Anti-freezing system of high altitude and high geothermal tunnel based on air-source heat extraction and case analysis

Abstract

The entrance section of high altitude and high geothermal tunnel is prone to severe freezing damage. Because of the high and stable rock temperature around the tunnel, it can provide favorable conditions for the efficient working of air-source heat pump (ASHP). Therefore, the anti-freezing system of high altitude and high geothermal tunnel based on air-source heat extraction is proposed. Taking the Zilashan tunnel as an example, the steady heat conduction calculation method of the heat extraction tunnel (HET) is derived, and then verified and modified by CFD numerical model. The size of the HET is discussed as well. The results show that the ambient temperature of the HET increases with the increase of the HET length, and the coefficient of performance (COP) of the ASHP increases as well, but the increase rate gradually becomes slow. The influence range of the ASHP decreases with the increase of the HET length, and the actual working ambient temperature is about 4.4 °C lower than the average ambient temperature calculated by the steady heat conduction formula. Considering the actual ambient temperature of the ASHP, the HET length of the Zilashan tunnel is recommended to be 50 m, and the corresponding COP is 4.14. The COP increases with the increase of the volume and the surface-volume ratio of the HET. It can be considered to excavate the branch HET on both sides of the main HET, or further excavate the assistant HET, to improve the COP of the ASHP.