Using heat flux sensors for a contribution to experimental analysis of heat transfers on a triple-glazed supply-air window

An experimental set-up of triple-glazed supply-air window is developed in this research in order to characterize the window’s thermal performance without solar radiation. By simultaneously measuring the local temperatures and heat fluxes with thermocouples and heat fluxmeters, the heat fluxes determination requires no longer using the correlations of heat transfer coefficients around the window, which are often the source of high uncertainties. Results show that the use of fluxmeters brought a more accurate measure of heat transfers around and in the window. Thereafter, the heat transfer coefficients can be correctly estimated by empirical evidence. Uncertainty analysis is then presented to highlight the reliability of the experimental method. Afterwards, the obtained experimental data are compared with those of numerical model developed by using Fluent® software. A thorough comparison analysis is provided to explain which parameters play a role in deviating the results between the two methods, leading to conclude the validity of numerical model assumptions with respect to the real conditions of experimental set-up

Experimental investigation of thermal characteristics of a mortar with or without a micro-encapsulated phase change material

International audienc

Modeling a triple-glazed supply-air window

The supply-air window is a passive system of heat recovery contributing to the building ventilation. It allows the air renewal to circulate between glasses before entering the inside environment. Based on this principle, a part of heat transfer through the glasses is recovered by the airflow. In order to study the thermal performance of this window, Computational Fluid Dynamics (CFD) simulations are firstly conducted. Then, this article proposes simplified models which can be implemented easily in building simulation codes. The models are based on analytical solution of the problem of air circulating between isothermal panes differentially heated. It has been carried out in Modelica language using the Buildings library. A special attention is paid to the convective heat transfer coefficient between air space and glasses. The results obtained from these simplified models are compared to those obtained from the CFD model

Study of the Performances of A Supply-Air Window for Air Renewal Pre-Heating

1876-6102 © 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).Peer-review under responsibility of the CENTRO CONGRESSI INTERNAZIONALE SRLdoi: 10.1016/j.egypro.2015.11.727 Energy Procedia 78 ( 2015 ) 525 – 530 ScienceDirect6th International Building Physics Conference, IBPC 2015 Study Of The Performances Of A Supply-Air Window For Air Renewal Pre-Heating François Glorianta, Pierre Titteleina, Annabelle Joulina, Stéphane Lassuea,* aUniversité Lille Nord de France, Université d’Artois, LGCgE-EA 4515, Laboratoire de Génie Civil et géo-Environnement, Technoparc Futura, 62400 Béthune, FranceAbstractThe principle of a supply-air window is based on the air renewal circulation between the glazings of a window before entering home. We study in this work the Paziaud® window composed of three glazings forming a U-shaped channel. The air warms up by recovering some part of the heat losses from the building and also by solar radiation absorbed through the glasses. This system generally works in forced convection by association with an air extraction system. This type of component is not embedded in usual dynamic tools for building thermal simulation. A major reason of this lack is that the heat transfers through the walls and the air exchange are treated separately. Moreover, this particular system is characterized by different heat fluxes if we consider the inner or the outer surface of the component. Our contribution is based on an original and appropriate representation of convective heat transfer in asymmetrically heated air layers. We offer a "simplified" model that can be easily implemented in dynamic simulation tools. This model is compared CFD simulations. From this model, parametric studies are performed to look for the parameters influencing the performance of the Paziaud® window: we show here that boundary conditions in temperatures, the thickness of the cavities, low emissivity coatings and the glazing area have significant effects on the performance criteria. We perform the parametric study on the basis of indicators specifically defined for the supply-air window

Experimental and numerical investigation of a phase change material: Thermal-energy storage and release

The application of phase change materials (PCMs) for solar thermal-energy storage capacities has received considerable attention in recent years due to their large storage capacity and isothermal nature of the storage process. This study deals with the comparison of numerical and experimental results for a PCM conditioned in a parallelepipedic polyefin envelope to be used in passive solar walls. The experimental results were obtained by use of a genuine set-up involving heat flux sensors and thermocouples mounted on two vertical aluminium exchanger plates squeezing the samples. Numerical predictions were obtained with a custom one-dimensional Fortran code and a two-dimensional use of Fluent. Both methods showed a very good agreement with experimental observations for the melting process ([less-than-or-equals, slant]5%). However during solidification, both numerical codes failed to predict the phase change process accurately, the maximal relative error was as high as 57% (with an average of 8%).Phase change material Energy storage Supercooling Enthalpy method Fluxmetric experiments

Modeling phase change materials behavior in building applications: Comments on material characterization and model validation

