Research On The Thermal Characteristics Of Hybrid Tank And Its Impact On The Performance Of Solar Heating Systems In Plateau Areas

The use of renewable energy such as solar energy for heating is an urgent need to achieve energy conservation and emission reduction in China’s construction industry,and to reasonably control the total amount of energy consumption.However,for most urban areas in China,where buildings are dense,land resources and available space are limited,using phase change materials to enhance the heat storage capacity of heat storage tanks is an effective way to improve their heat storage capacity.However,there are still some issues:firstly,there is still controversy over whether adding phase change materials to water tanks can improve the performance of solar heating systems.Secondly,during the heating process,the water supply temperature of the water tank is generally low.In addition to the high heat storage capacity of the water tank,a relatively high water supply temperature should also be considered.Finally,phase change materials cannot completely melt or solidify during the heat storage/release process,resulting in their heat storage capacity not being fully utilized.To address the above issues,this study designed a sensible and latent heat hybrid heat storage tank by adding double-layer phase change materials to the water tank.Based on this,this article proposes a solar heating sensible and latent heat hybrid heat storage tank,which not only ensures the economic efficiency of the heat storage system,but also improves the heat storage capacity and quality of the heat storage tank,achieving efficient operation of the solar heating system.(1)Compared with ordinary water tanks,the heat storage capacity of the sensible-latent heat hybrid heat storage tank is increased,and the heat storage time is extended.When the volume ratio of phase change materials is 7.87%,the heat storage capacity of the sensible-latent heat hybrid heat storage tank is increased by 9.7%compared to ordinary water tanks;In the early stage of heat storage,the thermal stratification effect of the sensible and latent heat hybrid heat storage tank is poor,while in the later stage of heat storage,the gain effect of phase change materials on the thermal stratification of the tank will only be apparent.(2)The effects of inlet water temperature,inlet flow rate,and the height to diameter ratio of the water tank on the thermal performance of the water tank during the heat storage process were studied.Increasing the inlet water temperature can improve the thermal stratification effect of the sensible and latent heat hybrid heat storage tank,and the gain effect is more obvious than that of the ordinary water tank;When the inlet flow rate is low,the thermal stratification effect of the sensible and latent heat hybrid heat storage tank is poor;The larger the aspect ratio of the water tank,the better the thermal stratification effect of the water tank.(3)Design the internal structure and materials of the sensible-latent heat hybrid heat storage water tank,and improve the thermal conductivity of the phase change material to effectively improve the thermal stratification effect of the water tank.When the thermal conductivity of the phase change material is increased to 3W/(m·K),the mixing number at t*=0.50 is reduced by 0.049,and the heat storage efficiency is increased by 0.039.Increasing the volume fraction of phase change materials in the water tank can increase the heat storage capacity,but the thermal stratification effect deteriorates;When the position of the double-layer phase change material is further away from the inlet and outlet,that is,the closer the phase change material is to the center of the water tank,the better the thermal stratification effect of the water tank;The specific surface area of phase change materials under different packaging forms is an important factor affecting the thermal analysis characteristics of water tanks.Increasing the specific surface area of phase change materials can improve the thermal performance of water tanks during operation.Among them,the use of circular packaging forms has the best thermal stratification effect of water tanks,and the melting speed of phase change materials is the fastest.Compared with cylindrical packaging forms,the melting time of upper and lower double-layer phase change materials has decreased by 0.19 and 0.11 dimensionless times,respectively.(4)By analyzing the heating characteristics of solar sensible and latent heat hybrid storage water tanks under different operating conditions such as water tank volume,number of phase change material layers,and heating mode,it was found that when the water tank volume was 33.3～66.7 L/m~2,compared with ordinary water tanks,the solar heating system with sensible and latent heat hybrid storage water tanks increased the heat collection by 7.0%～15.6%,and the heating capacity by 13.5%～25.2%.As the water tank volume continued to increase,The effect of adding phase change materials on the performance of solar heating systems is weakened;When using double-layer phase change materials with different melting points,both the heat collection and heating temperature of the solar heating system with sensible and latent heat hybrid heat storage tanks are taken into account;The heating effect of the sensible and latent heat hybrid heat storage tank is poor in the daytime heating mode,while in the all-day heating mode,the heat collection and heating capacity of the solar heating system with the sensible and latent heat hybrid heat storage tank have increased by 15.6%and16.9%,and at night,the heat collection and heating capacity have increased by 13.5%and 15.5%,respectively.Based on the above research,the heating characteristics of the solar sensible and latent heat hybrid storage water tank have been mastered,and the optimization design method for key parameters of the sensible and latent heat hybrid storage water tank has been obtained,providing a theoretical basis for the promotion and application of solar heating sensible and latent heat hybrid storage technology.