The long-term operation and thermal disturbance investigation on a four-borehole-heat-exchanger residential heating system

In this research, a residential heating system consisting of four squared-arranged DBHEs (deep borehole heat exchangers) is investigated to reveal its long-term performance and thermal disturbance on the surrounding rocks. A simulation model was developed and verified to specifically analyze the effect of layout spacings of DBHEs array. The simulation revealed that the system with smaller layout spacings performs worse in a long term. After a 15-year operation, the average outlet temperature rises within the heating season decays to 8.15 °C for the DBHEs array with a layout spacing of 20 m, 11.0 % lower than that of the DBHEs array with a layout spacing of 56 m. During a 15-year operation, the total heat extraction of the DBHEs array with a layout spacing of 20 m (9.55 GWh) is 7.1 % less than that of the array with a layout spacing of 56 m (10.28 GWh). The DBHEs array with smaller layout spacings disturbs the surrounding rocks more dramatically, accounting for more severe performance deterioration. For the DBHEs array with a layout spacing of 20 m, the temperature drop of the surrounding rock (5 m from the borehole) reaches 14.82 °C at the depth of 2400 m, 7.28 °C higher than that for the array with a layout spacing of 56 m. The unevenness in the temperature drop reaches 3.17 °C of surrounding rocks (5 m from the borehole) for the array with a layout spacing of 20 m, presenting stronger anisotropy compared with that for the array with a layout spacing of 56 m (0.36 °C). Additionally, the “dead zone” phenomenon within the interior region explains the performance deterioration and the unevenness of thermal disturbance. These results of the current work can offer references to the design and further investigation of ground source systems with different DBHEs arrays.