A Numerical Study on the Impact of Grouting Material on Borehole Heat Exchangers Performance in Aquifers

U-pipesforgroundsourceheatpump(GSHP)installationsaregenerallyinsertedinvertical boreholes back-filled with pumpable grouts. Grout thermal conductivity is a crucial parameter, dominating the borehole thermal resistance and impacting the heat exchanger efficiency. In order to seal the borehole and prevent leakages of the heat carrier fluid, grouting materials are also hydraulicallyimpermeable,sothatgroundwaterflowinsidetheboreholeisinhibited. Theinfluenceof groundwater flow on the borehole heat exchangers (BHE) performance has recently been highlighted by several authors. However groundwater impact and grouting materials influence are usually evaluated separately, disregarding any combined effect. Therefore simulation is used to investigate the role of the thermal and hydraulic conductivities of the grout when the BHE operates in an aquiferwitharelevantgroundwaterflow. Here3maincasesforasingleU-pipeinasandyaquiferare compared. InCase1theboreholeisback-filledwiththesurroundingsoilformation,whileathermally enhancedgroutandalowthermalconductivitygroutareconsideredinCase2andCase3respectively. Simulations are carried out maintaining the inlet temperature constant in order to reproduce the yearly operation of the GSHP system. For each of the 3 cases three different groundwater flow velocities are considered. The results show that a high thermal conductivity grout further enhances the effects of a significant groundwater flow. The conditions when neglecting the grout material in the numerical model does not lead to relevant errors are also identified.