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Recently, utilization of building foundation piles as the ground heat exchanger (GHE) received more and more attention since it can reduce the initial cost and land area requirement compared with the borehole GHE. This study designs a foundation pile GHE with spiral coil (FPGHE) by intertwing the circulating coil pipe tightly in spiral shape against the reinforcing steel of a pile. The distinct advantage of this proposed FPGHE is that it can offer higher heat transfer efficiency, reduce pipe connection complexity, prevent air blocking and decrease the thermal “short-circuit” between the feed and return pipes compared with other existing configurations. In order to analyze its heat transfer characteristic, analytical models are established for the proposed FPGHE. Analytical thermal analysis is carried out to simulate temperature responses of the coil pipe wall and the circulating water entering/effusing the FPGHE to the short time step heat transfer loads based on the established analytical model. Furthermore, the operation performance and heat exchange capacity of the FPGHE is investigated. INTRODUCTION The ground heat exchangers (GHE) with vertical boreholes (Bose, et al. 1985) have been the mainstream technology for the ground coupled heat pump systems, but the high initial cost and land area requirement to install the borehole GHE remain the major obstacles of this technology. Since the foundation pile is commonly used in high rise buildings, combining the heat exchanger and building foundation pile can eliminate the drilling expense and land area requirement of borehole GHE. Therefore, utilization of building foundation piles as the GHE has received more and more attention(Mehrizi, et al. 2016; Luo, et al. 2016; Huerta and Krarti 2015; Loveridge and Powrie 2014). In existing studies, pipes are usually buried in foundation piles in configurations of U-tubes, W-tubes or spiral coils. For the first two configurations, the heat transfer area inside foundation pile is small and the air blocking may occur in pipes. In order to overcome these drawbacks, this study focus on the third type and designs a foundation pile GHE with spiral coil (FPGHE). The circulation coil pipe is intertwined tightly in spiral shape against the reinforcing steel of a pile, and is disposed within about 0.1m of the pile’s outer surface. The distinct advantage of FPGHE is that it can offer higher heat transfer efficiency, reduce pipe connection complexity, prevent air blocking and decrease the thermal “short-circuit” between the feed and return pipes compared with other existing configurations. The schematic diagrams of a conventional single U-tube vertical borehole GHE and the FPGHE with spiral coils are compared in Figure 1. Figure 1 Schematic diagram of a vertical borehole and a FPGHE with spiral coil Modeling the FPGHE with spiral coils is complex and existing studies concentrated on the numerical or experimental methods. Suryatriyastuti carried out a 3-D numerical heat transfer analysis on the energy pile under the constant heat load case (Suryatriyastuti, et al. 2012). Xiang built a numerical model includes a 1-D transient convection–diffusion submodel for the fluid domain and a 1-D transient diffusion submodel for the solid domain (Xiang, et al. 2015). Luo conducted the thermal performance test to analyze the operation performance of energy pile under an intermittent condition (Luo, et al. 2016). In the present study, an analytical method is explored as it can provide a more practical and convenient tool for engineering design, as well as thermal analysis of the FPGHE, compared with existing numerical and experimental methods. For the proposed FPGHE, its diameter is much thicker and depth is usually shorter compared with the borehole GHE. Obviously, classical heat transfer models for the borehole GHE fail for the FPGHE. By analyzing the heat transfer process of proposed FPGHE, the analytical finite spiral heat source model is established in this study based on the Green’s function theory, the virtual heat source theory, and the superposition method. The temperature responses of the spiral heat source, the coil pipe wall, and the circulating water entering/effusing the FPGHE to the short time step heat transfer loads are deduced based on the established analytical model. Then the operation performance and the heat exchange capacity of the FPGHE is investigated. DESIGN OF FOUNDATION PILE GHE WITH SPIRAL COIL The high-density polyethylene (HDPE) pipe with exterior and interior diameters of 25mm and 20mm, respectively, are selected as the circulation pipe of proposed FPGHE. First hydrostatic test with pressure of 0.8Mpa and last for 15min are required in order to prevent the leakage before the pipe are installed. Then the pipe is intertwined tightly in spiral shape against the reinforcing steel cage of a pile, as shown in Figure 2(a). The second hydrostatic test with pressure of 0.8Mpa and last for 15min should be carried out followed. Then the combination of reinforcing steel cage and coil pipe are put into the hole of pile. After the third hydrostatic test with pressure of 0.8Mpa and last for 2 hours, the last step is the concrete pouring of the pile foundation, as shown in Figure 2(b). Extreme caution should be paid during the concrete pouring, and coarse aggregate in concrete must be smooth and non-angular particles. The conduit are utilized to lead the concrete into the bottom of pile hole, and it should be extracted gradually from the bottom to top according to grouting speed. During the placement and extraction process of conduit, it is important to keep the vertical and center for preventing the hanging cage of concrete, ensuring the compaction of pouring and decreasing the heat transfer resistance. The concrete pouring process is finished when the density of return slurry is identical with which of pouring concrete. grout U-tube concrete pile spiral coil

Design and Analytical Analysis of Foundation Pile Ground Heat Exchanger with Spiral Coils