Study On Energy Storage And Heat Transfer Characteristics Of The Underground Heat Exchanger Based On Phase Change Material Backfilling

The ground source heat pump system is a renewable energy utilization technology that uses shallow surface geothermal resources as a source of heating and cooling.It has been widely used in recent years in the context of increasingly serious energy consumption and environmental pollution.Underground heat exchanger as the only equipment to achieve heating exchange with underground soil in the use of ground source heat pump system,its heat storage and heat transfer problems have been a research hotspot,and the impact of backfill materials on the heat transfer performance of underground heat exchanger is crucial.The traditional backfill material has the disadvantages of large radius of heat influence and continuous increase or decrease of soil temperature in underground heat transfer area.Therefore,in this paper,phase change material(PCM)is used as a part of backfill material for numerical simulation and experimental test,and the influence of phase change process and thermal properties of phase change material on the energy storage and heat transfer characteristics of the underground heat exchanger is discussed.In theory,the numerical model of phase-change material backfilled ground heat exchanger is established by using CFD software.The influences of phase change process,thermal conductivity,phase change temperature,and latent heat on the energy storage and heat transfer characteristics of underground heat exchanger under winter and summer conditions were discussed.The results show that using PCM backfilling is beneficial to soil temperature recovery,when the summer conditions using mixed acid backfill material,the total heat exchange capacity of a single borehole is 1.16 times that of the original soil backfill,and the heat influence radius is only 95.65%of the original soil backfill.Under certain building load conditions,the spacing between boreholes and soil occupied area can be reduced.The increase of the thermal conductivity of PCM will lead to the increase of the heat influence radius,but it also increases the heat transfer significantly,which has advantages under the same load condition.Using phase change backfill material with lower phase change temperature in summer and higher phase change temperature in winter can obviously increase the heat storage efficiency of underground heat exchanger.Under the same conditions,the increase of latent heat of phase change can significantly enhance the energy storage capacity of underground heat exchanger.In the aspect of experiment,the model test bench of underground heat exchanger with phase change backfill material was built by using similarity theory,and the phase change process of PCM,different mixing ratios and enhanced heat transfer on the energy storage and heat transfer characteristics of the underground heat exchanger in summer and winter conditions were carried out.The results show that the underground heat exchanger with PCM backfill material can slow down the continuous increase or decrease of the temperature in the borehole,and keep higher temperature difference between the circulating fluid temperature of the ground heat exchanger and the surrounding backfill material,which is beneficial to the heat transfer process.At the same time,using PCM backfill material can effectively alleviate the variation of soil temperature outside the borehole,in addition,the heat exchange of unit well depth is greater than that of original soil backfill material,and the total heat storage of a single well under summer and winter conditions is 9.41and 28.27%more than that of original soil backfill material,respectively,which can enhance the energy storage effect of underground heat exchanger and reduce the radius of heat effect,and the land occupation area can be reduced under certain building load conditions.When the mixed acid and oleic acid are backfilled together,the physical properties of the two materials will affect each other.When the mixed ratio is 4:6,the heat affected radius is the smallest.When the actual building adopts PCM backfilling,regional adaptability should be taken into consideration,and the appropriate ratio should be selected according to the conditions of building cooling and heating load.Strengthening the heat transfer of PCM by adding iron chips will increase the radius of heat effect,but also increase the total heat storage of single well significantly,and improve the heat transfer performance of heat storage in underground heat exchanger.However,the greater thermal conductivity will increase the temperature of the soil during the operation time,and the phase transformation recovery process can not be completed in time.The purpose of this paper is to investigate the effect of phase change material backfill on the energy storage and heat transfer characteristics of the underground heat exchanger,and give the influencing factors and rules.In order to provide theoretical and experimental basis for the further study of phase change material backfilled ground heat exchanger.