Advances in the research and application of geothermal downhole heat exchangers

The up-to-date technologies and recent research works on geothermal Downhole Heat Exchanger(DHE) were summarized in this paper. Key points affecting the performance of DHE in a gethermal aquifer are analyzed, such as the heat loss from the return leg to the surrounding matrix, the casing and liner diameters, the installation of the convection promoter, and the inlet water temperature. DHEs are reccommended to be used with a surface heat pump system together, especially for the place where the well drilled having a low temperature geothermal aquifer. Since the inlet temeperature through the DHE can be decreased using heat pumps, the total extracted heat from the aquifer can be increased. Correspondingly, the upper temperature limitation in use of DHEs can be extended to the case with low temperature geothermal reservoirs. In addition, the perspective future market for DHE application in China is discussed.