Use of Identification Methods to characterise Phase Changes Materials – MICMCP

The buildings account for today approximately 42% of the final total energy consumed in France and 23% of the CO2 emissions. "Le Grenelle de l’Environnement" took the measurement of this effect, and laid down very ambitious objectives for the energy renovation: reduction of energy consumption of 20% in the tertiary buildings and 12% in the residential buildings in 5 years; on the horizon 2020, the objective is to have renewed 30 % of the residential park. Besides the energy aspects, it is also necessary to take into account and to improve the comfort of the occupants. Indeed, thermal comfort is mainly related to the thermal inertia of the building, especially in summer. However, the renovation of buildings by the inside cause a reduction of their thermal inertia which could be filled by Phase Change Materials. The other asset of these materials relates to the storage of energy resulting from the direct solar contributions during the interseason or winter days and their restitution in the night.
The use of the MCP is an interesting solution because they present a strong density of energy storage in a reduced volume, thanks to the latent heat of transformation. The modelling of the energy and thermal behaviour of the buildings integrating PCMs require a fine knowledge of the phase transition processes. The commercial computer codes used by the professionals of the building often use apparent characteristics which are badly evaluated by current practices. It is however essential to precisely determine some thermophysical characteristics (apparent specific heats including the latent heat in particular…) so that the modelling is representative of the physical phenomena.
As it is explained that the methods currently employed, calorimetry in particular, are not sufficiently exact; this is why, within the framework of project MICMCP, we propose to work on the characterization of Phase Change Materials by the mean of identification techniques which will give results in conformity with the physics of the phase change. The characterization will be made both in the heating for the fusion and in the cooling in the crystallization of supercooled liquids. The objective is to determine the apparent thermal characteristics, generally used with the commercial software. We suggest identifying them by experiments on samples of increasing sizes and complexity: small homogeneous samples studied mainly by calorimetry, macroscopic samples (fraction of liter) studied with finely instrumented measurement cells or on models allowing to study the PCM in their commercial configuration (for example 1mx1m). Finally, it will be evaluated the influence of a more accurated determination of the thermodynamical properties for applying in commercial codes for buildings.

The methods of identifications are completed by the determination of the errors due to the imperfect knowledge of certain parameters of the experiments or the measurement errors. The project is thus to develop numerical methods of analysis of experimental results. It will be a question of establishing, at first by direct models, the physics of the thermal phenomena, either in the calorimetric cells, or for the experiments on macroscopic samples, or on models. Then the main work will be to implement identification methods in these three cases and to deduct the associated precisions. The sensitivity analysis will be used to the design and then development of the experimental devices. Finally, these studies will allow defining a protocol of experiments allowing a procedure of technical evaluation of the industrial PCM.