCE ASSESSMENT AND CLINICAL FEASIBILITY Paolo Cabras 1,2 , Pierre Auloge 3 , Erik Dumont 2 , Benoit Wach 1 , Elodie Breton 1 , Afshin Gangi 1,3 , and Jonathan Vappou 1 1 Université de Strasbourg, CNRS, ICube, UMR7357, Strasbourg, France; 2 Image Guided Therapy, Pessac, France; 3 Department of Interventional Imaging, Strasbourg University Hospital, France Purpose: Originally developed for the treatment of uterine fibroids, whole-body MRgHIFU systems are not optimal for treating musculoskeletal tumors because of the difficulty in patient positioning 1 , as well as the need for a dedicated, expensive water-filled MR-table. We have developed a new portable MRgHIFU device in which the transducer can be directly positioned on the patient. Methods: The proposed device relies on: (1) An MR-safe flexible supporting device that allows free manipulation of the transducer. Up to 4 supporting legs with adaptable length can support the transducer holder. Once the transducer is in the desired position, the supporting device is rigidified using the granular jamming principle 2 . (2) A navigation software that guides the physician during the transducer positioning process outside the MRI bore, through a virtual environment, displaying the target, the planned configuration, and the actual position of the transducer in real time. Once the transducer has been positioned, the HIFU ablation is monitored in real time using Proton Resonance Frequency Shift (PRFS) Thermometry. Positioning accuracy was first evaluated in a phantom. First clinical results on a patient suffering from a bone metastasis in the forearm are also presented.

Results:

The average targeting error of the complete process was found to be equal to 5 ± 2 mm in terms of focus position, and 4 ± 2 degrees in terms of transducer orientation. In the clinical trial, the lesion to be treated in the first patient was an ellipsoid of 81×47×34 mm 3 . Five different transducer positions were necessary to cover the whole volume, in addition to using electronic steering (using a 128-element, 1MHz transducer) for each transducer pose. The patient was successfully treated: Pain score decreased from 8/10 to 3/10 four days after treatment, and 85% of the lesion was treated. Pain palliation was durable, and no adverse effects were reported.

Conclusion:

The proposed MRgHIFU device allows easy and accurate positioning of the HIFU transducer directly on the patient. This system is intended to be modular and flexible: It can be easily adapted to a variety of clinical applications that may require different HIFU transducers in terms of central frequency, aperture, number of elements and geometry. We expect this device to increase the accessibility to MRgHIFU therapies thanks to its versatility and its intrinsic low-cost nature.

  : 1. Joo, B. et al. Pain Palliation in Patients with Bone Metastases Using Magnetic Resonance-Guided Focused Ultrasound with Conformal Bone System: A Preliminary Report. Yonsei Med J 56, 503-509 (2015). 2.Behringer, R. P. & Chakraborty, B. The physics of jamming for granular materials: a review. Rep Prog Phys 82, 012601 (2019).

Fig. 1 :

 1 Fig. 1 : (A) General setup; (B) Experimental setup for the presented clinical case: the transducer is positioned over the forearm; (C) Transverse MR image showing the HIFU transducer, and the target on the ulna; (D) Real-time MR Thermometry during the ablation


Sonja Kallio

Monica Siroux
email: monica.siroux@insa-strasbourg.fr

Multi-objective design optimization of a hybrid renewable energy system

Keywords: renewable energy, photovoltaic-thermal, micro cogeneration, multi-objective optimization, hybrid energy system

A photovoltaic-thermal (PVT) collector is a solar-based micro-cogeneration system which generates simultaneously heat and power for buildings. Thermal and electrical energy storages are coupled with the PVT collectors to support fluctuating energy production in order to meet a certain energy demand. To maximize the self-consumption of renewable energy sources, the optimal size of the hybrid energy system components has to be determined. In this paper, a solar energy system is modelled and a genetic algorithm (NSGA-II) is used to obtain a Pareto front of optimal design solutions for decision makers.

Introduction

The building sector has a high impact on greenhouse gas emissions and final energy consumption in the European Union (EU) by being the largest energy end-use sector with a share of 41% [START_REF] Doroudchi | Approaching net zero energy housing through integrated EV[END_REF]. The micro combined heat and power (micro-CHP) units produce simultaneously heat and electricity from a single fuel source at high efficiency and close to the consumption point for building use. The units are referred up to 15 kW of electrical power [START_REF] Murugan | A review of micro combined heat and power systems for residential applications[END_REF]. The renewable energy based micro-CHP systems are in the key role in reaching the primary energy and pollutant emissions reduction targets of the EU [START_REF] González-Pino | Modelling and experimental characterization of a Stirling engine-based domestic micro-CHP device[END_REF].

