Impact of relative humidity on the optical properties of sol–gel processed black spinel oxide coatings for solar absorbers

This study investigates the influence of relative humidity on the sol–gel deposition process of a multilayer black spinel oxide coatings for solar selective absorbers. Changes in humidity conditions during deposition were found to affect porosity, thickness, and optical properties, ultimately influencing the performance and reproducibility of the coatings. Under optimized conditions, we achieved a solar absorptance of 0.95 and a thermal emissivity of 0.16 at 100 °C. When applied to solar tubular receivers, the coatings maintained high performance with 0.95 absorptance and 0.15 emissivity, demonstrating uniformity along the tube. In addition, the aging of the solution and its sensitivity to ambient humidity were also taken into account. The solution remained effective for one week, meeting industrial durability requirements. In general, optimizing relative humidity enables the scalable, efficient, and cost-effective production of coatings for low- to mid-temperature solar applications.