Thermal performance of a deep borehole heat exchanger: Insights from a synthetic coupled heat and flow model

Earth heat exchangers are drawing increasing attention and popularity due to their efficiency, sustainability and universality. Compared to shallow geothermal energy, these systems operate at greater depths, which could offer higher temperatures and more return of investment, despite greater investment costs. However, the functioning and numerical modeling of deep borehole heat exchangers (DBHE), in contrast to those of shallow conventional systems, remain poorly known. In this work, the influence of subsurface physical parameters, DBHE materials and operating settings had been investigated, in order to assess their impacts on deep-system performance. To this end, a numerical model was built, accounting for flow and mass/heat transport in a homogenous porous media, at the vicinity of a vertical 5-km coaxial DBHE. Fluid outlet temperature, specific heat extraction rate as well as thermal affected zone were observed over 6-month and 25-year periods of operation. Sensitivity studies highlighted the importance of parameters related to DBHE design and operational settings, such as thermal conductivity of the inner pipe, discharge rate of fluid carrier. It also emphasized the role of groundwater velocity as a catalyst for subsurface thermal recovery. Over 25-year periods of operation, average differential temperature from 3.8 to 13.9 degrees C, associated with circulation rate from 150 to 600 m(3) d(-1), led to a potential installed capacity from approximately 125-600 kW. Therefore, this study demonstrated the basis of a feasible and sustainable thermal use of DBHE. (C) 2015 Elsevier Ltd. All rights reserved.