Experimental enhancement of a double-slope solar still using external reflectors and nano-coated absorber plates for solar desalination

Freshwater scarcity, particularly in arid and off-grid regions, demands efficient and low-cost desalination technologies. While solar stills offer a sustainable solution, their low productivity remains a key limitation. Studies on the combined effect of external reflectors and nanoparticle-coated absorber plates in double-slope solar stills are scarce, and no such investigation has been conducted under the climatic conditions of Gondar, Ethiopia. This study addresses this gap by experimentally evaluating a double-slope solar still integrated with four externally mounted aluminum reflectors and interchangeable absorber plates coated with black paint, black paint + CuO nanoparticles (0.443 wt%), and black paint + Al 2 O 3 nanoparticles (0.443 wt%). Experiments were conducted in December 2023 and May 2024 at water depths of 2 cm, 2.5 cm, and 3 cm to optimize performance. A water depth of 2 cm consistently produced the highest distillate output across all configurations. The CuO-coated absorber plate with reflectors achieved the maximum productivity of 4.95 L day −1 (3.30 L m −2 day −1 ) with a thermal efficiency of 38.38%, substantially exceeding the typical efficiency range for conventional double-slope stills (25–34%). Reflectors alone increased productivity by 130.7% (1.37 to 3.16 L day −1 ), while the combined use of reflectors and CuO coating improved productivity by 261.3% (1.37 to 4.95 L day −1 ) and thermal efficiency by 27.10% points. The CuO coating outperformed Al 2 O 3 by 6.7% after normalization for irradiance variations. Theoretical predictions agreed closely with experimental results for the CuO configuration (difference < 0.5 L day −1 ), though the model over-predicted the black-paint case by 49%, highlighting the importance of accurate absorptance inputs. The proposed system offers a passive, low-cost solution for decentralized water production in off-grid communities, with demonstrated potential for significant productivity enhancement. Water quality testing is recommended to confirm compliance with drinking water standards in future studies.