Investigating scalability of deep borehole heat exchangers: Numerical modelling of arrays with varied modes of operation

Abstract

Deep Borehole Heat Exchangers (DBHEs) are a potentially important method of developing geothermal resources through closed-loop systems for carbon neutral, spatial heating. Past research has primarily focused on single-well systems, with few investigating arrays of multiple DBHEs as a method of extracting more thermal energy. In this study, a series of arrays were modelled using OpenGeoSys software, with the aim of understanding the influence of array geometry, inter-borehole spacing and the mode of operation on the thermal performance and system efficiency. OpenGeoSys software is a finite-element model which solves thermal fluxes through the wellbore and surrounding rock using the dual-continuum method. Simulations were undertaken for the lifetime of an array (20 years) with modes of operation testing 1) long-term constant heat load application and 2) intermittent operation with 6 months of extraction followed by a recovery period. Results indicate geometry and mode of operation had a significant impact on inter-borehole spacing and system performance. For long term constant heat load application of 50 kW per DBHE, the minimal spacing required for line and square arrays should be 40 and 30 m. When considering intermittent operation, recovery periods allow replenishment of heat around the borehole, meaning smaller spacing can be utilised.