Optimal Integration Of Backfill Material Of Ground-source Heat Pump And Research On Thermal Characteristics

Ground-source heat pump is the use of shallow geothermal energy for heating and cooling of buildings with energy-efficient and environmental air conditioning.The vertical soil heat transfer of buried pipe systems due to the economical use of land,excellent performance,not depleting groundwater,has been most widely used in our country.Ground-source heat pump is used as a response to the global energy crisis,global warming,the important way to develop a low-carbon economy.The key technology of designing is the underground heat exchanger in the vertical buried pipe system for ground-source heat pumps.Underground heat is transferred from the soil to the vertical U-tube and the tube of liquid by the backfill material in the heat transfer hole.Excellent heat conduction ability of backfill material can reduce the thermal resistance of heat transfer hole,reduce the depth of borehole and the length of U-tube,thereby,reduce the initial investment of a ground-source heat pump,enhance to promote the use of ground source heat pump.This article,had taken into account the basis of the thermal conduction characteristic,engineering performance,cost,stability and environmental protection of backfill materials for ground-source heat pump,proportionated test and optimized integration to bentonite-based and cement-based backfill material,has achieved the following results:(1)The rule of thermal conductivity of bentonite-based backfill materials is effected by the quartz sand content,dry density,moisture content and porosity.(2)The best water-cement ratio is 0.55 and the optimal sand-cement ratio is 2.0 of cement-based backfill material.Be doped with natural graphite powder,the thermal conductivity is increased by 17.8.Be doped with retarding expansive material can significantly improve the expansion characteristics,and 11% of water is reduced.The thermal conduct ivity is also increased.(3)The thermal conductivity of bentonite-based and cement-based backfill material increase with the rise in temperature.The change of temperature gradient helps to increase the thermal conductivity of backfill material.(4)Add anti-cracking polypropylene fibers can significantly improve cracking resistance of backfill material.Fissure is restrained after cyclic wetting-drying.(5)Optimized integration came up with two suitable backfill material ratio with good thermal conductivity and engineering performance,could significantly reduce the depth of the heat transfer borehole and the length of U-tube,reduce construction costs.