Water storage in underground mined-out space as a geothermal reservoir: Heat extraction performance and temperature evolution

• Optimal mechanical strength of solid waste backfill is achieved with 10‰ glass fiber content and 6 mm fiber length. • Permeability of solid waste backfill exhibits a quadratic relationship with both axial and confining pressure. • Geothermal energy extraction occurs via convective heat transfer between water and rock, with temperature in the heat extraction space remaining lower than surrounding rock during recovery. • Heat extraction amount increases with water flow velocity but decreases with higher initial water temperature.