| Heat storage technology is the main method of storing energy.By storing and releasing the heat flexibly,heat storage equipment can be effectively used in development of clean energy,such as solar energy and geothermal energy,and improvements of thermal imbalance across different time and spatial area.This can improve efficiency of energy usage.Phase change heat storage has the advantages of high energy storage density and stability,thus can be widely used in the field of heat storing.This paper conducted theoretical and experimental research on thermal transmission of low temperature heat storage device,with low temperature and barium hydroxide as heat storage medium.In this paper,the optimization design of low temperature heat storage device mainly includes the following aspects:(1)Measuring the phase transition properties and physical parameters of barium hydroxide,with the DSC curves,viscosity and thermal expansion of barium hydroxide.The phase transition process of barium hydroxide was tested by thermostatic water bath method,and the heat conducting oil and water was added to study their impact on the phase change process of barium hydroxide.(2)Constructing a mathematical model and analyzing the phase change of thermal storage device,considering the natural convection using Fluent heat transfer model.(3)Establishing the heat storage model of U-shape finned tube and U-shape smooth pipe,analyzing the heat storage properties of U-shape smooth pipe,and assessing impact of inlet velocity,natural convection and fin on heat transmission of heat storage device.The heat storage model of spiral coil is established and the heat storage characteristics of spiral coil tube are analyzed.(4)Establishing the spiral pipe heat storage device,and analyzing its heat storage process by experiment,and(5)Establishing the heat storage model of overall smooth pipe,analyzing the heat storage properties of the heat storage model.Constructing the circulation system of finned tube heat storage device,analyzing the effect of change of water to barium hydroxide ratio on the heat storage and release of the system,and assessing the impact of input and output water flow on the heat storage performance of the system.My results showed that :(1)Barium hydroxide has a high enthalpy of phase change,which is 258.2 J/g--264.6 J/g,and the barium hydroxide has a good cyclic property,which can be a candidate phase change material for the low temperature heat storage device.In addition,my experiments showed that it is very easy to loss water in the heat storage process when the pure barium hydroxide is used,which prevents the cycle process of the phase change.But this issue can be solved when the water with a third mass ratio and the heat conduction oil with a quarter mass ratio are added into the process.(2)The natural convection in the process of barium hydroxide accumulative heat is helpful to strengthen the natural convection in the process of heat transfer,which can shorten the thermal storage process by 37.2%.When the flow rate is higher than a certain value in the heat storage process,the flow rate of the heat storage process will not be changed any more.Then to optimize the heat transfer process,one should consider the heat transfer of the phase change material,rather than the flow rate.(3)Fin can effectively strengthen the phase transition heat transfer process.The time needed to complete the phase change process can be shortened by 15.3%,and the time of heat storage can be shortened by 21.4%,as the spacing of the fins is equal to the diameter of the tube and the outer diameter of the fin is twice the diameter of the tube.(4)In the process of the heat storage,temperature difference is different at different positions.The center of the coil is the worst place for the heat exchange,and spiral coil regenerative form is not good for the large heat storage device.(5)The change of material temperature in the heat storage device of overall smooth pipe causes the density change,which causes the natural convection in the gravity effect,and the temperature stratification is obvious in the vertical direction of the heat storage device.The thermal storage period of solid materials is longer,accounting for 79.4% of the total heat storage period.The average heat storage rate is 5468.9W.The material melting process in the heat storage device is actually the process of moving the solid liquid boundary layer from top to bottom.(6)In the experiment of the heat storage,the heat storage rate increases as the inflow increases but reaches to a saturate value at a large inflow.The mass storage capacity of the mixture unit is 356.3kJ/kg with the added mass ratio of 1/4 extra water.The capacity of the water pump increases as the inflow increases.Then there is an optimizing inflow,which leads to the largest rate of the heat storage and a proper capacity of the water pump.In the process of the heat release,the average heat release rate is proportional to the inflow.At last,I compared the average heat release rate and the average storage rate,in the same storage heat cycle process.I found that when the inflow is the same in both processes and the temperature of the inflow is the same as the initial temperature in regenerative process,the average heat release rate is 218.75% times as the average storage rate. |
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