Calorimetric and theoretical determination of the concentration dependent enthalpy change around CaBr 2 6H 2 O Henri Schmit *, Simon Pöllinger, Werner Pfeffer, Stefan Hiebler Bavarian Center for Applied Energy Research – ZAE Bayern, Walther-Meissner-Strasse 6, 85748 Garching, Germany A R T I C L E I N F O Article history: Received 3 December 2014 Received in revised form 18 March 2015 Accepted 21 March 2015 Available online 18 April 2015 Keywords: Phase change material (PCM) Salt hydrate Differential scanning calorimetry (DSC) Specific heat capacity Concentration determination Enthalpy model A B S T R A C T In large latent heat storages based with the phase change material (PCM) being a salt hydrate, it is difficult to assure the stoichiometrically correct hydrate concentration of the salt hydrate. In this study the impact of the concentration of CaBr 2 + H 2 O in a concentration range of about 2.6 wt% around the congruent melting CaBr 2 6H 2 O on the melting enthalpy and the maximum storage capacity is exemplary investigated via differential scanning calorimetry (DSC) and via a model. The melting enthalpy of CaBr 2 6H 2 O is found to be 142.1 J g 1 , which is significantly larger than the value indicated in literature (115.5 J g 1 ). For a CaBr 2 concentration around 0.8 wt% lower than the concentration of CaBr 2 6H 2 O, a decrease of over 17% is found for both the melting enthalpy and the maximum storage capacity. A good agreement is found for the maximum storage capacity between the experimentally determined values and the values determined by the model. ã 2015 Elsevier B.V. All rights reserved. 1. Introduction Phase change materials (PCM) allow storing of large amounts of heat in narrow temperature ranges [1]. Therefore, PCM are attractive in many applications, e.g. for HVAC in buildings, which consume 20– 40% of the total final energy consumption in developed countries [2]. While organic PCM are normally less corrosive and show higher thermal cycle stability in terms of phase separation than inorganic PCM, they are generally more expensive than inorganic PCM, which makes them less attractive for usage in large thermal storages. Calcium chloride hexahydrate (CaCl 2 6H 2 O) is an inorganic PCM that received a lot of attention in the past due to its large melting enthalpy (190.8 J g 1 ) and its low costs [3]. However, CaCl 2 6H 2 O melts semicongruently, and is thus subject to phase separation with an associated loss of latent heat with increasing heating/cooling cycles. Therefore, a lot of the research on this salt hydrate has been focused on modifications to render the melting congruently and thus make CaCl 2 6H 2 O thermally stable under repeated heating and cooling [4]. An important aspect in the usage of a salt hydrate as PCM, the impact of the mixing of the stoichiometricallycorrect hydrate concentration on the latent heat, has not yet been investigated in literature. For example, for large thermal storages of 1 m 3 as described in [5], CaCl 2 6H 2 O is mixed from CaCl 2 2H 2 O by adding water. The dissolution of CaCl 2 2H 2 O respectively the hydration reaction is exothermal [6], which leads to evaporation of water and the concentration of the obtained CaCl 2 + H 2 O mixture being too low for CaCl 2 6H 2 O. In such a case, it is difficult to obtain the stoichiometri- cally correct hexahydrate concentration and also to verify the concentration. In order to investigate the impact of small deviations from the stoichiometrically correct hydrate concentration on the enthalpy change, CaBr 2 6H 2 O was chosen, which is isomorphous to CaCl 2 6H 2 O, but, in contrast to CaCl 2 6H 2 O, congruently melting. Both hexahydrates crystallise in the hexagonal crystal system, with the two crystal axes of the same length being 3.5% largerand the third crystal axis of the unit cell being 3.7% larger in the case of CaBr 2 6H 2 O, respectively. Therefore, CaCl 2 6H 2 O and CaBr 2 6H 2 O can be de- scribed as isomorphous (for further information refer to [3]). An investigation of the concentration dependent enthalpy change around the stoichiometrically correct hexahydrate concentration of CaBr 2 6H 2 O (in the following just referred to as CaBr 2 6H 2 O) is thus possible, without the effect of the semicongruent melting potentially biasing the results. While a congruent melting salt hydrate of the stoichiometrically correct hydrate concentration (also known as a dystectic) crystallises and melts at a defined temperature, deviations from this concentration lead to a certain temperature range in which crystallisation and melting occur [7]. Even though CaBr 2 6H 2 O is congruently melting, it has so far received less attention than CaCl 2 6H 2 O, which is due to its higher costs and its lower melting * Corresponding author. +49 8932944222. E-mail address: henri.schmit@zae-bayern.de (H. Schmit). http://dx.doi.org/10.1016/j.tca.2015.03.027 0040-6031/ ã 2015 Elsevier B.V. All rights reserved. Thermochimica Acta 609 (2015) 20–30 Contents lists available at ScienceDirect Thermochimica Acta journal homepa ge: www.elsev ier.com/locate/tca