The photovoltaic-thermal (PVT) collector is a solar-based micro-cogeneration unit which produces electricity by the PV module and useful heat by cooling the PV module with a coolant fluid. This leads to increase overall system efficiency but also an increasing electrical efficiency due to the decreased operation temperature of the PV module [START_REF] Tamayo | Performance evaluation and multi-objective optimization of hybrid photovoltaic-thermal collectors[END_REF]. Liang et al. [START_REF] Liang | Performance evaluation of sheet-and-tube hybrid photovoltaic/thermal (PVT) collectors connected in series[END_REF] studied the performance of PVT collectors connected in series. They performed numerical and experimental study, in which 6 PVT collectors where connected in series resulting to total amount of 36 collectors. They analyzed coolant fluid outlet temperature, PV temperature, and electrical and thermal efficiencies. The serial connection of PVT collectors increases coolant outlet temperature until certain point but decreases the electrical efficiency of the last panels due to higher operation temperature. Buonomano et. al [START_REF] Buonomano | Dynamic analysis of the integration of electric vehicles in efficient buildings fed by renewables[END_REF] performed a dynamic analysis of the hybrid energy system including PVT collectors, energy storages and electric vehicle. They studied energy, environmental and economic performance of the system powering a building. They also carried out a sensitivity analysis to optimize the system configuration.

The PVT collectors can be connected in series with multiple rows, and be integrated with electrical and thermal storages to provide energy for residential building use. The optimal size of the PVT system components depends mainly on the electrical and thermal energy demand of the building but also on the weather conditions and amount of solar radiation during the day. In order to maximize self-consumption of the solar system, the most of the produced energy has to be stored on-site for the later use of the building.

In recent years, the design methods have got alongside new design methods using nature inspired artificial intelligence methods, such as genetic algorithm (GA), evolutionary algorithm and particle swarm optimization (PSO). The objective function describes the key design criteria of the optimization problem. In many optimization cases, multiple objectives have to be satisfied in order to find a trade-off solution for the design. Due to this, instead of a single-objective optimization, the multi-objective optimization approach is recommended resulting in more accurate and realistic system design. [START_REF] Ko | Multi-Objective Optimization Design for Indirect Forced-Circulation Solar Water Heating System Using NSGA-II[END_REF] In this given framework, a research contribution is provided by demonstrating multiobjective optimization of the PVT/battery/thermal storage system with the evolutionary NSGA-II approach. The objectives of the 3-dimensional optimization problem are to maximize the thermal and electrical reliability of the system and to minimize the initial investment costs. The Pareto optimal set of design solutions is derived and analyzed. The sensitivity analyze of the selected inputs is carried out.

Methodology

Photovoltaic-thermal (PVT) collectors provide low-grade heat for domestic hot water (DHW) production and electricity for building appliances. In this section, the model description of the hybrid renewable energy system in Fig. 3 including PVT collectors, thermal storage, battery system, thermal load and electrical load is presented. A residential building is selected to present the energy consumption side of the system. The hourly yearly meteorological data of Strasbourg, including global solar radiation, ambient temperature and wind speed, is used to simulate PVT collector thermal and electrical energy production. Strasbourg is located in the Central Europe in Northern France and has oceanic and semicontinental climate. The system is modelled into Matlab/Simulink environment and the opensource CARNOT Toolbox [START_REF] Wohlfeil | CARNOT Toolbox[END_REF] is used to model the system components, such as thermal and electrical storages.

The solar system provides the energy demand of the building and an auxiliary boiler is used to support DHW production. The electricity production of the PVT collectors is supported by the electric grid connection and the surplus production is first stored to the battery system or sold to the grid.

PVT collectors

The dynamic model of the covered PVT collector have been presented in the literature [START_REF] Tamayo | Performance evaluation and multi-objective optimization of hybrid photovoltaic-thermal collectors[END_REF][START_REF] Chow | Performance analysis of photovoltaic-thermal collector by explicit dynamic model[END_REF][START_REF] Kallio | Energy Analysis and Exergy Optimization of Photovoltaic-Thermal Collector[END_REF][START_REF] Da Silva | Hybrid photovoltaic/thermal (PV/T) solar systems simulation with Simulink/Matlab[END_REF][START_REF] Evola | Exergy and thermoeconomic optimization of a water-cooled glazed hybrid photovoltaic/thermal (PVT) collector[END_REF] and used to evaluate the thermal and electrical performance of the collector. The following energy balance governing equations of various collector layers are used to model the single water-based flat plate PVT collector.

The glass cover (g):

(1) The PV layer (pv):

(2)

In which electrical efficiency η pv(T) depends on the photovoltaic cell/absorber layer temperature T pv . Due to this the electrical efficiency of the PV depends linearly on the temperature T pv , the temperature coefficient β pv and on the efficiency at standard conditions T ref . The efficiency is calculated according to the following relation:

(

The absorber layer (a):

(4)
The tube bonding (t):

(5)

The insulation layer (i): [START_REF] Buonomano | Dynamic analysis of the integration of electric vehicles in efficient buildings fed by renewables[END_REF] The coolant fluid (f): [START_REF] Ko | Multi-Objective Optimization Design for Indirect Forced-Circulation Solar Water Heating System Using NSGA-II[END_REF] The governing equations (1-7) are implemented to the Matlab/Simulink. The more detailed description of the equations and model validation are presented in the Author's previous work [START_REF] Kallio | Energy Analysis and Exergy Optimization of Photovoltaic-Thermal Collector[END_REF]. The model of the single PVT collector is used to model the PVT field with the number of PVT collectors in series N and the number of PVT rows M.

