How a Sensitive Analysis Coupling Geology and Borehole Heat Exchangers Characteristics Can Improve Efficiency and Production of Geothermal Plants

Abstract

Knowledge of thermal behaviour around and throughout Borehole Heat Exchangers (BHEs) is essential in designing a low enthalpy geothermal plant. In particular, the the kind of grout used in sealing the space between BHE walls and the pipes is fundamental to optimize the heat transfer and minimize the thermal resistance, promoting the reduction of total drilling lengths and installation costs. A comparison between grouts with different thermal conductivities coupled with simplified hydro-geological contexts were modelled over 212 days (typical one-year heating for continental climates). These data allowed for a sensitive analysis taking into account a different flow rate through pipes, as well. In groundwater transient conditions, porous lithologies enabled obtaining obtaining greater heat power extractions with increasing the thermal conductivity of grouts (until 105.91 W/m) if compared with limestone (until 32.09 W/m) or clayey silt deposits (until 6.74 W/m). Increasing of the inlet flow rates through the pipe greatly improves the final heat power extraction. In other hydro-geological contexts the situation is different and may depend on different factors for which using more performing grouts does not increase the final heat power extraction.