Transient heat transfer model of a three-dimensional spiral heat source in an energy pile

An energy pile is a new type of ground source heat pump system. A heat exchanger is casted into the concrete pile foundation of a building structure for the purpose of heating or cooling the building through heat exchange between the pile foundation and surrounding soil. An energy pile can be developed rapidly because of its high heat transfer efficiency and stable structure and because it requires no additional drilling requirements. In the long-term operation, energy piles have to bear both the overlying and thermal loads caused by changes in the temperature field. Thus, accurately evaluating the temperature field of an energy pile and its surrounding soil is one of the key problems in the design and application of energy piles. To improve the heat transfer efficiency of energy piles, U-type, W-type, spiral type, and similar types of coils have been developed to be casted into the energy pile. Results of thermal efficiency analysis show that the spiral type coil has the best heating and cooling performance and was nearly 150% more thermally efficient than the double U-type coil. Thus, a spiral coil is selected as the main coils’ form in the current practical application. However, due to the complex heat exchange structure of the spiral pipe, the present analytical model had to be simplified to accurately characterize the temperature field characteristics of a spiral pipe casted in an energy pile. In this paper, the spiral pipe was regarded as a three-dimensional spiral heat source. Considering the existing heat transfer model, an analytical solution of the temperature field was obtained by integrating green’s function and the first curve function; then, the high-precision, three-dimensional (3-D) heat transfer model of the spiral pipe was established considering the time, space, buried pipe parameters, and thermal property of host soil. In addition, a 3-D model of a spiral pipe casted in an energy pile was created in the numerical simulation software COMSOL; after simulation, the numerical solution of the temperature field was obtained. The contrastive results showed that the built 3-D spiral heat source model has high analytical accuracy. Finally, based on the analytical model, the spatial distribution and time effect of a spiral pipe casted in an energy pile were discussed.