Analysis Evaluation of Downhole Heat Exchanger for the promotion of Geothermal Energy Utilization

为了直接从浅层地热资源中提取热量，人们提出了热虹吸式热管、同心管式热虹吸、u型管式井下换热器、同轴式井下换热器等几种井下换热器。在这些井下换热器中，u型管井下换热器以其性价比高、施工方便等优点在日本乃至世界范围内得到了广泛的应用。使用同轴换热器的另一个原因是，有报道称，井下同轴换热器在抽热过程中，通过工作流体的反循环方式，可以实现高效的抽热。本研究的目的是通过实验和计算机模拟来评估地热换热器的抽放热量性能，包括单u管换热器、双u管换热器和三u管换热器以及井下同轴换热器。实验结果表明，提高抽热工质的流量和入口温度，可以提高单u型管换热器的放热速率。通过三维数值计算对双u管和三u管井下换热器的抽热性能进行了比较，发现在一定抽热工质流量下，三u管井下换热器的抽热速率大于双u管井下换热器。以井下同轴换热器为例，通过计算机模拟发现，抽热性能在很大程度上取决于内管材料、换热器长度和抽热工质循环方式等因素。

Several types of downhole heat exchangers, for example thermosyphon type heat pipe, concentric tube thermosyphon, U-tube downhole heat exchanger, coaxial downhole heat exchanger, as well as others, have been proposed in order to extract heat directly from shallow geothermal resources. Among these downhole heat exchangers, the U-tube downhole heat exchanger is widely used in Japan as well as around the world, because of its cost performance and easy construction. Another reason it is used is because it has been reported that highly efficient heat extraction can be achieved with the downhole coaxial heat exchanger through its reverse circulation mode of working fluid in heat extraction process.The aim of this study is to estimate the heat extraction and heat release performance of the geothermal heat exchangers including single, double and triple U-tube heat exchangers and downhole coaxial heat exchangers by experiments and computer simulation.From the experimental results it was found that increasing both the flow rate and inlet temperature of the working fluid of the heat extraction increases the heat release rate with a single U-tube heat exchanger. Furthermore, the comparison of the heat extraction performance between double and triple U-tube downhole heat exchangers was made by three-dimensional numerical calculation, The finding revealed that the heat extraction rate using a triple U-tube downhole heat exchanger is greater than that using a double U-tube under a fixed flow rate of the working fluid of heat extraction. In the case of downhole coaxial heat exchanger, it was found from the computer simulation that heat extraction performance greatly depended on the factors of the inner tube material, length of heat exchanger and circulation modes of working fluid of heat extraction.