Electrical storage

The battery system is used to store produced electricity when the electricity demand of the building is lower than the production from the PVT collectors. The use of battery storage improves the matching between the electricity production and demand in order to maximize the self-consumption.

The battery model is based on the load balance between the electricity production and demand. If the electricity production is higher than demand, the battery is charged and discharged if the demand is higher. If the battery is full and the demand is low, the electricity is sold to the grid. P dis [START_REF] Wohlfeil | CARNOT Toolbox[END_REF] where SOC B is the battery state of charge. [START_REF] Chow | Performance analysis of photovoltaic-thermal collector by explicit dynamic model[END_REF] where P ch and P dis are charging and discharging powers and C B is the battery capacity. The battery block of the CARNOT Toolbox is used to simulate the battery behavior.

Thermal storage

The considered solar energy system includes a stratified thermal storage for hot water to improve the system efficiency and enable the matching between the heat production and demand. The CARNOT Toolbox was used to model the stratified thermal storage of the system. The volume of the storage is divided into nodes to present the stratification. For each node, the energy balance is calculated in order to find the current temperature of each node. This modelling approach is widely used in the literature [START_REF] Tamayo | Multi-objective optimization of hybrid photovoltaicthermal collectors integrated in a DHW heating system[END_REF][START_REF] Uchman | An Experimental Data-Driven Model of a Micro-Cogeneration Installation for Time-Domain Simulation and System Analysis[END_REF]. The CARNOT tank model called "Storage_Type_1" was used in the simulation. The model present a heat buffer with 4 nodes in which solar heat is injected and transferred to the cold DHW flow by heat exchangers. More detailed description of the tank model is presented in [START_REF] Wohlfeil | CARNOT Toolbox[END_REF].

Energy demand profiles

The residential building is selected to present the demand side of the model in the optimization problem. The normalized standard electricity demand profiles generated by German Association of Energy and Water Industries, BDEW (Bundesverband der Energieund Wasserwirtschaft) [START_REF]BDEW Standardlastprofile Strom[END_REF] are used to simulate electricity demand in every 15 min in the residential building. The profile is based on the measured data and takes into account electrical appliances excluding special applications, such as heat pumps and electrical storage heaters. The used electricity profile includes 3 different time periods of the year: winter, summer and transition. Additionally, 3 different day types are presented: work day, Saturday and Sunday/holiday.

In addition to the electricity demand, the system is used to produce domestic hot water (DHW). The household is assumed to include 4 inhabitants and daily DHW demand profile is used according to [START_REF] Herrando | A UK-based assessment of hybrid PV and solarthermal systems for domestic heating and power: System performance[END_REF].

Multi-objective optimization

In this study, the Matlab function called gamultiobj of the Global Optimization Toolbox is used to run the multi-objective design optimization of the renewable energy system. The gamultiobj-function creates a set of optimal solutions in the space of decision variables on the Pareto front. The function uses a controlled, elitist genetic algorithm, which is a variant of NSGA-II. The detailed description of the algorithm is presented by Deb in [START_REF] Deb | Multi-Objective Optimization using Evolutionary Algorithms[END_REF]. Generally, the multi-objective optimization problem of two or more conflicting objectives is described mathematically as follows [START_REF] Tamayo | Performance evaluation and multi-objective optimization of hybrid photovoltaic-thermal collectors[END_REF]:

Minimized/Maximized f m (x), m = 1, 2,…, M; Subject to g j (x) ≥ 0, j = 1, 2,…, J; h k (x) = 0, k = 1, 2,…, K; x i (L) ≤ x ≤ x i (U) , i = 1, 2,…, n;
In this formulation, the objective functions are minimized or maximized subject to certain constraints.

The flow chart of the NSGA-II optimization process is presented in Fig. 1. First, the objective functions and optimization variables are defined for the optimization problem. The initial population (the parent population) and score matrices for all decision variables are created with the simulation model of the system and based on the input data and bounds of the decision variables. Next, the created matrices are given for the gamultiobj-function, which starts the iterative process by evaluating the fitness function with the simulation model and making selection, crossover and mutation of the population in order to generate a better generation of the population [START_REF] Kallio | Energy Analysis and Exergy Optimization of Photovoltaic-Thermal Collector[END_REF].

