Heat Transfer Enhancement of Graphite–modified Concrete Energy Piles

Haoran Guo, Lan Qiao, Yunyang Xiao, Xiangyu Ren

Abstract


Designed for utilizing the ground-source systems for heating and cooling, the use of energy piles in commercial and residential
buildings has increased exponentially especially in Europe. The heat transfer efficiency of energy piles may directly influence
the energy-saving effect on buildings. Apart from the optimization of pipe laying, many other factors can also influence the
heat transfer efficiency of energy piles. In this study, a new method that can increase the heat transfer efficiency of energy
piles was proposed to explore the influences of adding graphite powder with high thermal conductivity to pile concrete on the
heat transfer efficiency of energy piles. The thermal resistance models of energy piles in three different pipe-burying modes
were constructed by combining the 2D plane method and the heat transfer mechanism of energy piles. The internal heat
transfer characteristics of energy piles at different temperatures, graphite contents, and pipe-burying modes were discussed
by combining the indoor thermal conductivity test of graphite-modified concrete. The external heat transfer characteristics
of graphite-modified concrete energy piles were analyzed through numerical simulation analysis. Results demonstrate that
the increase in graphite contents is beneficial to heat transfer in energy piles. In particular, thermal conductivity significantly
increases when the graphite content exceeds 5%. The high temperature in the pipe is also conducive to the thermal conductivity
of the energy pile. The thermal conductivity of the concrete samples with 8% graphite content in an environment at 40°C
is 1.35 times that at 20°C. The heat transfer efficiency of the spiral coil-type energy pile is higher than those of single-U and
double-U tube energy piles. The proposed method provides a certain reference for improving the heat transfer efficiency of
energy piles and constructing the internal and external heat transfer models in energy piles.


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