Abstract: In order to study the longterm heat recharge performance of energy tunnel under different hydrogeological conditions, the temperature fields of the surrounding strata of energy tunnel under three heat exchange schemes of annual cycle, daily cycle, and superposition of annual and daily cycles are simulated by finite element method. In the numerical model, the temperature fields of strata, lining and air, as well as underground water seepage and air flow are considered. For simplicity, the heat exchanger is equivalent to a surface heat source, and the different operation schemes of energy tunnel are simulated by adjusting the heat exchange power.  The calculation results show that: (1) The larger the underground water seepage velocity, the higher the thermal replenishment capacity of the strata, and the larger the range of temperature affected by the energy tunnel; (2) Under the annual cycle and daily cycle operation schemes, the larger the underground water seepage velocity, the faster the temperature field around the tunnel reaches a stable state; (3) When compared with the annual cycle heat exchange scheme, the daily cycle heat exchange scheme can significantly reduce the temperature variation range of the formation, although it cannot effectively improve the thermal recharge capacity of the formation; (4) By optimizing the cooling or heating power and duration, the temperature of the surrounding strata of the energy tunnel can be restored to initial state after one year cycle, thus improving the operation efficiency of energy tunnel and reducing the impact of energy tunnel on the strata environment.

Key words: energy tunnel, heat recharge capability, heat exchange, water seepage velocity, operational scheme