J. Thakkar, N. Bowen, P. Horvath, A. Chang, J. Kosny, M.J. Sobkowicz
University of Massachusetts, Lowell,
Summary:The building sector is a front runner in energy consumption, with energy demand rapidly increasing. One way to reduce this demand is to improve thermal performance. Latent heat storage is a promising approach that makes use of phase change materials (PCMs) to smooth out temperature fluctuations. Inorganic salt hydrate PCM calcium chloride hexahydrate (CaCl2.6H2O) has a moderate melting point (25-28℃), high latent heat storage capacity (170-190 J/g) and is very low cost, all of which make it attractive for the building application. However, like other salt hydrate PCMs, CaCl2.6H2O exhibits non-ideal behaviors such as supercooling and phase separation (semi-congruent melting), which need to be addressed for reliable long-term use. Semi-congruent melting results from the formation of lower hydrates of CaCl2 and affects the energy storage capacity. Supercooling prevents reliable heat release during solidification and can also exacerbate the semi-congruent melting effect. In this study, we evaluated additives and preparation methods to stabilize CaCl2.6H2O by promoting congruent melting and suppressing supercooling. Strontium chloride hexahydrate (SrCl2.6H2O) was added as a nucleating agent and sodium chloride and potassium chloride (NaCl, KCl) were added as stabilizers against incongruent melting. The effects of additive concentrations on supercooling and enthalpy were determined for up to 25 thermal cycles. X-ray diffraction (XRD) and differential scanning calorimetry (DSC) results showed that heating during the mixing of formulations was detrimental to the stability, and the heat of mixing was sufficient for complete dissolution. Addition of SrCl2.6H2O (2 wt%) reduced supercooling to 0.5℃ from 12℃ and increased the latent heat by 5% compared to the CaCl2.6H2O without SrCl2.6H2O over 15 cycles. Addition of more than 2 wt% SrCl2.6H2O reduced the latent heat and increased supercooling. NaCl/KCl (5 wt%) with SrCl2.6H2O (2 wt%) were found to suppress the phase separation and maintain congruent melting of this PCM over 25 cycles with no supercooling and latent heat almost similar to neat CaCl2.6H2O (172 J/g). NaCl and KCl lower than 5 wt% resulted in formation of CaCl2.4H2O which led to reduced energy storage capacity. XRD analysis was done to compare the structure of CaCl2.6H2O with and without additives; it showed that the crystalline structure of CaCl2.6H2O was preserved with the additives. In conclusion, this work found two formulations were stable after 25 cycles: CaCl2.6H2O with 5 wt% KCl and 2 wt% SrCl2.6H2O and CaCl2.6H2O with 5 wt% NaCl and 2 wt% SrCl2.6H2O with no supercooling and latent heat of 169 J/g and 178 J/g, respectively.