Figure 1: The simulation-based multi-objective optimization process. [START_REF] Kallio | Energy Analysis and Exergy Optimization of Photovoltaic-Thermal Collector[END_REF] In this study, the electrical and thermal reliability of the system and the initial investment costs are three objectives to be optimized. The goal of the optimization is to find a Pareto optimal set of system designs to maximize self-consumption and autonomy of the system over year with the minimum initial investment costs. The 3-dimensional optimization problem of the system design is formulated as follows. To minimize: [START_REF] Kallio | Energy Analysis and Exergy Optimization of Photovoltaic-Thermal Collector[END_REF] (11) [START_REF] Evola | Exergy and thermoeconomic optimization of a water-cooled glazed hybrid photovoltaic/thermal (PVT) collector[END_REF] Where R electric and R thermal are the electrical and thermal reliability of the system, respectively. The reliability shows the share of the load that cannot be met by the solar system. I total is the total initial investment of the system including the initial investment costs of PVT collectors (I PVT ), battery system (I B ) and thermal storage (I TS ). SOC B , P L and P PVT represent battery power, electrical power of load and PVT generation, respectively. Q aux , Q DHW , m L , T set and T top are the generated thermal power by auxiliary device, required thermal power of DHW, DHW demand, the required temperature of DHW and temperature of water in the top of heat storage, respectively. Table 1 presents the selected decision variables of the system to be optimized by minimizing the objective functions. Table 2 present the estimated prices of the system components used in the optimization.

Results and discussion

The yearly electricity and DHW demand profiles of the residential building with 4 inhabitants were used to perform the multi-objective optimal component sizing optimization of the hybrid energy system including PVT collectors, thermal storage and battery system with the NSGA-II algorithm. The hourly yearly meteorological data of Strasbourg was used to simulate PVT collector thermal and electrical energy production. As a result the Pareto front was obtained for the optimization problem and is presented in Fig. 2. The goal of the optimization was to maximize self-consumption and autonomy of the system over year with the minimum initial investment costs. The optimization algorithm found 14 non-dominating optimal solutions for the component sizing of the hybrid energy system. The solutions are presented in Fig. 2. Within the optimal solutions, the yearly thermal reliability R thermal was between 0.83 and 0.59. On the other words, at least 59% of the yearly thermal energy demand had to be covered by an auxiliary boiler and 41% is covered by the PVT collectors. At the same time, the yearly electrical reliability R electric was between 0.82 and 0.39, and at least 39% of the yearly electricity demand had to be bought from the electric grid. The initial investment costs of the optimal solutions were between 5517 € and 30 480 €. However, the best reliability of the system was reached with the investment costs of 22 600 € including 3 PVT collectors in series and 5 rows, the battery capacity of 9.5 kWh and thermal storage volume of 0.4 m 3 .

The Pareto front presents the optimal set of solutions in the objective space and can be used by the decision-makers to select the most reasonable solution for each application. Table 3 presents

Conclusion

In this paper, a PVT/battery/thermal storage hybrid energy system was modelled into Matlab/Simulink environment and the optimal sizing problem was solved using NSGA-II algorithm. The system was optimized for a residential building electricity and DHW demand, and the weather conditions of Strasbourg, were used to estimate PVT energy production.

The main objective of the optimization was to maximize self-consumption and autonomy of the system over year with the minimum initial investment costs. The objective functions were to minimize thermal and electrical reliability, and the initial investment costs. The Pareto front was obtained containing a set of the optimal solutions within the objective space. Within the Pareto front, the higher investment costs resulted in the better reliability of the system. The results revealed that the studied energy system cannot cover all energy demand under defined constraints but an additional energy source is required and the considered energy system should be extended to be a hybrid renewable energy system that integrates different energy conversion and storage technologies in order to form a sustainable and energy efficient set-up to satisfy a certain energy demand. The optimization results revealed that the multi-objective optimization with the genetic algorithm is suitable for the hybrid energy system optimal component sizing problem.

  ≤ x(1) ≤ 0.024 kg/s N Nr. of PVT in series 1 ≤ x(2) ≤ 6 -M Nr. of PVT in rows 1 ≤ x(3) ≤ 5 -C bat Battery capacity 4 ≤ x(4) ≤ 10 kWh V TS Thermal storage volume 0.1 ≤ x(5) ≤ 0.4 m 3

Figure 2 :

 2 Figure 2: The Pareto front in the objective space

  3 solutions with different range of investment costs selected from the Pareto front. PVT in series PVT rows C B (kWh) V TS (m 3 ) R th R el Inv. (€) Sol.

Figure 3 :

 3 Figure 3: The system layout of the hybrid renewable energy system.

Table 1 :

 1 Decision variables for multi-objective optimization problem.

	Component	Price	Ref.
	PVT collector	325 €/m 2	DualSun company
	Thermal storage 2.95 €/m 3	[13]
	Electrical storage 1143 €/kWh Enphase company

Table 2 :

 2 The component prices of the system

Table 3 :

 3 The selected solutions of the Pareto front

Acknowledgements

The authors would like to thank Interreg V Rhin supérieur ACA-MODES project for their support and funding of this research.


