Gilles Lefebvre

Using model size reduction techniques for thermal control applications in buildings

This paper deals with buildings indoor air temperature control problems. It specially aims applications concerning actual heating devices testing and control strategies analysis. In both cases, accurate but low-dimension dynamical models representing the thermal behaviour of the building are required. Knowledge models satisfy accuracy requirements but they usually include a great amount of differential equations to solve. Using model size reduction techniques becomes thus essential for control applications. Different model size reduction methods are presented and tested in PID control and optimal control problems. Their performances are shown through an example of application.

Mechanical Properties and Melting Heat Transfer Characteristics of Shape-Stabilized Paraffin Slurry

This article is presented in the framework of the increasing interest for the use of latent heat transfer slurries for cooling processes. Paraffin serves as a latent heat storage material, and a polymer network acts as the supporting material. The phase change material melts around 7°C with a latent heat of fusion of 115 kJ/kg. Differential scanning calorimetry and indentation analysis were employed to investigate the thermal properties and mechanical stability of the material under freezing–thawing cycles. Results indicated that the form-stable phase change material with the advantages of no liquid leakage constitutes a potential material in the field of low-temperature thermal energy storage. Experimental investigation is made to study the melting process of a single slurry in an agitated bath. Analysis of data permits the development of a phenomenological correlation adapted to the millimetric dimension of the particle, leading to an estimation of the melting phase change time duration as a function of the main parameters of the problem.

SUBSTRUCTURED MODELING OF LINEAR THERMAL SYSTEMS: THE MODEL SYNTHESIS

A general and systematic procedure for generating thermal state space models is presented. It allows one to build models of complex thermal systems by assembling elementary models which caneachbe generated with specific methods or tools. A size reduction procedure may be applied on every elementary model. This method is particularly useful when modeling large systems requiring detailed meshing, as it allows one to distribute the calculation effort along a process which does not require handling the whole global, nonreduced problem. This approach is coherent with a reusability strategy in which models of elementary parts are stored in libraries.A general and systematic procedure for generating thermal state space models is presented. It allows one to build models of complex thermal systems by assembling elementary models which caneachbe generated with specific methods or tools. A size reduction procedure may be applied on every elementary model. This method is particularly useful when modeling large systems requiring detailed meshing, as it allows one to distribute the calculation effort along a process which does not require handling the whole global, nonreduced problem. This approach is coherent with a reusability strategy in which models of elementary parts are stored in libraries.

A New Inverse Method Using Model Synthesis: Application to Thermal Systems

We propose a new inverse method which consists of building an “inverse model.” The model synthesis [1], which allows us to gather several coupled elementary models in a single one, can be used to permute some outputs with some inputs of the direct model, solving a special coupling between the direct model and a particular one called “inversor.” The model synthesis then provides an inverse model which can be used to perform usual simulations. We show the principles of the method, the practical “ticks,” and demonstrate with an example that it is an easy-to-use and efficient method.

Thermomechanical Characterization of a Mortar Reinforced by Animal Fibers

To meet the needs always more accurate and demanding in the construction industry, mechanical and thermal or acoustic features of building materials have known significant improvements over the last two decades. Researchers in materials science and civil engineering are constantly listening to the industry and continue to innovate in this field. The current trend is the search for new materials, called intelligent, where several properties as mechanical and physico-chemical are combined. In our case, the aim was to develop a new construction material reaching the construction standards, --- i.e. with acceptable mechanical properties --- but which is also able to perform other functions such as thermal insulation or sound insulation. To do this, we chose to strengthen a cement mortar with natural fibers obtained from poultry feather. A physical and chemical stability behavior is obtained thanks to a treatment performed before their incorporation into the composite matrix. The fibers are introduced in a mortar matrix as a substitute for mineral or synthetic fibers which are traditionally used for this purpose. The cylindrical and prismatic specimens were then prepared with the composite in order to determine the mechanical characteristics of this composite. Compression tests and three-point bending were carried out for this purpose. To determine the thermal conductivity of composites, several plates with different percentages of fiber, whose size is 300x300x10 mm3, was chosen in order to be adaptable to the experimental device, were fabricated.

Different Phase Change Material Implementations for Thermal Energy Storage

This paper presents the principal methods available for phase change material (PCM) implementation in different storage applications. The first part is devoted to a non-exhaustive overview of the various chemical processes used to develop stable PCM (such as microencapsulation, emulsion polymerization or suspension polycondensation, polyaddition, etc.) based on the available literature. The second part deals with shape-stabilized PCM, developed from an intimate combination of a polymer matrix and a phase change element. Materials able to include more thermal energy as usual ones are interesting as they increase the thermal inertia of the system that presents by this way advantages. The energy efficiency of buildings may be improved including PCMs that store and provide enthalpy from one hand and without any significant temperature modification during the phase change process on the other hand. If the solid phase of the PCM does not present any problem, it is not the same for the liquid phase which must be maintained mechanically at its assigned location. Furthermore, the PCM in the solid (and furthermore in the liquid phase) does not have mechanical properties which allow to use it as a structural material able to support charge loads. This paper presents different methods to distribute and maintain the PCM in the thermal solid matrix.