Tailored Modelling Approaches to Assess the Feasibility of Closed-loop Systems

The use of the phrase ‘Geothermal Everywhere’ has grown as deep closed-loop systems have emerged. These include Advanced Geothermal Systems, defined by IRENA as “deep, large, artificial closed-loop circuits in which a working fluid is circulated and heated by sub-surface rocks through conductive heat transfer”, and Deep Borehole Heat Exchangers, which employ coaxial pipe configurations typically. Closed-loop systems are not only for heat extraction applications, but can provide effective heat storage. Their use in connection with geological facilities for nuclear waste disposal has also been explored. Through a range of recent case studies, this paper demonstrates the key role of tailored modelling frameworks and approaches in assessing the pre-feasibility of closed-loop projects by identifying key impacts on overall system performance and estimating its associated uncertainty. Depending on the geological settings, system engineering design, and intended use of the energy, the paper offers examples of how analytical and numerical models can assist with estimating - for example - the radius of the thermal plume around closed-loop sections, the decay heat released from nuclear waste canisters and interceptable by closed-loop systems, the effect of ground water flow, how much and how quickly thermal energy can be discharged from/charged to the system, and the long-term sustainability of the energy extraction or storage. The paper also offers example of when potentially complex and time-consuming numerical simulation can be replaced by simpler and more transparent analytical models and nomograms to support project developers and investors with quick look analyses.