Experimental study on the thermal imbalance and soil temperature recovery performance of horizontal stainless-steel ground heat exchanger

Soil thermal imbalance has become a common problem for ground source heat pump (GSHP) system, leading to the fluctuation of ground temperature and decline of heat transfer efficiency. However, compared with vertical ground heat exchanger (GHE), there is a lack of research and evaluation on the soil thermal imbalance for horizontal GHE. Therefore, in this paper, to investigate the extent of soil thermal imbalance and its effects on performance of GSHP system, a sandbox experiment embedded with horizontal stainless-steel GHE was implemented. Various operation conditions were designed to explore effects of intermittent time, water content (rainfall) and buried depth of pipes on soil temperature distribution and restoration performance. The experimental data was validated by a numerical simulation. The results indicated that 5-day intermittent operation of GSHP leads to an increase of 6.41 ℃ in soil temperature. After 2-day natural recovery, soil temperature recovery rate can only reach 0.72. Increasing soil moisture content and intermittent control are effective solutions to mitigate thermal accumulation and improve thermal performance. The heat transfer performance of GHE with an intermittent ratio of 6:18 which GSHP system runs for 6 h and stops for 18 h during a day is 33% higher than that with the intermittent ratio of 14:10 and 10:14. The higher the water content, the lower the soil temperature fluctuation and the stronger the soil temperature recovery capacity. This can be attributed to the increase of soil thermal conductivity and capacity, while the decrease of thermal diffusivity.