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 in literature highlight as key parameter the residence time of the fluid into the device. At fixed flow rate the residence time of the fluid in the WBHX is function of the well diameter. From analyzed papers, it raises the need of the insulation of the upward pipe in order to avoid heat losses. The range of produced thermal power is 0.15÷۲٫۵ MW and of electrical power is 0.25÷۳۶۴ MW. The WBHX is a promising technology if and only if is applied in the more convenient geothermal assets. The continuous study of the possible designing solutions and the improvements to enhance heat transfer is fundamental to allow this technology ready to market.

Introduction

The customary method to extract energy from a geothermal system at medium or high enthalpy is the production of geothermal fluids using wells. When the temperature of the extracted brines is greater than 150
C, they are used to produce electricity in flash or dry steam geothermal power plants. When the temperature is lower than 150
C, direct uses or electricity production using a binary power plant are the possible applications. The brines have properties not suitable to the terrestrial ecosystems. Therefore, after the use and before to inject it back into the ground the geothermal fluids must be treated. The extraction and reinjection of the brines entail some technical and environmental risks, also in the closed loop binary power plants: corrosion and scaling of pipes, groundwater pollution, land subsidence and induced seismicity. Therefore, the production of the brines involve high economic costs. The investment can become non-profitable in unconventional geothermal systems where there are fluids with characteristics requesting special techniques. The possibility of negative impacts on the environment and the risk of induced seismicity, however of low intensity, can cause a negative social response to the geothermal development in areas with a high urbanization or with a high natural seismicity.