Influencing factors of energy efficiency for heat pump coupled with mountain tunnel lining ground heat exchangers

To investigate the energy efficiency of the heat pump coupled with tunnel lining ground heat exchangers (GHEs) in a mountain environment, a 3D numerical model is developed to simulate the transient heat transfer of the tunnel lining GHEs. The energy efficiency of the heat pump coupled with tunnel lining GHEs in the cooling mode is evaluated. The results demonstrate that the EER of the ground-source heat pump decreases and then increases with the increase of wind speed. The EER increases with the increase of thermal conductivities of the surrounding rocks, whereas the growth rate decreases gradually. The EER decreases lineally with the increase of initial ground temperature. The results of the sensitivity analysis show that the wind speed has the least influence and the initial ground temperature has the greatest influence on the EER of the ground-source heat pump coupled with tunnel lining GHEs. As the absorber pipe length increases, the weight of the initial ground temperature on the EER increases, while the weights of the wind speed and thermal conductivity of the surrounding rock gradually decrease.