Rafik Belarbi

Development of feasibility approaches for studying the behavior of passive cooling systems in buildings

This study is a contribution to European projects Pascool/Joule II and Altener/Sink that deals with feasibility of passive cooling systems in Europe. The first aim of this work was to define a design methodology to evaluate natural cooling potential according to the climatic quantification criteria of the site, the cooling system performance, and comfort criteria defined by the couple of temperature and relative humidity set points. A simplified approach, based on climatic potential criteria as theoretical cooling potential index, the available potential index, the cooling need index, and the natural cooling normalized capacity, was developed. It was applied to 105 European sites for different types of evaporative cooling systems (direct and indirect), and for various temperature and relative humidity set points. During the second stage, a refined approach taking into account building characteristics and the cooling system performance, was developed. This method is based on the integration of numerical models of passive cooling systems in a thermal building software in order to consider interaction phenomena between cooling system and building. Application of this approach to one building has been done in order to assess energy consumption gain achieved by using passive cooling systems. These two complementary approaches provide helpful information dealing with the feasibility of a passive cooling technique based on comfort and energy saving criteria. They could be used by architects and building designers as helpful decision making tools during the different stages of building design.

Numerical Analysis of Hybrid Ventilation Performance Depending on Climate Characteristics

Abstract This study, which formed part of the Annex 35 “Hybrid Ventilation in New and Retrofitted Office Buildings” project, was completed at LEPTAB and supported by the French Research Ministry and the ADEME (Agence De l‘Environnement et de la Maîtrise de l‘Energie). It consisted of modelling a typical classroom and comparing different control strategies to estimate the performance of a hybrid ventilation system for different climates. The intention was that investigated classrooms were assumed to be on the middle level of a three-storey building, oriented South and surrounded by other classrooms subjected to the same conditions. Two mechanical ventilation systems were taken as references. These were: a mechanical exhaust system with a low consumption fan and without heat recovery, and a balanced mechanical ventilation system with two fans and some heat recovery. The hybrid ventilation approach investigated was a fan assisted natural ventilation system incorporating a demand control strategy based on indoor air temperature and CO concentration. The performance of this hybrid ventilation system was analysed in terms of energy consumption, indoor air and dry resultant temperatures, and CO concentration level. Simulations of specific weeks in the year were performed for ten French cities and gave quite detailed patterns of behaviour. The study was extended to include yearly mean values of energy consumption. The results for both short and long time periods showed the potential of this specific hybrid ventilation system according to climate and control strategy. Hybrid ventilation is shown to provide improved air quality. Also, in relation to delivered energy, energy savings are possible but, except for Mediterranean cities, are not as much as with a mechanical system with heat recovery.

Assessment of the variability of moisture transfer properties of High Performance Concrete from a multi-stage drying experiment

This article aims to assess the characterisation of the water vapour desorption process and its variability under site conditions. It presents a probabilistic approach allowing the assessment of the distribution laws for the water vapour desorption isotherms (WVDI) and the moisture diffusion coefficient. These two parameters were evaluated from multistage drying experiment. For each stage, the analysis of the equilibrium state allows the determination of WVDI whereas the analysis of the desiccation kinetic makes possible the evaluation of the moisture diffusion coefficient. In order to have a representative variability, the study was carried out on two different High Performance Concretes (HPC) during a large-scale construction: overall, more than 300 specimens were tested. Results indicate that Gaussian distribution can be used for modelling the statistical variability of desorption isotherm while log-normal distribution is more appropriate for describing the variability of moisture diffusion coefficient. Furthermore, whatever the concrete, the coefficient of variation on the water content at equilibrium remain very similar. Cet article a pour but la caractérisation du processus de désorption de la vapeur d’eau et de sa variabilité dans des conditions in-situ. Il présente une approche probabiliste permettant l’évaluation des lois de distribution des Isothermes de Désorption de la Vapeur d’Eau (IDVE) ainsi que celles des coefficients de diffusion d’humidité. Ces deux paramètres ont été évalués expérimentalement à partir d’essais de séchage par palier réalisés sur des bétons à haute performance (BHP), confectionnés in-situ. Pour chaque palier de séchage, l’exploitation des états d’équilibres hydriques a permis la détermination de l’IDVE. De plus, l’analyse des cinétiques de transfert a permis l’évaluation du coefficient de diffusion d’humidité. La campagne expérimentale a été réalisée sur plus de 300 échantillons. Les résultats obtenus ont révélés que la variabilité des isothermes de désorption peuvent être modélisés par une distribution Gaussienne tandis qu’une distribution de type log-normale s’avère plus appropriée pour décrire la variabilité du coefficient de diffusion d’humidité. En outre, quel que soit le béton testé, les coefficients de variation de la teneur en eau pour les différents états d’équilibre hydrique demeurent proches.

