Abstract: Energy tunnels effectively harness geothermal energy to mitigate frost damage in cold-region tunnels while enhancing energy efficiency and reducing carbon emissions.However,traditional ground source heat pump systems experience a decline in heat transfer efficiency over time.Nanofluids,which possess superior thermal conductivity,show promise as a replacement for water in these systems,potentially improving heat transfer performance and frost protection in cold climates.This paper investigates nanofluid-enhanced energy tunnels by selecting an efficient nanofluid heat transfer system.The results indicate that the thermal conductivity and viscosity of nanofluids are higher than those of water.As preparation time increases,the relationship between thermal conductivity and the nanofluid’s mass fraction shifts from linear to nonlinear,while viscosity increases exponentially with mass fraction.Under identical particle size and mass fraction conditions,the time-dependent thermal conductivity and stability of CuO nanofluids outperform those of Al₂O₃ nanofluids.CuO nanofluids with mass fractions of 0.5% and 1%,and a particle size of 20 nm,are selected as the heat transfer medium for the energy tunnel.Compared to water,the heat transfer per unit length for these two media increased by 8.66% and 14.38%,respectively.The mechanism enhancing the thermal conductivity of nanofluids involves changes in the base liquid structure,microconvection effects,and the formation of thermal conduction bridges.These findings highlight the significant potential of nanofluids as heat transfer media in energy tunnels and provide a foundation for addressing freezing damage in cold regions.