Conclusions
The analysis of literature and the results obtained by the authors demonstrates the feasibility of the extraction of geothermal energy without brine production by means of the WBHX. The low efficiency in heat recovery respect to the conventional geothermal plants makes this solution not profitable for the hydrothermal systems. Anyway, considering the advantage of avoiding risks and costs related to the extraction and reinjection of the fluids, the WBHX may encourage a positive social response to geothermal plants and it could be an opportunity for the exploitation of conventional and unconventional geothermal systems. Therefore, a research to improve the performance and the applicability of the WBHX is worthwhile. In details, the aspects to be deepen are the heat carrier fluid, the pipes materials, the design and the conversion systems. Regarding the heat carrier fluid, the evaluation of using nanoparticles in the heat carrier fluid is undergoing. The basic idea is the increase of the volumetric heat capacity of the pure fluid. First results, obtained based on available correlations, suggest a 1% increase of the heat exchange adding a 3% in volume of Aluminum oxide in water. There is a lack of specific studies on materials that are made up the pipes of the WBHX. In order to improve the heat recovery of the device, the research should focus on the possibility to use for the internal pipe a material able to isolate it, reducing the heat losses during the recovery of heated fluid. Instead, the grouting material mainly affects the heat exchange with the formation. Therefore, improvements are needed more on the grouting materials than in materials constituting the external casing.
