Researchers from PROMES - CNRS, UPVD and LMAP - UPPA have demonstrated how graphene-based nanofluids can significantly enhance the performance of direct absorption solar collectors (DASC), while also revealing the complex stability challenges these fluids face under real operating conditions.
Using a parabolic trough pilot system, the team combined on-sun and off-sun experiments to evaluate both the optical and thermal behavior of graphene-water nanofluids. In controlled environments, the nanofluid maintained stable optical properties for over two and a half months, even when exposed to temperatures up to 80 °C. However, when used under real solar operating conditions in a closed-loop DASC, the nanofluid exhibited notable degradation in optical performance due to changes in pH linked to corrosion within the hydraulic circuit.
Despite this degradation, the researchers achieved photo-thermal conversion efficiencies of 62.3 ± 0.6 % and 74.3 ± 0.8 % with graphene concentrations of 0.2 g/L and 0.3 g/L, respectively - almost tripling the 24.5 ± 0.5 % efficiency recorded with demineralized water. These results place graphene-based DASCs at the upper limit of conventional solar thermal technologies, such as flat-plate, evacuated-tube, and small-scale parabolic trough collectors.
The study’s hybrid methodological framework - comparing long-term stability in both controlled and on-sun conditions - offers a path to isolating the factors that trigger nanofluid degradation. The findings underscore that material compatibility in the hydraulic system is critical to minimizing corrosion, preserving pH stability, and maintaining optical performance.
Future work will aim to refine system design and materials to ensure long-term stability at higher operating temperatures (150-200 °C). The researchers highlight that selecting non-corrosive materials such as brass and exploring low-emissivity coatings could further enhance nanofluid stability while reducing thermal losses. By bridging controlled laboratory analysis with real-world operational testing, this study sets the foundation for developing robust, graphene-based DASCs capable of reliable, large-scale solar energy harvesting.
