Preliminary design method for absorber pipe length of tunnel lining ground heat exchangers based on energy efficiency of heat pump

Abstract

Many ongoing tunnel projects provide a favorable opportunity for the investigation and application of tunnel lining ground heat exchangers (GHEs). Tunnel lining GHEs can be connected to a heat pump to extract geothermal energy for heating and cooling buildings. Numerous studies have focused on the thermal performance of tunnel lining GHEs; however, the studies on the interaction between heat pumps and tunnel lining GHEs are relatively rare. In this study, a coupled heat transfer model of heat pumps and tunnel lining GHEs was proposed and then calibrated based on in situ test results. The model was used to evaluate the energy efficiency of a heat pump with tunnel lining GHEs under different conditions. The results show that the energy efficiency ratio (EER) increases exponentially with the absorber pipe length and thermal conductivity of the surrounding rock. The EER is governed by the convection heat transfer coefficient, which varies exponentially; meanwhile, the EER decreases approximately linearly with the annual average air temperature in the tunnel. Different types of heat pumps affect the EER significantly, and the EER of a Type-3 heat pump is higher than that of a Type-1 heat pump by 27.1%. Based on the aforementioned results, an empirical formula for the EER and absorber pipe length was established. Moreover, a preliminary design method for the absorber pipe length based on this empirical formula was developed. The method was employed to determine the appropriate absorber pipe length for the tunnel lining GHEs in the Shapu tunnel in Shenzhen, China. Finally, groups of absorber pipe layouts with a pipe spacing of 0.5 m, area of 135 m2, and length of 293.5 m were preliminarily determined.