Enhancing the performance of flat plate solar collector in forced circulation: the influence of rifle tube geometry and hybrid nanofluids

Extensive efforts have enhanced flat plate solar collectors through active and passive strategies to advance clean energy adoption aligned with SDG7. In this context, a compact domestic solar hot water system was developed using a rifled riser tube flat plate collector and CuO/La2O3 nanofluids under forced circulation. A 1 m² collector coupled with a 50 L day − 1 storage tank was experimentally evaluated using deionized water, CuO, La2O3 mono nanofluids, and an 80:20 CuO/La2O3 hybrid nanofluid at 0.5 wt% concentration and flow rates of 1, 2, and 3 LPM. At 3 LPM, maximum thermal efficiencies of 70.24%, 75.73%, 79.95%, and 81.96% were achieved for deionized water, La2O3, hybrid, and CuO nanofluids, respectively. The rifled tube produced heat transfer coefficients of 8836 W m − 2 K − 1 for the hybrid and 9027 W m − 2 K − 1 for CuO nanofluids. La2O3 improved colloidal stability, while CuO enhanced thermal conductivity, yielding synergistic performance. Compared with a plain tube collector using deionized water, average efficiency improved by 30.56% and 32.36% for hybrid and CuO nanofluids. Enhanced heat transfer increased exergy efficiency to 22.54% and 25.45% for hybrid and CuO nanofluids, respectively. The combined use of rifled tubes and hybrid nanofluid enabled a 24.86% reduction in collector area domestically.