Effect of intermittent operation on the thermal performance of a solar seasonal thermal storage heating system in a cold-region tunnel

Abstract

The efficiency of tunnel heat exchangers often diminishes annually due to thermal imbalances, particularly in heating-dominated cold regions. For addressing frost damage in the entrance sections of cold-region tunnels, a novel and cost-effective solution: the Seasonal Solar Thermal Storage Heating (SSTSH) system, is proposed and applied in Tianshan Victory Tunnel in Xinjiang, China. Effect of intermittent operation on the thermal performance of the SSTSH system in Tianshan Victory Tunnel is investigated. Initially, field experiments are conducted to test the separate operation of thermal storage and extraction. Subsequently, a transient 3D numerical model is developed and validated by using experimental data to investigate the thermal performance of mixed operation modes under varying intermittent thermal storage ratios. Field experiment results reveal that increased intermittent operating time significantly enhances the single thermal storage and extraction performance of the SSTSH system, thereby improving operational efficiency. Simulations demonstrate that in the SSTSH system, higher intermittent thermal storage ratios lead to improved coefficients of performance (COP) and average thermal extraction during thermal extraction. After 12 h of thermal extraction during the 48h trial period, intermittent thermal storage ratios of 1/3, 1, and 3 yield respective increases in extracted heat by 25.88 %, 39.26 %, and 48.28 % compared with continuous thermal extraction operations. Moreover, intermittent thermal storage operations result in lower surrounding rock temperature drops and longer recovery periods, further enhancing system efficiency. Employing the intermittent operation strategy in the SSTSH system presents a promising solution to mitigate the thermal imbalance of ground source heat pumps, thereby facilitating their application in cold-region tunnels.