Mitigation of long-term heat extraction attenuation of U-type medium-deep borehole heat exchanger by climate change

Abstract

U-type medium-deep borehole heat exchanger (U-MDBHE) is a sustainable building heating technology. Current studies assess the long-term thermal performance of U-MDBHE using typical meteorological year weather data. The conclusions indicate a discernible deterioration in the thermal performance of U-MDBHE attributed to heat extraction attenuation. The thermal performance deterioration leads to the oversize of U-MDBHE and hinders the widespread application of U-MDBHE. This study introduces a novel idea that the long-term thermal performance of U-MDBHE should be evaluated considering climate change (CC) and verifies that the favorable effects of CC on the thermal performance of U-MDBHE can effectively mitigate the adverse effect of heat extraction attenuation. The favorable effects of CC include reducing the heating demand (due to the reduced building heating load (BHL) caused by CC) and improving the heating supply capacity (due to the enhanced outlet temperature caused by CC). In addition, the reduced BHL under CC enhances the inlet temperature of U-MDBHE, thereby improving its operation safety. CC mitigates the heat extraction attenuation of U-MDBHE, with the strongest effect in the ascending well, followed by the descending well, and then the butted well. Case studies using experimentally validated simulations on the 30-year operation of U-MDBHE demonstrate that by mitigating the adverse effect of the heat extraction attenuation, CC reduces the accumulated energy consumption by 14.31%–26.59% and improves the operation safety by up to 100% in Harbin (severe cold region) and Beijing (cold region). This study significantly contributes to improving the long-term thermal performance of U-MDBHE.