International audienceIn a recent meeting of IEA’s Annex 23, several members presented their conclusions on the modeling of phase change materials behavior in the context of building applications. These conclusions were in agreement with those of a vast review, involving the survey of more than 250 journal papers, undertaken earlier by the group of École de technologie supérieure. In brief, it can be stated that, at this point, the confidence in reviewed models is too low to use them to predict the future behavior of a building with confidence. Moreover, it was found that overall thermal behaviors of phase change material are poorly known, which by itself creates an intrinsic unknown in any model. Models themselves are most of time suspicious as they are often not tested in a very stringent or exhaustive way. In addition, it also appears that modeling parameters are somewhat too simplified to realistically describe the complete physics needed to predict the real life performance of PCMs added to a building. As a result, steps are now taken to create standard model benchmarks that will improve the confidence of the users. Hopefully, following these efforts, confidence will increase and usage of PCM in buildings should be eased

Modeling phase change materials behaviour in building applications: selected comments

Abstract: In a recent meeting of IEA's Annex 23, several members presented their conclusions on the modeling of phase change materials behavior in the context of building applications. These conclusions were in agreement with those of a vast review, involving the survey of more than 250 journal papers, undertaken earlier by the group of École de technologie supérieure. In brief, it can be stated that, at this point, the confidence in reviewed models is too low to use them to predict the future behavior of a building with confidence. Moreover, it was found that overall thermal behaviors of PCM are poorly known, which by itself creates an intrinsic unknown in any model. Models themselves are most of time suspicious as they are often not tested in a very stringent or exhaustive way. In addition, it also appears that modeling parameters are somewhat too simplified to realistically describe the complete physics needed to predict the real life performance of PCMs added to a b uilding. As a result, steps are now taken to create standard model benchmarks that will improve the confidence of the users. Hopefully, following these efforts, confidence will increase and usage of PCM in buildings should be eased. Keywords: Phase change material, PCM characterization, Mathematical model, Model validation Context The ever increasing level of greenhouse gas emissions combined with the overall rise in fuel prices (although fluctuations occur) are today's main reasons behind efforts devoted to improve the use of various sources of energy. Economists, scientists, and engineers throughout the world are nowadays in search of: 1) strategies to reduce the demand; 2) methods to ensure the security of the supplies; 3) technologies to increase the energy efficiency of power systems; and 4) new and renewable sources of energy to replace the limited and harmful fossil fuels. One of the options to improve energy efficiency is to develop energy storage devices and systems in order to reduce the mismatch between supply and demand. In this context, latent heat storage could be considered. Indeed, it is particularly attractive since it provides a highenergy storage density and has the capacity to store energy at a constant temperature -or over a limited range of temperature variation -which is the temperature that corresponds to the phase transition temperature of the material. For instance, it takes 80 times as much energy to melt a g iven mass of water (ice) than to raise the same amount of water by 1°C. For the interested reader, excellent global reviews that pertain to phase change materials and their various applications were proposed by Zalba et al. Modeling in building applications A better management of the fluctuations of the external temperatures, wind, solar load, and heating or cooling needs is possible by the use of phase change materials. In building applications, these materials undergo a phase change close to the desired room temperature, which allow storing a large amount of heat in a relatively small volume compared to liquid water, brick or concrete. This results in direct energy savings as the solar gains can be used when needed, thus reducing the energy consumption for heating in the winter and cooling in the summer. Moreover, in many countries, these materials could also be used to reduce the peak consumption leading to money savings in this particular case. Nevertheless, high fidelity models are needed to guide the decisions of the architects and/or HVAC engineers in choosing optimum designs. Unfortunately, to formulate, implement, and validate such models is a rather difficult task mainly due to the non-linear nature of the problem. In addition, other technical issues add complexity to this problem. Here, we will discuss two of the most significant problems that should be addressed by the scientific community: phase change material characterization and model validation PCM characterization The first problem faced even before beginning the modeling process is the characterization of the phase change materials (PCMs) themselves. In building applications, composite PCMs are the favored packaging method for inner walls applications. In this form, PCMs can be integrated into a building using the same techniques used for gypsum panel, which would provide a seamless integration. However, this type of material is rather difficult to characterize. The key problem comes from the interaction with the substrate and the PCM in confined pores. This interaction affects both the melting and freezing temperatures as their respective enthalpy. To our knowledge, this phenomenon was first observed in building application by Hawes et al. Many physical models have been proposed to explain this behavior Even then, adequate characterization of PCM is a d ifficult task. For example, we have observed that reported enthalpy of melting and freezing can differ by more than as 15% in composite PCMs (ex: construction material In practice, the broad width of the composite PCM freezing/melting curve impairs the separation between latent and sensible heat. In addition, in some cases, there is an indication in many published measurements that at least a part of the PCM stays in supercooling state during the whole thermal cycle. In addition, heat capacity value and conductivity are different between liquid and solid phases. All these problems make very difficult to define a meaningful baseline to extract the latent heat curve. In addition, hysteresis in the