EXPERIMENTAL STUDY OF THE EFFECT OF BACKFILLS ON THE THERMAL PERFORMANCE OF A VERTICAL U-TUBE GROUND HEAT EXCHANGER

Geothermal or ground coupled heat pump(GCHP) have been shown to be a very efficient method of providing heating and cooling for buildings. In GCHP systems, heat is extracted from or rejected to the ground by ground heat exchanger (GHE). The GHE is usually a single U-tube or double U-tube and the borehole is usually drilled to a depth between 30 and 200 m with a diameter of 100-200mm. The annular space between the GHE and borehole wall is usually grouted with a high thermal conductivity backfill material which provides a strong bond and ensures good thermal contact between the GHE and soil. Therefore, the selection of backfill material is very important for the heat transfer enhancement between GHE and surrounding soil. In this paper, a model experimental facility was set up based on the similarity principle. Three different backfills, which include the native soil, the soil mixed with pebbles and the soil mixed with scrap iron, were selected to test the effects of backfill on the thermal performance of U-tube GHE. The temperature distribution of the U-tube wall and the soil surrounding the U-tube and heat transfer rate of GHE with the three backfills were tested by the experimental device. The experimental results show that a backfill with high thermal conductivity can improve the thermal performance of U-tube GHE, but the thermal conductivity should not be increased unlimitedly considering the thermal interference of two adjacent legs of U-tube.