An experimental investigation of CuO/water nanofluid heat transfer in geothermal heat exchanger

The energy efficiency of geothermal heat exchanger (GHE) systems can be significantly impacted by the thermal performance of heat transfer fluid inside. In the present study, nanofluid (CuO/water) was experimentally used in the GHE to investigate its thermal performance as circuit fluid. The main part of experimental system consisted of a 0.8 m x 0.5 m x 0.58 m box, two double U-tubes, thermocouples and thermal resistors. A three-dimensional discrete phase model was built to simulate the flow process. By using nanofluid, the heat transfer rate and pumping power consumption of the GHE system increased by 39.84% and 16.75%. Moreover, the heat load-to-pumping power ratio had an enhancement of 20.2%. Furthermore, the previous literature showed that the nanofluids, which had a significant effect on the heat transfer of other types of heat exchangers, had an insignificant effect (less than 5%) on the energy efficiency of GHEs. The simulation result showed that the special structure of traditional GHEs may be the main reason for the lower possibility of collision between nanoparticles and the insignificant effect of nanofluid on its heat transfer enhancement. Therefore, the length of every straight tube segment should be restricted to achieve the better thermal performance of GHEs using nanofluids as heat transfer fluid. (C) 2020 Elsevier B.V. All rights reserved.