The Heat Transfer Study And Engineering Application Of Energy Piles

The main restriction against the ground-coupled heat pump (GCHP) application in urban areas is that the system requires a large plot of land for installation of geothermal heat exchangers (GHEs), and this is usually difficult in cities. Besides, compared to the traditional air conditioning system, the higher initial cost of the geothermal heat exchanger (GHE) installation cost has hindered applications of this technology. To overcome these obstacles, it is desirable to find a new alternative to the borehole heat exchangers prevailing in the GCHP applications. This novel GHE system is known as the "Pile Geothermal Heat Exchanger (PGHE) system", also known as energy piles. In the PGHE system pipes may be buried in the foundation piles of a building. Besides supporting the building, the piles can act as heat exchangers.It is vital to model the heat transfer in the PGHEs for proper design and energy analysis of the systems. This article aims to develop a transient analytical model which can describe the heat transfer process in the PGHEs. Optimal design methods of PGHE can improve heat efficiency of the system, and ensure the GCHP system to run in an effective and reliable way. Therefore, the study is a key to application the new technology.In the study of heat transfer mechanism of energy piles, the "solid" cylindrical heat source model is first proposed by the research group in Shandong Jianzhu University. In the analysis of heat transfer process of the PGHE with spiral coils and the borehole GHE, this model is much better than the line-source and the "hollow" cylindrical source models. But the solid cylindrical heat source model fails to distinguish the effect of the spiral pitches because the coil is simplified as a continuous cylindrical surface. So an improved analytical model of ring-coil heat source has been established, which is part of this study. Based on this model, detailed study of heat transfer process of the PGHE with spiral coils has been made. Improvements have been made here by introducing more appropriate assumptions for the spiral heat source. For the finite length of ring coil heat source and the spiral coil heat source models, the length and the buried depth of PGHE have been taken into account. The research indicates that when the heating time approaches infinity, the temperature of the two finite models will approach a steady-state. And the time for the temperature distribution to stabilize depends on both the length and depth of the PGHEs.The work here indicate that both the ring coil heat source and the spiral coil heat source models can illustrate adequately the heat transfer process of the PGHE with spiral coils. The temperature distributions computed with the two models have been discussed for the sake of comparison. The results of the two models are consistent when other conditions are equal. And the maximum relative error remains within the acceptable range. The temperature field of spiral coil heat source model is three-dimensional while the ring coil heat source model results in a two-dimensional temperature response. However, as the temperature distributions are computed, the spiral coil heat source model does not increase the workload of calculation. Therefore, the spiral coil heat source model was recommended to discuss the heat conduction in energy piles.In previous models used to illustrate the heat transfer process of the energy piles, which were made by our group, the dimension of the coil pipe is neglected. As a result, the temperature on the points where the ring sources locate tends to infinity. This paper introduces the spiral pipe radius rp into the model. Then the spiral coil heat source model can be used to analyze temperature changes of the pipe wall and the fluid in the PGHE with spiral coils.Finally, a real project of a pile GHE system was developed in Qingdao, and it is used to simulate the inlet and outlet temperatures of pure water or anti-freezing solution of the heat pump.The study presented in this thesis helps to lay a foundation for proper applications of the pile energy technology.