The wellbore heat exchangers. A technical review

The available literature on the WellBore Heat eXchangers (WBHX) has been analyzed giving prominence to three aspects. First, the heat transfer through the geothermal reservoir and between the formation and the well has been analyzed. Then, the design of the WBHX and the modelling of the heat exchange has been reviewed. Lastly, the analysis of the performance of the WBHX in the production of thermal and/or electrical energy has been focused. Regarding the modelling of the heat transfer in the reservoir and between the wellbore and the formation, the sensitivity studies have highlighted that the key parameter of the heat transfer is the residence time of the fluid in the device. At fixed flow rate the residence time of the fluid in the WBHX is function of the well diameter. The diameters of the coaxial pipes have a secondary influence on the heat transfer. The insulation of the upward pipe is necessary in order to avoid heat losses. The performance of the WBHX is proportional to the geothermal gradient and the thermal properties of the formation. The analyzed papers have heterogeneous results. The range of produced thermal power is 150 kW÷2.5 MW and of electrical power is 25 kW÷364 MW.