D Sukamto

F Gloriant

M Siroux

Improved value of temperature different on climatic chamber climatic chamber for thermal conditions

Climatic chamber machine at Climatherm laboratory was used for thermal test on wall model. This climatic chamber machine allows the measurement of different environmental variables. In this research, thermal performance tests were conducted on wall model imposing boundary condition vary the cavity distance between two wall elements and then these tests were performed in steady-state conditions. Wall model prepared from aluminum composite and was maintaining controlling the air temperature difference between the two chambers 20°C and the air debit constant. In the first part of this paper, a description of the climatic chamber machine, operation ranges, and theoretical work principles of the climatic chamber are presented. Then, the second part shows the results for the thermal test on wall model. The thermal test performed with the climatic chamber machine allows simulating environmental conditions accurately during in certain distance of cavity and specific air debit. As a result, the configuration settings value at the climatic chamber machine has to correction to reach the air temperature difference as planned. The air temperature difference at the beginning of the thermal conditions on wall model is less than the planned air temperature difference. However, this correction value of temperature shows agreement result with the value 20°C in temperature different.

Introduction

Contribution of this research, in general, is to contribute to the reduction of energy consumption and greenhouse gas emissions. The focus of this research is contributing to the energy performance of buildings through the study of a bioclimatic wall (ventilated wall or parietodynamically insulated wall). In the basic principle of the ventilated wall cavity have an essential role in generating a convection exchange among wall model axis and disturbance of the radial heat flux between the indoor and outdoor environment. Many authors have investigated influence cavity distance to energy performance; principally, these studies deal with the mathematical model and experimental tests [START_REF] Balocco | A simple model to study ventilated facades energy performance[END_REF][START_REF] Ciampi | Ventilated facades energy performance in summer cooling of buildings Sol[END_REF][START_REF] Marinosci | Experimental analysis of the summer thermal performances of a naturally ventilated rainscreen facade building Energy Build[END_REF][START_REF] Gagliano | Evaporation, Condensation and Heat transfer-Computational Fluid Dynamic Simulations of Natural Convection in Ventilated Facades[END_REF][START_REF] Gagliano | Thermodynamic analysis of ventilated façades under different wind conditions in summer period[END_REF][START_REF] Sanjuan | Energy performance of an open-joint ventilated façade compared with a conventional sealed cavity façade[END_REF][START_REF] Sánchez | Experimental assessment of the performance of open joint ventilated façades with buoyancy-driven airflow Sol[END_REF][START_REF] Stazi | Experimental evaluation of ventilated walls with an external clay cladding Renew[END_REF][START_REF] Sukamto | Numerical study of a ventilated facade for the preheating of air (COlloque FRancophone en Energie[END_REF]. However, little research has been made in laboratory tests to find the proportional value of the input variable, which has an agreement result with the reference value of temperature.

The first tests activity of the climatic chamber climatic chamber in 2017 for thermal conditions of wall model classic and 2018 for the calculation coefficient of h on wall model surface [10,[START_REF] Sukamto | Development of ventilated facade for energy and building applications[END_REF]. This apparatus has shown possibilities of controlling some atmospheric variables but also had limitations on ambiance air temperature measurement. This activity has an improved method of the previous one; the ambient air temperature has a good agreement with the reference value of air temperature difference and was used for modeling the coefficient of h on wall model of the test; the ventilated cavity wall.

The climatic chamber has two climatic chambers and a specimen holder in between of two climatic chambers. In each climatic chamber, they can guaranty heat production by the electric radiator, and they can guaranty cooling production by compressor cycle. This paper presents two main items: (i) the climatic chamber machine instrumentations; (ii) preliminary results of improvement to temperature ambiance of the climatic chamber.

General diagram of the acquisition system

General diagram of the acquisition system Figure 1 at this studies was used two sensitive acquisition equipment, Keithley 2700 with multiplexer and data acquisition from National Instrument. First one Keithley 2700 with multiplexer was used to communicate data record between the flux meter instrument, type K thermocouple and the computer as the center of data processing. Another acquisition is data acquisition NI-9219 used to communicate data record between the hot-wire anemometer and the computer.

Instruments of measurement

The climatic chamber machine equipped with instrument thermo-resistance PT-100 Figure 2 at each climatic chamber for control temperature evolution of the climatic chamber machine.

Test condition value and calculation coefficient of h

Testing condition use to this study was (0°C, 75%) for climatic chamber cold side and (20°C, 55%) for climatic chamber hot side. Air temperature difference use between two climatic chambers is 20°C.

Otherwise, based on Newton's law as shown in Equation ( 1) to do a calculation coefficient of h, there are variables which mandatory to measure; measure the value of ambience air temperature (Ta) of each climatic chamber, measure the value of surface wall temperature (Tp) each side of wall model and measure the value of heat flux (q) which cross to wall model.

(1)

Preliminary results

Cold junction correction

Cold junction correction does correct measurement result of data recorder Keithley 2700 data acquisition. Method to use by drowning end section of the type K thermocouple into a jar of mixed water and ice, Error! Reference source not found.. Based on the measurement result in Error! Reference source not found. for temperature 0°C of mixed water and ice, Keithley 2700 showed the measurement result at 0.03°C. Its mean 0°C actual Keithley 2700 resulted in the measurement 0.03°C, so this value has an excellent agreement to the study.

