Influence of different basic raw materials from technical to laboratory grade on the maximum storage capacity of CaBr2·6H2O

Abstract

In this study, the influence of different basic raw materials from technical to laboratory grade on the maximum storage capacity of CaBr2·6H2O is investigated via differential scanning calorimetry (DSC). Five samples of CaBr2·6H2O are prepared from laboratory grade basic raw materials, 1 CaBr2·6H2O sample is prepared from grown CaBr2·6H2O crystals and 1 sample is prepared from a technical grade CaBr2 brine. To exclude differences in the maximum storage capacity due to deviations in the water content of the samples, the water content was adjusted for each sample until it matched the stoichiometrically correct water content of CaBr2·6H2O of 35.10 wt% within an interval of ±0.05 wt%. The CaBr2·6H2O sample obtained from the grown crystals serves as reference sample and presents a melting enthalpy of (143 ± 1) J g−1 and a maximum storage capacity of (166 ± 1) J g−1 in the temperature interval between 30 and 45 °C. The decrease in maximum storage capacity of the laboratory grade samples relative to the reference sample ranges between −1% and −18%. For the technical grade CaBr2·6H2O sample the decrease in maximum storage capacity is 45%. However, if considering a temperature interval between 25 and 40 °C for the evaluation of the maximum storage capacity, the difference between reference and technical grade CaBr2·6H2O is noticeably reduced to −18%. The considerable differences in maximum storage capacity for the CaBr2·6H2O samples from different grade basic raw materials were obtained via DSC measurements with a sample size typically orders of magnitude smaller than in potential applications. For the use of certain grade basic raw materials to prepare CaBr2·6H2O as phase change material (PCM) in view of an application, it should be checked, if the CaBr2·6H2O samples show a volume-dependent behaviour.