۱- Introduction

۲- Realization of enhanced heat transfer concrete

۳- Analysis of heat transfer law of single energy pile

۴- Heat transfer model considering seepage

۵- Effect on pile group

۶- Analysis of simulation results

۷- Conclusion

References

Abstract

Energy pile is a new type of building component which combines ground source heat pump technology with pile foundation. It can be used to heat or cool the buildings by utilizing the stable heat storage characteristics of soil. After 30 years of development, the methods of optimizing buried pipe and other ways to improve the heat transfer efficiency have met bottleneck. The methods of improving the heat transfer efficiency of energy pile need to find a new way. In this study, the graphite powder with high heat transfer characteristics was added into the concrete to prepare enhanced heat transfer concrete. After testing, the heat transfer coefficient of the concrete can be enhanced from 1.71 W/(m·K) to 2.84 W/(m·K). On this basis, the heat transfer models of single pile and pile group in summer working condition are established by using COMSOL Multiphysics software. The simulation results show that the heat transfer capacity of enhanced heat transfer energy pile is obviously higher than that of common energy pile, and the heat transfer capacity is increased by 6.5%. At the same time, the influence scope of soil has increased. Under the condition of pile group condition, the heat transfer enhanced effect of energy piles are affected by the location and the space, the corner pile is the most effective, the edge pile is the second, the center pile is the least; and the larger the space between the pile, the better effect.

Introduction

Nowadays, energy development and environmental protection issues are increasingly becoming an important factor for human development. Green, clean and environmentally friendly energy has become the preferential research in many countries. Energy pile system is a new type of building component which combines ground source heat pump with pile foundation. It not only acts as a bearing component to support the building, but also acts as a part of building air conditioning system to exchange heat with surrounding soil. It is due to the own the advantages of energy saving, energy pile has broad prospects for development and popularization. Morino et al. took the lead in burying tubular heat exchangers in steel pipe piles [1]. Pahud et al. buried Ushaped tubular heat exchangers in concrete piles and applied them in more than 500 piles of Munich Airport Building [2]. Laloui et al. designed the ground energy conversion pile, and showed the construction technology, results of field test and numerical model of the pile [3]. Hamada et al. [4] carried out the ground energy conversion test of friction piles in Japan, and recorded the test data and the thermal load of buildings [4]. So, the energy pile was developed gradually. On the basis of previous studies, many scholars have paid attention on improving the heat transfer capability. Some ideas of optimizing the form of buried pipe and the factors affecting the heat transfer capacity of energy pile have been proposed. Li et al. [5] applied the U-shaped buried pipe to the borehole and pile. The experimental results showed that the heat transfer of the double Ue pipes is much larger than that of the single Ue pipes [5]. Gao et al. [6] studied Other geometrical arrangements of U shapes (W-shaped and triple U-shaped) and concluded that W-shaped pipes were more efficient than single, double, and triple U-shaped pipes [6,7]. Zarrella et al. [8] analyzed the heat transfer performance of two kinds of buried tube (triple U-tube and spiral pipe) energy pile by numerical simulation and in-situ experiment. The heat exchange of piral pipes was higher than that of triple U-pipes about 23% [8]. Batini et al. [9] studied the effects of different structure of buried tube heat exchanger, fluid velocity and liquid mixture composition on the heat transfer performance of energy pile. The results show that the structure of buried pipe has a very strong influence on the heat transfer performance of energy pile [9]. Cecinato et al. [10] performed parameter analysis by numerical model and found that increasing the amount of buried pipe and the diameter of the pile within a reasonable range is very beneficial to increase the thermal conductivity of the energy pile. At the same time, the length of the pile and the thickness of the protective layer of the concrete also have a significant effect on the heat transfer efficiency [10].