Temperature measurement at the climatic chamber cold side and climatic chamber hot side

The settings value for the climatic chamber was (0°C, 75%) for climatic chamber cold side, and (20°C, 55%). That value was in condition steady-state, another result there is measurement result by the climatic chamber machine, thermo-resistance instrument PT-100 inside each climatic chamber. Moreover, this study will show measurement result ambiance air temperature from type K thermocouple instruments. Measurement result showed that the setting process of temperature hot side climatic chamber at the hot side and cold side are stabilized at the point 20°C and 0°C respectively. Measurement result from thermo-resistance instrument PT-100 at each climatic chamber side respectively showed average value of 19.77°C and 0.06°C, those average value are evolving near the setting process of temperature hot side climatic chamber. Measurement results from type K thermocouples installed in the center of the climatic chamber have a homogeny result at hot side chamber and cold side chamber, the average value of the ambient air temperature respectively are 15.71°C and -2.63°C. The air temperature difference between the two climatic chambers are 18.34°C; this value needs to be corrected to the value 20°C in air temperature difference.

Corrected value temperature control to ambience air temperature

In order to result in the expecting value from the air temperature difference between two climatic chambers, setting points of temperature to cold side chamber have to change from 0°C to -2°C. So the variable use to input to the climatic chamber machine, (-2°C, 75%) for cold side chamber and (20°C, 55%) for hot side chamber. The result has shown the average value of air temperature control are show at climatic chamber hot side in Figure 9 and cold side in Figure 10 respectively 15.71°C and -4.52°C, temperature gain is 20.23°C, the result showed a good agreement with the reference temperature value.

Calculation to coefficient of h on wall model surface

Calculation to coefficient of h on wall model surface for condition wall cavity distance 5 mm and constant air debit 10m 3 /h with the configuration condition setting to climatic chamber (-2°C, 75%) and (20°C, 55%) resulted the coefficient of h to cold side climatic chamber and to hot side climatic chamber respectively 19 W.m -2 .K -1 and 21 W.m -2 .K -1 .

From the calculation result to the coefficient of h, showed that the climatic chamber is capable of conducting the next test with other variables of the study parametric.

Conclusion

The climatic chamber at Climatherm laboratory was used in order to perform thermal conditions to wall model. This climatic chamber allows the user to impose specific environmental conditions through a closed circuit of air. Preliminary results of thermal conditions lead to the next conclusions:  The cold junction correction value showed a good agreement value of measurement by the Keithley 2700 data acquisitions.  The value on configuration condition settings to climatic chamber must be corrected to get an excellent agreement value to reference parametric value. This device will be used in the future to conduct a parametric study on thermal conditions on the ventilated cavity wall in the variable of different wall cavity distance and different value of constant air debit.

Figure 1 .

 1 Figure 1. General diagram of the acquisition system

Figure 2 .

 2 Figure 2. Measuring instrument at each climatic chamber

Figure 3 .

 3 Figure 3. Measuring instrument to measure ambiance air temperature

Figure 4 .

 4 Figure 4. Instruments flux meter and type k thermocouple on wall model surface

Figure 5 .

 5 Figure 5. Cold junction correction test

Figure 6 .

 6 Figure 6. Measurement result from cold junction correction test

Figure 7 .Figure 8 .

 78 Figure 7. Temperature measurement at the climatic chamber hot side

Figure 9 .Figure 10 .

 910 Figure 9. Temperature measurement at the climatic chamber hot side

Broad

  Exposure to Science and Technology 2019 (BEST2019) IOP Conf. Series: Materials Science and Engineering 673 (2019) 012116 IOP Publishing doi:10.1088/1757-899X/673/1/012116 7


Dwinanto Sukamto
email: dwinanto.sukamto@insa-strasbourg.fr

Camille Lefoul

François Gloriant

Monica Siroux

COFRET'18 NUMERICAL STUDY OF A VENTILATED FAÇADE FOR THE PREHEATING OF AIR

Keywords: Heat transfer, Building, Ventilated Façade, Computational Fluid Dynamics (CFD), Mixed Convection

Nowadays, external isolation is highly recommended for both building renovation and new building construction. This isolation requires the installation of structure protecting the insulating materials from the humidity and weather variations. One of the current techniques is the installation of ventilated façades. This study aims to show the use of ventilated façades as a preheating system of the air, based on parietodynamic principle. Literature studies have highlighted that this principle is interesting with regard to the heat recovery. The basic study of the thermal system revealed the difficulty of the simplified models in obtaining the internal convective exchange coefficients. Therefore, CFD simulations are chosen to study a 2D turbulent steady-state airflow, driven by mixed convection, in the cavity of the ventilated façade. In order to characterize the system performance, a series of simulations has been carried out in function of seasons, solar flux conditions and pressure differences between the inlet and outlet of the air gap.

