Closed-loop geothermal systems: Critical review of technologies, performance enhancement, and emerging solutions

Closed-loop geothermal systems (CLGSs) represent an emerging technology in geothermal energy production that offers significant advantages over traditional enhanced geothermal systems (EGSs), including reduced environmental impact and broader geographical applicability. This comprehensive review analyzes the current state of CLGS technology, examining both coaxial and U-shaped system configurations while highlighting their key operational principles and advantages. We identify and discuss four major challenges facing CLGS development: (1) limited heat transfer efficiency due to conduction-dominated heat exchange and restricted wellbore-formation contact area, (2) thermal depletion in near-wellbore regions affecting long-term performance, (3) material durability issues under extreme temperature and pressure conditions, and (4) economic viability constraints particularly for power generation applications. Critical shortcomings that currently hinder commercialization include insufficient heat production rates for power generation, high initial capital costs, and uncertainties in long-term performance prediction. The review evaluates various innovative solutions to address these challenges, including near-wellbore thermal conductivity enhancement, utilization of convection in reservoirs, advanced wellbore designs with multiple laterals, and adoption of superior working fluids. We also examine the potential for repurposing abandoned oil and gas wells for geothermal energy production to reduce capital costs. The paper concludes by outlining future research needs, emphasizing the importance of developing enhanced heat transfer methods, improving wellbore materials, and establishing predictive models for long-term performance. While CLGSs currently show greater promise for direct heating applications, addressing these technological gaps and economic challenges could expand their viability for power generation in diverse geological settings.