Numerical Analysis of Enhanced Conductive Deep Borehole Heat Exchangers

Abstract

Geothermal energy is a reliable and mature energy source, but it represents less than 1% of the total renewable energy mix. While the enhanced geothermal system (EGS) concept faces technical validation challenges and suffers from public acceptance issues, the development of unconventional deep-well designs can help to improve their efficiency and reliability. Modelling single-EGS-well designs is key to assessing their long-term thermal performances, particularly in unconventional geological settings. Numerical results obtained with the T2WELL/EOS1 code have been validated with available experimental data from a deep borehole heat exchanger (DBHE), where a temperature of 358 ∘C has been measured at a depth of 1962 m. Based on a calibrated model, the thermal performances of two enhanced thermal conductive DBHEs with graphite were compared for high geothermal gradients. The analysis highlights the potential recovery of a variable fraction of vapour. Graphite used along the well appears to be the most suitable solution to enhance the thermal output by 5 to 8% when compared to conventional wells. The theoretical implementation of such well in the Newberry volcano field was investigated with a single and doublet DBHE. The findings provide a robust methodology to assess alternative engineering solutions to current geothermal practices.

Repurposing a deep geothermal exploration well for borehole thermal energy storage: Implications from statistical modelling and sensitivity analysis

“…Recent work has evaluated the potential of deep borehole heat exchangers (DBHE) for extraction only (e.g., [21,5,20,26,31,46,24,37,48,46]), whilst few have evaluated BTES for depths in excess of 1000 m. DBHEs operate by circulating fluid in the subsurface within a closed-circuit system. Typically, for coaxial DBHEs, a concentric tube is inserted within the borehole.…”

Investigating Reynolds number effects in turbulent concentric coaxial pipe flow using stochastic one‐dimensional turbulence modeling

“…This enhances the transfer to the wall while retaining a high mass flow rate with a weak additional pressure drop along the pipe. This is exploited, for example, in borehole heat exchangers [2] and electrostatic precipitators used in gas cleaning [3]. In contrast to the flow through a plane channel or circular pipe, annular pipe flow exhibits a more complex geometry that encompasses additional internal curvature imposed as boundary condition to the flow.…”

“…This enhances the transfer to the wall while retaining a high mass flow rate with a weak additional pressure drop along the pipe. This is exploited, for example, in borehole heat exchangers [2] and electrostatic precipitators used in gas cleaning [3].…”