Interpretation of calorimetry experiments to characterise phase change materials Jean-Pierre Dumas a, * , Stéphane Gibout a , Laurent Zalewski b , Kévyn Johannes c , Erwin Franquet a , Stéphane Lassue b , Jean-Pierre Bédécarrats a , Pierre Tittelein b , Frédéric Kuznik c a Université de Pau & Pays de l'Adour, LaTEP-EA 1932, Laboratoire de Thermique, Energétique et Procédés, ENSGTI, rue Jules Ferry, BP 7511, 64075 Pau cedex, France b Université Lille Nord de France, U. d’Artois, LGCgE-EA 4515, Laboratoire de Génie Civil et géo-Environnement, Tecnoparc Futura, 62400 Béthune, France c Université de Lyon 1, CETHIL CNRS UMR 5008, Centre de Thermique de Lyon, 9 rue de la Physique, 69621 Villeurbanne Cedex, France article info Article history: Received 23 November 2012 Received in revised form 21 November 2013 Accepted 21 November 2013 Available online 25 December 2013 Keywords: Phase change material (PCM) Energy storage DSC abstract In the building field, the topic of thermal storage is generally studied with assistance from dedicated software programs. To generate transient thermal simulations, these software programs use enthalpy functions h (T) to describe the thermal behaviour of the different parts of a modelled structure. Unfor- tunately, the mathematical form of these functions is often extremely unrealistic due to an erroneous interpretation of the calorimetric experiments that were performed to determine these functions. The purpose of this study was to evaluate the energy-related errors that occur if a misinterpreted enthalpy function is used and to thereby assess the impact that these inaccurate functions generate with respect to thermal simulations of buildings. Ó 2013 Elsevier Masson SAS. All rights reserved. 1. Introduction The use of phase change materials (PCMs) is increasingly rec- ommended in many contexts, such as industrial applications [1,2], transportation [3,4], electronics [5,6], electric systems [7,8], storage devices that are used for solar heating or cooling [9,10] and build- ings [11e 14]. The software that is generally encountered in these industries is often designed for generic calculations that are not always adapted to providing a correct description of the solide liquid phase changes. In particular, melting involves (at a fixed pressure) a sudden change (for pure substances) or a variation more or less rapid depending on the concentrations (for solutions), of the specific enthalpy h (T) as a function of temperature and only of temperature. However, to model this process, the aforemen- tioned software programs often utilise the so-called “equivalent capacity” curves that are obtained from calorimetry experiments through the simple integration of the thermogram generated by scans that are performed at a constant rate. Several commercial building energy simulation software programs have used and continue to employ these “equivalent capacity” curves to represent the thermal behaviour of buildings that contain PCM. Simulation programs that adopt this approach include Esp-r [15], CoDyBa [16], TRNSYS [17e19] and EnergyPlus [20]. We will explain why the direct analysis of a thermogram does not permit the enthalpy h (T) to be correctly defined as a function of temperature. In fact, a thermogram, which represents the heat flow rate between a plate and a sample over time, reflects the transient energetic behaviour of the sample as a whole. This behaviour is governed not only by thermodynamic processes (phase changes) but also by thermal transfers within the sample. Thus, the sample temperature is not uniform during the experiment, particularly during phase changes, even in cells that contain a mass of only a few mg. Consequently, heat transfers within the sample are important and explain the apparent change in thermograms under different experimental conditions (e.g., heating rate and sample mass) [21e24]. As an example, Figs. 1 and 2 illustrate the thermograms that are obtained for two materials (a pure substance and a binary solution) at different heating rates. For emphasis, the derivative of the enthalpy with respect to the temperature dh/dT is also depicted; this derivative demonstrates Dirac behaviours at the melting temperature for the pure substance and at the eutectic temperature for the saline-type solution. * Corresponding author. E-mail address: jean-pierre.dumas@univ-pau.fr (J.-P. Dumas). Contents lists available at ScienceDirect International Journal of Thermal Sciences journal homepage: www.elsevier.com/locate/ijts 1290-0729/$ e see front matter Ó 2013 Elsevier Masson SAS. All rights reserved. http://dx.doi.org/10.1016/j.ijthermalsci.2013.11.014 International Journal of Thermal Sciences 78 (2014) 48e55