Field test of the performance of an energy tunnel employing capillary heat exchangers

• Evaluated the performance of tunnel lining CHEs under diverse conditions • Remains consistently high performance under various operating conditions • Offers valuable design and operational reference data for engineering applications The integration of capillary heat exchangers (CHEs) with tunnel lining to form an energy tunnel represents a viable and effective approach for the efficient utilization of shallow geothermal energy. Nonetheless, empirical field studies assessing the performance of tunnel lining CHEs are still scarce. To address this gap, a comprehensive testing system has been developed to evaluate the performance of tunnel lining CHEs under diverse conditions, encompassing variable heat flux, variable flow rates, periodically fluctuating inlet temperatures, and distinct installation positions. The results demonstrate that the heat transfer performance of the tunnel lining CHEs is highly efficient. At heating power levels of 200 W and 500 W, the stable temperatures of the CHE outlet water were 29.1°C and 36.8°C, respectively. The heat release rate of the CHEs increased progressively with the rise in flow rate. Specifically, at flow rates of 0.070, 0.085, and 0.100 m³/h, the average heat release rate was 61, 87, and 115 W/m², respectively. Over 7 periodic heating cycles (1505 minutes), the peak heat release rate decreased from 171 W/m² to 148 W/m², while the minimum heat release rate fell from 88 W/m² to 65 W/m². This research offers significant design guidelines for the engineering implementation of tunnel lining CHEs.