Investigation Of Latent Heat Storage Unit Using Low Temperature Phase Change Paraffin

With the rapid development of economy and the continuous promotion of industrialization process,the proportion of industrial manufacturing industry in the GDP is rising,which greatly increases the global unified energy demand.Fossil energy's excessive development and use directly or indirectly lead to a series of related environmental and ecological problems.Therefore,in order to solve the serious problems of energy shortage and environmental pollution,various countries have begun to advocate the development and utilization of clean energy.However,due to the lack of solar seasonal and day night periodicity,there is a fundamental contradiction between the utilization of solar energy in the demand time and the law of solar radiation energy.Heat storage technology is an effective way to solve this contradiction.At present,there are three kinds of thermal storage methods,i.e.sensible thermal storage,latent thermal storage and chemical reaction storage.This project is based on the "13th five year plan" major special project of the Ministry of science and technology,"Research on Key Technologies of multi energy complementary heating based on air source heat pump and post demonstration evaluation"(Project No.:2016YFC0700406-02),and has carried out relevant research on latent heat storage.In this paper,a new type of spiral coil heat exchanger is designed,and the paraffin with low temperature phase change is used as the phase change material to store the solar energy in winter in the solar poor area,so as to preheat the fresh air of air conditioning at night.A multi-functional experimental platform is built to couple the heat exchanger to test various working conditions,including temperature monitoring,flow monitoring,electric heater heat storage,water tank heat release and other functions.Then according to the thermophysical parameters of phase change materials and the size parameters of latent heat storage device,a three-dimensional numerical model of latent heat storage device is established based on the enthalpy porous medium method.The dynamic melting and solidification law of the low temperature phase change paraffin was studied by numerical calculation,and the reliability of the numerical model was verified by comparing with the experimental data.The error between the two models was less than ±5%,which provided a reference for further optimizing the design of the latent heat storage device.Finally,according to the numerical model calculation,the three design conditions of each coil section corresponding to different shell length are compared and analyzed.The results show that the shell length has no significant effect on the dynamic melting and solidification of PCM,but the average temperature of 100 mm shell is about 1.5?and 2? lower than that of 200 mm and 300 mm shell,and the average liquid phase ratio is about 30% and 40% lower,under the heat storage condition,100 mm shell is about30% and 40% lower than that of 200 mm and 300 mm shell,respectively The shell length,the average temperature is about 2? and 3?,and the average liquid phase ratio is about 40% and 60% higher,Considering that the single plate heat exchanger is limited by its heat transfer capacity,with the increase of the distance between the heat exchange coils,the heat storage and release capacity of the phase change material will be reduced,resulting in the reduction of the cost-effectiveness ratio of the phase change material.Therefore,properly shortening the shell length of each coil can not only make more effective use of the heat exchange capacity of the heat exchange fluid,but also reduce the filling amount of phase change materials and improve the cost-effectiveness ratio of the whole energy storage device.According to the analysis,after increasing the number of heat exchange coils,the increase of the total heat exchange length may cause the phenomenon of uneven phase change in the axial direction of the shell side.Under the condition of heat storage,the temperature of the heat exchange fluid in the axial direction decreases gradually due to the heat exchange between the heat exchanger and the phase change material,and the phase change material in the rear part may not reach the phase change temperature due to the lack of heat in the heat exchanger Achieve heat storage.However,with the increase of heat storage time,the heat exchange between the heat exchanger and the front part of the device decreases,and the temperature drop decreases.After reaching the rear part,there is the ability to realize phase change of phase change materials.The research results of this paper lay a foundation for further research on the overall heat transfer law of the device,the flow change and heat transfer characteristics of fresh air in the device,and can be used as a reference for the application of heat storage design in engineering.