cooling/heating curve has been observed In general, thermophysical properties measurements are done on a small sample. However, due to the non classical behavior of composite PCM, it is unclear whether these measurements are representative of the macroscopic thermal properties of the material. A more detailed study is under investigation, which consists on the consideration of the heat transfers within the calorimeter cells. The goal is to determine the true value of specific enthalpy regardless of experimental conditions (sample mass, heating and cooling rates) In conclusion, improved thermal characterisation procedures are needed and will be certainly welcomed by modellers. Model validation The validation of modelling algorithms is also troublesome. While not restricted to building applications However, as time went by, the authors relied more and more on ot her studies, mostly numerical ones, to validate their own numerical results. Many of the recent studies discuss their results qualitatively only, as the comparison with a graph taken from a publication may be somehow hazardous. And, interestingly, among the numerous -more than 250 -references and studies reported in Statements are almost never made on t he agreement or disagreement with previous results. This may be explained partly by the engineering scientific culture, where challenging or trying to duplicate previous works is not a common practice. As an illustration of this observation, we noticed that the work of Heim and Clark However, engineering sociology merely reflects the practical constraint of doing such crossvalidations. Materials, geometries, testing conditions and models are almost always different from one study to another. In such conditions, even for the most dedicated researcher, it is very hard to validate previous work. In our mind, this is a serious issue. Without a common ruler, it is impossible to formulate a meaningful recommendation about a technology. Finally, we found that there is little comparison between various models and experiments. Every research group seems to have its own numerical model. To our knowledge, all these models were claimed to work well. Nevertheless, recent works Further steps To address some of these problems, the IEA annex 23 has prepared two standard cases to test numerical models The existence of such divergence with a simple situation is by itself a strong warning about the models reliability. A second benchmark is now proposed. This benchmark is based on a small cubicle using PCM in its walls. In that case, high quality experimental data are used as a reference. To populate a database of benchmark, members of the annex 23 are invited to submit there own experimental data. These initiatives are certainly a step in the right direction. Their use as a validation tool should be considered by any researchers working into application of PCM in building. Nevertheless, results are too fragmentary at this point to produce general guideline for researchers. Conclusion While the applications of PCM in building are promising as a tool to reduce energy consumption, there are still many roadblocks on the widespread utilization. To optimize their utilization in buildings, reliable models are needed. At this point, the confidence in models is too low to be use to predict the future behavior of a building. However, thermal behavior of PCM themselves are poorly known, which by itself create a huge unknown in model. Models themselves are suspicious as they are rarely tested in a very stringent way. 932 In addition, it a lso appears to us that modeling parameters are somewhat too simplified to realistically describe the real life performance of PCM addition into buildings. For example, seldom complete meteorological information (solar irradiation, external temperature and wind) are used as inputs. However, correlation and anti-correlation between these factors could strongly affect the results. In addition, in most systems modeled, thermal loads are restricted to solar heating. Additional heat from appliances will certainly affect the results. Also most of the time modeling is done on individual rooms or few rooms aligned in a perfect east-west alignment and empty. In real life, most houses are not perfectly oriented, have additional room with little solar heating, are equipped with furniture, and are occupied by people. This will both modify the thermal loading and the effective storage mass of the building. From our analysis of the literature, typical gain in energy efficiency by the utilization of PCM is expected to be roughly about 10-15%. In consequence, the factors not included in models could easily change the overall conclusion about the pertinence of PCM in building application. The steps taken now by the IEA ECES IA Annex 23 to create standard model benchmark will improve the confidence of the users. Phase change material characterization is still an unresolved issue, but many research teams work on it. Hopefully, following these efforts, confidence will increase and usage of PCM in building will be more straightforward

Phase change materials characterisation and applications to the thermal simulation of buildings

International audienceThis study aims at presenting the main results of the Stock-E MICMCP project, fund by the French National Research Agency (ANR). The principal goal of this project is the correct characterisation of the thermophysical properties of phase change materials (PCMs) in order to have reliable inputs when considering their use in numerical simulations of thermal behaviour of buildings. Firstly, the method developed to determine the dependency of the enthalpy function with respect to the temperature of the material is presented. It is based on the use of experimental measurements together with an inverse method combined with a numerical modelling. By assuming an a priori formulation of the enthalpy, based on some basic thermodynamic constraints, a simulated heat flux may be computed. It is then compared to the measured one, which permits to define an objective function. Its minimization thus allow to determine the value of the parameters involved in the equation of state. This step is first tested with microscopic samples thanks to differential scanning calorimetry (DSC). Secondly, it will be shown that this method can be extended to macroscopic and heterogeneous materials, which are more representative of real samples. Eventually, some examples of thermal simulations of buildings are done so as to highlight the necessity to correctly represent the PCM behaviour. This is particularly important since they appeared to be a promising way to save energy and achieve a better comfort in buildings, therefore an incorrect determination of their properties may lead to wrong conclusions on their real benefits