Laboratory investigations of the thermal performance of an energy pile with spiral coil ground heat exchanger

Spiral coil energy pile is a new type of ground heat exchanger (GHE) used in the ground coupled heat pump (GCHP) system. In this manuscript, a model experimental apparatus of energy pile with spiral coil was set up based on the similarity principle. The experimental investigations of influences of inlet water temperature, intermittent operation mode, spiral pitch and pile material on the thermal behavior of the spiral coil energy pile and soil temperature distribution around it were performed on the experimental apparatus. The experimental results indicate that increasing the inlet temperature contributes to the increase of heat release rate of coil, and the heat release rate increases approximately linearly with the inlet temperature. At the same time, the higher the inlet temperature, the more the heat storage in the pile. For a given inlet temperature, the soil temperature at a level gradually increases from center to outer surface of the pile, and then drops sharply across the outer surface. Under the intermittent operation mode, the soil temperature rise can be stayed effectively by a reasonable intermittent operation control, and thus the performance of GCHP can be improved. As for the spiral pitch, reducing the spiral pitch can increase the total heat release rate of the coil, but also result in the decrease of heat release rate per unit pipe length. Thus, the spiral pitch cannot be reduced unlimitedly and should be optimized through considering the total heat release rate, the heat release rate per unit pipe length, available pile area, and the costs of installation and material. Additionally, increasing the thermal effusivity of pile material can enhance the heat release rate of coil, but also reduce the temperature restoration effect of the soil surrounding the pile. Thus, the selection of pile materials should be overall considered according to the construction requirement of building foundation pile and the heat transfer optimization of energy pile.