Study on soil temperature restoration around vertical double U-type ground heat exchanger in Chongqing

Tests continuously the soil temperature around the ground heat exchanger of a demonstration building in Chongqing for the whole year, analyses the influences of the different backfill materials and the increased insulation resistance of return water pipe on the temperature restoration and the thermal accumulation effect of the soil around the ground heat exchanger. The test results show that the soil temperature around the ground heat exchanger rises by 0.26 to 0.94 ℃ in a typical day in summer and by 0.10 to 0.39 ℃ in an operation cycle, and they are all gradually weakened with the increase of depth. In the transition season, except that the soil temperature in depth of 1.5 m changes with the weather, the soil temperature recovers mainly concentrate in first two days before with the speed of 1.5 to 2 ℃ per day. The smaller coefficient of thermal conductivity of the backfill material and return water pipe thermal insulation are harmful to the soil restoration in a short time. But this effect is weakened as the time goes on.