Numerical Simulation Study on Heat Transfer Mechanism of Excavation Based Enhanced Geothermal System

Abstract

The innovative excavation based EGS (EGS-E) scheme uses mining techniques to form deep underground access for creating artificial heat reservoir with multi-level fracture networks using excavation, blasting, and caving. The two-level heat transfer mechanism of distributed artificial heat reservoir has a dominant influence on the performance of excavation based Enhanced Geothermal System(EGS-E). However, due to the large-scale field testing and laboratory physical experiment are not yet realistic, the process of natural convection heat transfer within the surrounding rock enhanced permeability and water filled zones hasn’t been effectively studied. The simulation software COMSOL Multiphysics was used to numerically simulate the natural convection heat transfer within the enhanced stimulated surrounding rock around a single straight roadway with its circular opening, which should be an elemental unit for mining heat in deep underground engineering structure. The simulation results demonstrated that the temperature difference inside the surrounding rock of a roadway causes the density of working fluid to vary. Driven by buoyancy, the natural convection of working fluid occurs in the surrounding rock. It enhances the heat transfer inside hot dry rock, and its efficiency is significantly more remarkable than that of the conduction through hot dry rock itself, which can improve the efficiency of heat recovery with regards to EGS-E. Due to the low solid heat conduction efficiency of the surrounding dry hot rock, adjusting the speed of the water flow inside the pipe can effectively control the temperature of the water outlet to improve the heat collection life of the system.