SUSTAINABILITY OF PRODUCTION FROM BOREHOLE HEAT EXCHANGER FIELDS

摘要

Sustainability and renewability aspects of a group of borehole heat exchangers (BHE) are treated by numerical model simulations. The software FRACTure (Kohl and Hopkirk, 1995) is used as the modeling tool which implies transient, coupled heat and mass transfer, rock mechanics and rock/water interaction in 3D. The tool is equipped with a semiautomatic mesh generator. The long-term thermal behavior (heat extraction / recovery) of an array of six 100 m deep BHEs has been simulated over 100 years. The distance between the BHEs is 7.5 m. The model simulations are based on load profiles with monthly differing heating demands, with a total of 1800 h/a heat pump runtime. The results (ground temperatures and BHE delivery temperatures) have been compared to a single BHE of the same length. Single and multiple BHEs show the same cooling and recovery characteristics: the cooling is strong at the beginning and slows later down asymptotically; also the recovery is strong in the beginning and with time it levels off. The BHE array spacing is a critical parameter; the minimum distance shall not fall short of 8 m to provide sustainable production. In a BHE array the recovery time is longer than for a single BHE; the lower temperatures of the produced fluid can be compensated for by additional drilling meters. Numerical values are given for the latter. Sustainable production from a BHE field can be achieved by proper design.