Introduction

Currently, there are various systems of ventilated façade, all of which are characterized by incorporating a ventilated air chamber between the outer skin and the insulation (which is continuous along the entire façade). The main purpose of this type of façade is to protect insulating materials by dissipating humidity. Nevertheless, this disposal could be also used for the preheating of the renewal air in winter and for the dissipating of the solar heat in summer. The operation is based on natural air convection by temperature differences and wind effect. Thus, an upward flow of air is created through the chamber, which dissipates heat from the interior wall and/or from outer skin and is finally expelled out through the top of the façade. The more important the heat recovery is, the higher the air velocity is. This phenomenon can be valorised in 2 ways depending on the season: • Minimising the need of heating in winter: By circulating the air in the cavity, it collects the heat from the interior (surface heat loss) and from the exterior (solar gain).

Consequently, this preheated air can be used for the air renewal of the building. • Minimising the need of air conditioning in summer: The outer skin absorbs solar radiation that is dissipated by the airflow. The preheated air can be evacuated [START_REF] Gagliano | Thermodynamic analysis of ventilated façades under different wind conditions in summer period[END_REF].

Ventilated façade with open joint allows free circulation of air between the chamber and the outside. In this type of façade, the air usually enters from outside into the chamber through the joints of the lower-half of the façade, and is gradually out to the outside through the upper joints [START_REF] Gratia | Greenhouse effect in double-skin façade[END_REF]. The performance of this type of façade has been previously studied by the

COFRET'18

Strasbourg _France/28_29 juin 2018 development of CFD numerical models [START_REF] Gloriant | Modeling numerical study of the thermal performance of a ventilaeted façade[END_REF], [START_REF] Sanjuan | Energy performance of an open-joint ventilated façade compared with a conventional sealed cavity façade[END_REF] as well as by the nonintrusive experimental of airflow techniques [START_REF] Sanjuan | Experimental assessment of the performance of open joint ventilated fac ß ades with buoyancy-driven airflow[END_REF].

On one hand, ventilated façades are particularly adapted in summer periods and where orientations are exposed to direct sunlight. Under these conditions, solar radiation can increase the surface temperature of a traditional façade by 60°C, so this heat dissipation through the ventilated chamber represents a significant energy saving in air conditioning systems. On the other hand, during cold periods, the heated air from the cavity can be introduced into the building in order to reduce the heat losses due to ventilation.

In general, a ventilated façade consists of brick, insulation, cavity and outer layer [START_REF] Romila | General Principles for the Design and Construction of Ventilated Façades[END_REF], [START_REF] Zhang | The application of air layers in building envelopes : A review[END_REF]. Based on experiment results conducted by Christina Sanjuan et.al, 2011 [START_REF] Sanjuan | Energy performance of an open-joint ventilated façade compared with a conventional sealed cavity façade[END_REF], the open joint ventilated façade shows better performance in thermal phenomena rather than the sealed cavity. Figure 1 shows the principle of airflow circulation in glazing windows. This principle represents the same airflow circulation for the ventilated façade. Model (A) "supply air" shows to introduce a renewal air into the house. This model is particularly effective to reduce thermal losses in building during the heating season. Model (B) "exhaust air" is used to evacuate the air from the interior. Model (C) and (D) are respectively "indoor air circulation" and "outdoor air circulation". It should be noted that indoor and outdoor air are separated, so they have different functional models system ventilation of the house. [START_REF] Zhang | The application of air layers in building envelopes : A review[END_REF] In this research, the model (A) in heating season and the model (D) in cooling season are considered. In the first time, the variations of the different heat fluxes are analyzed in function of the depression between the inlet and the outlet of the ventilated façade and in function of the presence or absence of solar radiation. In the second time, the thermal performance of the disposal is studied and compared to a façade without outer skin.

Figure 1. Different types of airflow

-Problem description

Geometry of the model

In this study, the bidimensional model considered consists of a brick wall 3 m high, insulated from the exterior by glass-wool. The insulating material is protected by a ventilated façade of aluminium and polyethylene. The width of cavity wall is about 6 cm. The proposed system is detailed below (Figure 2). The ventilated façade system is composed (from outside to inside) of the following: outer layers (2 mm), air chamber (60 mm), thermal insulation (200 mm), brick and plaster (200 mm). The thermal properties of the materials used are presented in Table 1. According to ISO 15099 [START_REF] Iso | Thermal performance of windows, doors and shading devices -detailed calculations[END_REF], the air properties variation is based on temperature polynomial functions and the density variation is governed by the ideal-gas law.

COFRET'18

Strasbourg _France/28_29 juin 2018

Hypotheses and boundary conditions

The equations of numerical model were solved using finite volume method. The simulations were performed at steady-state by the Euler and Navier-Stokes equations [START_REF] Coirier | Based Scheme for the Euler and Navier-Stokes Equations by[END_REF]. The effects of the turbulence is taken into account by the standard k-epsilon turbulence model [START_REF] Buratti | Mean age of air in a naturally ventilated office: Experimental data and simulations[END_REF], [START_REF] Kwon | Analysing ventilation efficiency in a test chamber using age-of-air concept and CFD technology[END_REF], [START_REF] Mohammed | A simplified method for modeling of round and square ceiling diffusers[END_REF]. Transfer radiation has been simulated with the discrete ordinates (DO) model, allowing the determination of radiative transfer in the air chamber. The criteria of convergence are set at 10 -3 .

