Solar energy storage has become more attractive in recent years. In particular, solar energy with the characteristics of being abundance, cost-free and clean to the environment is a prospective renewable energy resource, but the extensive utilization is impeded by its intermittent characteristic [1, 2]. An efficient and reliable energy storage system is one of the promising solutions for this time-dependent limitation [3, 4]. Compared with other materials of heat storage, molten salts have thermal energy storage and heat transfer properties . Three major groups of molten salts mixtures (nitrate/nitrite group, carbonate group and chloride/fluoride group) are mostly studied [2–7]. Generally, compared to the other two groups, molten nitrate salts are being used as media for the thermal energy storage because of the low-melting point, low unit cost, high heat capacity and high thermal stability, etc. [8–10].
To date, NaNO3-KNO3 system  and NaNO3-KNO3-NaNO2 system  are being widely used as medium for the heat storage solar thermal power stations. These molten salts are similar to water at a high temperature, so they are suitable heat transfer-thermal storage materials used for solar thermal power [13, 14]. Besides, their thermal conductivities are two times higher than other organic heat transfer fluids . Unfortunately, these molten salts are not stable enough at high temperatures in the presence of oxygen and the working temperature range of these molten salts is not wide enough . In addition, in the process of solar thermal power generation, high impurity content of the molten salts will corrode various metal materials and molten salts piping, thus shortening the service life of equipment . Besides, equipment fouling will severely reduce heat transfer coefficient and heat transfer efficiency of the system . So there are very stringent requirements on the purity of molten salts.
In order to solve the problems mentioned above, it must be preparing high-purity molten nitrate salts and focused on reducing its melting point, increase its upper limitation temperatures and reinforce its stability. We chose potassium nitrate (KNO3) and sodium nitrate (NaNO3) [19, 20]. Besides, a kind of molten salts (purified Solar Salt [NaNO3 (60%) and purified KNO3 (40%)]) was sufficiently stable at high temperatures. All thermo-physical properties of molten salt were determined in order to examine whether it was suitable for heat transfer-thermal storage material.
Materials and preparation
Potassium nitrate (KNO3, industrial grade, purity >99%, Qinghai Salt Lake Industry Group Company Limited) and sodium nitrate (NaNO3, industrial grade, purity >99%, Qinghai Salt Lake Industry Group Company Limited) were purified by recrystallization (Industrial-grade NaNO3 is dissolved in the smallest amount of hot solvent at a definite temperature to make saturated solution. Saturated solution filtered to remove insoluble substance, cooling and crystallization, filtration, washing high-purity of products obtained after the operation.) and dried in an air oven at 85°C for 24 h. Solar Salt [NaNO3 (60%) and KNO3 (40%)] were made by statically mixing method . NaNO3 (60%) and KNO3 (40%) were put into a mortar and mixed uniformly in a macroscopic scale by the pestle. Subsequently, these molten salts were placed in the Muffle furnace and heated statically to melt completely at about 350°C (heating rate of 7°C /min) and then kept for 3 h. The Solar Salt was then cooled to room temperature, ground to powders, and kept in a dryer.
Apparatus and calibration
Melting point and latent heat of molten salts were measured by SDT Q600 thermo gravimetric analysis (The United States thermoelectric company); molten salts were identified by X-ray powder diffractometry (Cu-Kα radiation and a scanning range of 20–75(2θ)); the contents of impurity ions were determined by chemical analysis and ICAP 6500 DUO (The United States thermoelectric company). In the experiment of thermal cycling molten salts were put into a Muffle furnace and melted completely at different temperatures: 350°C, 400°C, 450°C, 500°C, 550°C, 600°C (7°C/min), respectively, then kept for 3 h and cooled to room temperature naturally. The weights of cooled solids were measured by electronic scales and recorded. Repeated 10 times in this way, finally, these data were used in the weight loss vs time curve of molten salts at different temperatures.
Results and discussion
The component analysis of the sample was measured by chemical analysis and ICAP as shown in Table 1. Table 1 showed the levels of impurity ions of NaNO3 and KNO3 before/after purification. The levels of Cl–, SO42–, Mg2+ and K+ in purified NaNO3 were reduced by 74.2%, 61.3%, 86.1% and 4.2%, respectively. Furthermore, the levels of Cl–, SO42– and Mg2+ in purified KNO3 were reduced by 68.5%, 81.5% and 73.9%, respectively. In the study, few of Fe3+ and Ca2+ can be neglected.