Sensitivity analyses of convective and diffusive driving potentials on combined heat air and mass transfer in hygroscopic materials

ABSTRACT The focus of this paper was to examine the contribution of two key mechanisms—moisture convection and diffusion–on heated air and moisture transfer in porous building envelopes and to define the validity of the sub-models. A numerical simulation was performed and is focused on the one-dimensional problem for drying test boundary conditions. Thereafter, a detailed parametric analysis was carried out in order to investigate the influence of typical nondimensional parameters. Results show that convection is a prominent driving potential with respect to the diffusion process when the hygric state is stable between the environment and the envelope.

Development of simplified approach to model the moisture transfer in building materials

ABSTRACT A simple but effective gravimetric method for determining the moisture diffusivity of concrete and cement paste is given in this paper. The method allows calculating the moisture diffusion coefficient and moisture distribution inside building materials purely based on the isothermal gravimetric measurements. The method was also applied to research into the properties of concrete and cement paste. Results indicate that the increase of water-cement ratio in the cement pastes will result in an increase of moisture diffusion coefficient. Furthermore, the experiment also points out the fact that moisture coefficient of concrete based on cement with fly ash additions is lower than that of concrete with OPC.

Minimization of indoor temperatures and total solar insolation by optimizing the building orientation in hot climate

AbstractIn order to reduce the energy load, understanding the overall architectural design features and optimizing building orientation are important. They are guided by natural elements like sunlight and its intensity, direction of the wind, seasons of the year and temperature variations. The main aim of presented analysis is to give solutions for architects to design standard and low energy buildings in a proper way. The orientation effect of a non-air-conditioned building on its thermal performance has been analyzed in terms of direct solar gain and temperature index for hot-dry climates. This paper aims at introducing an improved methodology for the dynamic modeling of buildings by the thermal nodal method. The study is carried out using computer simulation. This study examines also the effect of geometric shapes on the total solar insolation received by a real building. As a result, the influence of orientation changing depends on the floors and exterior walls construction materials, the insulation l...

Simultaneous heat and moisture transport in porous building materials: evaluation of nonisothermal moisture transport properties

The paper presents a mathematical model for calculating the nonisothermal moisture transfer in porous building materials. The simultaneous heat and moisture transfer problem was modeled. Vapor content and temperature were chosen as principal driving potentials. The coupled equations were solved by a numerical method. An experimental methodology for determining the temperature gradient coefficient for building materials was also proposed. Both the moisture diffusion coefficient and the temperature gradient coefficient for building material were experimentally evaluated. Using the measured moisture transport coefficients, the temperature and vapor content distribution inside building materials were predicted by the new model. The results were compared with experimental data. A good agreement was obtained.

Optimal settings of residential oil burners

Abstract Residential oil burners are capable of almost complete burning of the fuel oil, without visible smoke, when they are operated to deliver approximately 12% CO 2 in the flue gases. The positions of the air damper and of the combustion nozzle are adjusted at start-up and during operation in order to maximize the combustion efficiency. In practice, one factor at a time is varied, starting with the air damper. However, this method fails to detect the interaction between air excess and nozzle position and results in non-optimal settings. Optimal designed experiments allow obtaining local regression models and statistical analyses indicate if experiment augmentation is required. The air damper and combustion nozzle settings are changed in the direction of local gradient until a second order model that contains the optimal point in its experimental region is obtained. The gain in combustion efficiency thus obtained may be up to 5% as compared with the classical approach.

Periodic homogenization for heat, air, and moisture transfer of porous building materials

ABSTRACT In this work, a macroscopic model of hygrothermal transfers in porous building materials was developed, using periodic homogenization, where the air infiltration was added to the classical mass and energy balance equations written at the microscopic scale. The corresponding infiltration, hygric, and thermal input parameters were carefully identified. Numerical calculations of thermal and diffusion tensors were performed on a representative concrete elementary cell. Further, the diffusion tensor was compared to the equivalent experimental results available in the literature, and its sensitivity to the water content variations and porosity has been evaluated on the concerned elementary cell.

CONVECTIVE AND CONDUCTIVE THERMAL HOMOGENIZATION FOR NONSTURATED POROUS BUILDING MATERIALS: APPLICATION ON THE THERMAL CONDUCTIVITY TENSOR

Porous materials possess a complex structure on a microscopic scale and present strong heterogeneities, which makes their precise study extremely complex. In fact, the macroscopic behavior of these materials is strongly dependent on mechanisms that act to the scale of their components. The present work focus on the development of a macroscopic conductive; and convective fluid heat transfer model, with a heat source in the unsaturated porous materials. This model is established by periodic homogenization of energy conservation equations written on a microscopic scale in each phase (solid, liquid and gas). The resulting input parameters formulations of the sub model were explicitly identified. Numerical calculations of the homogenized thermal conductivity tensor are performed on a representative three-dimensional elementary cell of the porous medium. Finally, a sensitivity study of this tensor depending of the variation of the water content and porosity of the concerned elementary cell has been performed. This sensitivity is required to be considered in simulations to better understand the behavior of building materials and improve the prediction of energy performance.