Study On Drilling Slurry Treatment And Backfilling Of Ground Source Heat Pump

Ground source heat pump(GSHP)system has been widely valued because of its wide application,high efficiency and energy saving.It has been applied in a large number of engineering projects and has become one of the important choices for new projects.During the drilling process of GSHP,massive drilling slurry is inevitably generated.When the slurry has large shrinkage and many pores,the operation of the GSHP may be affected if the slurry is directly used as a backfill material.At this time,the drilling slurry becomes construction waste and needs to be disposed.The backfilling materials need to be purchased and transported by the external site,which increases the cost and wastes resources.In order to make full use of resources and reduce unnecessary waste,research on the drilling slurry can well ensure the performance of GSHP.By analyzing the effect of GSHP backfill materials and reviewing related literatures,the parameters that can characterize their performance for testing are selected.The drilling process destroyes the original geological structure,which increases the pores of the drilling slurry,strengthens the shrinkage,causes poor internal cohesiveness and easy loss,and reduces the thermal conductivity.In order to ensure the effect of GSHP,some additives should be added to treat the slurry.In this paper,cement,water reducer,expansion agent,natural graphite powder and other materials are used as additives.The thermal conductivity,porosity,fluidity,compressive strength and shrinkage of the drill slurry under different additive ratios were tested and analyzed.The ratio of additives in each part of the backfill material was determined: the ratio of cement to drill paper was 0.6,the ratio of liquid mass to solid mass was 0.3,and the addition amount of water reducing agent and expansion agent was 2% and 6% of cement quality,respectively.The amount of graphite added is 1% of the total mass of the solid.The CFD numerical heat transfer model was established and verified by the experimental bench.This model was used to study the effects of buried pipe form,flow rate inside the pipe,borehole size,geotechnical temperature and water inlet temperature on different backfilling methods on direct backfilling of drilling slurry(DBDS),backfilling of drilling slurry treatment(BDST)and backfilling of cement mortar(BCM)respectively.Backfilling The simulation shows that in addition to the size of the borehole,other factors have an effect on the heat transfer process.In order to comprehensively analyze the factors that have certain influence,orthogonal experiments are designed.Orthogonal experimental design and analysis of results were carried out for different backfill materials without groundwater and with sufficient groundwater,and for changing conditions of cooling and heating.It was found that different backfill materials had different factors affecting the ranking and factor significance levels varies in winter & summer seasons and underground wet & dry conditions.The significant ordering of the backfill materials with the same factors is generally: BDST > BCM > DBDS.Compared with the case of no groundwater,there is a small difference in the significant results between different backfill materials when the groundwater is sufficient.According to an actual project,the building load model is built and the dynamic load simulation is performed.Through the calculation and analysis of the load,the scheme and the number of boreholes of the ground source heat pump system are determined.And the load is input into the CFD model for dynamic calculation.According to the results,the initial investment,operation cost and life cycle cost of the cold and heat source side of the project are calculated.The calculation found that the operating costs differenc of each program is small,main of which is reflected in the initial investment.It is found that in the absence of groundwater,the initial investment of BDTS can be saved by 24.53% and 5.87% respectively and the life cycle can be saved by 8.01% and 1.93% respectively compared with DBDS and BCM.BDTS has certain economic advantages.When the groundwater is sufficient,compared with DBDS and BCM,the initial investment of BDTS will save 6.40% and 2.79% respectively,and the life cycle cost will be reduced by 0.84% and 1.82%.Its economic advantage has become less obvious,and it has little difference with DBDS.However,DBDS may cause a significant risk of shrinkage,which affects heat transfer.Therefore,it is recommended that when the groundwater is rich,graphite is unnecessary for improving thermal conductivity,but cement,water reducing agent and expansion agent still need to be added to ensure the compaction of backfilling and guarantee the utilization result.