Research On Heat Conductive Property Of Backfill Material Used In GSPH

Energy has become the bottle neck of the economy development of our country, and alsothe whole world. Every country has put their eyes on new energy exploitation in succession ,since the conflict between supply and demand of fossil energy become more and moreprominent. Geothermal source as one of the renewable sources, especially shallow geothermalsource, is caught the attention of every country because of its widely spread, and is widely usedfor building energy supply, industry process and agriculture. In the building energy supply field,Ground source heat pump (GSHP), as a shallow ground geothermal collection technology, isnamed as "Green air condition technology in 21 century".The application of GSPH technology began at the end of last century. After more than tenyears of developing, this technique began to go to the market step by step. The research work tothis technique never stopped during this period, but there are still some problems, such as highinitial cost and unstable of long term running effect till now. All these problems areconcentrated on Underground Heat Exchanger. The capability of the whole system depends onthe design effect of underground heat exchanger. There are many factors affect the performanceof the underground heat exchanger, and the thesis discussed and studied from the point ofthermal conductive performance of borehole backfill material. On the one hand, backfillmaterial with good thermal conductivity can improve the heat transfer efficiency of GSHP,shorten the design depth of heat exchanging hole and reduce the initial cost, on the other hand,the backfill material should have stable performance and wear well, to insure a stable long termrunning system.The thesis studied the thermal conductive performance of borehole backfill materialthrough a field simulation test which based on self-develop test equipment. The thermalconductive performance of two different backfill materials, bentonite based grout and cementbased grout, were tested and compared. The experiment found that bentonite based groutbackfill material has low thermal conductive capability, and is easy to lose water and shrinkseverely in a dry situation. It is not suitable to use this kind of backfill material in a dry area oran area with a sharp groundwater changing level;it will affect the system running stability.Cement based grout backfill material was found to have better thermal conductive capabilityand low shrinkage performance, and belong to these material with stable performance. And itwas thought to be suitable to different ground conditions, especially dry and rock base area. Tothese two kinds of backfill material, there are one common tendency. It was that the thermalconductive capability of these two backfill material were improved with the increase of quartzsand percentage. The experiment also tested and analyzed W/C rate affection to thermalconductivity, and another tendency was found that the thermal conductivity of cement basedgrout was increased with the decrease of W/C rate. Finally, the thesis combine the practicalwork in Beijing area, and give feasible suggestions to choose suitable borehole backfill materialwhich used in GSHP system.