Hybrid Al₂O₃-MWCNT nano-coatings for advanced solar thermal systems: enhancing energy conversion and sustainability

Evacuated tube collectors (ETCs) are advanced solar thermal systems designed to capture and retain solar energy for heating, offering reliable performance even in low-temperature environments. This study is motivated to improve the thermal performance and efficiency of evacuated tube solar collectors by developing a durable, high-absorptivity coating. Conventional coatings often suffer from heat loss and limited energy conversion. By introducing a hybrid Al₂O₃–MWCNT nano-enhanced coating, this study aims to enhance solar absorption, thermal conductivity, and overall energy utilization, contributing to more efficient and sustainable solar energy systems. The hybrid nano-enhanced coatings were applied using the spray pyrolysis technique at 300 °C, with two thickness variations: 1 mm and 2 mm. The results reveal that the 2 mm hybrid Al₂O₃/MWCNT coating significantly enhances ETC performance. The maximum fluid temperature reached 93.6 °C, with a heat absorption of 623.1 W. The heat transfer coefficient improved to 561.9 W/m²K, achieving a thermal efficiency of 82.7 %. Exergy efficiency increased to 20.7 %, indicating better energy utilization. The enviro-economic analysis demonstrated an energy output of 548.1 kWh, CO₂ savings of 5.12 kWh/$, and an annual CO₂ reduction cost of $64.71. These findings highlight the potential of hybrid nano-coatings in optimizing solar thermal systems for sustainable energy solutions.