Investigation of the Thermal and Transport Properties of NaCl-KCl-MgCl2- CaCl2 Molten Salt for Application as HTF and TES Media in CSP Systems

Abstract

Solar energy attracts more and more attention to serve as a substitute energy source of traditional fossil fuel due to the fast increase of demand for global energy and rapid reduction of the reserve of fossil fuel. As an energy source, solar energy is clean and renewable without the generation of greenhouse gases or air pollution, which are the productions of the burning of fossil fuel. Meanwhile, it has huge potential and is available in many areas in the world. Therefore, Solar energy is universally considered as one of the best renewable energy sources with great unexploited potential and minimum adverse effects on the environment. CSP (concentrated solar power) technology is considered the most feasible method to economically provide power converted from. Heat transfer fluid is the key factor to carrier heat from solar receiver to heat exchanger, power turbine, or thermal storage system. Thermal oil and Nitrite/nitrate have been successfully using as heat transfer fluid (HTF) in CSP plants for decades. However, they are not stable and will decompose at a temperature higher than 400 oC and 560 oC, correspondingly. With the development of material of science, it is available to manufacture better solar receiver that withstands a temperature as high as 1000 oC. Therefore, the new challenge is to find better material with good and desirable thermal and transport properties to serve as HTFs to increase the efficiency and performance of the CSP plant. Eutectic mixtures of chloride molten salts have been considered as suitable heat transfer fluid (HTFs) and thermal storage media in large scale CSP application. NaCl, KCl, MgCl2, and CaCl2 are the candidates for chloride salts mixture formation due to their physical and chemical stability and low vapor pressure at ultra-high temperatures up to 1400 oC and the relatively lower cost compared to other salts. After investigation of the phase diagrams of six binary molten salts from above four single pure chloride salt, The KCl-MgCl2 was found to have a eutectic composition with the lowest melting point at 423 oC at the mole ratio of 68%-32%. Furthermore, to lower the melting temperature and decrease the cost, NaCl was introduced and four ternary molten NaCl-KCl-MgCl2 compositions with different molar ratios were selected to study in this work. Their remarked names, compositions and theoretical melting point are NKM-#1 (Mole: 27.5%-32.5%-40%) at 383 oC, NKM-#2 (Mole: 30.9%-21.2%-47.9%) at 390 oC, NKM-#3 (Mole: 28.39%-27.25%-44.36%) at 382 oC and NKM-#4 (Mole: 20%-52.5%-27.5%) at 390 oC. However, the NKM-#4 salt was not considered for further study in this work due to the melting point is found to be not a eutectic point from DSC experiments. Moreover, A new eutectic chloride molten salt, MgCl2-KCl-NaCl (wt.% 45.98-38.91-15.11), has been recognized as one of the most promising high-temperature heat transfer fluids (HTF) for both heat transfer and thermal storage for the 3rd Generation concentrated solar thermal power (CSP) systems were studied in this work. For the first time, some essential thermophysical and transport properties of these eutectic chloride molten salt needed for basic heat transfer and energy storage analysis in the application of concentrating solar power systems have been experimentally tested in the range from 450 oC to 800 oC. The studied properties include heat capacity, melting point, the heat of fusion, viscosity, vapor pressure, density, and thermal conductivity. The experimental results indicate the following properties: melting points are found to be from 386.7 oC to 424.4 oC; The heat of fusion is in the range of 207 - 284.38 kJ/kg. the heat capacity is in the range of 0.9-1.2 J/(g•K). The density decreases linearly with the increase of temperature and falls in the range of 1471 - 1751 kg/m3. The viscosity decreases exponentially from 6 cP to 2.2 cP with the increment of temperature. The thermal conductivity was calculated by the experimental data of thermal diffusivity, density, and specific heat capacity and in the range of 0.32 - 0.55 W/(m•K). The vapor pressure of those chloride molten salt mixtures is lower than 20 kPa at a temperature as high as 1000 oC. The correlations of those properties as functions of temperature are also provided. The property equations provide an essential database for engineers to use to calculate convective heat transfer in concentrated solar receivers, heat exchangers, and thermal storage for concentrated solar power plants. Theoretical and empirical correlations for the prediction of those properties are also investigated and the calculated properties are used to compare with the experimental results. The existence of water in the salt is found to be the major reason for the corrosion of molten chloride salts to the piping materials. The candidate salts MgCl2 and ZnCl2 from previous work have the hygroscopic property and it is proven that the happening of the hydrolysis reaction of MgCl2 and ZnCl2 in the existence of moisture during the heating process. Therefore, newly designed experiments were conducted to study the kinetic process of water uptake and water removal on the molten salt mixtures. The results imply that the wet binary or ternary chloride molten salts need to be dried at least 240 oC to remove all the absorbed water. Acid production is generated during the drying process. The prediction of heat transfer coefficient (h) from four correlations of Nusselt number indicates were conducted in this work. The Reynolds number was found to increase with the increase of temperature for the five candidate chloride eutectic molten salts in the temperature range of 450 - 850 oC. There is a peak value of the heat transfer coefficient for each candidate molten salt in the considered temperature range, which means h increases with temperature from 450 oC and reaches the highest value at some temperature, then decreases with the increase of temperate. The corresponding temperature is in the range of 600 - 650 oC, 550 - 650 oC, 450 - 500 oC, 500 - 600 oC for KM, NKM-#1, NKM-#2, and NREL salt, separately. The heat transfer coefficient of the ternary Na-K-Mg chloride molten salts is in the range of 2000 - 2500 W/(m•K), which are calculated when the velocity and pipe diameter set as 1.2 m/s and 0.025m, and higher than the binary KCl-MgCl2 molten salt from 450 - 800 oC. For NKM-#1 salt, the heat transfer coefficient increase with the increment of working fluid velocity from 0.6 m/s to 2 m/s, and the agreement of values from the different correlations of Nu becomes worse when the velocity increases. Increasing the pipe size from 0.025 m to 0.1 m will cause a reduction in the heat transfer coefficient. In the temperature range of 450 - 750 oC, the five chloride molten salts have better heat transfer performance than the NaCl-KCl-MgCl2 and Alkali carbonate salts.