On-sun performance and stability of graphene nanofluids in concentrating direct absorption solar collectors

• In a controlled environment, the nanofluid optical properties remained constant at temperatures up to 80 °C. • On-sun parabolic trough DASC efficiency reaches 74.3 ± 0.8 % with a 0.3 g/L graphene concentration. • Construction materials of the solar energy collection loop have a significant effect on the nanofluid stability. • Corrosion is the main mechanism affecting the pH and the optical absorption degradation of the nanofluid. The Performances of direct absorption solar collectors (DASC) are limited by the stability of the nanofluid’s optical properties. This study investigates the performance and long-term stability of a graphene-based nanofluid under real on-sun operating conditions using an experimental parabolic trough pilot. The on-sun experiments were complemented with a detailed off-sun experimental evaluation of the nanofluid stability with temperature. Experimental results show that while the nanofluid optical properties remained stable over long periods (over two and a half months) and across varying temperatures (up to 80 °C) in a controlled environment, exposure to actual operational conditions in a parabolic trough collector working in closed loop caused significant degradation of optical properties, particularly due to pH changes due to corrosion in the collector hydraulic circuit. Despite this, overall photo-thermal conversion efficiencies of 62.3 ± 0.6 % and 74.3 ± 0.8 % were achieved with graphene concentrations of 0.2 and 0.3 g/L respectively, a substantial improvement over the 24.5 ± 0.5 % achieved using demineralized water. These findings highlight the critical role of material compatibility in hydraulic systems with graphene/water nanofluids to minimize corrosion, maintain particle stability and preserve collector performance in practical applications.