Influence and economic analysis of heat storage in the non-heating season on the heat extraction capacity of mid-deep borehole heat exchangers

Abstract

This study investigates the effect of heat storage in the non-heating season on the heat extraction capacity and the economy of heat storage in a coaxial borehole mid-deep borehole heat exchanger (MBHE). A heat transfer model of the MBHE containing heat extraction and storage modes is presented and validated using experimental data reported in the literature; the results show that the established model has high accuracy. The effects of the operation time, annual heat storage time, inlet temperature of heat storage fluid, and heat storage fluid flow rate on the heat extraction capacity of the MBHE and the ground temperature are studied, and the economy of heat storage is analyzed. The results show that long-term heat storage can significantly improve the heat extraction capacity of the MBHE. Furthermore, heat storage can improve the heat extraction capacity of the MBHE only when the inlet temperature of heat storage fluid is greater than the critical heat storage temperature. The increase in the annual heat storage time, inlet temperature of the heat storage fluid, and heat storage fluid flow rate can improve the heat extraction capacity of the MBHE. The heat extraction capacity of the MBHE increases approximately linearly with the increase of the inlet temperature of the heat storage fluid; moreover, the MBHE heat extraction capacity increases with the increase in the annual heat storage time and the heat storage fluid flow rate, but the heat extraction capacity increase rate gradually decreases. The higher the inlet temperature of the heat storage fluid, the better the heat extraction capacity of the MBHE. The recommended annual heat storage time range and fluid flow rate ranges are 3–4 months and 4–6 kg/s, respectively.