Explorations On Heat Transfer Performance Of Energy Piles And The Settlement Characteristics Aroused By Cyclic Temperature Variations

Development and utilization of shallow underground geothermal resources by ground source heat pump technology is an important measures in building energy conservation and carbon emission reduction.It has become a national focus in the promotion of energy saving and ecological construction of new technologies.Pile heat exchanger uses characters that building foundation concrete has better heat conduction properties and a larger heat transfer area of the building foundation and underground rock, which improve the heat transfer performance of ground heat exchanger dramatically. Because that it saves a large amount of drilling cost, uses underground space more rational, and operate more safely and reliably, there is a trend in the developing ground source heat pump in the recent years.The difference of heat transfer performance of seven kinds buried tube heat exchangers and economic performance of four kinds of buried tube heat exchangers were analyzed, choosing the heat transfer rate per unit borehole depth as the index of heat transfer performance combining with economic index E, based on data tested on the inlet, outlet water temperature and flow rate of a single U-tube, parallel double U-tube, pile foundation with parallel double U-tube and parallel double helix-tube heat exchangers in a certain engineering projects in Jiangsu Province. The influence of cold and heat accumulation on the heat transfer performance of the energy piles was also analyzed. In addition, loads were exerted on two energy piles in different geometries and they were heated or cooled to simulate their heat rejection in summer and heat absorption in winter. During these processes, transient temperatures and the displacements of the piles’ top surfaces were monitored by thermal couples and stress sensors. The coupled effect of temperature variations and conventional static loads were evaluated and the variation patterns of the piles tops displacements caused by temperature variations was illustrated. It indicates that the accumulation of the displacements may lead to pile instability, which should be taken into consideration at the design stage of energy piles.In order to further explore the influence of various factors on the heat transfer performance of the pile foundation and the influence of the heat exchanger on the soil temperature field, On the basis of engineering measurement, this paper establish the numerical models of the perfusion pile foundation buried tube heat exchanger and the prefabricated pile foundation buried tube heat exchanger.Frist, the numerical model was validated using the measured data. On this basis, the effects of flow rate, the length of the pile, backfill material properties, and initial temperature of soil on the pile borehole heat exchanger performance were studied, then the effect of the distance between two tubes of the prefabricated pile foundation on heat exchange were analyzed, and the optimal flow rate was determined. The difference between the best heat transfer performance of the two pile foundation was compared. Simulation results show that the influence of heat capacity of backfill material on heat transfer performance is greater than thermal conductivity; The length of pile body is increased from 30 m to 40 m, the heat transfer rate per unit borehole depth increased 29.88W/m;The total heat transfer rate of perfusion pile foundation heat exchanger with parallel double U buried pipe is higher than prefabricated pile foundation heat exchanger 2452.34W; The optimal flow rate of the perfusion pile foundation and prefabricated pile foundation heat exchanger with parallel double U buried pipe are recommended for 0.5m/s and 0.4m/s.In addition, the numerical model of three pile groups of the buried pipe heat exchanger of the cast-in-place pile was established, and the corresponding measuring points were set up to study the change of the soil temperature field in the pile group under the intermittent operation mode.Finally, soil layering numerical model of the precast pile buried tube heat exchanger was established based on the site engineering geological prospecting data, and the measuring points in the homogeneous model and hierarchical model temperature field changes were compared. The results show that the volume of soil specific heat capacity plays a major role in the soil temperature and whether the soil were stratified or not has little effect on the heat exchange unit depth, but the temperature rise of the soil will produce a certain deviation after the long-term operation of the heat exchanger.