Modelling the Change of Heat Exchange Rate of Borehole Heat Exchanger in Large GSHP System

Abstract

As a low-carbon technology of providing heating and cooling to buildings, utilizing shallow geothermal energy through Ground Source Heat Pump (GSHP) system is increasingly applied all over the world. A recent trend in the industry is to build large GSHP system targeting commercial buildings and small neighborhood. Such systems are typically constructed by connecting and linking hundreds of Borehole Heat Exchangers (BHE) through a pipe network. In such applications, the heat exchange rate of individual BHE is strongly dependent on the hydraulic and thermal processes in the pipe network. In this study, a numerical model has been further developed to simulate this coupling effect. Two existing open-source softwares, namely the finite element simulator OpenGeoSys, and the Thermal Engineering Systems in Python (Tespy), are linked together in this work to simulate large GSHP system equipped with hundreds of BHEs. The coupled model allows the inflow temperature of each BHE to be dynamically calculated based on the hydro-thermal flow process within the pipeline network. Compared to the super-imposition analytical approach, in which the heat extraction rate of each BHE is imposed as a fixed boundary condition, the newly developed model is more realistic and capable of predicting the dynamic behavior of system efficiency. Preliminary modelling results demonstrates that the heat extraction rate in the center of the BHE array will be lowered after a period of operation, and the thermal load will be shifted to the outer skirt of the reservoir. The heat exchange rates will be considerably different as soon as the cold/heat starts to accumulate in the system. The newly developed model provides a more accurate approach in predicting the subsurface temperature evolution, as well as the dynamics of large GSHP system performance.