Development and Application of Temperature Simulator for Borehole Heat Exchangers of Geothermal Heat Pump Systems

Abstract

To simulate geothermal heat transport associated with the vertical closed-loop geothermal heat pump (GHP) system, a numerical model named gModified TOUGHREACTh is developed and applied to sensitivity analysis and evaluation of performance of the GHP system in this study. It can simulate temperature changes in borehole heat exchangers with circulating water through the pipe as well as it computes groundwater flow and aquifer temperature changes. The proposed model is based on TOUGHREACT, a widely accepted three-dimensional numerical simulator for heat and water flow and geochemical reactions in geothermal systems. In order to take circulating pipe flow in the BHE and heat flux from the heat pump to the BHE into account, three modules are developed and added to TOUGHREACT. The proposed model is applied to analyze the sensitivities of design parameters that can affect the performance of the vertical closed-loop GHP and to evaluate performance of the GHP system installed at Korea Institute of Geoscience and Mineral (KIGAM). The most sensitive parameters on the system are the thermal conductivity of aquifer and the Darcian groundwater velocity considering acceptable distribution range in the realm of nature. Maximum change of the circulating fluid temperature at the outlet is about 4Ž when thermal conductivity of the aquifer changes from 2 W/m-K to 5 W/m-K and the Darcian groundwater velocity changes from 10-8 m/s to 10-6 m/s, respectively. Performance of field application at the KIGAM GHP site is under evaluation process by using the developed model.