Energy conversion through deep borehole heat exchanger systems: Heat storage analysis and assessment of threshold inlet temperature

The heating capacity of deep borehole heat exchanger (DBHE) systems gradually attenuates due to the cold accumulation of rock and soil during long-term operation. Therefore, heat storage in rock and soil is crucial to alleviate thermal attenuation and ensure stable operation. However, the mechanism and design methods for heat storage are unclear. In this paper, heat extraction and storage models for DBHE systems are established, the effects of heat storage on system operation are revealed, and a design method for the threshold inlet water temperature of heat storage is proposed. The results show that the fluid in the DBHE should inflow from the inner pipe to release more heat into the rock and soil when adopting heat storage. The heat storage power under fluid inflow from the inner pipe is 10.81 kW higher than that from the annular space under the same operating conditions. Increasing the inlet water temperature is more beneficial than increasing the flow rate to enhance the heat storage power. After 2880 h of operation, the heating power of the DBHE is increased by 3.44% for the next heating season when the storage flow rate is increased from 8 m3/h to 38 m3/h. Thus, a high heat storage inlet temperature with a low flow rate should be adopted during the heat storage period. Furthermore, the threshold inlet water temperature of heat storage is highly dependent on the depth of the DBHE and the geothermal gradient, with percentage contributions of 57.14% and 39.26%, respectively. The multiple linear regression model (R2 = 0.972) has shown high predictive accuracy in the assessment of threshold inlet water temperatures, with a maximum relative error of -5.60%. The findings of this paper provide technical support for designing heat storage using DBHE systems.