Research On Preparation And Thermophysical Properties Of Molten Nitrate Salt Energy Storage Materials

Solar Salt molten salt is widely used in the field of solar thermal power generation,because of its wide range of working temperature,good thermal stability,low corrosion,good thermal storage and low cost,etc.Solar Salt molten salt is suitable for high-temperature energy storage material,because its working temperature is 300-550?.Thermal properties are diversity,because purity of raw materials is different.Resources of sodium,potassium and magnesium in Saline Lake in Qinghai province of China are wealthy.The use of potassium chloride,sodium sulfate and magnesium chloride hexahydrate from saline lake to prepare potassium nitrate,sodium nitrate,magnesium nitrate hexahydrate respectively can improve the value of resources in saline lake and promote the development of nitrate molten salt heat storage technology.Based on Solar Salt molten salt,ternary molten nitrate salt was prepared with additive magnesium nitrate hexahydrate.Effect of impurity ions on thermophysical properties,structure and corrosion resistance of Solar Salt molten salt were studied first.Preparation and thermal properties of ternary molten nitrate salt heat storage materials were studied secondly.The process of preparation,purification of nitrate salt and effect of purity on thermophysical properties of ternary molten nitrate salt were also studied.The main work is summarized as follows:?1?Preparation of high purity nitrates and impurity ions regulationThe industrial grade potassium nitrate and sodium nitrate were purified by recrystallization method and product purity was greater than 99.9%.Using magnesium chloride hexahydrate as raw materials,magnesium nitrate hexahydrate was prepared by ion exchange method and its purity was greater than 99.0%.High-purity nitrates can be directly applied in preparing energy storage materials for solar thermal power generation.?2?Effects of Cl^-and SO₄^2-on thermophysical properties,structure and corrosion resistance of Solar Salt molten saltThe effects of Cl^-and SO₄^2-on the thermal properties,structure and corrosion resistance of Solar Salt molten salt were studied and effect laws were obtained.The change of melting point,latent heat capacity and upper limitation temperature are small with additive Cl^-and SO₄^2-ions less than 1%in Solar Salt molten salt.Thermal stability of Solar Salt molten salt is extremely unstable with additive Cl^-and SO₄^2-ions more than 0.25%and corrosion of 304 stainless steel is weak with additive Cl^-(SO₄^2-)ions less than 0.75%?1.0%?after 576 hours recycling at 500?.?3?Preparation and thermophysical properties of low melting point Na NO₃-KNO₃-Mg?NO₃?₂ ternary molten saltTernary molten salt was prepared by the method of melt-blending.The process parameters and conditions were optimized,thermophysical properties of ternary moltensaltwereobtained.Thebestratiois49.5%Na NO₃-33%KNO₃-17.5%Mg?NO₃?₂ and preparation condition is 250?,2 h;300?,18 h.Melting point,latent heat capacity and upper limitation temperature of ternary molten salt are 157.4?,74 J/g and 451.7?respectively.?4?Effect of purity on thermophysical properties of Na NO₃-KNO₃-Mg?NO₃?₂ternary molten saltIndustrial grade and high-purity potassium nitrate,sodium nitrate and magnesium nitrate hexahydrate were used to preparing ternary molten salt respectively.Effects of purity on thermophysical properties of ternary molten salt was investigated,such as melting point,latent heat capacity,upper limitation temperature,density,viscosity,thermal conductivity and so on.High-purity ternary molten salt has a lower melting point,higher latent heat capacity,higher upper limitation temperature and better thermal stability compare with industrial grade ternary molten salt,so purification of materials can prolong service life of ternary molten salt.The density,viscosity,heat capacity and thermal conductivity of ternary molten salt were measured by archimedes method,rotating method,differential scanning calorimetry and laser flash method respectively.Compared with industrial grade ternary molten salt,high-purity ternary molten salt has higher density,lower viscosity,lower heat capacity and thermal conductivity.High-purity ternary molten salt had better thermophysical properties.