Nanofluids as Next-Generation Geothermal Working Fluids: Benchmarking Against Water-Based Systems, Phase Change Materials (PCMs), and Supercritical CO2 (SC-CO2)

Geothermal energy is a promising clean energy source because of its ample reserves and minimal carbon footprint. However, its effective extraction relies on the stimulated reservoir's size and the thermophysical properties of the working fluid. Mainly water and, in some cases, SC-CO 2 are used as a working fluid, but PCMs and nanofluids could be an alternative. Most existing reviews treat these candidate fluids separately, making it difficult to compare options using consistent criteria that account for thermal performance, hydraulic penalties, operational risks, and deployment feasibility. Therefore, this review evaluates thermophysical properties of geothermal fluids, including water/brine, SC-CO 2 , PCMs, and nanofluids, across various reservoir and wellbore configurations, using a framework that clarifies tradeoffs and readiness barriers to help identify the most suitable fluid for each geothermal application. Water/brines have high specific heat and conductivity but face corrosion and scaling problems. SC-CO 2 ,’s low viscosity and high expansivity boost heat extraction by about 50% in moderate-temperature reservoirs, but its low thermal conductivity can limit heat uptake and reduce delivered temperature in practice. PCMs store latent heat for thermal management, yet phase segregation and cycling degradation limit their use in geothermal systems. Nanofluids can boost thermal conductivity by up to 40%, improving heat transfer, but face high viscosity and pumping power costs. Nevertheless, with suitable nanoparticle type and concentration, nanofluids can greatly improve heat extraction, showing promise as next-generation geothermal fluids. Accordingly, this review discusses feasible solutions to key challenges of nanofluids and identifies research gaps and future directions to support their ongoing development.