On the practical use of carbon black nanofluids for solar thermal energy applications

This study explores the use of carbon black nanofluids in solar thermal energy systems, specifically focusing on their application in direct absorption solar collectors, where the working fluid directly absorbs incoming radiation. This work presents a novel combination of long-term stability testing, geometrical effects, and multi-method analysis (experimental, theoretical, and numerical) to evaluate the thermal performance of nanofluids. Uniquely, carbon black nanofluids stored under ambient conditions for two years were tested and compared with freshly prepared samples, a comparison not previously reported in the research literature. The influence of nanoparticle concentration, irradiation distance, and system geometry was investigated using two simplistic container configurations: a test tube and a volumetric flask. Results demonstrated that the optimal concentration depended on geometry and irradiation conditions. Stability tests showed minimal performance degradation over time, confirming their viability for long-term operation. A theoretical model and CFD simulations further supported the findings, offering insight into heat transfer mechanisms and the sensitivity of system performance to physical parameters. The results highlight the practical potential of low-concentration carbon black nanofluids in scalable solar thermal systems and underscore the importance of system design in maximizing efficiency.