Preparation And Performance Control Of Composite Nitrate Molten Salt As Heat Transfer And Storage Material

Energy is one of the keys to the future economic and technological development of China and the world.It is of great significance to vigorously develop renewable and clean energy and to further promote energy technology innovation.Concentrating solar thermal power generation as a high-efficiency power generation technology has been developed by many countries.However,in the heat transfer and storage system of solar thermal power generation,there is still a problem of low specific heat capacity of the molten salt heat transfer and storage medium.This not only affects the efficiency of heat transfer,but also increases the cost of heat storage.Many studies have proved through experiments and some theories that adding nanoparticles into molten nitrate salt can effectively enhance the specific heat capacity,but the mechanism of the enhancement remains unclear.Therefore,it is very important to study the mechanism of enhancing the specific heat capacity of molten salt by adding nanoparticles and to prepare nano-particle composite molten salt materials which can effectively improve the thermophysical properties.The effects of the addition amount of SiO₂ nanoparticles,the particle size and the dispersion degree of nanoparticles on the thermal properties of binary nitrate molten salt were studied by molecular dynamic simulation.The simulation results show that with the increase of SiO₂ content,the specific heat capacity increases first and then decreases,and there is an optimal ratio.The analysis of the interaction energy of the composite molten salt system shows that the increase of Coulomb energy with the addtion of SiO₂ is the main factor for the enhancement of specific heat capacity;while the long-range k-space energy decreases with the increase of SiO₂,along with the change trend of Coulomb energy is the reasons for the presence of best ratio.The study on the microstructure of molten salt shows that the nitrate molten salt and its nanofluid composite with SiO₂ present short-range order and long-range disorder.For molten salt,the addition of nanoparticles will reduce the distance between the anions pair Na-N,Na-O,K-N,K-O.And the closer the distance between the anions,the more the specific heat increases.For the molten salt/particle interface,there is a circumstance that the molten salt ions are arranged on the surface of the particle to form a compressed layer.These two phenomena are the reasons for the increase of the Coulomb energy of the system.The ions need more energy to overcome the coulombic force,so the specific heat capacity increases during heating.In the composite molten salt system,when the SiO₂ nanoparticles are added in 2.5wt.% and 3.5wt.%SiO₂,the specific heat capacity simulation results in the dispersed and aggregated state show that the nanofluid with a higher degree of dispersion has a greater increase in specific heat capacity.It shows that the degree of dispersion is high,and the solid/liquid interface effect accounts for a large proportion,which can effectively increase the specific heat capacity.Under the guidance of the above-mentioned simulation results,the high-dispersibility SiO₂/ternary calcium-containing molten nitrate composite material was prepared by the in-situ generation method using the highly dispersed characteristics of the precursor in the water phase.When the precursor is added less,Scanning electron microscopy characterization of the composite sample shows that the generated SiO₂ nanoparticles are extremely uniformly distributed in the molten salt.A series of thermophysical tests including density,specific heat,thermal diffusivity,thermal conductivity,viscosity,was adopted to the prepared composite material.It is found that when the addition of SiO₂ is 0.5wt.%,the specific heat capacity of the composite material is increased by about 55% compared with the base salt.The thermal diffusivity is increased by about 10%under all content.Identically when the SiO₂ addition amount is 0.5wt.%,the thermal conductivity is increased by about 67% on average compared to the base salt.In the low-medium temperature section,the viscosity of molten salt is high,and is lower in the high temperature section.The microstructure simulation of the ternary nitrate molten salt shows that when the molten salt is in the liquid state,it also presents the characteristics of short-range order and long-range disorder.The coordination structure of calcium-containing ion pairs with different signs is more stable than containing sodium/potassium.With temperature increasing,the distance between Ca-Ca,Ca-Na,Ca-N,and Ca-O ion pairs decreases.Calcium nitrate contains more nitrate and Ca²⁺ is a divalent ion,which makes the peak shape of N-N ion pair different from the peak shape of sodium potassium unit and binary nitrate molten salt.The distance between O-O ion pairs increases with temperature increasing,which indicates that the distance between different nitrates become larger.With the increase in the amount of nano-SiO₂ added,the coordination number of the Ca-Ca ion pair decreases,and the coordination number of other anion and cation pairs increases.The thermophysical properties of ternary nitrate molten salt and its composite materials were also simulated,and it was found to be in good agreement with the experimental values.In order to further improve the specific heat capacity and thermal conductivity of calcium-containing ternary nitrate molten salt,Mg(NO₃)₂·6H₂O and MgCl₂·6H₂O were used as precursors to prepare MgO composite ternary nitrate molten salt materials.The experimental results confirm that only the Mg O nanoparticles produced by using magnesium chloride hexahydrate as the precursor can be well-dispersed and can be suspended well,and the nanoparticles settled only a very small amount.The specific heat capacity measurement results show that the specific heat capacity of the composite molten salt liquid prepared with MgCl₂·6H₂O as the precursor is increased by 18.7%;while Mg(NO₃)₂·6H₂O is used as the precursor,there is even decline than the base salt.After the prepared Mg O nanoparticles are surface-modified with carbonate salt and then dispersed in molten nitrate salt,the specific heat capacity test results confirm that this material prepared by this method can further increase the specific heat capacity of ternary nitrate molten salt,which increases the specific heat energy by 35% compared to the base salt.