COFRET'18

Strasbourg _France/28_29 juin 2018

Boundary conditions of the simulations are detailed in Figure 3. The boundary conditions at the inner side (CL1) and the outer side (CL3) are fixed by a heat transfer coefficient, an ambiant temperature and a radiative temperature. Their values are typically used in building codes for interior and exterior conditions, and are based on ISO 15099 [START_REF] Iso | Thermal performance of windows, doors and shading devices -detailed calculations[END_REF]. For the air inlet (CL5) and the air outlet (CL6), a difference pressure ∆P is set. The entrance temperature of the air is imposed at the exterior temperature. Finaly, solar radiation on the plate is simulated as an internal heat source on the exterior surface of the ventilated façade.

-Results and Discussion

2.1 Temperature profiles in the air chamber with and without solar radiation Figure 4 shows the horizontal profiles of temperature inside the cavity at different height in winter season and the bulk temperature for each section. The temperature gradually increases with the upward air movement through the chamber. The profiles highlight a thermal disturbance at throttling section due to the assembly of the two facings (green curve, Figure 4). It is worth noting that the temperature varies slower after throttling section of cladding. Without solar radiation, the temperature at different height is decreased while reaching the outer skin. With the presence of solar radiation, the solar energy reheats the outer skin thus the air is preheated from both faces.

Depression effect on the heat fluxes

From the obtained numerical results, the heat fluxes in the inner and outer faces of the ventilated façade and the heat recovered by the airflow have been estimated. Figure 5 shows the evolution of these fluxes in function of seasons, solar radiation and pressure difference.

At the first sight, it should bear in mind that the presence of solar radiation has a significant impact on the quantity of heat exchanged inside ventilated chamber; up to 200W in summer and 90W in winter. Without solar radiation, fluxes values are lower.

In any climatic conditions, the interior heat flux varies slightly due to the high performance of the insulation. In winter season, the heat loss is about 20W and in summer season, there is a heat gain of about 5W from the cavity.

COFRET'18

Strasbourg _France/28_29 juin 2018

In the cavity, the more important the airflow is, the higher the heat recovery is. Note that this trend is opposed in summer without solar radiation. For this case, the air temperature decreases in the cavity. The variation of the external flux leads directly to the variation of flux exchanged by energy conservation principle.

Thermal performance analysis

From the heat balances presented in Figure 6 and realized for a 2 Pa depression, it is clear that air cavity bring a positive impact on the thermal performance of wall. In summer season, with solar radiation, the cavity helps to dissipate 90% of the heat received by radiation, which allows to reduce by more than half of the heat gain in comparison with the same wall without cladding. Without solar radiation, the air cavity seems not to be interesting. It does not have a positive influence on the thermal behaviour of the wall because the air inside ventilation chamber cools by heat transfer to the indoor environment. In winter season, if the air cavity is used for the ventilation of the building, the heat balance can be enhanced. Without solar radiation, the cladding makes it possible to reduce thermal losses by 17%. The case with solar radiation is more interesting. The ventilated façade makes it possible to recover more heat that there are no losses, thanks to the heating of the cavity on both sides of it. In comparison with the wall without cladding, the heat gain represents more than 30%.

Conclusion

There are several passive solar façade systems. The studied one consists of the existence of an air chamber behind a ventilated cladding that is not isolated from the outside environment. Thus, the air can circulate in the cavity, be preheated before being distributed in the building or dissipated outside. The study shows that the cavity can be enhanced. In winter, it can be used for the preheating of the air ventilation by recovering the heat loss from interior and the solar radiation. In summer, the cavity can be used to evacuate the excessive heat gain.

This work shows also that the behaviour of the ventilated façade depends strongly on the pressure difference between the inlet and outlet. A further study should be conducted in order to evaluate the obtained airflow in the cavity.

Figure 2 .

 2 Figure 2. Geometry of the model; A) Air inlet, B) Assembly of two shell, C) Air outlet

Figure 3 .

 3 Figure 3. Boundary conditions

Figure 4 .

 4 Figure 4. Temperature profiles inside ventilated chamber in winter season, condition pressure 2 Pa. A) With solar radiation B) Without solar radiation

Figure 5 .

 5 Figure 5. Heat fluxes inside ventilation chamber in summer and winter conditions, with and without solar radiation

Figure 6 .

 6 Figure 6. Heat balances in summer and winter conditions, with and without solar radiation for a 2Pa depression

Acknowledgments

This study has been carried out as part of the ARCONIC research project, funded by INSA Strasbourg, laboratory ICUBE, University of Strasbourg.