Heat transfer performance of energy piles in seasonally frozen soil areas

Abstract

Energy piles constitute a combination of ground source heat pump and building pile foundation technology. Energy piles can not only exchange heat with soil and extract geothermal energy but can also bear upper loads. However, due to the complex geological and climatic conditions in seasonally frozen soil areas, research on the heat transfer performance of energy piles in these areas is limited. According to the characteristics of the ground temperature distribution in seasonally frozen soil areas, the soil region can be divided into frozen and nonfrozen layers. First, a laboratory model test was carried out to investigate the heat transfer performance in these two layers. Then, a three-dimensional heat transfer model of an energy pile suitable for seasonally frozen soil areas was established. The actual working conditions of the real-scale model were simulated, and parametric analysis was then conducted. The results indicated that in annual energy pile operation, the energy pile maximum heat exchange amount per unit length can reach 125 W/m. In the performed parametric analysis, the pile length, concrete thermal conductivity, fluid volumetric flowrate and pipe type impose the greatest influence on the energy pile heat transfer performance, while the pile diameter and concrete cover thickness impose relatively little influence. Moreover, the influence of the pipe diameter should be further studied.