Laboratory Experiments Research On Backfill Material For Ground-Source Heat Pump

Ground-source heat pump(GSHP) systems use the shallow geothermal energy for heating and air-conditioned in buildings, has a good energy-saving and environmental benefits. In recent years, China have been increasingly wide range of GSHP applications. Vertical U-pipe ground-source heat pump system because of its excellent performance and broad scope of application, gradually become the dominant form in source heat pump heating and refrigeration technology. Because of the complexity of heat exchange, geological, climatic conditions, as well as variability of air conditioning system in buildings, GSHP system technology is still exploring the theory.GSHP technology because of its relatively high installation costs, its application has been restricted. Underground heat exchanger directly determines the design of the entire GSHP system. Backfill material will affect the underground heat exchange and the installation costs. High performance backfill material can enhance the heat transfer efficient, reduce the bore length, and reduce the installation costs of GSHP.This paper analysis the characters of backfill material of GSHP, choose the raw materials of backfill material according to the characters of backfill material. Test the thermal conductivity, liquidity, expansion and compressive strength of different proportion of raw material in laboratory. Choose the appropriate proportion of raw material of backfill material. This paper got the following conclusions:(1) Backfill material should be possible to meet the construction site requirement on the basis of high thermal conductivity, to enhance the capacity of underground heat exchanger.(2) Cement-based backfill material has better performance than traditional backfill material, from laboratory experiments, the recommended W/C is 0.55, and S/C is 2. According to adding additives can make the backfill material has a good performance to adapt to ground-source heat pump system.(3) According to the laboratory experiments, this paper got one kind of backfill material, compare to traditional backfill material, it can reduce 7% of the heat exchanger length.(4) Use the GSHP design software, the suitable thermal conductivity of backfill material should be slightly higher or equal to the thermal conductivity of the surrounding rocks.(5) High performance backfill material can reduce the drilling length for the heat exchanger, decrease the installation cost as well as the maintain cost.