Experimental and numerical investigations of the energy performance of a solar seasonal thermal storage heating system under different operation modes in cold-region tunnels

Solar seasonal thermal storage heating (SSTSH) system is a new type of energy-efficient and environment- friendly anti-freezing technology in cold-region tunnels. The purpose of this study is to investigate the energy performance of an SSTSH system operated in different modes by using field experimental tests and numerical simulation. The experimental system is installed in Tianshan Shengli Tunnel of Xinjiang, China. Three operation modes including single heat storage, single heat extraction, and combined operation mode are investigated under different flow rates and inlet temperatures. The thermal performance, system operational efficiency and borehole wall temperature are analyzed and compared under various modes. The experimental results indicate that for the single heat storage operation mode, the thermal storage performance can be improved by enhancing the flow rate and inlet temperature. However, when ensuring the thermal storage amounts, a lower flow rate or inlet temperature should be employed to improve system efficiency. For single heat extraction operation mode, the heat extraction performance is effectively improved by lowering the inlet temperature, while the coefficient of performance (COP) is decreased. It is advisable to maintain a higher inlet temperature to improve operational efficiency of heat extraction. For the combined operation mode, the simulation shows the heat extraction performance of the SSTSH system can be greatly improved by the increasing the heat storage inlet temperature. When the heat storage inlet temperature increases by 5 degrees C, the growth rate of the average heat extraction amount is 2.45 W/m during the combined operation mode. The seasonal heat stored raises the surrounding rock temperature to a higher level, which is beneficial for increasing the COP of the heat pump, and effectively addresses the thermal imbalance in cold regions. The combined operation mode of the SSTSH system is recommended for the tunnel heating in cold regions.