Heat Transfer Analysis Of Energy Piles With Parallel Connected U-Tubes

Due to the advantages of energy saving and environmental protection, ground source heat pump technology is more and more widely used in energy-saving buildings under the policy of the state to advocate the renewable energy applications. As a kind of clean renewable energy sources, some novel technologies of geothermal energy applications have been promoted under the current background of energy crisis. Under this circumstance, the research and development of the pile heat exchanger has been raised some concerns by the researchers. The pile foundation heat exchanger which is also known as energy pile can make full use of the building underground area and reduce the outside drilling area and drilling costs. These advantages greatly improve the applications of the energy pile in engineering projects. The common configurations of the buried pipes in piles are the U-tube, W-type tube, and spiral tube. Among these configurations, the type of multiple U-tubes connected in parallel has been widely used in pile heat exchanger because of its simple construction and high heat transfer efficiency with large heat transfer area.The majority studies on the pile heat exchanger with multiple U-tubes in parallel were focused on the numerical model and some analytic models with the assumption of same thermal properties of the pile and the surrounding soil considering the complexity of the heat transfer process of this kind of configuration. In this dissertation, the heat transfer model of the parallel connected U-tubes with different thermal properties between the pile and the soil has been developed which can provide a reliable and theoretical guidance for the engineering design of the pile heat exchanger with parallel connected U-tubes in ground source heat pump system.Firstly, the transient heat transfer model of the single U-tube buried in a pile foundation has been established under the assumption that the thermal properties of the pile foundation and the soil are the same. Secondly, on the basis of the heat transfer model of a single U buried tube, the heat transfer model of the multiple U-tubes connected in parallel has been developed under the different thermal properties between the pile and soil. The analytic solution of the temperature field has been obtained by means of the Laplace transform method and linear superposition principle. The numerical heat transfer model has also been established in order to validate the analytic model. According to the comparison results, the analytic solution is slightly larger than the numerical simulation results. Some possible reasons have been discussed in the dissertation. As a whole, the proposed analytic model has the similar temperature field as the numerical data, which demonstrates that the analytic model has enough accuracy from the viewpoint of the engineering applications. By comparing the temperature fields obtained from the two cases (the first case is the same thermal properties and the second case is the different thermal properties), the circulating fluid temperature in 1 st case is lower than that of the 2rd case. This shows that the thermal properties of the pile material are better than those of the surrounding soil. Therefore, it is suggested that the different thermal properties should be considered in the practical engineering design to obtain more accurate results.There are a number of factors which can influence the accuracy of the heat transfer model of the pile heat exchanger with multiple u-tubes, among which the thermal properties of the pile and the soil are the two main factors. Therefore, the reasonable assumptions and comprehensive considerations among all the factors are the key points which can guarantee the accuracy of the heat transfer model. This dissertation only developed the transient heat transfer model considering the different thermal properties of the pile and the soil. The conclusions can provide a useful and theoretical method to design pile heat exchanger with U-tubes in parallel. However, there is still a lot work which needs to be done. It is suggested that the future work may focus on the experimental study and the engineering problems.