Design and Operational Strategies for Enhancing Thermal Output in Coaxial Closed-Loop Geothermal Systems

Abstract

Coaxial closed-loop geothermal systems, increasingly recognized as scalable and low-impact geothermal solutions, remain limited by conductive heat transfer between the reservoir and wellbore. This study investigates three strategies to enhance thermal output: (i) dynamic operation scheduling, (ii) substitution of conventional fluids with Organic Rankine Cycle (ORC) working fluids, and (iii) targeted conductive enhancements near the well. Using a CMG STARS simulation framework, system performance was evaluated over 1- to 20-year horizons, introducing a characteristic thermal recovery curve as a tool for analyzing long-term behavior. Results show that extended recovery durations raise outlet temperatures but with diminishing returns, identifying approximately 80% recovery as a practical optimization point. Fluids such as n-pentane and R245fa deliver substantially greater ORC-compatible heat than water, with thermo-siphoning observed under low-flow conditions. Conductive enhancement geometries, namely ring and fishbone configurations, exhibit distinct performance profiles, with rings outperforming fishbones due to larger injected volumes and greater advantage due to reservoir reach. One-year gains range from 4.5–9.4% for rings and 0.65–1.37% for fishbones, stabilizing at 3.7–7.8% and 0.55–1.18% after 20 years. These findings provide design and operational guidance for advancing coaxial closed-loop systems in low-carbon energy deployment.