Design And Research On Performance Optimization Of Small Capacity Molten Salt Heat Storage Device

At present,the energy consumption structure dominated by coal has caused the haze weather to intensify,and energy reforms focusing on clean substitution and electric energy substitution are imperative.However,the power load in some areas of China is seriously unbalanced.The contradiction between the “peak and valley” is increasing.In addition,for places such as northeast and northwest,where wind power and photovoltaics are sufficient,elimination on the spot is difficult,the efficiency of wind power and photovoltaic are not high,and wind and light are seriously abandoned,which seriously affects the economics of wind power and solar power generation.it is necessary to vigorously develop low valley electric energy storage technology,and the traditional water heat storage has a series of problems such as large floor space,large changes in outlet steam temperature,and low efficiency.New technology of new solid heat storage,heat storage by phase change needs technology of heat storage and other technologies has a high initial investment.In order to solve the above problems,this paper proposes a small-capacity molten salt regenerative electric steam boiler that can ensure that the steam outlet temperature is stable within a certain range.The brine heat exchanger of the system is built inside the tank.The system uses a built-in multi-stage heat exchanger structure to change the heat exchanger area according to the molten salt temperature.At the same time,theoretical analysis and numerical simulation were used to analyze the temperature field distribution characteristics of the molten salt in the heat storage tank during the exothermic process,and the heat exchanger structure was optimized.The main research work is as follows:(1)Design and calculate the heat storage device and establish an economic model.The study found that under the premise of ensuring a certain volume of the heat storage tank,the total cost of the heat storage device is first decreased in the height direction and then increased.(2)The mathematical model of molten salt heat storage tank including U-shaped tube is established.The trend of tube length and molten salt temperature in each phase of heat exchanger is simulated by MATLAB,and different mass flow rates and different initial temperatures are analyzed.The effect of the lower heat transfer performance indicates that the temperature of the molten salt decreases linearly with time,and the rate of change is 0.49°C/min.The length of each phase increases with time,and the initial increase is slow,then the increase is faster,the length of the pipe in superheating zone changed the most with time.The larger the inlet flow rate,the greater the heat flux density,and the greater the influence on the heat transfer characteristics;the influence of the inlet temperature on the average heat flux density is smaller.(3)Based on the above calculation,the tank heat exchange was simplified into the closed space natural convection problem with a negative internal heat source,and the average heat flux density equation was input the Fluent solver through the UDF.Temperature field in the heat storage tank of different heat exchange areas under the condition of internal heat source is simulated.In addition,the influence of heat exchanger structure,initial temperature of exothermic heat and arrangement on the thermal performance of the tank was evaluated by temperature field distribution and Nusselt number.It was found that reducing the inner diameter of the heat exchanger can reduce the average temperature of the molten salt in the tank,reduce the temperature difference in the axial direction,and make the temperature distribution more uniform.When the initial temperature was increased,the average Nusselt number increases,but the increase in temperature is small;When the U-tube heat exchanger is placed above,the temperature distribution of the molten salt in the tank become more uniform,the average flow rate of the molten salt in the tank becomes larger,and the temperature stratification is gradually eliminated.(4)The life cycle model was established,and the economics of various types of boilers were compared and analyzed.It can be seen that the whole life cycle cost of the molten salt heat storage electric steam boiler was 50.84% lower than that of the electric heat boiler and 4.35% lower than that of the water heat storage boiler.Compared with the direct-heating electric boiler,the net recovery period of the energy storage electric boiler equipment was 1.24 years,so the economic benefit was remarkable.