| The present energy crisis caused by economic development is more and more serious. How to increase utilization ratio of energy and save energy attracts worldwide attentions. Phase change material (PCM), which can store or release latent heat during phase change process and solve the unbalance between supply and demand of energy shortage, has become hot topic in the research. Conventionally, a formula method is used widely for looking for PCM, which is difficult to find real eutectic points. In this paper, thermodynamic model has been chosen for calculating the phase diagrams of lithium and magnesium hydrated salt systems and finding phase change materials with suitable temperature. The main research contents and results are:1. Several thermodynamic models commonly used were evaluated, and a Pitzer-Simonson-Clegg model (PSC model) and a BET model were chosen for calculating phase diagrams of seven ternary systems including CaCl2-LiCl-H2O, LiCl-LiNO3-H2O, LiNO3-Mg(NO3)2-H2O, NaNO3-Mg(NO3)2-H2O, KNO3-LiNO3-H2O, KNO3-Mg(NO3)2-H2O and CuCl2-LiCl-H2O, and the calculated values are compared with experimental data. The results show that the BET model is more appropriate than the PSC model to predict the properties of highly soluble hydrated salt systems. For salt-water systems consisting of lowly soluble salt and highly soluble salt, the PSC model, which includes long range electrostatic terms and interaction between solvent water and ions, is more suitable than the BET model.2. Suitable model was applied to calculate phase diagrams of salt-water systems containing lithium (magnesium), and then eutectic points with appropriate temperature were found.(1) For LiNO3-KNO3-M(NO3)n-H2O (M=Na (n=1), Mg (n=2)) systems containing lowly soluble salt KNO3and highly soluble salt LiNO3, combining with experimental data of its binary and ternary systems, the PSC model was used to calculate phase diagrams of quaternary systems and their sub-systems. In these phase diagrams, we have found three binary eutectic points:KNO3-LiNO3-3H2O, NaNO3-LiNO3-3H2O, LiNO3-3H2O-Mg(NO3)2-6H2O, and five ternary eutectic points:KNO3-LiNO3-3H2O-LiNO3, NaNO3-LiNO3-3H2O-LiNO3, LiNO3·3H2O-LiNO3-Mg(NO3)2-6H2O, LiNO3·3H2O-NaNO3-KNO3, KNO3-LiNO3-3H2O-Mg(NO3)2-6H2O, with melting temperature range from294K to301K.(2) The BET model is applied to calculate phase diagrams of systems LiNO3-NaNO3-NH4NO3-H2O, Mg(NO3)2-MgCl2-H2O and their sub-systems, and then three binary eutectic points including NaNO3-LiNO3-3H2O, NH4NO3-LiNO3-3H2O, Mg(NO3)2-6H2O-MgCl2-6H2O, and four ternary eutectic points including NaNO3-LiNO3·3H2O-LiNO3, NH4NO3-LiNO3·3H2O-LiNO3, Mg(NO3)2-6H2O-Mg(NO3)2-2H2O-MgCl2-6H2O and LiNO3·3H2O-NaNO3-NH4NO3were found.3. The solubility isotherm of the KNO3-LiNO3-H2O system at283.1K has been determined using method of isothermal solution, and the solubilities of the KNO3-LiNO3-NaNO3-H2O system at298.1K and308.5K as well as its sub-system LiNO3-NaNO3-H2O at323.1K have been measured by means of isothermal solid-disappearance method. The experimental solubility data are in excellent agreement with the calculated values.4. Melting and crystallization behavior of the aforementioned predicted eutectic points have been measured using our device, and DSC or DTA was used to measure the fusion heat and fusion temperature. The results show that:(1) The binary eutectic points KNO3(17.5%)-LiNO3·3H2O (82.5%) and the ternary eutectic point KNO3(20.9%)-LiNO3·3H2O (64.4%)-LiNO3(14.7%), LiNO3·3H2O (76.2%)-NaNO3(5.9%)-KNO3(17.9%) can be taken as potential room temperature PCM with high stored energy effect, and the differences of melting temperature between experimental data and predicted ones are less than±1K.(2) The ternary eutectic point LiNO3·3H2O (63.4%)-NaNO3(5.5%)-NH4NO3(31.3%) which has excellent heat storage behavior, could be used as potential low temperature PCM.(3) The binary eutectic point Mg(NO3)2·6H2O (61.6%)-MgCl2·6H2O (38.4%) whose melting point is333.5K and phase change heat is137J·g-1, could be regarded as potential PCM. However, ternary eutectic point Mg(NO3)2-6H2O (45.6%)-Mg(NO3)2·2H2O (29.6%)-MgCl2·6H2O (24.8%) possesses less storage energy ability. |
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