Abstract:
Abstract: The new trends in energy savings and greenhouse gas reductions are expecting to explore the utilization of shallow geothermal energy. The most popular way to exploit shallow geothermal energy resources is the ground coupled heat pump (GCHP) system with using ground as a heat source. Because underground temperature is rather constant compared with ambient air temperature, the GCHP could achieve higher efficiency as well as more stable performance compared with traditional air source heat pumps. Thus the GCHP system becomes increasingly popular in commercial and institutional buildings. In general, a vertical borehole with ground heat exchanger (GHE) is used as the mainstream of GCHP system. However, the wide application of this type of GCHP technology has been limited by its higher initial cost and substantial land areas required to install the GHE. For this reason, the foundation piles of buildings have been used as part of GHE in recent years to reduce the cost of drilling borehole and save the required land area. This innovative idea of utilizing what are usually called "energy piles", has led to notable progress in the field of GCHP systems. It has become particularly attractive because it lowers total cost and spatial requirements, and offers the higher renewable contribution. In this paper, a novel configuration of an energy pile with a spiral coil was proposed. In order to investigate the effects of various factors on heat exchange performance of the pile spiral coil GHE, a numerical model of the pile with a spiral coil was developed. Based on the numerical solution of the model, the effects of pile diameter, pile depth, spiral coil group number and soil type on the heat exchange rate and soil temperature distribution of the spiral pile GHE were analyzed. The results indicated that increasing foundation pile diameter can improve the thermal storage capacity and thus enhance heat exchange rate of pile. But increase in foundation pile diameter can also result in the decrease of heat exchange rate per unit pipe length. So the pile diameter cannot be increased unlimitedly. At the same time, increasing the pile depth can improve the heat exchange rate of pile, and have little influence on heat exchange rate per unit pipe length. Thus the thermal performance of pile foundation can be improved by increasing pile depth. As for soil type, among clay, sand and sandstone, the sandstone was most conductive to the pile heat transfer and thus the soil temperature rise rate was minimum. On the contrary, the clay was the worst for heat transfer of pile foundation and soil temperature rise rate was the fastest among the three soil types. Additionally, increasing spiral pipe group number helped to improve heat exchange rate, but the heat exchange rate per unit length can be reduced largely. The experimental validation showed that the heat transfer rate and soil temperature predicted by the model were in good agreement with the corresponding experimental data, and the maximum relative errors were within 9.7% and 9.2% for heat transfer rate and soil temperature, respectively.