Christian Inard

Prediction of air temperature distribution in buildings with a zonal model

Abstract This study presents a zonal model used for predicting the air temperature distribution inside a room. This model is original in the calculation of mass air flows between two zones. To do so, we distinguish zones where the momentum is small and for which we calculate the flow rates with the aid of a pressure field, and the driving zones described using appropriate specific flow laws. A comparison between experimental results and those obtained with the model proves that they are fully consistent with each other. This allows us to consider the integration of this type of model into a general building thermal code.

Short-term storage systems of thermal energy for buildings: a review

Thermal energy storage in buildings is essential to reduce energy consumption, to switch electrical consumption from on-peak period to off-peak period and to develop the use of intermittent renewable energy sources. Several systems designed to store thermal energy on a short-term scale (maximum a few days of storage) are presented in previous publications. However, there are no available comparisons of these systems and their conditions of use. This paper details these different designs for short-term scale thermal energy storage, regarding (i) their passive or active nature, (ii) their usage conditions and (iii) their performances. In the first section, the thermal properties of materials are listed. In particular, advantages and problems associated with phase change materials are presented. Subsequently, thermal storage systems are presented in two parts, on the one hand, passive systems and on the other hand active systems, according to the fluid used. For each system, the advantages of substituting sensible storage with latent storage are highlighted. Furthermore, an original and comparative analysis of published studies attempts to define some criteria and requirements for efficient use of latent storage. This review demonstrates that an exhaustive comparison of the systems encounters difficulties, due to the differences between the studies with respect to experimental measurements and weather data and the lack of similar comparison criteria, such as decrement factor, efficiency and cost.

Energy consumption and thermal comfort in dwelling-cells: A zonal-model approach

Abstract In this study, we present a simplified model of the thermal behaviour of dwelling-cells, with a view to evaluating the performances of various heating systems that are commonly used in such environments. This model is based on a zonal-method representation of thermal exchanges in enclosed spaces. Following the validation of the model, we carried out a numerical study on two types of heat source, i.e. localized (a hot-water radiator and an electrical convector) and distributed (a hot-water heated floor and an electrical heated ceiling). The models were used to predict the heat losses specific to each system, as well as the indoor thermal ambience that the different systems induced. It was found that, for the configurations studied, the distributed heat sources presented a slight advantage over the localized sources, with regard to the criteria of energy consumption and thermal comfort.

Microclimate and building energy consumption: study of different coupling methods

The aims of this study are to highlight the microclimatic phenomena to which the energy consumption of a building is the most sensitive and to compare different approaches to coupling microclimate and building energy simulation models. We first present a study in which spatial variations in outdoor air temperature are not taken into account so as to compare the relative effect of convective and radiative heat flux on the outer surface energy balance. Then, several coupling methods are implemented, for which energy consumption in winter and indoor temperature in summer are compared between insulated and non-insulated buildings. Results show that for the urban context, taking into account long-wave radiative heat fluxes is crucial. Moreover, representing local modifications in outdoor air temperature is necessary for non-insulated buildings in summer, but can be neglected in winter. In conclusion, an intermediate coupling method that can be used under certain assumptions is proposed.

Modelling the radiative exchanges in urban areas: A review

In the consideration of building energy performance and urban climate models, irradiation is a key parameter. From the city scale to the detailed building interchange, different approaches and models with various levels of details are explained in this review, and their scope and usefulness considered. The first part of the study deals with solar and longwave sky irradiance models. Six models of sky diffuse irradiance are presented, together with details of the models of longwave sky irradiance. At city scale, the influence of an urban area on the atmospheric radiative budget requires knowledge of the radiative flux leaving the urban area. Albedo and longwave reflectivity models are detailed in the second part to determine shortwave and longwave fluxes from urban areas. Solar access quantifies the shortwave radiative energy received by an urban surface for a specified period, determined by the buildings scale and type of facade. Depending on the urban morphology, tridimensional city models are presented in this section to compute the solar access. In order to calculate the energy budget of building walls, the radiation interchanges in the district have to be coupled with other physical phenomenon. Thus, some simplifications and various models to reduce computation time are discussed.

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.

PERTURBATION OF THE INPUT DATA OF MODELS USED FOR THE PREDICTION OF TURBULENT AIR FLOW IN AN ENCLOSURE

The aim of this article is to put forward an economical method for the determination of the uncertainty domain of the numerical results compared with a computational fluid dynamics (CFD) code due to data uncertainties In the first part of the study, we compare two methods of determining the domain of uncertainty: the quasi-Monte Carlo (QMC) method, and the finite-differences differential analysis (FDDA) method. This comparison is based on the two-dimensional modeling of laminar airflow in natural convection, in a small, thermally driven enclosure. The FDDA method is shown to be much more economical, in terms of computing time, than the QMC method.

Effects of Coupled Heat and Moisture Transfers through Walls upon Indoor Environment Predictions

Abstract The non-uniform behaviour of the air inside a room, which is important in comfort analysis, can be evaluated by zonal models. While not as fine-grained as CFD simulation, they do give useful information about temperature and moisture distributions that is not available from lumped-parameter models. Therefore, we have developed a tool, called SimSPARK, to automatically build dynamic zonal simulations of a building zone. Its model library includes different models to describe heat and moisture transfers across the building zone envelope, with two of them taking into account moisture adsorption/desorption by building materials. To illustrate the applicability of this tool, we compare two zonal models including adsorption and desorption processes with one that ignores these phenomena, in a ventilated room modelled using 27 cells. The results indicate that adsorption/desorption by building materials does affect indoor air behaviour in a hot and humid climate.

Structure moyenne et analyse intégrale du panache thermique des convecteurs électriques

In this study, we present an experimental and numerical analysis of thermal plumes issued from electric convectors. The study of the mean velocity and temperature profiles measured within the thermal plume shows that the self-similarity of the profiles is only partly achieved. We developed an integral model that takes into account the settlement flow region by the variations of the entrainment coefficient and the ratio between the temperature and velocity profile widths. A comparison between computed and experimental results proves that the model gives a satisfactory prediction of the physical phenomena. We propose finally a simplified thermal plume model for an integration into a thermal building code.

Numerical Evaluation of Earth to Air Heat Exchangers and Heat Recovery Ventilation Systems

Abstract In France there is an increasing demand for energy efficient and environmentally friendly buildings of high thermal comfort. Balanced ventilation systems with heat recovery on the exhaust air and earth to air heat exchangers (EAHEX) are interesting techniques which can reduce the heating and cooling demand of buildings, and improve internal thermal comfort. A numerical study was carried out to evaluate the impact of these two systems on the energy performance and internal thermal comfort of a dwelling, with respect to the French climate characteristics. The impact on CO2 emission was also analyzed. The results showed that a balanced ventilation system with heat recovery is a better preheating technique than the earth to air heat exchanger. The additional heating gain obtained from coupling an earth to air heat exchanger to a balanced ventilation system is rather marginal. However, earth to air heat exchangers present a significant potential for cooling. Regarded in this way, winter preheating becomes an additional free service, which can be coupled to other preheating techniques.