Decomposition curves of molten salts are shown in Figures 1–6. Figure 1 showed that the weight loss of sodium nitrate molten salt (SNMS), respectively, was 0.098%, 1.21% and 2.75% at 350°C, 450°C and 500°C. The weight loss of purified SNMS slightly decreases which was shown in Figure 2. Figure 2 showed the weight loss of purified SNMS was 0.055% after 27 h at 350°C, 1.04% at 450°C and 2.01% at 500°C.
Whereas the weight loss of purified potassium nitrate molten salt (PNMS) increases which were shown in Figures 3 and 4. Figure 3 showed the weight loss of PNMS was 1.74% after 27 h at 350°C, 2.56% at 450°C and 4.29% at 550°C. These indicated PNMS was more stable below 450°C (when the weight loss was less than 3%, molten salts are defined as the steady state). The optimum operating temperature of purified PNMS was increased from 450°C to 550°C. These were shown in Figure 4. The weight loss of purified PNMS was 1.63% at 350°C, 1.60% at 450°C, 2.6% at 550°C and 15.12% at 600°C.
In addition, Figures 5 and 6 showed the weight loss of Solar Salt and purified Solar Salt. The weight loss of Solar Salt and purified Solar Salt was 3.54% and 2.65% at 550°C respectively, and the weight loss increased when the temperature was higher than 550°C. It was clear that the optimum operating temperature of purified Solar Salt was increased to 550°C. Because of oxidation of the nitrites or other reactions resulted in the increasing of the weight of purified Solar Salt at the early stage.
Melting point and latent heat
The melting point is an important physical property for heat transfer materials. Lower melting point requires less heat insulation measures between heat storage containers and pipe valves. The fuel gas or electric heating system to prevent the solidification of molten salt in entire experiment loop was reduced. Therefore, molten salts with lower melting point were suitable for heat transfer and thermal storage materials. Besides, latent heat of molten salts was also one of the important characteristics for thermal storage materials. The melting point of molten salts was analyzed by SDT-Q600 thermo gravimetric analysis and DSC test results were shown in Figures 7–9. Compared with unpurified SNMS, the melting point of purified SNMS was increased from 306.4°C to 308.4°C and latent heat of melting also increased from 159.2 J/g to 195.2 J/g. While that of purified PNMS was sharply decreased from 322.2°C to 228.9°C and latent heat of melting was increased from 161.5 J/g to 171.51 J/g. Figure 9 showed the melting point of purified Solar Salt was decreased to 223.8°C and the latent heat of melting was increased from 74.39 J/g to 80.79 J/g compared with Solar Salt. It was clear that the purification effect of molten salts on their thermal properties was significant. The first endothermic peak is caused by the water to vaporize.
Generally, thermal storage material should be used repeatedly and corresponding properties should be stable. After 10 cycles, melting point of Solar Salt was reduced and latent heat was increased as DSC test results were shown in Figures 10 and 11. Melting point of purified Solar Salt was decreased by 0.44% at 350°C, 0.93% at 450°C and 23.2% at 550°C. Loss of latent heat for purified Solar Salt was increased by 3.5% at 400°C, 21.5% at 500°C and 30.6% at 600°C, compared with unpurified Solar Salt, so that was favorable to reduce latent heat loss when purifying NaNO3 and KNO3.
Upper limitation temperature
The results for the TG experiments of different molten salts are shown in Figures 12–14. Figure 12 showed the upper limitation temperature of purified SNMS was decreased from 613.4°C to 607.4°C and Figure 13 showed the upper limitation temperature of purified PNMS was decreased from 613.7°C to 583.9°C, while Figure 14 showed the upper limitation temperature of purified Solar Salt was increased from 577.5°C to 592°C.
Accumulation of heat or release of heat cycle test can reflect the performance of the heat storage materials, in other words, the materials could maintain their original characteristics. Figure 15 showed that the upper limitation temperature of Solar Salt and purified Solar Salt after 27 h at different temperatures. In addition, the upper limitation temperature of purified Solar Salt was higher than unpurified. The upper limitation temperature of purified Solar Salt was increased from 577.5°C to 593.7°C at 350°C, 594.2°C to 595.2°C at 450°C and 583.3°C to 584.8°C at 550°C. It was concluded that the purification of NaNO3 and KNO3 could raise the upper limitation temperature of Solar Salt after several cycles.
