Can deep borehole heat exchangers operate stably in long-term operation? Simulation analysis and design method

Abstract

Deep borehole heat exchanger (DBHE) extracts heat from medium-depth geothermal energy through heat transfer process, and its annual accumulative heat extraction directly influences the stability and safety in long-term operation. To calculate the maximum annual accumulative heat extraction ( Q a,max ) per DBHE rapidly and accurately at design stage, a method combining numerical simulation, modified finite linear heat source model (FLM), and analytical solutions is proposed in this paper. Based on the superposition feature of ground excess temperature, this method divides heat flow of DBHE to one constant value and one extra periodic value. The effect of constant heat flow on ground temperature is calculated by modified FLM rapidly, and the extra periodic effect in first year is calculated by numerical simulation. Then the actual ground temperature variation at any time can be rapidly calculated with the superposition of these two effects. After obtaining ground temperature, the water temperature distribution in DBHE is directly calculated by analytical solutions. Results show that Q a,max per DBHE with depth of 2500 m and normal conditions could reach about 3000 GJ per heating season. However, the specific value is significantly influenced by geothermal conditions, DBHE parameters and heating season duration. This paper then studies the effects of these key factors quantitatively and further raises a system design method for DBHE based on Q a,max , so as to guarantee the stability, safety, and feasibility in long-term operation. • Long-term operational performance of deep borehole heat exchanger (DBHE) is studied. • Maximum annual accumulative heat extraction ( Q a,max ) is key design parameter. • Q a,max is affected by geothermal conditions, DBHE parameters, heating season duration. • System design method based on Q a,max is raised to design the number of DBHEs.