Study On Characteristics Of Seasonal Underground Energy Storage Based On Borehole

Seasonal Borehole Thermal Energy Storage is a technology that uses solar energy,industrial residual heat,etc.as heat sources,underground energy storage body as heat storage carrier,meanwhile,heat sources in the non-heating season are stored in the underground space through borehole heat exchanger and extracted and utilized during the heating season.It is widely used because it can solve the problem of mismatch between energy demand and supply and improve the efficiency of renewable energy utilization.It is of great significance for energy conservation and emission reduction and the realization of the ’double carbon’ goal.In practical the process of seasonal borehole heat transfer is complicated,and different factors and operation control strategies have a great influence on its energy storage process,which makes its operation characteristics need to be further optimized.Therefore,in this paper,the experimental platform for the borehole energy storage is built,the model of borehole energy storage is also established.The effects of different influencing factors and control strategies on the seasonal underground energy storage characteristics of borehole are discussed theoretically and experimentally.In numerical simulation,the model of borehole energy storage is established by using CFD software.The influences of the physical parameters of the structure,operation parameters and control strategy of the energy storage body on the borehole thermal energy storage system were discussed.The change laws of soil temperature distribution,thermal characteristic index and heat storage and extraction were analyzed.The results show that the rectangular layout has the largest heat loss and is not suitable for the seasonal operation of borehole,The hexagonal layout has the highest energy storage efficiency,energy storage efficiency of the circular layout is slightly higher than that of the square layout,while the exergy efficiency of the circular layout is slightly greater than that of the hexagon.Increasing the depth of borehole will increase the heat storage and extraction capacity and system operation efficiency,but the increase continues to decrease.And when the borehole depth increases to a certain extent,the impact of the depth change on the energy storage body is very small.The heat storage and extraction capacity at the borehole depth of 100 m are 1.04 and 1.07 times that of 50 m,respectively,the energy storage and exergy efficiency only increase by 0.83%and 0.54%.Using non-equal spacing layout can increase the heat exchange borne by the internal boreholes,At the end of heat storage,the heat transfer capacity of the inner borehole in the non-equal spacing layout one and two is 13.5 and 28.5%higher than that in the equidistant arrangement,respectively.For the physical parameters,the smaller the specific heat capacity of the soil,the greater the initial temperature and thermal conductivity,the larger the radius of the thermal action of the soil,and the higher the energy storage efficiency of the system.From the perspective of heat extraction,increasing the inlet temperature of heat storage not only cannot effectively increase heat extraction,but also leads to heat accumulation and waste.At the same heat storage temperature,the lower the heat extraction temperature,the more heat is extracted,and the higher energy storage and exergy efficiency of the system.If the heat storage time is longer than the heat extraction time,a lower heat extraction temperature can be selected.Increasing the inlet flow rate does not significantly improve the heat storage and extraction,When the inlet flow rate is between 0.2 and 0.6 m/s,the energy storage and exergy efficiency increase with the increase of the inlet flow rate.However,when the inlet flow rate increases to 0.8 m/s,the system energy storage and exergy efficiency decrease instead.Among the four zonal operation modes,considering the temperature distribution of the energy storage body and the soil temperature recovery rate,Mode two is better.Considering the system operation efficiency,Mode one has the highest energy storage and energy efficiency.In the non-uniform heat storage and extraction intensity mode,after five years operation,the average temperature rise of the energy storage body in mode one is the largest,while the heat loss is also the largest.From the perspective of system energy storage and energy efficiency,Mode four is superior to other three operating modes.In the experimental research,the experimental platform for the borehole was built,and the experimental research were carried out on different start-stop ratios,different storage and extraction time ratios,storage and extraction synchronous mode and focused energy storage mode.The results show that increasing start-stop ratio can increase the heat storage and heat extraction,but due to the reduction of recovery time,the soil temperature difference at different positions increase after heat extraction,and the energy storage efficiency is reduced.When the difference between heat storage and extraction time is greater,the thermal imbalance of the energy storage body is greater,and long-term operation can lead to a gradual increase or decrease in the temperature of the the energy storage body,which is not conducive to system operation.The storage and extraction synchronous mode can effectively reduce the average temperature level of the energy storage body and improve the borehole heat exchangers efficiency during the heat storage period.At the same time,adopting the focused energy storage mode can improve the energy storage efficiency of the system,and optimize the seasonal operation of the borehole.In the focused energy storage mode,increasing the inlet temperature of the borehole during the heat storage period can effectively improve the heat storage capacity of the system.The purpose of this paper is to study the characteristics of the seasonal operation of the borehole under different influencing factors and control strategies,in order to provide theoretical basis and experimental guidance for further in-depth study of the seasonal operation of borehole.