Research On Temperature Recovery Characteirstics Influenced By Layered Rock-soil Body Around The Single U-tube

Compared with conventional heating and cooling systems, GSHPsystems have been developing rapidly due to its significant advantage insaving energy and reducing environmental pollution. However, theunbalanced load of hot and cold caused by impact of climatecharacteristics and load characteristics of the building is a seriousproblem when the GSHP system used in most parts of China. And theintermittent operation of GSHP system can reduce the drastic change ofthe rock-soil body temperature caused by unbalanced load.In this paper, a rock-soil temperature field monitoring system ofGSHP system in cold region was installed, the differences of rock-soilbody temperature field changes between the ground heat exchanger wellsand test wells during the recovering season were analyzed and thevariation laws of rock-soil body temperature field were obtained. TheFLUENT software was used to build model based on the actual rock-soiltemperature field recovery of GSHP system, and the impact of layered rock-soil body and geothermal was considered. Research was made onrock-soil body axial and radial stratification.The test results showed that the axially layered of rock-soil body hasa great influence on the temperature recovery. At the beginning of therecovering season, the recovery of soil temperature in normal temperaturezone mainly driven by geothermal energy spreading, while inheterothermo zone the recovery is affected by both geothermal and solarradiation. The temperature recovered at the early and middle of therecovering season and reached a stable value meaning the recoverycomplete in the10th day with the recovery rate of82%-94%. Overall,after the annual intermittent operation, rock-soil body temperature ishigher than the original, the average difference is0.47℃.In this paper, a hierarchical model and non-hierarchical model werecompared and the simulation results showed that each layer of rock-soilbody temperature recovery rate was different due to its thermal properties,and temperature recovery rate sorted in descending order: highlyweathered sandstone, pebbles, moderately weathered sandstone,collapsible loess (silt), loess (silt). So layer of collapsible loess (silt),loess (silt) were more likely to accumulation heat. The simulation resultsof two models had an enormous range, the final temperature distributionobtained by hierarchical model were closer to the actual than thenon-hierarchical model because the temperature recovery rate obtained by non-hierarchical model was unchanged and the final temperature wasslightly above the actual temperature.Artificial selection of different backfill material leads to radiallystratified in rock-soil body due to its thermal properties. In this paper,four different thermal conductivity of backfill material were selected tosimulate and analyze its impact to rock-soil body temperature restoration.It is known that with recovery time increasing, the impact of conductivityof the backfill material were different. In the beginning of the recoveringseason, the increasing of thermal conductivity of backfill material wereconducive to strengthen the heat transfer between fluid within the u-tubeand the surrounding rock-soil body. The variances of temperaturesimulated by four different backfill materials were becoming smaller astime goes on, and they almost decreased to zero at12hours of recoveringseason. Accordingly, thermal conductivity of backfill material had lessinfluence on rock-soil body temperature restoration with recovery timeincreasing. Therefore only in recovering season, selected high thermalconductivity to strengthen the heat transfer had little significance.