Research On Heat Treatment,Modification And Heat Storage And Release Performance Of Nitrate Heat Transfer And Storage Material With Low Phase Change Temperature

Heat storage materials and technique have a wide application prospect in fields of solar energy application and waste heat recovery.Solar Salt witch is composed of NaNO₃ and KNO₃ has already been used in many solar thermal power plant for great advantage in heat storage density and cost.Its melting point is 220^oC,and the high melting point makes the salt block pipes and valves easily.To reduce its melting point,NaNO₃-KNO₃ was heated at high temperature,and then the relevant operation technology and chemical reaction mechanism were studied.NaNO₃-KNO₃?Solar Salt?was prepared.It was heated at high temperature to change its compositions by decomposition reactions,aiming to reducephase change temperature in a certain extent.High temperatures and calcination times were discussed as two factors for solidus temperature.Temperatures include 600^oC,750^oC and 800^oC.Calcination times include 30 min,60min,90min,120min,150min and180min.DSC was used to obtain melting and solidification curves,and the law that operation technology affects solidus temperature was summarized.The change of valency and phase was measured by XPS,XRD and FTIR.The process of decomposition reaction and reasons why high temperature can affect solidus temperature were analyzed.The material obtained through heat treatment was placed in simulated conditions to test its durability.Results show that solidus temperature of Solar Salt can be changed by proper heat treatment technology.Aiming to enhance operation temperature,the oxidation reaction between nitrite salt and oxygen in KNO₃-KNO₃?NaNO₃-NaNO₃ and KNO₃-NaNO₃ microcells were studied by Density Functional Theory?DFT?respectively.The main tool used in this research is Gaussian09,adopting B3LYP function and 6-31+G*as the base set.Reactants and productions were obtained by geometry optimization.TS method was used to find intermediate and transition state,and frequency calculation was conducted to verify the correctness.Transition state IRC calculation was used to verify the reaction pass.As for KNO₃-KNO₃ andNaNO₃-NaNO₃ system,there is only one reaction path.In NaNO₃-KNO₃,there are two reaction paths due to two kinds of positive metal ions and one of them is the main path.The temperature at which the decomposition reaction can happen is higher than the temperature at which the oxidation reaction can happen.Increase of Specific heat of nitrate PCM can improve quantity of sensible heat storage.In this research,nanoparticles and nitrate salt was dissolved into deionized water,and the solution was dried after ultrasonic dispersion.Effect of nanoparticles,grain size and additive amount on specific heat was studied.Grain sizes include 30nm and 50nm mainly.Additive amounts distributed between 0.5wt.%and 2wt.%.The specific heat of nitrate and micro structure were studied by DSC and SEM.It is found that nanoparticles can raise the specific heat.Nano-SiO₂and nano-MgO lead to generation of spherical structure needlelike structure.To understand heat storage and release property of modified nitrate PCM in heat storage unit,an evaluation system was set up.The nitrate PCM was modified by additionof 30nm-SiO₂ with 1wt.%.Heat storage and releaseefficiency was proposed to evaluate the property of heat storage unit.First,a curve that can stand for the temperature of the entire system should be calculated.Next step,the initial state and terminal state are determined based on temperatures on the curve made in step one.At the end,the efficiency can be calculated by quantities of heat storage and release over this stage.Results show that bending number of heat exchange pipe can affect the rate of heat transfer.Higher speed of heat transfer leads to a higher efficiency and more heat transmitting between PCM and heat transfer,which means less heat is lost to outer environment.Heat storage quantity of the unit can be increased by specific heat modification of nitrate PCM.