Thermal extraction and recovery performance of a deep coaxial borehole heat exchanger under intermittent operation for building heating

Deep coaxial borehole heat exchangers (DCBHEs) exhibit superior thermal performance compared to conventional shallow systems, particularly in cold regions. This study investigates the intermittent operation of a DCBHE for building heating through an integrated approach of field testing and numerical simulations. A comprehensive monitoring system, including distributed optical fiber temperature sensors, thermal resistance sensors, and an ultrasonic flowmeter, was deployed to capture real-time fluid and formation thermal behaviors. Results demonstrate a thermal recovery rate of 96.96% during the first non-heating period. Fluid temperature profiles with depth follow exponential and linear trends during heating and non-heating periods, respectively. The average coefficient of system performance (COPsys) reached 6.14 under intermittent operation, representing a 21.34% improvement in energy efficiency compared to continuous operation. Prolonged operation adversely affects long-term thermal sustainability, while intermittent operation exhibits distinct impacts on thermal influence zones at shallow versus deep depths. Significant thermal loss was identified at the borehole bottom, underscoring the need for optimized operational strategies. This study provides practical insights for sustainable geothermal heating system design and operation.