Content of impurities analysis
The analysis above indicated that the melting point of the purified Solar Salt was reduced, the upper limitation temperature was increased and the thermal stability was enhanced. The compositions of the molten salts and the preparation conditions above were similar, except for the impurities. Therefore, it was necessary to research the levels of impurity ions of Solar Salt and purified Solar Salt. Table 1 showed the main impurity ions of NaNO3 and KNO3 were Cl–, SO42– and Mg2+. Figure 16 showed the impurity contents of Solar Salt and purified Solar Salt after 10 cycles at different temperatures. The ions of Cl–, SO42– and Mg2+ are all decreased in either Solar Salt or purified Solar Salt. It can be preliminarily determined that the interaction force of different ions was decreased with the decrease in impurities ions and the energy required for the reactions was also decreased. The effects of impurities are explored from the point view of crystal structure and mechanics. The vibration patterns of NaNO3-KNO3 system increased with the increase in impurities ions, and the consistency of whole-vibration can be broken. Thus it affected the melting point and the thermal stability of molten salt . Thermodynamically, the total Gibbs energy of NaNO3-KNO3 system is given by the following equation : (1)where is the molar standard Gibbs energy of pure end-members, and are the mole fractions of NaNO3 and KNO3, respectively. The excess enthalpy of mixture indicates the degree to which will vary from the ideal mixture. The full optimized excess Gibbs energy of the (Na1−x, Kx)NO3 liquid solution obtained in our assessment is given below : (2)That was based on the mixing enthalpies experimentally determined. The excess enthalpy of the (Na1−x, Kx)NO3 liquid has been measured at 618–723 K by Kleppa , (3)The values of XNaNO3 and XKNO3 were increased with decreasing the contents of Cl–, SO42– and Mg2+ respectively. From eq. (3), the absolute values of were decreased with increasing the values of XNaNO3 and XKNO3. This proves that purified Solar Salt was closer to the ideal mixture. So it was concluded that purification could improve the thermal stability, reduce the melting point, promote the upper limitation temperature and raise the latent heat of Solar Salt. But our results and explanations need to be proved by further experiments.
The stability of purified Solar Salt was identified by XRD radiation. The tests showed that the main content of molten salts was NaNO3 and KNO3, indicating that there was good chemical compatibility between molten salts. Figure 17 showed XRD pattern of unpurified Solar Salt without cycle and after 10 cycles at 550°C. Compared with raw nitrate molten salts, some new diffraction characteristic peak of unpurified Solar Salt appeared after 10 cycles at 550°C, giving rise to new substances. Figure 18 showed XRD pattern of purified Solar Salt without cycle and after 10 cycles at 550°C. No new diffraction characteristic peak appeared for Solar Salt after 10 cycles, which was similar with purified Solar Salt without cycle. It was indicated that the composition of the purified Solar Salt could keep stable in heat cycle. Therefore, the upper limitation temperature of purified Solar Salt was 550°C, which was higher than unpurified.
In this paper, a new heat transfer-thermal storage material [purified NaNO3 (60%) and KNO3 (40%)] was prepared by statical mixing method. In order to reduce equipment corrosion and extend service life of Solar Salt, purification was used during the modification process. The tentative conclusions can be summarized as follows:
Compared with unpurified Solar Salt, purified Solar Salt possessed better high-temperature thermal stability. The optimum operating temperature was increased from 450°C to 550°C.
The melting point of purified PNMS was sharply decreased from 322.2°C to 228.6°C. That of purified Solar Salt was decreased to 223.8°C and the latent heat was increased from 74.39 J/g to 80.79 J/g. The upper limitation temperature was increased from 577.5°C to 593.7°C, indicating it was suitable for heat transfer–thermal storage.
The properties of Solar Salt were promoted with the levels of the impurity decreasing. After 10 time thermal cycle test, the composition of the purified Solar Salt could keep stable. The temperature scope is range from 224.9°C to 584.8°C and the latent heat was 81.15 J/g. So the purified Solar Salt has an excellent and stable thermal storage performance.
Financial support from the Natural Science Foundation of Qinghai province of China (No. 2014-ZJ-909) is gratefully acknowledged. Prof. Min Wang, M. YouJing Zhao, M. Jinli Li, M. Lijie Shi in Chinese Academy of Sciences are appreciated for helpful suggestions.
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Published Online: 2015-02-28
Published in Print: 2015-12-01